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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ADDRESS SPECIFICATIONS | ADDRESS TYPES | ADDRESS OPTIONS | DATA VALUES | EXAMPLES | DIAGNOSTICS | FILES | SIGNALS | ENVIRONMENT VARIABLES | CREDITS | VERSION | BUGS | SEE ALSO | AUTHOR | COLOPHON |
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socat(1) socat(1)
socat - Multipurpose relay (SOcket CAT)
socat [options] <address> <address>
socat -V
socat -h[h[h]] | -?[?[?]]
filan
procan
Socat is a command line based utility that establishes two
bidirectional byte streams and transfers data between them.
Because the streams can be constructed from a large set of
different types of data sinks and sources (see address types), and
because lots of address options may be applied to the streams,
socat can be used for many different purposes.
Filan is a utility that prints information about its active file
descriptors to stdout. It has been written for debugging socat,
but might be useful for other purposes too. Use the -h option to
find more infos.
Procan is a utility that prints information about process
parameters to stdout. It has been written to better understand
some UNIX process properties and for debugging socat, but might be
useful for other purposes too.
The life cycle of a socat instance typically consists of four
phases.
In the init phase, the command line options are parsed and logging
is initialized.
During the open phase, socat opens the first address and
afterwards the second address. These steps are usually blocking;
thus, especially for complex address types like socks, connection
requests or authentication dialogs must be completed before the
next step is started.
In the transfer phase, socat watches both streams’ read and write
file descriptors via select() , and, when data is available on one
side and can be written to the other side, socat reads it,
performs newline character conversions if required, and writes the
data to the write file descriptor of the other stream, then
continues waiting for more data in both directions.
When one of the streams effectively reaches EOF, the closing phase
begins. Socat transfers the EOF condition to the other stream,
i.e. tries to shutdown only its write stream, giving it a chance
to terminate gracefully. For a defined time socat continues to
transfer data in the other direction, but then closes all
remaining channels and terminates.
Socat provides some command line options that modify the behaviour
of the program. They have nothing to do with so called address
options that are used as parts of address specifications.
-V Print version and available feature information to stdout,
and exit.
-h | -?
Print a help text to stdout describing command line options
and available address types, and exit.
-hh | -??
Like -h, plus a list of the short names of all available
address options. Some options are platform dependent, so
this output is helpful for checking the particular
implementation.
-hhh | -???
Like -hh, plus a list of all available address option
names.
-d Without this option, only fatal, error, and warning
messages are printed; applying this option also prints
notice messages. See DIAGNOSTICS for more information.
-d0 With this option, only fatal and error messages are
printed; this restores the behaviour of socat up to version
1.7.4.
-d -d | -dd | -d2
Prints fatal, error, warning, and notice messages.
-d -d -d | -ddd | -d3
Prints fatal, error, warning, notice, and info messages.
-d -d -d -d | -dddd | -d4
Prints fatal, error, warning, notice, info, and debug
messages.
-D Logs information about file descriptors before starting the
transfer phase.
--experimental
New features that are not well tested or are subject to
change in the future must be explicitly enabled using this
option.
-ly[<facility>]
Writes messages to syslog instead of stderr; severity as
defined with -d option. With optional <facility>, the
syslog type can be selected, default is "daemon". Third
party libraries might not obey this option.
-lf <logfile>
Writes messages to <logfile> [filename] instead of stderr.
Some third party libraries, in particular libwrap, might
not obey this option.
-ls Writes messages to stderr (this is the default). Some third
party libraries might not obey this option, in particular
libwrap appears to only log to syslog.
-lp<progname>
Overrides the program name printed in error messages and
used for constructing environment variable names.
-lu Extends the timestamp of error messages to microsecond
resolution. Does not work when logging to syslog.
-lm[<facility>]
Mixed log mode. During startup messages are printed to
stderr; when socat starts the transfer phase loop or daemon
mode (i.e. after opening all streams and before starting
data transfer, or, with listening sockets with fork option,
before the first accept call), it switches logging to
syslog. With optional <facility>, the syslog type can be
selected, default is "daemon".
-lh Adds hostname to log messages. Uses the value from
environment variable HOSTNAME or the value retrieved with
uname() if HOSTNAME is not set.
-v Writes the transferred data not only to their target
streams, but also to stderr. The output format is text with
some conversions for readability, and prefixed with "> " or
"< " indicating flow directions.
-x Writes the transferred data not only to their target
streams, but also to stderr. The output format is
hexadecimal, prefixed with "> " or "< " indicating flow
directions. Can be combined with -v .
-r <file>
Dumps the raw (binary) data flowing from left to right
address to the given file. The file name may contain
references to environment variables and $$ (pid), $PROGNAME
(see option option -lp), $TIMESTAMP (uses format
%Y%m%dT%H%M%S), and MICROS (microseconds of daytime). These
references have to be protected from shell expansion of
course.
-R <file>
Dumps the raw (binary) data flowing from right to left
address to the given file. See option -r for customization
of file name.
-b<size>
Sets the data transfer block <size> [size_t]. At most
<size> bytes are transferred per step. Default is 8192
bytes.
-s By default, socat terminates when an error occurred to
prevent the process from running when some option could not
be applied. With this option, socat is sloppy with errors
and tries to continue. Even with this option, socat will
exit on fatals, and will abort connection attempts when
security checks failed.
-S<signals-bitmap>
Changes the set of signals that are caught by socat just
for printing an log message. This catching is useful to get
the information about the signal into socats log, but
prevents core dump or other standard actions. The default
set of these signals is SIGHUP, SIGINT, SIGQUIT, SIGILL,
SIGABRT, SIGBUS, SIGFPE, SIGSEGV, and SIGTERM; replace this
set (0x89de on Linux) with a bitmap (e.g., SIGFPE has value
8 and its bit is 0x0080).
Note: Signals SIGHUP, SIGINT, SIGQUIT, SIGUSR1, SIGPIPE,
SIGALRM, SIGTERM, and SIGCHLD may be handled specially
anyway.
-t<timeout>
When one channel has reached EOF, the write part of the
other channel is shut down. Then, socat waits <timeout>
[timeval] seconds before terminating. Default is 0.5
seconds. This timeout only applies to addresses where write
and read part can be closed independently. When during the
timeout interval the read part gives EOF, socat terminates
without awaiting the timeout.
-T<timeout>
Total inactivity timeout: when socat is already in the
transfer loop and nothing has happened for <timeout>
[timeval] seconds (no data arrived, no interrupt
occurred...) then it terminates. Up to version 1.8.0.0 "0"
meant infinite"; since version 1.8.0.1 "0" means 0 and
values <0 mean infinite.
Useful with protocols like UDP that cannot transfer EOF.
-u Uses unidirectional mode. The first address is only used
for reading, and the second address is only used for
writing (example).
-U Uses unidirectional mode in reverse direction. The first
address is only used for writing, and the second address is
only used for reading.
-g During address option parsing, don’t check if the option is
considered useful in the given address environment. Use it
if you want to force, e.g., appliance of a socket option to
a serial device.
-L<lockfile>
If lockfile exists, exits with error. If lockfile does not
exist, creates it and continues, unlinks lockfile on exit.
-W<lockfile>
If lockfile exists, waits until it disappears. When
lockfile does not exist, creates it and continues, unlinks
lockfile on exit.
-4 Use IP version 4 in case the addresses do not implicitly or
explicitly specify a version. Since version 1.8.0.1 this is
the default.
-6 Use IP version 6 in case the addresses do not implicitly or
explicitly specify a version.
-0 Do not prefer a particular IP version; this lets passive
addresses (LISTEN, RECV, ...) serve both versions on some
platforms (not BSD).
--statistics
-S Logs transfer statistics (bytes and blocks counters for
both directions) before terminating socat.
See also signal USR1.
This feature is experimental and might change in future
versions.
With the address command line arguments, the user gives socat
instructions and the necessary information for establishing the
byte streams.
An address specification usually consists of an address type
keyword, zero or more required address parameters separated by ’:’
from the keyword and from each other, and zero or more address
options separated by ’,’.
The keyword specifies the address type (e.g., TCP4, OPEN, EXEC).
For some keywords there exist synonyms (’-’ for STDIO, TCP for
TCP4). Keywords are case insensitive. For a few special address
types, the keyword may be omitted: Address specifications starting
with a number are assumed to be FD (raw file descriptor)
addresses; if a ’/’ is found before the first ’:’ or ’,’, GOPEN
(generic file open) is assumed.
The required number and type of address parameters depend on the
address type. E.g., TCP4 requires a server specification (name or
address), and a port specification (number or service name).
Zero or more address options may be given with each address. They
influence the address in some ways. Options consist of an option
keyword or an option keyword and a value, separated by ’=’. Option
keywords are case insensitive. For filtering the options that are
useful with an address type, each option is member of one option
group. For each address type there is a set of option groups
allowed. Only options belonging to one of these address groups may
be used (except with option -g).
Address specifications following the above schema are also called
single address specifications. Two single addresses can be
combined with "!!" to form a dual type address for one channel.
Here, the first address is used by socat for reading data, and the
second address for writing data. There is no way to specify an
option only once for being applied to both single addresses.
Usually, addresses are opened in read/write mode. When an address
is part of a dual address specification, or when option -u or -U
is used, an address might be used only for reading or for writing.
Considering this is important with some address types.
With socat version 1.5.0 and higher, the lexical analysis tries to
handle quotes and parenthesis meaningfully and allows escaping of
special characters. If one of the characters ( { [ ’ is found,
the corresponding closing character - ) } ] ’ - is looked for;
they may also be nested. Within these constructs, socats special
characters and strings : , !! are not handled specially. All those
characters and strings can be escaped with \ or within ""
This section describes the available address types with their
keywords, parameters, and semantics.
CREATE:<filename>
Opens <filename> with creat() and uses the file descriptor
for writing. This is a write-only address because a file
opened with creat cannot be read from. See options -u and
-U, and dual addresses.
Flags like O_LARGEFILE cannot be applied. If you need them
use OPEN with options create,create.
<filename> must be a valid existing or not existing path.
If <filename> is a named pipe, creat() might block; if
<filename> refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options: mode, mode, user, group, unlink-early,
unlink-late, append
See also: OPEN, GOPEN
DCCP-CONNECT:<host>:<port> (DCCP:<host>:<port>)
Establishes a DCCP connect to the specified <host> [IP
address] and <port> [DCCP service] using IP version 4 or 6
depending on address specification, name resolution, or
option pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, connect-timeout, tos, dccp-set-ccid,
nonblock, sourceport, retry, readbytes
See also: DCCP4-CONNECT, DCCP6-CONNECT, DCCP-LISTEN,
TCP-CONNECT SCTP-CONNECT
DCCP4-CONNECT:<host>:<port> (DCCP4:<host>:<port>)
Like DCCP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,DCCP,CHILD,RETRY
DCCP6-CONNECT:<host>:<port> (DCCP6:<host>:<port>)
Like DCCP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,DCCP,CHILD,RETRY
DCCP-LISTEN:<port> (DCCP-L:<port>)
Listens on <port> [DCCP service] and accepts an DCCP
connection. The IP version is 4 or the one specified with
address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this
address usually blocks until a client connects.
Option groups:
FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,DCCP,RETRY
Useful options: fork, bind, range, max-children, backlog,
accept-timeout, dccp-set-sid, su, reuseaddr, retry
See also: DCCP4-LISTEN, DCCP6-LISTEN, TCP-LISTEN,
SCTP-LISTEN, DCCP-CONNECT
DCCP4-LISTEN:<port> (DCCP4-L:<port>)
Like DCCP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,DCCP,RETRY
DCCP6-LISTEN:<port> (DCCP6-L:<port>)
Like DCCP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,DCCP,RETRY
EXEC:<command-line>
Forks a sub process that establishes communication with its
parent process and invokes the specified program with
execvp() . <command-line> is a simple command with
arguments separated by single spaces. If the program name
contains a ’/’, the part after the last ’/’ is taken as
ARGV[0]. If the program name is a relative path, the
execvp() semantics for finding the program via $PATH apply.
After successful program start, socat writes data to stdin
of the process and reads from its stdout using a UNIX
domain socket generated by socketpair() per default.
(example)
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d,
nofork, socktype, pty, stderr, ctty, setsid, pipes, umask,
login, sigint, sigquit, netns
See also: SYSTEM,SHELL
FD:<fdnum>
Uses the file descriptor <fdnum>. It must already exist as
valid UN*X file descriptor.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: STDIO, STDIN, STDOUT, STDERR
GOPEN:<filename>
(Generic open) This address type tries to handle any file
system entry except directories usefully. <filename> may be
a relative or absolute path. If it already exists, its type
is checked. In case of a UNIX domain socket, socat
connects; if connecting fails, socat assumes a datagram
socket and uses sendto() calls. If the entry is not a
socket, socat opens it applying the O_APPEND flag. If it
does not exist, it is opened with flag O_CREAT as a regular
file (example).
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also: OPEN, CREATE, UNIX-CONNECT
IP-SENDTO:<host>:<protocol>
Opens a raw IP socket. Depending on host specification or
option pf, IP protocol version 4 or 6 is used. It uses
<protocol> to send packets to <host> [IP address] and
receives packets from host, ignores packets from other
hosts. Protocol 255 uses the raw socket with the IP header
being part of the data.
Option groups: FD,SOCKET,IP4,IP6
Useful options: pf, ttl
See also: IP4-SENDTO, IP6-SENDTO, IP-RECVFROM, IP-RECV,
UDP-SENDTO, UNIX-SENDTO
INTERFACE:<interface>
Communicates with a network connected on an interface using
raw packets including link level data. <interface> is the
name of the network interface. Currently only available on
Linux.
Option groups: FD,SOCKET
Useful options: pf, type
See also: ip-recv
IP4-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv4.
Option groups: FD,SOCKET,IP4
IP6-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv6.
Option groups: FD,SOCKET,IP6
IP-DATAGRAM:<address>:<protocol>
Sends outgoing data to the specified address which may in
particular be a broadcast or multicast address. Packets
arriving on the local socket are checked if their source
addresses match RANGE or TCPWRAP options. This address type
can for example be used for implementing symmetric or
asymmetric broadcast or multicast communications.
Option groups: FD, SOCKET, IP4, IP6, RANGE
Useful options: bind, range, tcpwrap, broadcast,
ip-multicast-loop, ip-multicast-ttl, ip-multicast-if,
ip-add-membership, ip-add-source-membership,
ipv6-join-group, ipv6-join-source-group, ttl, tos, pf
See also: IP4-DATAGRAM, IP6-DATAGRAM, IP-SENDTO,
IP-RECVFROM, IP-RECV, UDP-DATAGRAM
IP4-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv4. (example)
Option groups: FD,SOCKET,IP4,RANGE
IP6-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv6. Please note that
IPv6 does not know broadcasts.
Option groups: FD,SOCKET,IP6,RANGE
IP-RECVFROM:<protocol>
Opens a raw IP socket of <protocol>. Depending on option
pf, IP protocol version 4 or 6 is used. It receives one
packet from an unspecified peer and may send one or more
answer packets to that peer. This mode is particularly
useful with fork option where each arriving packet - from
arbitrary peers - is handled by its own sub process. This
allows a behaviour similar to typical UDP based servers
like ntpd or named.
Please note that the reply packets might be fetched as
incoming traffic when sender and receiver IP address are
identical because there is no port number to distinguish
the sockets.
This address works well with IP-SENDTO address peers (see
above). Protocol 255 uses the raw socket with the IP
header being part of the data.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: pf, fork, range, ttl, broadcast
See also: IP4-RECVFROM, IP6-RECVFROM, IP-SENDTO, IP-RECV,
UDP-RECVFROM, UNIX-RECVFROM
IP4-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv4.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
IP6-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv6.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
IP-RECV:<protocol>
Opens a raw IP socket of <protocol>. Depending on option
pf, IP protocol version 4 or 6 is used. It receives packets
from multiple unspecified peers and merges the data. No
replies are possible, this is a read-only address, see
options -u and -U, and dual addresses. It can be, e.g.,
addressed by socat IP-SENDTO address peers. Protocol 255
uses the raw socket with the IP header being part of the
data.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, range
See also: IP4-RECV, IP6-RECV, IP-SENDTO, IP-RECVFROM,
UDP-RECV, UNIX-RECV
IP4-RECV:<protocol>
Like IP-RECV, but always uses IPv4.
Option groups: FD,SOCKET,IP4,RANGE
IP6-RECV:<protocol>
Like IP-RECV, but always uses IPv6.
Option groups: FD,SOCKET,IP6,RANGE
OPEN:<filename>
Opens <filename> using the open() system call (example).
This operation fails on UNIX domain sockets.
Note: This address type is rarely useful in bidirectional
mode.
Option groups: FD,REG,NAMED,OPEN
Useful options: creat, excl, noatime, nofollow, append,
rdonly, wronly, lock, readbytes, ignoreeof
See also: CREATE, GOPEN, UNIX-CONNECT
OPENSSL:<host>:<port>
Tries to establish a SSL connection to <port> [TCP service]
on <host> [IP address] using TCP/IP version 4 or 6
depending on address specification, name resolution, or
option pf.
NOTE: Up to version 1.7.2.4 the server certificate was only
checked for validity against the system certificate store
or cafile or capath, but not for match with the server’s
name or its IP address. Since version 1.7.3.0 socat checks
the peer certificate for match with the <host> parameter or
the value of the openssl-commonname option. Socat tries to
match it against the certificates subject commonName, and
the certificates extension subjectAltName DNS names.
Wildcards in the certificate are supported.
Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY
Useful options: min-proto-version, cipher, verify,
commonname, cafile, capath, certificate, key, compress,
bind, pf, connect-timeout, sourceport, retry
See also: OPENSSL-LISTEN, TCP
OPENSSL-LISTEN:<port>
Listens on tcp <port> [TCP service]. The IP version is 4
or the one specified with pf. When a connection is
accepted, this address behaves as SSL server.
Note: You probably want to use the certificate option with
this address.
NOTE: The client certificate is only checked for validity
against cafile or capath, but not for match with the
client’s name or its IP address!
Option groups:
FD,SOCKET,IP4,IP6,TCP,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, min-proto-version, cipher, verify,
commonname, cafile, capath, certificate, key, compress,
fork, bind, range, tcpwrap, su, reuseaddr, retry
See also: OPENSSL, TCP-LISTEN
OPENSSL-DTLS-CLIENT:<host>:<port>
Tries to establish a DTLS connection to <port> [UDP
service] on <host> [IP address] using UDP/IP version 4 or 6
depending on address specification, name resolution, or
option pf.
Socat checks the peer certificates subjectAltName or
commonName against the addresses option openssl-commonname
or the host name. Wildcards in the certificate are
supported.
Use socat option -b to make datagrams small enough to fit
with overhead on the network. Use option -T to prevent
indefinite hanging when peer went down quietly.
Option groups: FD,SOCKET,IP4,IP6,OPENSSL,RETRY
Useful options: min-proto-version, cipher, verify,
commonname, cafile, capath, certificate, key, compress,
bind, pf, sourceport, retry, rcvtimeo
See also: OPENSSL-DTLS-SERVER, OPENSSL-CONNECT, UDP-CONNECT
OPENSSL-DTLS-SERVER:<port>
Listens on UDP <port> [UDP service]. The IP version is 4
or the one specified with pf. When a connection is
accepted, this address behaves as DTLS server.
Note: You probably want to use the certificate option with
this address.
NOTE: The client certificate is only checked for validity
against cafile or capath, but not for match with the
client’s name or its IP address! Use socat option -b to
make datagrams small enough to fit with overhead on the
network. Use option -T to prevent indefinite hanging when
peer went down quietly.
Option groups:
FD,SOCKET,IP4,IP6,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, min-proto-version, cipher, verify,
commonname, cafile, capath, certificate, key, compress,
fork, bind, range, tcpwrap, su, reuseaddr, retry
rcvtimeo
See also: OPENSSL-DTLS-CLIENT, OPENSSL-LISTEN, UDP-LISTEN
PIPE:<filename>
If <filename> already exists, it is opened. If it does not
exist, a named pipe is created and opened. Beginning with
socat version 1.4.3, the named pipe is removed when the
address is closed (but see option unlink-close
Note: When a pipe is used for both reading and writing, it
works as echo service.
Note: When a pipe is used for both reading and writing, and
socat tries to write more bytes than the pipe can buffer
(Linux 2.4: 2048 bytes), socat might block. Consider using
socat option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode,
unlink-early
See also: unnamed pipe
PIPE Creates an unnamed pipe and uses it for reading and
writing. It works as an echo, because everything written to
it appears immediately as read data.
Note: When socat tries to write more bytes than the pipe
can queue (Linux 2.4: 2048 bytes), socat might block.
Consider, e.g., using option -b 2048
Option groups: FD
See also: named pipe, SOCKETPAIR
SOCKETPAIR
Creates a socketpair and uses it for reading and writing.
It works as an echo, because everything written to it
appears immediately as read data. The default socket type
is datagram, so it keeps packet boundaries.
Option groups: FD
Useful options: socktype
See also: unnamed pipe
POSIXMQ-READ:/<mqueue>
Opens or creates the specified POSIX message queue and
reads messages (packets). It keeps the packet boundaries.
This is a read-only address, see options -u and -U and dual
addresses.
Socat provides this address type only on Linux because
POSIX MQ is based on UNIX filedescriptors there.
Useful options: posixmq-priority, unlink-early,
unlink-close, o-nonblock, o-creat, o-excl, umask
POSIXMQ-RECEIVE:/<mqueue>
POSIXMQ-RECV:/<mqueue>
Opens or creates the specified POSIX message queue and
reads one message (packet).
This is a read-only address. See POSIXMQ-READ for more
info.
Example: POSIX MQ recv with fork
Useful options: posixmq-priority, fork, max-children,
unlink-early, unlink-close, o-creat, o-excl, umask
POSIXMQ-SEND:/<mqueue>
Opens or creates the specified POSIX message queue and
writes messages (packets).
This is a write-only address. See POSIXMQ-READ for more
info.
(Example)
Useful options: posixmq-priority, posixmq-flush, fork,
max-children, unlink-early, unlink-close, o-creat, o-excl,
umask
POSIXMQ-WRITE:/<mqueue>
Does the same as POSIXMQ-SEND.
POSIXMQ-BIDIRECTIONAL:/mqueue
POSIXMQ:/mqueue
Opens or creates the specified POSIX message queue in read
and/or write mode depending on context, then reads and/or
writes messages (packets). In bidirectional mode this is
just another echo service.
See POSIXMQ-READ and POSIXMQ-SEND for more info.
PROXY:<proxy>:<hostname>:<port>
Connects to an HTTP proxy server on port 8080 using TCP/IP
version 4 or 6 depending on address specification, name
resolution, or option pf, and sends a CONNECT request for
hostname:port. If the proxy grants access and succeeds to
connect to the target, data transfer between socat and the
target can start (example). Note that the traffic need not
be HTTP but can be an arbitrary protocol.
Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY
Useful options: proxyport, ignorecr, proxyauth, resolve,
crnl, bind, connect-timeout, mss, sourceport, retry
See also: SOCKS, TCP
PTY Generates a pseudo terminal (pty) and uses its master side.
Another process may open the pty’s slave side using it like
a serial line or terminal. (example). If both the ptmx and
the openpty mechanisms are available, ptmx is used (POSIX).
Option groups: FD,NAMED,PTY,TERMIOS
Useful options: link, openpty, wait-slave, mode, user,
group
See also: UNIX-LISTEN, PIPE, EXEC, SYSTEM, SHELL
READLINE
Uses GNU readline and history on stdio to allow editing and
reusing input lines (example). This requires the GNU
readline and history libraries. Note that stdio should be a
(pseudo) terminal device, otherwise readline does not seem
to work.
Option groups: FD,READLINE,TERMIOS
Useful options: history, noecho
See also: STDIO
SCTP-CONNECT:<host>:<port>
Establishes an SCTP stream connection to the specified
<host> [IP address] and <port> [TCP service] using IP
version 4 or 6 depending on address specification, name
resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, pf, connect-timeout, tos,
mtudiscover, sctp-maxseg, sctp-nodelay, nonblock,
sourceport, retry, readbytes
See also: SCTP4-CONNECT, SCTP6-CONNECT, SCTP-LISTEN,
TCP-CONNECT
SCTP4-CONNECT:<host>:<port>
Like SCTP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,SCTP,CHILD,RETRY
SCTP6-CONNECT:<host>:<port>
Like SCTP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,SCTP,CHILD,RETRY
SCTP-LISTEN:<port>
Listens on <port> [TCP service] and accepts an SCTP
connection. The IP version is 4 or the one specified with
address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this
address usually blocks until a client connects.
Option groups:
FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,SCTP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf,
max-children, backlog, accept-timeout, sctp-maxseg,
sctp-nodelay, su, reuseaddr, retry
See also: SCTP4-LISTEN, SCTP6-LISTEN, TCP-LISTEN,
SCTP-CONNECT
SCTP4-LISTEN:<port>
Like SCTP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,SCTP,RETRY
SCTP6-LISTEN:<port>
Like SCTP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,SCTP,RETRY
SOCKET-CONNECT:<domain>:<protocol>:<remote-address>
Creates a stream socket using the first and second given
socket parameters and SOCK_STREAM (see man socket(2)) and
connects to the remote-address. The two socket parameters
have to be specified by int numbers. Consult your OS
documentation and include files to find the appropriate
values. The remote-address must be the data representation
of a sockaddr structure without sa_family and (BSD) sa_len
components.
Please note that you can - beyond the options of the
specified groups - also use options of higher level
protocols when you apply socat option -g.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, setsockopt,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN,
SOCKET-SENDTO
SOCKET-DATAGRAM:<domain>:<type>:<protocol>:<remote-address>
Creates a datagram socket using the first three given
socket parameters (see man socket(2)) and sends outgoing
data to the remote-address. The three socket parameters
have to be specified by int numbers. Consult your OS
documentation and include files to find the appropriate
values. The remote-address must be the data representation
of a sockaddr structure without sa_family and (BSD) sa_len
components.
Please note that you can - beyond the options of the
specified groups - also use options of higher level
protocols when you apply socat option -g.
Option groups: FD,SOCKET,RANGE
Useful options: bind, range, setsockopt,
See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO,
SOCKET-RECV, SOCKET-RECVFROM
SOCKET-LISTEN:<domain>:<protocol>:<local-address>
Creates a stream socket using the first and second given
socket parameters and SOCK_STREAM (see man socket(2)) and
waits for incoming connections on local-address. The two
socket parameters have to be specified by int numbers.
Consult your OS documentation and include files to find the
appropriate values. The local-address must be the data
representation of a sockaddr structure without sa_family
and (BSD) sa_len components.
Please note that you can - beyond the options of the
specified groups - also use options of higher level
protocols when you apply socat option -g.
Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY
Useful options: setsockopt, setsockopt-listen,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN,
SOCKET-SENDTO, SOCKET-SENDTO
SOCKET-RECV:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters
(see man socket(2)) and binds it to <local-address>.
Receives arriving data. The three parameters have to be
specified by int numbers. Consult your OS documentation and
include files to find the appropriate values. The
local-address must be the data representation of a sockaddr
structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET,RANGE
Useful options: range, setsockopt, setsockopt-listen
See also: UDP-RECV, IP-RECV, UNIX-RECV, SOCKET-DATAGRAM,
SOCKET-SENDTO, SOCKET-RECVFROM
SOCKET-RECVFROM:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters
(see man socket(2)) and binds it to <local-address>.
Receives arriving data and sends replies back to the
sender. The first three parameters have to be specified as
int numbers. Consult your OS documentation and include
files to find the appropriate values. The local-address
must be the data representation of a sockaddr structure
without sa_family and (BSD) sa_len components.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,CHILD,RANGE
Useful options: fork, range, setsockopt, setsockopt-listen
See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM,
SOCKET-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV
SOCKET-SENDTO:<domain>:<type>:<protocol>:<remote-address>
Creates a socket using the three given socket parameters
(see man socket(2)). Sends outgoing data to the given
address and receives replies. The three parameters have to
be specified as int numbers. Consult your OS documentation
and include files to find the appropriate values. The
remote-address must be the data representation of a
sockaddr structure without sa_family and (BSD) sa_len
components.
Option groups: FD,SOCKET
Useful options: bind, setsockopt, setsockopt-listen
See also: UDP-SENDTO, IP-SENDTO, UNIX-SENDTO,
SOCKET-DATAGRAM, SOCKET-RECV SOCKET-RECVFROM
ACCEPT-FD:<fdnum>
Expects a listening socket in <fdnum> and accepts one or
(with option fork) more connections. This address type is
useful under systemd control with "inetd mode".
Example: (example)
Option groups: FD, SOCKET, TCP, CHILD, RETRY
Useful options: fork, range, sourceport, lowport, tcpwrap
SOCKS4:<socks-server>:<host>:<port>
Connects via <socks-server> [IP address] to <host> [IPv4
address] on <port> [TCP service], using socks version 4
protocol over IP version 4 or 6 depending on address
specification, name resolution, or option pf (example).
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
Useful options: socksuser, socksport, sourceport, pf, retry
See also: SOCKS5, SOCKS4A, PROXY, TCP
SOCKS4A:<socks-server>:<host>:<port>
like SOCKS4, but uses socks protocol version 4a, thus
leaving host name resolution to the socks server.
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
SOCKS5-CONNECT:<socks-server>:<socks-port>:<target-host>:<target-port>
SOCKS5-CONNECT:<socks-server>:<target-host>:<target-port>
Connects via <socks-server> [IP address] to <target-host>
[IPv4 address] on <target-port> [TCP service], using socks
version 5 protocol over TCP. Currently no authentication
mechanism is provided.
Option groups: FD, SOCKET, IP4, IP6, TCP, CHILD, RETRY
Useful options: socksport, sourceport, pf, retry
See also: SOCKS5-LISTEN, SOCKS4, SOCKS4A, PROXY, TCP
SOCKS5-LISTEN:<socks-server>:<socks-port>:<listen-host>:<listen-port>
SOCKS5-LISTEN:<socks-server>:<listen-host>:<listen-port>
Connects to <socks-server> [IP address] using socks version
5 protocol over TCP and makes it listen for incoming
connections on <listen-port> [TCP service], binding to
<-listen-host> [IPv4 address] Currently not authentication
mechanism is provided.
Option groups: FD, SOCKET, IP4, IP6, TCP, CHILD, RETRY
Useful options: sourceport, pf, retry
See also: SOCKS5-CONNECT,
STDERR Uses file descriptor 2.
This is a write-only address, see options -u and -U, and
dual addresses.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
STDIN Uses file descriptor 0.
This is a read-only address, see options -u and -U, and
dual addresses.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
STDIO Uses file descriptor 0 for reading, and 1 for writing.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
STDOUT Uses file descriptor 1.
This is a write-only address, see options -u and -U, and
dual addresses.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
SHELL:<shell-command>
Forks a sub process that establishes communication with its
parent process and invokes the specified program with the
configured shell ($SHELL). Note that <shell-command>
[string] must not contain ’,’ or "!!", and that shell meta
characters may have to be protected. After successful
program start, socat writes data to stdin of the process
and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d,
nofork, socktype, pty, stderr, ctty, setsid, pipes, umask,
sigint, sigquit
See also: EXEC, SYSTEM
SYSTEM:<shell-command>
Forks a sub process that establishes communication with its
parent process and invokes the specified program with
system() . Please note that <shell-command> [string] must
not contain ’,’ or "!!", and that shell meta characters may
have to be protected. After successful program start,
socat writes data to stdin of the process and reads from
its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d,
nofork, socktype, pty, stderr, ctty, setsid, pipes, umask,
sigint, sigquit, netns
See also: EXEC, SHELL
TCP:<host>:<port>
Connects to <port> [TCP service] on <host> [IP address]
using TCP/IP version 4 or 6 depending on address
specification, name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,TCP,RETRY
Useful options: connect-timeout, retry, sourceport, netns,
crnl, bind, pf, tos, mtudiscover, mss, nodelay, nonblock,
readbytes
See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT,
UNIX-CONNECT
TCP4:<host>:<port>
Like TCP, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,IP4,TCP,RETRY
TCP6:<host>:<port>
Like TCP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,TCP,RETRY
TCP-LISTEN:<port>
Listens on <port> [TCP service] and accepts a TCP/IP
connection. The IP version is 4 or the one specified with
address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this
address usually blocks until a client connects.
Option groups:
FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,TCP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf,
max-children, backlog, accept-timeout, mss, su, reuseaddr,
retry, cool-write
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN,
SCTP-LISTEN, UNIX-LISTEN, OPENSSL-LISTEN, TCP-CONNECT
TCP4-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY
TCP6-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv6 protocol.
Additional useful option: ipv6only
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY
TUN[:<if-addr>/<bits>]
Creates a Linux TUN/TAP device and optionally assigns it
the address and netmask given by the parameters. The
resulting network interface is almost ready for use by
other processes; socat serves its "wire side". This address
requires read and write access to the tunnel cloning
device, usually /dev/net/tun , as well as permission to set
some ioctl()s. Option iff-up is required to immediately
activate the interface!
Note: If you intend to transfer packets between two Socat
"wire sides" you need a protocol that keeps packet
boundaries, e.g.UDP; TCP might work with option nodelay.
Option groups: FD,NAMED,OPEN,TUN
Useful options: iff-up, tun-device, tun-name, tun-type,
iff-no-pi, netns
See also: ip-recv
UDP:<host>:<port>
Connects to <port> [UDP service] on <host> [IP address]
using UDP/IP version 4 or 6 depending on address
specification, name resolution, or option pf.
Please note that, due to UDP protocol properties, no real
connection is established; data has to be sent for
`connecting’ to the server, and no end-of-file condition
can be transported.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4, UDP6, UDP-LISTEN, TCP, IP
UDP4:<host>:<port>
Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
UDP6:<host>:<port>
Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
UDP-DATAGRAM:<address>:<port>
Sends outgoing data to the specified address which may in
particular be a broadcast or multicast address. Packets
arriving on the local socket are checked for the correct
remote port only when option sourceport is used (this is a
change with Socat version 1.7.4.0) and if their source
addresses match RANGE or TCPWRAP options. This address type
can for example be used for implementing symmetric or
asymmetric broadcast or multicast communications.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: bind, range, tcpwrap, broadcast,
ip-multicast-loop, ip-multicast-ttl, ip-multicast-if,
ip-add-membership, ip-add-source-membership,
ipv6-join-group, ipv6-join-source-group, ttl, tos,
sourceport, pf
See also: UDP4-DATAGRAM, UDP6-DATAGRAM, UDP-SENDTO,
UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LISTEN,
IP-DATAGRAM
UDP4-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv4 protocol
(example1, example2).
Option groups: FD,SOCKET,IP4, RANGE
UDP6-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
UDP-LISTEN:<port>
Waits for a UDP/IP packet arriving on <port> [UDP service]
and `connects’ back to sender. The accepted IP version is
4 or the one specified with option pf. Please note that,
due to UDP protocol properties, no real connection is
established; data has to arrive from the peer first, and no
end-of-file condition can be transported. Note that opening
this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6
Useful options: fork, bind, range, pf
See also: UDP, UDP4-LISTEN, UDP6-LISTEN, TCP-LISTEN
UDP4-LISTEN:<port>
Like UDP-LISTEN, but only support IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4
UDP6-LISTEN:<port>
Like UDP-LISTEN, but only support IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6
UDP-SENDTO:<host>:<port>
Communicates with the specified peer socket, defined by
<port> [UDP service] on <host> [IP address], using UDP/IP
version 4 or 6 depending on address specification, name
resolution, or option pf. It sends packets to and receives
packets from that peer socket only. This address
effectively implements a datagram client. It works well
with socat UDP-RECVFROM and UDP-RECV address peers. When
the peer might send data first, UDP-DATAGRAM is preferable.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4-SENDTO, UDP6-SENDTO, UDP-RECVFROM, UDP-RECV,
UDP-CONNECT, UDP-LISTEN, IP-SENDTO
UDP4-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
UDP6-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
UDP-RECVFROM:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP
version 4 or 6 depending on option pf. It receives one
packet from an unspecified peer and may send one or more
answer packets to that peer. This mode is particularly
useful with fork option where each arriving packet - from
arbitrary peers - is handled by its own sub process. This
allows a behaviour similar to typical UDP based servers
like ntpd or named. This address works well with socat
UDP-SENDTO address peers.
Note: When the second address fails before entering the
transfer loop the packet is dropped. Use option retry or
forever on the second address to avoid data loss. When you
know the peer address, UDP-DATAGRAM is preferable.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: fork, ttl, tos, bind, sourceport, pf
See also: UDP4-RECVFROM, UDP6-RECVFROM, UDP-SENDTO,
UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-RECVFROM,
UNIX-RECVFROM
UDP4-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
UDP6-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
UDP-RECV:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP
version 4 or 6 depending on option pf. It receives packets
from multiple unspecified peers and merges the data. No
replies are possible. It works well with, e.g., socat
UDP-SENDTO address peers; it behaves similar to a syslog
server.
This is a read-only address, see options -u and -U, and
dual addresses.
Note: if you need the fork option, use UDP-RECVFROM in
unidirectional mode (with option -u) instead.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, bind, sourceport, ttl, tos
See also: UDP4-RECV, UDP6-RECV, UDP-SENDTO, UDP-RECVFROM,
UDP-CONNECT, UDP-LISTEN, IP-RECV, UNIX-RECV
UDP4-RECV:<port>
Like UDP-RECV, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,RANGE
UDP6-RECV:<port>
Like UDP-RECV, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
UDPLITE-CONNECT:<host>:<port>
UDPLITE4-CONNECT:<host>:<port>
UDPLITE6-CONNECT:<host>:<port>
UDPLITE-DATAGRAM:<address>:<port>
UDPLITE4-DATAGRAM:<address>:<port>
UDPLITE6-DATAGRAM:<address>:<port>
UDPLITE-LISTEN:<port>
UDPLITE4-LISTEN:<port>
UDPLITE6-LISTEN:<port>
UDPLITE-SENDTO:<host>:<port>
UDPLITE4-SENDTO:<host>:<port>
UDPLITE6-SENDTO:<host>:<port>
UDPLITE-RECVFROM:<port>
UDPLITE4-RECVFROM:<port>
UDPLITE6-RECVFROM:<port>
UDPLITE-RECV:<port>
UDPLITE4-RECV:<port>
UDPLITE6-RECV:<port>
The UDPLITE addresses are almost identical to the related
UDP addresses but they use UDP-Lite protocol and have the
additional UDPLITE option group.
UNIX-CONNECT:<filename>
Connects to <filename> assuming it is a UNIX domain socket.
If <filename> does not exist, this is an error; if
<filename> is not a UNIX domain socket, this is an error;
if <filename> is a UNIX domain socket, but no process is
listening, this is an error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX
) Useful options: bind
See also: UNIX-LISTEN, UNIX-SENDTO, TCP
UNIX-LISTEN:<filename>
Listens on <filename> using a UNIX domain stream socket and
accepts a connection. If <filename> exists and is not a
socket, this is an error. If <filename> exists and is a
UNIX domain socket, binding to the address fails (use
option unlink-early!). Note that opening this address
usually blocks until a client connects. Beginning with
socat version 1.4.3, the file system entry is removed when
this address is closed (but see option unlink-close)
(example).
Option groups: FD,SOCKET,NAMED,LISTEN,CHILD,RETRY,UNIX
Useful options: fork, umask, mode, user, group,
unlink-early
See also: UNIX-CONNECT, UNIX-RECVFROM, UNIX-RECV,
TCP-LISTEN
UNIX-SENDTO:<filename>
Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain datagram socket.
It sends packets to and receives packets from that peer
socket only. Please note that it might be necessary to
bind the local socket to an address (e.g. /tmp/sock1, which
must not exist before). This address type works well with
socat UNIX-RECVFROM and UNIX-RECV address peers.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-RECVFROM, UNIX-RECV, UNIX-CONNECT,
UDP-SENDTO, IP-SENDTO
UNIX-RECVFROM:<filename>
Creates a UNIX domain datagram socket [<filename>].
Receives one packet and may send one or more answer packets
to that peer. This mode is particularly useful with fork
option where each arriving packet - from arbitrary peers -
is handled by its own sub process. This address works well
with socat UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
See the note about RECVFROM addresses.
Useful options: fork
umask
See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN,
UDP-RECVFROM, IP-RECVFROM
UNIX-RECV:<filename>
Creates a UNIX domain datagram socket [<filename>].
Receives packets from multiple unspecified peers and merges
the data. No replies are possible, this is a read-only
address, see options -u and -U, and dual addresses. It can
be, e.g., addressed by socat UNIX-SENDTO address peers. It
behaves similar to a syslog server.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: umask
See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN,
UDP-RECV, IP-RECV
UNIX-CLIENT:<filename>
Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain socket. It first
tries to connect and, if that fails, assumes it is a
datagram socket, thus supporting both types.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-CONNECT, UNIX-SENDTO, GOPEN
VSOCK-CONNECT:<cid>:<port>
Establishes a VSOCK stream connection to the specified
<cid> [VSOCK cid] and <port> [VSOCK port].
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, connect-timeout, retry, readbytes
See also: VSOCK-LISTEN,
VSOCK-LISTEN:<port>
Listens on <port> [VSOCK port] and accepts a VSOCK
connection. Note that opening this address usually blocks
until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RETRY
Useful options: fork, bind, max-children, backlog, su,
reuseaddr, retry
See also: VSOCK-CONNECT
ABSTRACT-CONNECT:<string>
ABSTRACT-LISTEN:<string>
ABSTRACT-SENDTO:<string>
ABSTRACT-RECVFROM:<string>
ABSTRACT-RECV:<string>
ABSTRACT-CLIENT:<string>
The ABSTRACT addresses are almost identical to the related
UNIX addresses except that they do not address file system
based sockets but an alternate UNIX domain address space.
To achieve this the socket address strings are prefixed
with "\0" internally. This feature is available (only?) on
Linux. Option groups are the same as with the related UNIX
addresses, except that the ABSTRACT addresses are not
member of the NAMED group.
Useful options: netns
Address options can be applied to address specifications to
influence the process of opening the addresses and the properties
of the resulting data channels.
For technical reasons not every option can be applied to every
address type; e.g., applying a socket option to a regular file
will fail. To catch most useless combinations as early as in the
open phase, the concept of option groups was introduced. Each
option belongs to one or more option groups. Options can be used
only with address types that support at least one of their option
groups (but see option -g).
Address options have data types that their values must conform to.
Every address option consists of just a keyword or a keyword
followed by "=value", where value must conform to the options
type. Some address options manipulate parameters of system calls;
e.g., option sync sets the O_SYNC flag with the open() call.
Other options cause a system or library call; e.g., with option
`ttl=value’ the setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int))
call is applied. Other options set internal socat variables that
are used during data transfer; e.g., `crnl’ causes explicit
character conversions. A few options have more complex
implementations; e.g., su-d (substuser-delayed) inquires some user
and group infos, stores them, and applies them later after a
possible chroot() call.
If multiple options are given to an address, their sequence in the
address specification has (almost) no effect on the sequence of
their execution/application. Instead, socat has built in an option
phase model that tries to bring the options in a useful order.
Some options exist in different forms (e.g., unlink, unlink-early,
unlink-late) to control the time of their execution.
If the same option is specified more than once within one address
specification, with equal or different values, the effect depends
on the kind of option. Options resulting in function calls like
setsockopt() cause multiple invocations. With options that set
parameters for a required call like open() or set internal flags,
the value of the last option occurrence is effective.
The existence or semantics of many options are system dependent.
Socat usually does NOT try to emulate missing libc or kernel
features, it just provides an interface to the underlying system.
So, if an operating system lacks a feature, the related option is
simply not available on this platform.
The following paragraphs introduce just the more common address
options. For a more comprehensive reference and to find
information about canonical option names, alias names, option
phases, and platforms see file xio.help.
FD option group
This option group contains options that are applied to a UN*X
style file descriptor, no matter how it was generated. Because
all current socat address types are file descriptor based, these
options may be applied to any address.
Note: Some of these options are also member of another option
group, that provides another, non-fd based mechanism. For these
options, it depends on the actual address type and its option
groups which mechanism is used. The second, non-fd based mechanism
is prioritized.
cloexec[=<bool>]
Sets the FD_CLOEXEC flag with the fcntl() system call to
value <bool>. If set, the file descriptor is closed on
exec() family function calls. Socat internally handles this
flag for the fds it controls, so in most cases there will
be no need to apply this option.
setlk[=<bool>]
Tries to set a discretionary write lock to the whole file
using the fcntl(fd, F_SETLK, ...) system call. If the file
is already locked, this call results in an error. On
Linux, when the file permissions for group are "S"
(g-x,g+s), and the file system is locally mounted with the
"mand" option, the lock is mandatory, i.e. prevents other
processes from opening the file.
setlkw[=<bool>]
Tries to set a discretionary waiting write lock to the
whole file using the fcntl(fd, F_SETLKW, ...) system call.
If the file is already locked, this call blocks. See
option setlk for information about making this lock
mandatory.
setlk-rd[=<bool>]
Tries to set a discretionary read lock to the whole file
using the fcntl(fd, F_SETLK, ...) system call. If the file
is already write locked, this call results in an error.
See option setlk for information about making this lock
mandatory.
setlkw-rd[=<bool>]
Tries to set a discretionary waiting read lock to the whole
file using the fcntl(fd, F_SETLKW, ...) system call. If
the file is already write locked, this call blocks. See
option setlk for information about making this lock
mandatory.
flock-ex[=<bool>]
Tries to set a blocking exclusive advisory lock to the file
using the flock(fd, LOCK_EX) system call. Socat hangs in
this call if the file is locked by another process.
flock-ex-nb[=<bool>]
Tries to set a nonblocking exclusive advisory lock to the
file using the flock(fd, LOCK_EX|LOCK_NB) system call. If
the file is already locked, this option results in an
error.
flock-sh[=<bool>]
Tries to set a blocking shared advisory lock to the file
using the flock(fd, LOCK_SH) system call. Socat hangs in
this call if the file is locked by another process.
flock-sh-nb[=<bool>]
Tries to set a nonblocking shared advisory lock to the file
using the flock(fd, LOCK_SH|LOCK_NB) system call. If the
file is already locked, this option results in an error.
lock[=<bool>]
Sets a blocking lock on the file. Uses the setlk or flock
mechanism depending on availability on the particular
platform. If both are available, the POSIX variant (setlkw)
is used.
user=<user>
Sets the <user> (owner) of the stream. If the address is
member of the NAMED option group, socat uses the chown()
system call after opening the file or binding to the UNIX
domain socket (race condition!). Without filesystem entry,
socat sets the user of the stream using the fchown() system
call. These calls might require root privilege.
user-late=<user>
Sets the owner of the fd to <user> with the fchown() system
call after opening or connecting the channel. This is
useful only on file system entries.
group=<group>
Sets the <group> of the stream. If the address is member
of the NAMED option group, socat uses the chown() system
call after opening the file or binding to the UNIX domain
socket (race condition!). Without filesystem entry, socat
sets the group of the stream with the fchown() system call.
These calls might require group membership or root
privilege.
group-late=<group>
Sets the group of the fd to <group> with the fchown()
system call after opening or connecting the channel. This
is useful only on file system entries.
mode=<mode>
Sets the <mode> [mode_t] (permissions) of the stream. If
the address is member of the NAMED option group and uses
the open() or creat() call, the mode is applied with these.
If the address is member of the NAMED option group without
using these system calls, socat uses the chmod() system
call after opening the filesystem entry or binding to the
UNIX domain socket (race condition!). Otherwise, socat
sets the mode of the stream using fchmod() which, btw,
might not have any effect.
These calls might require ownership or root privilege.
Note: this option can only tighten the permissions implied
by processes umask. See option umask to loosen permissions.
perm-late=<mode>
Sets the permissions of the fd to value <mode> [mode_t]
using the fchmod() system call after opening or connecting
the channel. This is useful only on file system entries.
append[=<bool>]
Always writes data to the actual end of file. If the
address is member of the OPEN option group, socat uses the
O_APPEND flag with the open() system call (example).
Otherwise, socat applies the fcntl(fd, F_SETFL, O_APPEND)
call.
nonblock[=<bool>]
Tries to open or use file in nonblocking mode. Its only
effects are that the connect() call of TCP addresses does
not block, and that opening a named pipe for reading does
not block. If the address is member of the OPEN option
group, socat uses the O_NONBLOCK flag with the open()
system call. Otherwise, socat applies the fcntl(fd,
F_SETFL, O_NONBLOCK) call.
binary[=<bool>]
Opens the file in binary mode to avoid implicit line
terminator conversions (Cygwin).
text[=<bool>]
Opens the file in text mode to force implicit line
terminator conversions (Cygwin).
noinherit[=<bool>]
Does not keep this file open in a spawned process (Cygwin).
cool-write[=<bool>]
Takes it easy when write fails with EPIPE or ECONNRESET and
logs the message with notice level instead of error. This
prevents the log file from being filled with useless error
messages when socat is used as a high volume server or
proxy where clients often abort the connection. Use this
option only with option fork because otherwise it might
cause socat to exit with code 0 even on failure.
This option is deprecated, consider using option
children-shutup instead.
end-close[=<bool>]
Changes the (address dependent) method of ending a
connection to just close the file descriptors. This is
useful when the connection is to be reused by or shared
with other processes (example).
Normally, socket connections will be ended with shutdown(2)
which terminates the socket even if it is shared by
multiple processes. close(2) "unlinks" the socket from the
process but keeps it active as long as there are still
links from other processes.
Similarly, when an address of type EXEC or SYSTEM is ended,
socat usually will explicitly kill the sub process. With
this option, it will just close the file descriptors.
shut-none[=<bool>]
Changes the (address dependent) method of shutting down the
write part of a connection to not do anything.
shut-down[=<bool>]
Changes the (address dependent) method of shutting down the
write part of a connection to shutdown(fd, SHUT_WR). Is
only useful with sockets.
shut-close[=<bool>]
Changes the (address dependent) method of shutting down the
write part of a connection to close(fd).
shut-null[=<bool>]
When one address indicates EOF, socat will send a zero
sized packet to the write channel of the other address to
transfer the EOF condition. This is useful with UDP and
other datagram protocols. Has been tested against netcat
and socat with option null-eof.
null-eof[=<bool>]
Normally socat will ignore empty (zero size payload)
packets arriving on datagram sockets, so it survives port
scans. With this option socat interprets empty datagram
packets as EOF indicator (see shut-null).
ioctl-void=<request>
Calls ioctl() with the request value as second argument and
NULL as third argument. This option allows utilizing ioctls
that are not explicitly implemented in socat.
ioctl-int=<request>:<value>
Calls ioctl() with the request value as second argument and
the integer value as third argument.
ioctl-intp=<request>:<value>
Calls ioctl() with the request value as second argument and
a pointer to the integer value as third argument.
ioctl-bin=<request>:<value>
Calls ioctl() with the request value as second argument and
a pointer to the given data value as third argument. This
data must be specified in <dalan> form.
ioctl-string=<request>:<value>
Calls ioctl() with the request value as second argument and
a pointer to the given string as third argument. <dalan>
form.
NAMED option group
These options work on file system entries.
Please note that, with UNIX domain client addresses, this means
the bind entry, not the target/peer entry.
See also options user, group, and mode.
user-early=<user>
Changes the <user> (owner) of the file system entry before
accessing it, using the chown() system call. This call
might require root privilege.
group-early=<group>
Changes the <group> of the file system entry before
accessing it, using the chown() system call. This call
might require group membership or root privilege.
perm-early=<mode>
Changes the <mode> [mode_t] of the file system entry before
accessing it, using the chmod() system call. This call
might require ownership or root privilege.
unlink-early[=<bool>]
Unlinks (removes) the file before opening it and even
before applying user-early etc.
unlink[=<bool>]
Unlinks (removes) the file before accessing it, but after
user-early etc.
unlink-late[=<bool>]
Unlinks (removes) the file after opening it to make it
inaccessible for other processes after a short race
condition.
unlink-close[=<bool>]
Controls removal of the addresses file system entry when
closing the address. For named pipes, UNIX domain sockets,
and the symbolic links of pty addresses, the default is
remove (1); for created files, opened files, and generic
opened files the default is keep (0). Setting this option
to 1 removes the entry, 0 keeps it. No value means 1.
OPEN option group
The OPEN group options allow setting flags with the open() system
call. E.g., option `creat’ sets the O_CREAT flag. When the used
address does not use open() (e.g.STDIO), the fcntl(..., F_SETFL,
...) call is used instead.
See also options append and nonblock.
creat[=<bool>]
Creates the file if it does not exist (example).
dsync[=<bool>]
Blocks write() calls until metainfo is physically written
to media.
excl[=<bool>]
With option creat, if file exists this is an error.
largefile[=<bool>]
On 32 bit systems, allows a file larger than 2^31 bytes.
noatime[=<bool>]
Sets the O_NOATIME options, so reads do not change the
access timestamp.
noctty[=<bool>]
Does not make this file the controlling terminal.
nofollow[=<bool>]
Does not follow symbolic links.
nshare[=<bool>]
Does not allow sharing this file with other processes.
rshare[=<bool>]
Does not allow other processes to open this file for
writing.
rsync[=<bool>]
Blocks write() until metainfo is physically written to
media.
sync[=<bool>]
Blocks write() until data is physically written to media.
rdonly[=<bool>]
Opens the file for reading only.
wronly[=<bool>]
Opens the file for writing only.
trunc[=<bool>]
Truncates the file to size 0 during opening it.
REG and BLK option group
These options are usually applied to a UN*X file descriptor, but
their semantics make sense only on a file supporting random
access.
seek=<offset>
Applies the lseek(fd, <offset>, SEEK_SET) (or lseek64 )
system call, thus positioning the file pointer absolutely
to <offset> [off_t or off64_t]. Please note that a missing
value defaults to 1, not 0.
seek-cur=<offset>
Applies the lseek(fd, <offset>, SEEK_CUR) (or lseek64 )
system call, thus positioning the file pointer <offset>
[off_t or off64_t] bytes relatively to its current position
(which is usually 0). Please note that a missing value
defaults to 1, not 0.
seek-end=<offset>
Applies the lseek(fd, <offset>, SEEK_END) (or lseek64 )
system call, thus positioning the file pointer <offset>
[off_t or off64_t] bytes relatively to the files current
end. Please note that a missing value defaults to 1, not 0.
ftruncate=<offset>
Applies the ftruncate(fd, <offset>) (or ftruncate64 if
available) system call, thus truncating the file at the
position <offset> [off_t or off64_t]. Please note that a
missing value defaults to 1, not 0.
secrm[=<bool>]
unrm[=<bool>]
compr[=<bool>]
fs-sync[=<bool>]
immutable[=<bool>]
fs-append[=<bool>]
nodump[=<bool>]
fs-noatime[=<bool>]
journal-data[=<bool>]
notail[=<bool>]
dirsync[=<bool>]
These options change non standard file attributes on
operating systems and file systems that support these
features, like Linux with ext2fs and successors, xfs, or
reiserfs. See man 1 chattr for information on these
options. Please note that there might be a race condition
between creating the file and applying these options.
PIPE options
These options may be applied to pipes (fifos).
f-setpipe-sz=<int>
setpipe=<int>
Set the number of bytes a pipe can buffer. Where more bytes
are written the writing process might block. When more
bytes are written in a single write() the writing process
blocks and might never recover.
General address options
These options may be applied to all address types. They change
some process properties that are restored after opening the
address.
chdir=<filename>
cd=<filename>
Changes the working directory. After opening the address
the master process changes back to the original working
directory. Sub processes inherit the temporary setting.
umask=<mode>
Sets the umask of the process to <mode> [mode_t] before
opening the address. Useful when file system entries are
created or a shell or program is invoked. Usually the value
is specified as octal number with leading ’0’.
The processes umask value is inherited by child processes.
Note: umask is an inverted value: creating a file with
umask=0026 results in permissions 0640.
PROCESS option group
Options of this group change the process properties instead of
just affecting one data channel. For EXEC and SYSTEM addresses
and for LISTEN and CONNECT type addresses with option fork, these
options apply to the child processes instead of the main socat
process.
chroot=<directory>
Performs a chroot() operation to <directory> after
processing the address (example). This call might require
root privilege.
chroot-early=<directory>
Performs a chroot() operation to <directory> before opening
the address. This call might require root privilege.
setgid=<group>
Changes the primary <group> of the process after processing
the address. This call might require root privilege. Please
note that this option does not drop other group related
privileges.
setgid-early=<group>
Like setgid but is performed before opening the address.
setuid=<user>
Changes the <user> (owner) of the process after processing
the address. This call might require root privilege. Please
note that this option does not drop group related
privileges. Check if option su better fits your needs.
setuid-early=<user>
Like setuid but is performed before opening the address.
su=<user>
Changes the <user> (owner) and groups of the process after
processing the address (example). This call might require
root privilege.
su-d=<user>
Short name for substuser-delayed. Changes the <user>
(owner) and groups of the process after processing the
address (example). The user and his groups are retrieved
before a possible chroot() . This call might require root
privilege.
setpgid=<pid_t>
Makes the process a member of the specified process group
<pid_t>. If no value is given, or if the value is 0 or 1,
the process becomes leader of a new process group.
setsid Makes the process the leader of a new session (example).
netns=<net-namespace-name>
Before opening the address it tries to switch to the named
network namespace. After opening the address it switches
back to the previous namespace (example with TCP forwarder,
example with virtual network connection).
Only on Linux; requires root; use option --experimental.
READLINE option group
These options apply to the readline address type.
history=<filename>
Reads and writes history from/to <filename> (example).
noprompt
Since version 1.4.0, socat per default tries to determine a
prompt - that is then passed to the readline call - by
remembering the last incomplete line of the output. With
this option, socat does not pass a prompt to readline, so
it begins line editing in the first column of the terminal.
noecho=<pattern>
Specifies a regular pattern for a prompt that prevents the
following input line from being displayed on the screen and
from being added to the history. The prompt is defined as
the text that was output to the readline address after the
last newline character and before an input character was
typed. The pattern is a regular expression, e.g.
"^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See
regex(7) for details. (example)
prompt=<string>
Passes the string as prompt to the readline function.
readline prints this prompt when stepping through the
history. If this string matches a constant prompt issued by
an interactive program on the other socat address,
consistent look and feel can be achieved.
APPLICATION option group
This group contains options that work at data level. Note that
these options only apply to the "raw" data transferred by socat,
but not to protocol data used by addresses like PROXY.
cr Converts the default line termination character NL (’\n’,
0x0a) to/from CR (’\r’, 0x0d) when writing/reading on this
channel.
crnl Converts the default line termination character NL (’\n’,
0x0a) to/from CRNL ("\r\n", 0x0d0a) when writing/reading on
this channel (example). Note: socat simply strips all CR
characters.
ignoreeof
When EOF occurs on this channel, socat ignores it and tries
to read more data (like "tail -f") (example).
readbytes=<bytes>
socat reads only so many bytes from this address (the
address provides only so many bytes for transfer and
pretends to be at EOF afterwards). Must be greater than 0.
lockfile=<filename>
If lockfile exists, exits with error. If lockfile does not
exist, creates it and continues, unlinks lockfile on exit.
waitlock=<filename>
If lockfile exists, waits until it disappears. When
lockfile does not exist, creates it and continues, unlinks
lockfile on exit.
escape=<int>
Specifies the numeric code of a character that triggers EOF
on the input stream. It is useful with a terminal in raw
mode (example).
SOCKET option group
These options are intended for all kinds of sockets, e.g. IP or
UNIX domain. Most are applied with a setsockopt() call.
bind=<sockname>
Binds the socket to the given socket address using the
bind() system call. The form of <sockname> is socket domain
dependent: IP4 and IP6 allow the form
[hostname|hostaddress][:(service|port)] (example), VSOCK
allows the form [cid][:(port)].
See also: unix-bind-tempname
connect-timeout=<seconds>
Abort the connection attempt after <seconds> [timeval] with
error status.
so-bindtodevice=<interface>
Binds the socket to the given <interface>. This option
might require root privilege.
broadcast
For datagram sockets, allows sending to broadcast addresses
and receiving packets addressed to broadcast addresses.
debug Enables socket debugging.
dontroute
Only communicates with directly connected peers, does not
use routers.
keepalive
Enables sending keepalives on the socket.
linger=<seconds>
Blocks shutdown() or close() until data transfers have
finished or the given timeout [int] expired.
oobinline
Places out-of-band data in the input data stream.
priority=<priority>
Sets the protocol defined <priority> [<int>] for outgoing
packets.
rcvbuf=<bytes>
Sets the size of the receive buffer after the socket() call
to <bytes> [int]. With TCP sockets, this value corresponds
to the socket’s maximal window size.
rcvbuf-late=<bytes>
Sets the size of the receive buffer when the socket is
already connected to <bytes> [int]. With TCP sockets, this
value corresponds to the socket’s maximal window size.
so-rcvtimeo=<time>, rcvtimeo=<time>
Specifies the time [int] until recv() , read() etc.
functions timeout when no data is received. Note that in
the transfer phase socat only calls these functions when
select() has reported that data is available. However this
option is useful with DTLS addresses to timeout during
connection negotiation.
so-sndtimeo=<time>, sndtimeo=<time>
Like so-recvtimeo, but for send . Not usecase known.
rcvlowat=<bytes>
Specifies the minimum number of received bytes [int] until
the socket layer will pass the buffered data to socat.
reuseaddr[=[0|1]]
Allows other sockets to bind to an address even if parts of
it (e.g. the local port) are already in use by socat.
With version 1.8.0, this socket option is set automatically
for TCP LISTEN addresses. If you prefer the system default
(no related setsockopt(...SO_REUSEADDR...) call at all),
use form reuseaddr=.
(example).
sndbuf=<bytes>
Sets the size of the send buffer after the socket() call to
<bytes> [int].
sndbuf-late=<bytes>
Sets the size of the send buffer when the socket is
connected to <bytes> [int].
sndlowat=<bytes>
Specifies the minimum number of bytes in the send buffer
until the socket layer will send the data to <bytes> [int].
pf=<string>
Forces the use of the specified IP version or protocol.
<string> can be something like "ip4" or "ip6". The
resulting value is used as first argument to the socket()
or socketpair() calls. This option affects address
resolution and the required syntax of bind and range
options.
socktype=<type>
Sets the type of the socket, specified as second argument
to the socket() or socketpair() calls, to <type> [int].
Address resolution is not affected by this option. Under
Linux, 1 means stream oriented socket, 2 means datagram
socket, 3 means raw socket, and 5 seqpacket (stream keeping
packet boundaries). Datagrams are useful when you want to
keep packet boundaries.
protocol
Sets the protocol of the socket, specified as third
argument to the socket() or socketpair() calls, to
<protocol> [int]. Address resolution is not affected by
this option. 6 means TCP, 17 means UDP.
reuseport
Set the SO_REUSEPORT socket option.
so-timestamp
Sets the SO_TIMESTAMP socket option. This enables receiving
and logging of timestamp ancillary messages.
setsockopt=<level>:<optname>:<optval>
Invokes setsockopt() for the socket with the given
parameters. level [int] is used as second argument to
setsockopt() and specifies the layer, e.g. SOL_TCP for TCP
(6 on Linux), or SOL_SOCKET for the socket layer (1 on
Linux). optname [int] is the third argument to setsockopt()
and tells which socket option is to be set. For the actual
numbers you might have to look up the appropriate include
files of your system. For the 4th and 5th setsockopt()
parameters, value [dalan] specifies an arbitrary sequence
of bytes that are passed to the function per pointer, with
the automatically derived length parameter.
setsockopt-int=<level>:<optname>:<optval>
Like setsockopt, but <optval> is a pointer to int [int]
setsockopt-listen=<level>:<optname>:<optval>
Like setsockopt, but for listen type addresses it is
applied to the listening socket instead of the connected
socket.
setsockopt-string=<level>:<optname>:<optval>
Like setsockopt, but <optval> is a string. This string is
passed to the function with trailing null character, and
the length parameter is automatically derived from the
data.
setsockopt-socket=<level>:<optname>:<optval>
Like setsockopt, but is applied to the socket before other
operations ( bind() , connect() , accept() , ...)
setsockopt-connected=<level>:<optname>:<optval>
Like setsockopt, but is applied only when the socket has
been connected by a connect() or listen() call.
UNIX option group
These options apply to UNIX domain based addresses.
bind-tempname[=/tmp/pre-XXXXXX],
unix-bind-tempname[=/tmp/pre-XXXXXX]" Binds to a random
path or random address (on abstract namespace sockets).
This is useful with datagram client addresses (SENDTO, or
CLIENT) that are opened in child processes forked off from
a common parent process where the child processes cannot
have different bind options. In the path X ’s get replaced
with a random character sequence similar to tempnam(3).
When no argument is given socat takes a default like
/tmp/fileXXXXXX .
unix-tightsocklen[=(0|1)]
On socket operations, pass a socket address length that
does not include the whole struct sockaddr_un record but
(besides other components) only the relevant part of the
filename or abstract string. Default is 1.
IP4 and IP6 option groups
These options can be used with IPv4 and IPv6 based sockets.
tos=<tos>
Sets the TOS (type of service) field of outgoing packets to
<tos> [byte] (see RFC 791).
ttl=<ttl>
Sets the TTL (time to live) field of outgoing packets to
<ttl> [byte].
ip-options=<data>
Sets IP options like source routing. Must be given in
binary form, recommended format is a leading "x" followed
by an even number of hex digits. This option may be used
multiple times, data are appended. E.g., to connect to
host 10.0.0.1 via some gateway using a loose source route,
use the gateway as address parameter and set a loose source
route using the option ip-options=x8307040a000001 .
IP options are defined in RFC 791.
mtudiscover=<0|1|2>
Takes 0, 1, 2 to never, want, or always use path MTU
discover on this socket.
ip-pktinfo
Sets the IP_PKTINFO socket option. This enables receiving
and logging of ancillary messages containing destination
address and interface (Linux) (example).
ip-recverr
Sets the IP_RECVERR socket option. This enables receiving
and logging of ancillary messages containing detailed error
information.
ip-recvopts
Sets the IP_RECVOPTS socket option. This enables receiving
and logging of IP options ancillary messages (Linux, *BSD).
ip-recvtos
Sets the IP_RECVTOS socket option. This enables receiving
and logging of TOS (type of service) ancillary messages
(Linux).
ip-recvttl
Sets the IP_RECVTTL socket option. This enables receiving
and logging of TTL (time to live) ancillary messages
(Linux, *BSD).
ip-recvdstaddr
Sets the IP_RECVDSTADDR socket option. This enables
receiving and logging of ancillary messages containing
destination address (*BSD) (example).
ip-recvif
Sets the IP_RECVIF socket option. This enables receiving
and logging of interface ancillary messages (*BSD)
(example).
ip-add-membership=<multicast-address:interface-address>
ip-add-membership=<multicast-address:interface-name>
ip-add-membership=<multicast-address:interface-index>
ip-add-membership=<multicast-address:interface-address:interface-name>
ip-add-membership=<multicast-address:interface-address:interface-index>
Makes the socket member of the specified multicast group.
This only works for IPv4, see ipv6-join-group for the IPv6
variant. The option takes the IP address of the multicast
group and info about the desired network interface. The
most common syntax is the first one, while the others are
only available on systems that provide struct mreqn
(Linux).
The indices of active network interfaces can be shown using
the utility procan.
ip-add-source-membership=<multicast-address:interface-address:source-address>
Makes the socket member of the specified multicast group
for the specified source, i.e. only multicast traffic from
this address is to be delivered. This only works for IPv4,
see ipv6-join-source-group for the IPv6 variant. The option
takes the IP address of the multicast group, the IP address
of the desired network interface and the source IP address
of the multicast traffic.
ipv6-join-group=<multicast-address:interface-name>
ipv6-join-group=<multicast-address:interface-index>
Makes the socket member of the specified multicast group.
This only works for IPv6, see ip-add-membership for the
IPv4 variant. The option takes the IP address of the
multicast group and info about the desired network
interface. The indices of active network interfaces can be
shown using the utility procan.
ipv6-join-source-group=<multicast-address:interface-name:source-address>
ipv6-join-source-group=<multicast-address:interface-index:source-address>
Makes the socket member of the specified multicast group
for the specified source, i.e. only multicast traffic from
this address is to be delivered. This only works for IPv6,
see ip-add-source-membership for the IPv4 variant. The
option takes the IP address of the multicast group, info
about the desired network interface and the source IP
address of the multicast traffic. The indices of active
network interfaces can be shown using the utility procan.
This feature is experimental.
ip-multicast-if=<hostname>
Specifies hostname or address of the network interface to
be used for multicast traffic.
ip-multicast-loop[=<bool>]
Specifies if outgoing multicast traffic should loop back to
the interface.
ip-multicast-ttl=<byte>
Sets the TTL used for outgoing multicast traffic. Default
is 1.
ip-transparent
Sets the IP_TRANSPARENT socket option. This option might
require root privilege.
Resolver options
These options temporarily change the behaviour of hostname
resolution. The options of form ai-* affect behaviour of the
getaddrinfo() function that includes /etc/hosts and NIS based
lookups.
The addresses of form res-* only affect DNS lookups, and only when
the result is not cached in nscd . These options might not work on
all operating systems or libc implementations.
ai-addrconfig[=0|1]
addrconfig[=0|1]
Sets or unsets the AI_ADDRCONFIG flag to prevent name
resolution to address families that are not available on
the computer (e.g. IPv6). Default value is 1 in case the
resolver does not get an address family hint from Socat
address or defaults.
ai-passive[=0|1]
passive[=0|1]
Sets of unsets the AI_PASSIVE flag for getaddrinfo() calls.
Default is 1 for LISTEN, RECV, and RECVFROM type addresses,
and with bind option.
ai-v4mapped[=0|1]
v4mapped[=0|1]
Sets or unsets the AI_V4MAPPED flag for getaddrinfo() .
With socat addresses requiring IPv6 addresses, this
resolves IPv4 addresses to the appropriate IPv6 address
[::ffff:*:*]. For IPv6 socat addresses, the default is 1.
ai-all[=0|1]
Sets or unsets the AI_ALL flag for getaddrinfo() .
res-debug
res-aaonly
res-usevc
res-primary
res-igntc
res-recurse
res-defnames
res-stayopen
res-dnsrch
These options set the corresponding resolver (name
resolution) option flags. Append "=0" to clear a default
option. See man resolver(5) for more information on these
options. Socat restores the old values after finishing the
open phase of the address, so these options are valid just
for the address they are applied to.
Please note that these flags only affect DNS resolution,
but not hosts or NIS based name resolution, and they have
no effect when (g)libc retrieves the results from nscd .
res-retrans=<int>
Sets the retransmission time interval of the DNS resolver
(based on an undocumented feature).
res-retry=<int>
Sets the number of retransmits of the DNS resolver (based
on an undocumented feature).
res-nsaddr=<ipaddr>[:<port>]
Tries to overwrite nameserver settings loaded from
/etc/resolv.conf by writing the given IPv4 address into the
undocumented _res:nsaddr_list[0] field. /etc/hosts is
still checked by resolver. Please note that glibc’s nscd is
always queried first when it is running!
IP6 option group
These options can only be used on IPv6 based sockets. See IP
options for options that can be applied to both IPv4 and IPv6
sockets.
ipv6only[=<bool>]
Sets the IPV6_V6ONLY socket option. If 0, the TCP stack
will also accept connections using IPv4 protocol on the
same port. The default is system dependent.
ipv6-recvdstopts
Sets the IPV6_RECVDSTOPTS socket option. This enables
receiving and logging of ancillary messages containing the
destination options.
ipv6-recvhoplimit
Sets the IPV6_RECVHOPLIMIT socket option. This enables
receiving and logging of ancillary messages containing the
hoplimit.
ipv6-recvhopopts
Sets the IPV6_RECVHOPOPTS socket option. This enables
receiving and logging of ancillary messages containing the
hop options.
ipv6-recvpktinfo
Sets the IPV6_RECVPKTINFO socket option. This enables
receiving and logging of ancillary messages containing
destination address and interface.
ipv6-unicast-hops=link(TYPE_INT)(<int>)
Sets the IPV6_UNICAST_HOPS socket option. This sets the hop
count limit (TTL) for outgoing unicast packets.
ipv6-recvrthdr
Sets the IPV6_RECVRTHDR socket option. This enables
receiving and logging of ancillary messages containing
routing information.
ipv6-tclass
Sets the IPV6_TCLASS socket option. This sets the transfer
class of outgoing packets.
ipv6-recvtclass
Sets the IPV6_RECVTCLASS socket option. This enables
receiving and logging of ancillary messages containing the
transfer class.
TCP option group
These options may be applied to TCP sockets. They work by invoking
setsockopt() with the appropriate parameters.
cork Doesn’t send packets smaller than MSS (maximal segment
size).
defer-accept
While listening, accepts connections only when data from
the peer arrived.
keepcnt=<count>
Sets the number of keepalives before shutting down the
socket to <count> [int].
keepidle=<seconds>
Sets the idle time before sending the first keepalive to
<seconds> [int].
keepintvl=<seconds>
Sets the interval between two keepalives to <seconds>
[int].
linger2=<seconds>
Sets the time to keep the socket in FIN-WAIT-2 state to
<seconds> [int].
mss=<bytes>
Sets the MSS (maximum segment size) after the socket() call
to <bytes> [int]. This value is then proposed to the peer
with the SYN or SYN/ACK packet (example).
mss-late=<bytes>
Sets the MSS of the socket after connection has been
established to <bytes> [int].
nodelay
Turns off the Nagle algorithm for measuring the RTT (round
trip time).
rfc1323
Enables RFC1323 TCP options: TCP window scale, round-trip
time measurement (RTTM), and protect against wrapped
sequence numbers (PAWS) (AIX).
stdurg Enables RFC1122 compliant urgent pointer handling (AIX).
syncnt=<count>
Sets the maximal number of SYN retransmits during connect
to <count> [int].
md5sig Enables generation of MD5 digests on the packets (FreeBSD).
noopt Disables use of TCP options (FreeBSD, MacOSX).
nopush sets the TCP_NOPUSH socket option (FreeBSD, MacOSX).
sack-disable
Disables use the selective acknowledge feature (OpenBSD).
signature-enable
Enables generation of MD5 digests on the packets (OpenBSD).
abort-threshold=<milliseconds>
Sets the time to wait for an answer of the peer on an
established connection (HP-UX).
conn-abort-threshold=<milliseconds>
Sets the time to wait for an answer of the server during
the initial connect (HP-UX).
keepinit
Sets the time to wait for an answer of the server during
connect() before giving up. Value in half seconds, default
is 150 (75s) (Tru64).
paws Enables the "protect against wrapped sequence numbers"
feature (Tru64).
sackena
Enables selective acknowledge (Tru64).
tsoptena
Enables the time stamp option that allows RTT recalculation
on existing connections (Tru64).
UDP option group
This option may be applied to UDP datagram sockets.
udp-ignore-peerport>
Address UDP-DATAGRAM expects incoming responses to come
from the port specified in its second parameter. With this
option, it accepts packets coming from any port.
UDPLITE option group
These options may be applied to UDPLITE addresses:
udplite-send-cscov
Sets the number of bytes for which the checksum is
calculated and sent ("checksum coverage").
udplite-recv-cscov
Sets the number of bytes for which the checksum is checked
("checksum coverage").
SCTP option group
These options may be applied to SCTP stream sockets.
sctp-nodelay
Sets the SCTP_NODELAY socket option that disables the Nagle
algorithm.
sctp-maxseg=<bytes>
Sets the SCTP_MAXSEG socket option to <bytes> [int]. This
value is then proposed to the peer with the SYN or SYN/ACK
packet.
DCCP option group
These options may be applied to DCCP sockets.
dccp-set-ccid=<int>
ccid=<int>
Selects the desired congestion control mechanism (CCID).
UDP, TCP, SCTP, DCCP, and UDPLITE option group
Here we find options that are related to the network port
mechanism and thus can be used with UDP, TCP, SCTP, DCCP, and
UDP-Lite client and server addresses.
sourceport=<port>
For outgoing (client) connections, it sets the source
<port> using an extra bind() call. With TCP or UDP listen
addresses, socat immediately shuts down the connection if
the client does not use this sourceport. UDP-RECV,
UDP-RECVFROM, UDP-SENDTO, and UDP-DATAGRAM addresses ignore
the packet when it does not match. (example).
lowport
Outgoing (client) connections with this option use an
unused random source port between 640 and 1023 incl. On
UNIX class operating systems, this requires root privilege,
and thus indicates that the client process is authorized by
local root. TCP and UDP listen addresses with this option
immediately shut down the connection if the client does not
use a sourceport <= 1023. This mechanism can provide
limited authorization under some circumstances.
SOCKS option group
When using SOCKS type addresses, some socks specific options can
be set.
socksport=<tcp service>
Overrides the default "socks" service or port 1080 for the
socks server port with <TCP service>.
socksuser=<user>
Sends the <user> [string] in the username field to the
socks server. Default is the actual user name ($LOGNAME or
$USER) (example).
HTTP option group
Options that can be provided with HTTP type addresses. The only
HTTP address currently implemented is proxy-connect.
http-version=<string>
Changes the default "1.0" that is sent to the server in the
initial HTTP request. Currently it has not other effect, in
particular it does not provide any means to send a Host
header.
proxyport=<TCP service>
Overrides the default HTTP proxy port 8080 with <TCP
service>.
ignorecr
The HTTP protocol requires the use of CR+NL as line
terminator. When a proxy server violates this standard,
socat might not understand its answer. This option directs
socat to interpret NL as line terminator and to ignore CR
in the answer. Nevertheless, socat sends CR+NL to the
proxy.
proxy-authorization=<username>:<password>
Provide "basic" authentication to the proxy server. The
argument to the option is used with a "Proxy-Authorization:
Basic" header in base64 encoded form.
Note: username and password are visible for every user on
the local machine in the process list; username and
password are transferred to the proxy server unencrypted
(base64 encoded) and might be sniffed.
proxy-authorization-file=<filename>
Like option proxy-authorization, but the credentials are
read from the file and therefore not visible in the process
list.
resolve
Per default, socat sends to the proxy a CONNECT request
containing the target hostname. With this option, socat
resolves the hostname locally and sends the IP address.
Please note that, according to RFC 2396, only name
resolution to IPv4 addresses is implemented.
RANGE option group
These options check if a connecting client should be granted
access. They can be applied to listening and receiving network
sockets. tcp-wrappers options fall into this group.
range=<address-range>
After accepting a connection, tests if the peer is within
range. For IPv4 addresses, address-range takes the form
address/bits, e.g. 10.0.0.0/8, or address:mask, e.g.
10.0.0.0:255.0.0.0 (example); for IPv6, it is
[ip6-address]/bits, e.g. [::1]/128. If the client address
does not match, socat refuses the connection attempt,
issues a warning, and keeps listening/receiving.
tcpwrap[=<name>]
Uses Wietse Venema’s libwrap (tcpd) library to determine if
the client is allowed to connect. The configuration files
are /etc/hosts.allow and /etc/hosts.deny per default, see
"man 5 hosts_access" for more information. The optional
<name> (type string) is passed to the wrapper functions as
daemon process name (example). If omitted, the basename of
socats invocation (argv[0]) is passed. If both tcpwrap and
range options are applied to an address, both conditions
must be fulfilled to allow the connection.
allow-table=<filename>
Takes the specified file instead of /etc/hosts.allow.
deny-table=<filename>
Takes the specified file instead of /etc/hosts.deny.
tcpwrap-etc=<directoryname>
Looks for hosts.allow and hosts.deny in the specified
directory. Is overridden by options hosts-allow and
hosts-deny.
LISTEN option group
Options specific to listening sockets.
backlog=<count>
Sets the backlog value passed with the listen() system call
to <count> [int]. Default is 5.
accept-timeout=<seconds>
End waiting for a connection after <seconds> [timeval] with
error status.
CHILD option group
Addresses of LISTEN and CONNECT type take the fork option to
handle multiple connections via child processes.
fork After establishing a connection, handles its channel in a
child process and keeps the parent process attempting to
produce more connections, either by listening or by
connecting in a loop (example).
OPENSSL-CONNECT and OPENSSL-LISTEN differ in when they
actually fork off the child: OPENSSL-LISTEN forks before
the SSL handshake, while OPENSSL-CONNECT forks afterwards.
retry and forever options are not inherited by the child
process.
On some operating systems (e.g. FreeBSD) this option does
not work for UDP-LISTEN addresses.
max-children=<count>
Limits the number of concurrent child processes [int].
Default is no limit.
children-shutup[=1|2|..]
Decreases the severity of log messages produced by child
processes. For example, with value 1 notices are logged as
info (or dropped depending on option -dX), and errors are
logged as warnings but still cause termination of the child
process.
This option is intended to reduce logging of high volume
servers or proxies.
This option succeeds option cool-write.
EXEC option group
Options for addresses that invoke a program.
path=<string>
Overrides the PATH environment variable for searching the
program with <string>. This $PATH value is effective in the
child process too.
login Prefixes argv[0] for the execvp() call with ’-’, thus
making a shell behave as login shell.
FORK option group
EXEC or SYSTEM addresses invoke a program using a child process
and transfer data between socat and the program. The interprocess
communication mechanism can be influenced with the following
options. Per default, a socketpair() is created and assigned to
stdin and stdout of the child process, while stderr is inherited
from the socat process, and the child process uses file
descriptors 0 and 1 for communicating with the main socat process.
nofork Does not fork a subprocess for executing the program,
instead calls execvp() or system() directly from the actual
socat instance. This avoids the overhead of another process
between the program and its peer, but introduces a lot of
restrictions:
o this option can only be applied to the second socat
address.
o it cannot be applied to a part of a dual address.
o the first socat address cannot be OPENSSL or READLINE
o socat options -b, -t, -D, -l, -v, -x become useless
o for both addresses, options ignoreeof, cr, and crnl become
useless
o for both addresses, address specific end/shutdown handling
(e.g., graceful socket shutdown) and related options become
useless
o for the second address (the one with option nofork),
options append, cloexec, flock, user, group, mode,
nonblock, perm-late, setlk, and setpgid cannot be applied.
Some of these could be used on the first address though.
pipes Creates a pair of unnamed pipes for interprocess
communication instead of a socket pair.
openpty
Establishes communication with the sub process using a
pseudo terminal created with openpty() instead of the
default (socketpair or ptmx).
ptmx Establishes communication with the sub process using a
pseudo terminal created by opening /dev/ptmx or /dev/ptc
instead of the default (socketpair).
pty Establishes communication with the sub process using a
pseudo terminal instead of a socket pair. Creates the pty
with an available mechanism. If openpty and ptmx are both
available, it uses ptmx because this is POSIX compliant
(example).
ctty Makes the pty the controlling tty of the sub process
(example).
stderr Directs stderr of the sub process to its output channel by
making stderr a dup() of stdout (example).
fdin=<fdnum>
Assigns the sub processes input channel to its file
descriptor <fdnum> instead of stdin (0). The program
started from the subprocess has to use this fd for reading
data from socat (example).
fdout=<fdnum>
Assigns the sub processes output channel to its file
descriptor <fdnum> instead of stdout (1). The program
started from the subprocess has to use this fd for writing
data to socat (example).
sighup, sigint, sigquit
Has socat pass signals of this type to the sub process. If
no address has this option, socat terminates on these
signals.
Options for address SHELL
shell=<filename>
Overwrites use the default shell with the named executable,
e.g. /bin/dash. Also sets the SHELL environment variable.
TERMIOS option group
For addresses that work on a tty (e.g., stdio, file:/dev/tty,
exec:...,pty), the terminal parameters defined in the UN*X termios
mechanism are made available as address option parameters. Please
note that changes of the parameters of your interactive terminal
remain effective after socat’s termination, so you might have to
enter "reset" or "stty sane" in your shell afterwards. For EXEC
and SYSTEM addresses with option PTY, these options apply to the
pty by the child processes.
b0 Disconnects the terminal.
b19200 Sets the serial line speed to 19200 baud. Some other rates
are possible; use something like socat -hh |grep ’ b[1-9]’
to find all speeds supported by your implementation.
Note: On some operating systems, these options may not be
available. Use ispeed or ospeed instead.
echo[=<bool>]
Enables or disables local echo.
icanon[=<bool>]
Sets or clears canonical mode, enabling line buffering and
some special characters.
raw Sets raw mode, thus passing input and output almost
unprocessed. This option is obsolete, use option rawer or
cfmakeraw instead.
rawer Makes terminal rawer than raw option. This option
implicitly turns off echo. (example).
cfmakeraw
Sets raw mode by invoking cfmakeraw() or by simulating this
call. This option implicitly turns off echo.
ignbrk[=<bool>]
Ignores or interprets the BREAK character (e.g., ^C)
brkint[=<bool>]
bs0
bs1
bsdly=<0|1>
clocal[=<bool>]
cr0
cr1
cr2
cr3
Sets the carriage return delay to 0, 1, 2, or 3,
respectively. 0 means no delay, the other values are
terminal dependent.
crdly=<0|1|2|3>
cread[=<bool>]
crtscts[=<bool>]
cs5
cs6
cs7
cs8
Sets the character size to 5, 6, 7, or 8 bits,
respectively.
csize=<0|1|2|3>
cstopb[=<bool>]
Sets two stop bits, rather than one.
dsusp=<byte>
Sets the value for the VDSUSP character that suspends the
current foreground process and reactivates the shell (all
except Linux).
echoctl[=<bool>]
Echos control characters in hat notation (e.g. ^A)
echoe[=<bool>]
echok[=<bool>]
echoke[=<bool>]
echonl[=<bool>]
echoprt[=<bool>]
eof=<byte>
eol=<byte>
eol2=<byte>
erase=<byte>
discard=<byte>
ff0
ff1
ffdly[=<bool>]
flusho[=<bool>]
hupcl[=<bool>]
icrnl[=<bool>]
iexten[=<bool>]
igncr[=<bool>]
ignpar[=<bool>]
imaxbel[=<bool>]
inlcr[=<bool>]
inpck[=<bool>]
intr=<byte>
isig[=<bool>]
ispeed=<unsigned-int>
Set the baud rate for incoming data on this line.
See also: ospeed, b19200
istrip[=<bool>]
iuclc[=<bool>]
ixany[=<bool>]
ixoff[=<bool>]
ixon[=<bool>]
kill=<byte>
lnext=<byte>
min=<byte>
nl0 Sets the newline delay to 0.
nl1
nldly[=<bool>]
noflsh[=<bool>]
ocrnl[=<bool>]
ofdel[=<bool>]
ofill[=<bool>]
olcuc[=<bool>]
onlcr[=<bool>]
onlret[=<bool>]
onocr[=<bool>]
opost[=<bool>]
Enables or disables output processing; e.g., converts NL to
CR-NL.
ospeed=<unsigned-int>
Set the baud rate for outgoing data on this line.
See also: ispeed, b19200
parenb[=<bool>]
Enable parity generation on output and parity checking for
input.
parmrk[=<bool>]
parodd[=<bool>]
pendin[=<bool>]
quit=<byte>
reprint=<byte>
sane Brings the terminal to something like a useful default
state.
start=<byte>
stop=<byte>
susp=<byte>
swtc=<byte>
tab0
tab1
tab2
tab3
tabdly=<unsigned-int>
time=<byte>
tostop[=<bool>]
vt0
vt1
vtdly[=<bool>]
werase=<byte>
xcase[=<bool>]
xtabs
i-pop-all
With UNIX System V STREAMS, removes all drivers from the
stack.
i-push=<string>
With UNIX System V STREAMS, pushes the driver (module) with
the given name (string) onto the stack. For example, to
make sure that a character device on Solaris supports
termios etc, use the following options:
i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat
PTY option group
These options are intended for use with the pty address type.
link=<filename>
Generates a symbolic link that points to the actual pseudo
terminal (pty). This might help to solve the problem that
ptys are generated with more or less unpredictable names,
making it difficult to directly access the socat generated
pty automatically. With this option, the user can specify a
"fix" point in the file hierarchy that helps him to access
the actual pty (example). Beginning with socat version
1.4.3, the symbolic link is removed when the address is
closed (but see option unlink-close).
wait-slave
Blocks the open phase until a process opens the slave side
of the pty. Usually, socat continues after generating the
pty with opening the next address or with entering the
transfer loop. With the wait-slave option, socat waits
until some process opens the slave side of the pty before
continuing. This option only works if the operating system
provides the poll() system call. And it depends on an
undocumented behaviour of pty’s, so it does not work on all
operating systems. It has successfully been tested on
Linux, FreeBSD, NetBSD, and on Tru64 with openpty.
pty-interval=<seconds>
When the wait-slave option is set, socat periodically
checks the HUP condition using poll() to find if the pty’s
slave side has been opened. The default polling interval is
1s. Use the pty-interval option [timeval] to change this
value.
sitout-eio=<timeval>
The login program in Linux closes its tty/pty and reopens
it for security reasons. During this time the pty master
would get EIO on I/O operations and might terminate. With
this option socat tolerates EIO for the specified time.
Please note that in this state socat blocks traffic in both
directions, even when it is not related to this channel.
OPENSSL option group
These options apply to the openssl and openssl-listen address
types.
cipher=<cipherlist>
Specifies the list of ciphers that may be used for the
connection. See the man page of ciphers , section CIPHER
LIST FORMAT, for detailed information about syntax, values,
and default of <cipherlist>.
Several cipher strings may be given, separated by ’:’.
Some simple cipher strings:
3DES Uses a cipher suite with triple DES.
MD5 Uses a cipher suite with MD5.
aNULL Uses a cipher suite without authentication.
NULL Does not use encryption.
HIGH Uses a cipher suite with "high" encryption. Note that the
peer must support the selected property, or the negotiation
will fail.
method=<ssl-method>
This option is based on deprecated functions and is only
available when socat was build with option
--with-openssl-method. Use option min-proto-version and
maybe max-proto-version instead. Sets the protocol version
to be used. Valid strings (not case sensitive) are:
SSL2 Select SSL protocol version 2.
SSL3 Select SSL protocol version 3.
SSL23 Select the best available SSL or TLS protocol.
TLS1 Select TLS protocol version 1.
TLS1.1 Select TLS protocol version 1.1.
TLS1.2 Select TLS protocol version 1.2. When this option is not
provided OpenSSL negotiates the method with its peer.
min-proto-version
This option tells OpenSSL to use this or a later SSL/TLS
protocol version and refuses to accept a lower/older
protocol. Valid syntax is:
SSL2 Select SSL protocol version 2.
SSL3 Select SSL protocol version 3.
TLS1
TLS1.0 Select TLS protocol version 1.
TLS1.1 Select TLS protocol version 1.1.
TLS1.2 Select TLS protocol version 1.2.
TLS1.3 Select TLS protocol version 1.3.
openssl-max-proto-version
This option is similar to min-proto-version, however, it
disallows use of a higher protocol version. Useful for
testing the peer.
verify[=<bool>]
Controls check of the peer’s certificate. Default is 1
(true). Disabling verify might open your socket for
everyone, making the encryption useless!
cert=<filename>
Specifies the file with the certificate and private key for
authentication. The certificate must be in OpenSSL format
(*.pem). With openssl-listen, use of this option is
strongly recommended. Except with cipher aNULL, "no shared
ciphers" error will occur when no certificate is given.
key=<filename>
Specifies the file with the private key. The private key
may be in this file or in the file given with the cert
option. The party that has to proof that it is the owner of
a certificate needs the private key.
dhparams=<filename>
Specifies the file with the Diffie Hellman parameters.
These parameters may also be in the file given with the
cert option in which case the dhparams option is not
needed.
cafile=<filename>
Specifies the file with the trusted (root) authority
certificates. The file must be in PEM format and should
contain one or more certificates. The party that checks the
authentication of its peer trusts only certificates that
are in this file.
capath=<dirname>
Specifies the directory with the trusted (root)
certificates. The directory must contain certificates in
PEM format and their hashes (see OpenSSL documentation)
egd=<filename>
On some systems, openssl requires an explicit source of
random data. Specify the socket name where an entropy
gathering daemon like egd provides random data, e.g.
/dev/egd-pool.
openssl-maxfraglen=<int>, maxfraglen=<int>
For client connections, make a Max Fragment Length
Negotiation Request to the server to limit the maximum size
fragment the server will send to us. Supported lengths are:
512, 1024, 2048, or 4096. Note that this option is not
applicable for OPENSSL-LISTEN.
openssl-maxsendfrag=<int>, maxsendfrag=<int>
Limit the maximum size of the fragment we will send to the
other side. Supported length range: 512 - 16384. Note that
under OPENSSL-LISTEN, the maximum fragment size may be
further limited by the client’s Maximum Fragment Length
Negotiation Request, if it makes one.
pseudo On systems where openssl cannot find an entropy source and
where no entropy gathering daemon can be utilized, this
option activates a mechanism for providing pseudo entropy.
This is achieved by taking the current time in microseconds
for feeding the libc pseudo random number generator with an
initial value. openssl is then fed with output from
random() calls.
NOTE:This mechanism is not sufficient for generation of
secure keys!
compress
Enable or disable the use of compression for a connection.
Setting this to "none" disables compression, setting it to
"auto" lets OpenSSL choose the best available algorithm
supported by both parties. The default is to not touch any
compression-related settings. NOTE: Requires OpenSSL 0.9.8
or higher and disabling compression with OpenSSL 0.9.8
affects all new connections in the process.
commonname=<string>
Specify the commonname that the peer certificate must
match. With OPENSSL-CONNECT address this overrides the
given hostname or IP target address; with OPENSSL-LISTEN
this turns on check of peer certificates commonname. This
option has only meaning when option verify is not disabled
and the chosen cipher provides a peer certificate.
no-sni[=<bool>]
Do not use the client side Server Name Indication (SNI)
feature that selects the desired server certificate.
Note: SNI is automatically used since socat version 1.7.4.0
and uses commonname or the given host name.
snihost=<string>
Set the client side Server Name Indication (SNI) host name
different from the addressed server name or common name.
This might be useful when the server certificate has
multiple host names or wildcard names because the SNI host
name is passed in cleartext to the server and might be
eavesdropped; with this option a mock name of the desired
certificate may be transferred.
fips Enables FIPS mode if compiled in. For info about the FIPS
encryption implementation standard see
http://oss-institute.org/fips-faq.html. This mode might
require that the involved certificates are generated with a
FIPS enabled version of openssl. Setting or clearing this
option on one socat address affects all OpenSSL addresses
of this process.
RETRY option group
Options that control retry of some system calls, especially
connection attempts.
retry=<num>
Number of retries before the connection or listen attempt
is aborted. Default is 0, which means just one attempt.
interval=<timespec>
Time between consecutive attempts (seconds, [timespec]).
Default is 1 second.
forever
Performs an unlimited number of retry attempts.
INTERFACE option group
These options may be applied to addresses INTERFACE and TUN. These
address types and options are currently only implemented on Linux
operating system.
Note regarding VLANs: On incoming packets the Linux kernel strips
off the VLAN tag before passing the data to the user space program
on raw sockets. Special measures are required to get the VLAN
information, see packet(7) PACKET_AUXDATA, and to optionally
insert the tag into the packet again, use option retrieve-vlan
when you need this.
retrieve-vlan
On packets incoming on raw sockets, retrieve the VLAN
information and insert it into the packets for further
processing (Linux)
iff-up Sets the TUN network interface status UP. Strongly
recommended.
iff-broadcast
Sets the BROADCAST flag of the TUN network interface.
iff-debug
Sets the DEBUG flag of the TUN network interface.
iff-loopback
Sets the LOOPBACK flag of the TUN network interface.
iff-pointopoint
Sets the POINTOPOINT flag of the TUN device.
iff-notrailers
Sets the NOTRAILERS flag of the TUN device.
iff-running
Sets the RUNNING flag of the TUN device.
iff-noarp
Sets the NOARP flag of the TUN device.
iff-promisc
Sets the PROMISC flag of the TUN device.
iff-allmulti
Sets the ALLMULTI flag of the TUN device.
iff-master
Sets the MASTER flag of the TUN device.
iff-slave
Sets the SLAVE flag of the TUN device.
iff-multicast
Sets the MULTICAST flag of the TUN device.
iff-portsel
Sets the PORTSEL flag of the TUN device.
iff-automedia
Sets the AUTOMEDIA flag of the TUN device.
iff-dynamic
Sets the DYNAMIC flag of the TUN device.
TUN option group
Options that control Linux TUN/TAP interface device addresses.
tun-device=<device-file>
Instructs socat to take another path for the TUN clone
device. Default is /dev/net/tun.
tun-name=<if-name>
Gives the resulting network interface a specific name
instead of the system generated (tun0, tun1, etc.)
tun-type=[tun|tap]
Sets the type of the TUN device; use this option to
generate a TAP device. See the Linux docu for the
difference between these types. When you try to establish
a tunnel between two TUN devices, their types should be the
same.
iff-no-pi
Sets the IFF_NO_PI flag which controls if the device
includes additional packet information in the tunnel. When
you try to establish a tunnel between two TUN devices,
these flags should have the same values.
POSIX-MQ option group
Options that may be applied to POSIX-MQ addresses.
posixmq-priority (mq-prio)
Sets the priority of messages (packets) written to the
queue, or the minimal priority of packet read from the
queue.
posixmq-flush (mq-flush)
"Consumes" (drops) all messages currently in the queue
before starting transfers.
posixmq-maxmsg (mq-maxmsg)
Sets the maxmsg parameter of the POSIX message queue when
creating it.
Note: This option applies only when the queue does not
already exist.
posixmq-msgsize (mq-msgsize)
Sets the msgsize parameter of the POSIX message queue when
creating it.
Note: This option applies only when the queue does not
already exist.
This section explains the different data types that address
parameters and address options can take.
address-range
Is currently only implemented for IPv4 and IPv6. See
address-option `range’
bool "0" or "1"; if value is omitted, "1" is taken.
byte An unsigned int number, read with strtoul() , lower or
equal to UCHAR_MAX .
command-line
A string specifying a program name and its arguments,
separated by single spaces.
data This is a more general data specification, "dalan" (low
level data description language). The given text string
contains information about the target data type and value.
Generally a leading character specifies the type of the
following data item. In its specific context a default data
type may exist.
Currently only the following specifications are
implemented:
i A signed integer number, stored in host byte order.
Example: i-1000 (Integer number -1000)
I An unsigned integer number, stored in host byte order.
l A signed long integer number, stored in host byte order.
L An unsigned long integer number, stored in host byte order.
s A signed short integer number, stored in host byte order.
S An unsigned short integer number, stored in host byte
order.
b A signed byte (signed char).
B An unsigned byte (unsigned char).
x Following is an even number of hex digits, stored as
sequence of bytes, the data length is the resulting number
of bytes.
Example: x7f000001 (IP address 127.0.0.1)
" Following is a string that is used with the common
conversions \n \r \t \f \b \a \e \0; the string must be
closed with ’"’. Please note that the quotes and
backslashes need to be escaped from shell and socat
conversion. No implicit \0 is appended.
Example: "Hello world!\n"
’ A single char, with the usual conversions. Please note that
the quotes and backslashes need to be escaped from shell
and socat conversion.
Example: ’a’ Data items may be separated with white
space without need to repeat the type specifier again.
directory
A string with usual UN*X directory name semantics.
facility
The name of a syslog facility in lower case characters.
fdnum An unsigned int type, read with strtoul() , specifying a
UN*X file descriptor.
filename
A string with usual UN*X filename semantics.
group If the first character is a decimal digit, the value is
read with strtoul() as unsigned integer specifying a group
id. Otherwise, it must be an existing group name.
int A number following the rules of the strtol() function with
base "0", i.e. decimal number, octal number with leading
"0", or hexadecimal number with leading "0x". The value
must fit into a C int.
interface
A string specifying the device name of a network interface
as shown by ifconfig or procan, e.g. "eth0".
IP address
An IPv4 address in numbers-and-dots notation, an IPv6
address in hex notation enclosed in brackets, or a hostname
that resolves to an IPv4 or an IPv6 address.
Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1
IPv4 address
An IPv4 address in numbers-and-dots notation or a hostname
that resolves to an IPv4 address.
Examples: 127.0.0.1, www.dest-unreach.org, dns2
IPv6 address
An IPv6 address in hexnumbers-and-colons notation enclosed
in brackets, or a hostname that resolves to an IPv6
address.
Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0],
ip6name.domain.org
long A number read with strtol() . The value must fit into a C
long.
long long
A number read with strtoll() . The value must fit into a C
long long.
off_t An implementation dependent signed number, usually 32 bits,
read with strtol or strtoll.
off64_t
An implementation dependent signed number, usually 64 bits,
read with strtol or strtoll.
mode_t An unsigned integer, read with strtoul() , specifying mode
(permission) bits.
pid_t A number, read with strtol() , specifying a process id.
port A uint16_t (16 bit unsigned number) specifying a TCP or UDP
port, read with strtoul() .
protocol
An unsigned 8 bit number, read with strtoul() .
size_t An unsigned number with size_t limitations, read with
strtoul .
sockname
A socket address. See address-option `bind’
string A sequence of characters, not containing ’\0’ and,
depending on the position within the command line, ’:’,
’,’, or "!!". Note that you might have to escape shell meta
characters in the command line.
TCP service
A service name, not starting with a digit, that is resolved
by getservbyname() , or an unsigned int 16 bit number read
with strtoul() .
timeval
A double float specifying seconds; the number is mapped
into a struct timeval, consisting of seconds and
microseconds.
timespec
A double float specifying seconds; the number is mapped
into a struct timespec, consisting of seconds and
nanoseconds.
UDP service
A service name, not starting with a digit, that is resolved
by getservbyname() , or an unsigned int 16 bit number read
with strtoul() .
unsigned int
A number read with strtoul() . The value must fit into a C
unsigned int.
user If the first character is a decimal digit, the value is
read with strtoul() as unsigned integer specifying a user
id. Otherwise, it must be an existing user name.
VSOCK cid
A uint32_t (32 bit unsigned number) specifying a VSOCK
Context Identifier (CID), read with strtoul() . There are
several special addresses: VMADDR_CID_ANY (-1U) means any
address for binding; VMADDR_CID_HOST (2) is the well-known
address of the host.
VSOCK port
A uint32_t (32 bit unsigned number) specifying a VSOCK
port, read with strtoul() .
socat - TCP4:www.domain.org:80
transfers data between STDIO (-) and a TCP4 connection to
port 80 of host www.domain.org. This example results in an
interactive connection similar to telnet or netcat. The
stdin terminal parameters are not changed, so you may close
the relay with ^D or abort it with ^C.
socat -d -d \
READLINE,history=$HOME/.http_history \
TCP4:www.domain.org:www,crnl
this is similar to the previous example, but you can edit
the current line in a bash like manner (READLINE) and use
the history file .http_history; socat prints messages about
progress (-d -d). The port is specified by service name
(www), and correct network line termination characters
(crnl) instead of NL are used.
socat \
TCP4-LISTEN:www \
TCP4:www.domain.org:www
installs a simple TCP port forwarder. With TCP4-LISTEN it
listens on local port "www" until a connection comes in,
accepts it, then connects to the remote host (TCP4) and
starts data transfer. It will not accept a second
connection.
socat -d -d -lmlocal2 \
TCP4-LISTEN:80,bind=myaddr1,reuseaddr,fork,su=nobody,range=10.0.0.0/8 \
TCP4:www.domain.org:80,bind=myaddr2
TCP port forwarder, each side bound to another local IP
address (bind). This example handles an almost arbitrary
number of parallel or consecutive connections by fork’ing a
new process after each accept() . It provides a little
security by su’ing to user nobody after forking; it only
permits connections from the private 10 network (range);
due to reuseaddr, it allows immediate restart after master
process’s termination, even if some child sockets are not
completely shut down. With -lmlocal2, socat logs to stderr
until successfully reaching the accept loop. Further
logging is directed to syslog with facility local2.
socat \
TCP4-LISTEN:5555,fork,tcpwrap=script \
EXEC:/bin/myscript,chroot=/home/sandbox,su-d=sandbox,pty,stderr
a simple server that accepts connections (TCP4-LISTEN) and
fork’s a new child process for each connection; every child
acts as single relay. The client must match the rules for
daemon process name "script" in /etc/hosts.allow and
/etc/hosts.deny, otherwise it is refused access (see "man 5
hosts_access"). For EXEC’uting the program, the child
process chroot’s to /home/sandbox, su’s to user sandbox,
and then starts the program /home/sandbox/bin/myscript.
Socat and myscript communicate via a pseudo tty (pty);
myscript’s stderr is redirected to stdout, so its error
messages are transferred via socat to the connected client.
socat \
EXEC:"mail.sh target@domain.com",fdin=3,fdout=4 \
TCP4:mail.relay.org:25,crnl,bind=alias1.server.org,mss=512
mail.sh is a shell script, distributed with socat, that
implements a simple SMTP client. It is programmed to
"speak" SMTP on its FDs 3 (in) and 4 (out). The fdin and
fdout options tell socat to use these FDs for communication
with the program. Because mail.sh inherits stdin and stdout
while socat does not use them, the script can read a mail
body from stdin. Socat makes alias1 your local source
address (bind), cares for correct network line termination
(crnl) and sends at most 512 data bytes per packet (mss).
socat \
-,escape=0x0f \
/dev/ttyS0,rawer,crnl
opens an interactive connection via the serial line, e.g.
for talking with a modem. rawer sets the console’s and
ttyS0’s terminal parameters to practicable values, crnl
converts to correct newline characters. escape allows
terminating the socat process with character control-O.
Consider using READLINE instead of the first address.
socat \
UNIX-LISTEN:/tmp/.X11-unix/X1,fork \
SOCKS4:host.victim.org:127.0.0.1:6000,socksuser=nobody,sourceport=20
with UNIX-LISTEN, socat opens a listening UNIX domain
socket /tmp/.X11-unix/X1. This path corresponds to local
XWindow display :1 on your machine, so XWindow client
connections to DISPLAY=:1 are accepted. Socat then speaks
with the SOCKS4 server host.victim.org that might permit
sourceport 20 based connections due to an FTP related
weakness in its static IP filters. Socat pretends to be
invoked by socksuser nobody, and requests to be connected
to loopback port 6000 (only weak sockd configurations will
allow this). So we get a connection to the victims XWindow
server and, if it does not require MIT cookies or Kerberos
authentication, we can start work. Please note that there
can only be one connection at a time, because TCP can
establish only one session with a given set of addresses
and ports.
socat -u \
/tmp/readdata,seek-end=0,ignoreeof \
STDIO
this is an example for unidirectional data transfer (-u).
Socat transfers data from file /tmp/readdata (implicit
address GOPEN), starting at its current end (seek-end=0
lets socat start reading at current end of file; use seek=0
or no seek option to first read the existing data) in a
"tail -f" like mode (ignoreeof). The "file" might also be a
listening UNIX domain socket (do not use a seek option
then).
(sleep 5; echo PASSWORD; sleep 5; echo ls; sleep 1) | \
socat - \
EXEC:'ssh -l user server',pty,setsid,ctty
EXEC’utes an ssh session to server. Uses a pty for
communication between socat and ssh, makes it ssh’s
controlling tty (ctty), and makes this pty the owner of a
new process group (setsid), so ssh accepts the password
from socat.
socat -u \
TCP4-LISTEN:3334,reuseaddr,fork \
OPEN:/tmp/in.log,creat,append
implements a simple network based message collector. For
each client connecting to port 3334, a new child process is
generated (option fork). All data sent by the clients are
append’ed to the file /tmp/in.log. If the file does not
exist, socat creat’s it. Option reuseaddr allows immediate
restart of the server process.
socat \
READLINE,noecho='[Pp]assword:' \
EXEC:'ftp ftp.server.com',pty,setsid,ctty
wraps a command line history (READLINE) around the
EXEC’uted ftp client utility. This allows editing and
reuse of FTP commands for relatively comfortable browsing
through the ftp directory hierarchy. The password is
echoed! pty is required to have ftp issue a prompt.
Nevertheless, there may occur some confusion with the
password and FTP prompts.
socat \
PTY,link=$HOME/dev/vmodem0,rawer,wait-slave \
EXEC:'"ssh modemserver.us.org socat - /dev/ttyS0,nonblock,rawer"'
generates a pseudo terminal device (PTY) on the client that
can be reached under the symbolic link $HOME/dev/vmodem0.
An application that expects a serial line or modem can be
configured to use $HOME/dev/vmodem0; its traffic will be
directed to a modemserver via ssh where another socat
instance links it to /dev/ttyS0.
sudo socat --experimental \
TCP4-LISTEN:8000,reuseaddr,fork,netns=namespace1 \
TCP4-CONNECT:server2:8000
creates a listener in the given network namespace that
accepts TCP connections on port 8000 and forwards them to
server2.
sudo socat --experimental \
TUN:192.168.2.1/24,up \
TUN:192.168.2.2/24,up,netns=namespace2
creates two virtual network interfaces, one in default
namespace, the other one in namespace2, and forwards
packets between them, acting as a virtual network
connection.
socat \
TCP4-LISTEN:2022,reuseaddr,fork \
PROXY:proxy.local:www.domain.org:22,proxyport=3128,proxyauth=username:s3cr3t
starts a forwarder that accepts connections on port 2022,
and directs them through the proxy daemon listening on port
3128 (proxyport) on host proxy.local, using the CONNECT
method, where they are authenticated as "username" with
"s3cr3t" (proxyauth). proxy.local should establish
connections to host www.domain.org on port 22 then.
socat - \
SSL:server:4443,cafile=./server.crt,cert=./client.pem
is an OpenSSL client that tries to establish a secure
connection to an SSL server. Option cafile specifies a file
that contains trust certificates: we trust the server only
when it presents one of these certificates and proofs that
it owns the related private key. Otherwise the connection
is terminated. With cert a file containing the client
certificate and the associated private key is specified.
This is required in case the server wishes a client
authentication; many Internet servers do not.
The first address (’-’) can be replaced by almost any other
socat address.
socat \
OPENSSL-LISTEN:4443,reuseaddr,pf=ip4,fork,cert=./server.pem,cafile=./client.crt \
PIPE
is an OpenSSL server that accepts TCP connections, presents
the certificate from the file server.pem and forces the
client to present a certificate that is verified against
cafile.crt.
The second address (’PIPE’) can be replaced by almost any
other socat address.
For instructions on generating and distributing OpenSSL
keys and certificates see the additional socat docu
socat-openssl.txt.
echo |
socat -u - \
FILE:/tmp/bigfile,create,largefile,seek=100000000000
creates a 100GB+1B sparse file; this requires a file system
type that supports this (ext2, ext3, ext4, reiserfs, xfs;
not minix, vfat). The operation of writing 1 byte might
take long (reiserfs: some minutes; ext2: "no" time), and
the resulting file can consume some disk space with just
its inodes (reiserfs: 2MB; ext2: 16KB).
socat \
TCP-L:7777,reuseaddr,fork \
SYSTEM:'filan -i 0 -s >&2',nofork
listens for incoming TCP connections on port 7777. For each
accepted connection, invokes a shell. This shell has its
stdin and stdout directly connected to the TCP socket
(nofork). The shell starts filan and lets it print the
socket addresses to stderr (your terminal window).
echo -e "\0\14\0\0\c" |
socat -u - \
FILE:/usr/bin/squid.exe,seek=0x00074420
functions as primitive binary editor: it writes the 4 bytes
000 014 000 000 to the executable /usr/bin/squid.exe at
offset 0x00074420 (this was a real world patch to make the
squid executable from Cygwin run under Windows, in 2004).
socat - \
TCP:www.blackhat.org:31337,readbytes=1000
connects to an unknown service and prevents being flooded.
socat -U \
TCP:target:9999,end-close \
TCP-L:8888,reuseaddr,fork
merges data arriving from different TCP streams on port
8888 to just one stream to target:9999. The end-close
option prevents the child processes forked off by the
second address from terminating the shared connection to
9999 (close(2) just unlinks the inode which stays active as
long as the parent process lives; shutdown(2) would
actively terminate the connection).
socat \
TCP-LISTEN:10021,reuseaddr,socktype=6,protocol=33,fork PIPE
is a simple DCCP echo server. DCCP is now directly
provisioned in socat, however this example shows how use
socats TCP procedures and change the socket type to
SOCK_DCCP=6 (on Linux) and the IP protocol to
IPPROTO_DCCP=33.
socat - \
TCP:<server>:10021,reuseaddr,socktype=6,protocol=33,fork
is a simple DCCP client. DCCP is now directly provisioned
in socat, however this example shows how use socats TCP
procedures, but changes the socket type to SOCK_DCCP=6 (on
Linux) and the IP protocol to IPPROTO_DCCP=33.
socat - \
UDP4-DATAGRAM:192.168.1.0:123,sp=123,broadcast,range=192.168.1.0/24
sends a broadcast to the network 192.168.1.0/24 and
receives the replies of the timeservers there. Ignores NTP
packets from hosts outside this network.
socat - \
SOCKET-DATAGRAM:2:2:17:x007bxc0a80100x0000000000000000,bind=x007bx00000000x0000000000000000,setsockopt-int=1:6:1,range=x0000xc0a80100x0000000000000000:x0000xffffff00x0000000000000000
is semantically equivalent to the previous example, but all
parameters are specified in generic form. the value 6 of
setsockopt-int is the Linux value for SO_BROADCAST.
socat - \
IP4-DATAGRAM:255.255.255.255:44,broadcast,range=10.0.0.0/8
sends a broadcast to the local network(s) using protocol
44. Accepts replies from the private address range only.
socat - \
UDP4-DATAGRAM:224.255.0.1:6666,bind=:6666,ip-add-membership=224.255.0.1:eth0
transfers data from stdin to the specified multicast
address using UDP. Both local and remote ports are 6666.
Tells the interface eth0 to also accept multicast packets
of the given group. Multiple hosts on the local network can
run this command, so all data sent by any of the hosts will
be received by all the other ones. Note that there are many
possible reasons for failure, including IP-filters, routing
issues, wrong interface selection by the operating system,
bridges, or a badly configured switch.
socat \
UDP:host2:4443 \
TUN:192.168.255.1/24,up
establishes one side of a virtual (but not private!)
network with host2 where a similar process might run, with
UDP-L and tun address 192.168.255.2. They can reach each
other using the addresses 192.168.255.1 and 192.168.255.2.
Note that streaming eg.via TCP or SSL does not guarantee to
retain packet boundaries and might thus cause packet loss.
socat - \
VSOCK-CONNECT:2:1234
establishes a VSOCK connection with the host (host is
always reachable with the well-know CID=2) on 1234 port.
socat - \
VSOCK-LISTEN:1234
listens for a VSOCK connection on 1234 port.
socat - \
VSOCK-CONNECT:31:4321,bind:5555
establishes a VSOCK connection with the guest that have
CID=31 on 1234 port, binding the local socket to the 5555
port.
socat \
VSOCK-LISTEN:3333,reuseaddr,fork \
VSOCK-CONNECT:42,3333
starts a forwarder that accepts VSOCK connections on port
3333, and directs them to the guest with CID=42 on the same
port.
socat \
VSOCK-LISTEN:22,reuseaddr,fork \
TCP:localhost:22
forwards VSOCK connections from 22 port to the local SSH
server. Running this in a VM allows you to connect via SSH
from the host using VSOCK, as in the example below.
socat \
TCP4-LISTEN:22222,reuseaddr,fork \
VSOCK-CONNECT:33:22
forwards TCP connections from 22222 port to the guest with
CID=33 listening on VSOCK port 22. Running this in the
host, allows you to connect via SSH running "ssh -p 22222
user@localhost", if the guest runs the example above.
socat \
PTY,link=/var/run/ppp,rawer \
INTERFACE:hdlc0
circumvents the problem that pppd requires a serial device
and thus might not be able to work on a synchronous line
that is represented by a network device. socat creates a
PTY to make pppd happy, binds to the network interface
hdlc0, and can transfer data between both devices. Use pppd
on device /var/run/ppp then.
socat -u \
STDIO \
POSIXMQ-SEND:/queue1,unlink-early,mq-prio=10
Writes packets read from stdio (i.e., lines of input when
run interactively) into POSIX message queue, with priority
10.
socat -u \
POSIXMQ-RECV:/queue1,fork,max-children=3 \
SYSTEM:"worker.sh"
Receives messages (packets) from POSIX message queue and,
for each message, forks a sub process that reads and
processes the message. At most 3 sub processes are allowed
at the same time.
socat -T 1 -d -d \
TCP-L:10081,reuseaddr,fork,crlf \
SYSTEM:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType: text/plain\\\n\\\ndate: \$\(date\)\\\nserver:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT\\\nclient: \$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n\\\"\"; cat; echo -e \"\\\"\\\n\\\"\""
creates a very primitive HTTP echo server: each HTTP client
that connects gets a valid HTTP reply that contains
information about the client address and port as it is seen
by the server host, the host address (which might vary on
multihomed servers), and the original client request.
socat -d -d \
UDP4-RECVFROM:9999,so-broadcast,so-timestamp,ip-pktinfo,ip-recverr,ip-recvopts,ip-recvtos,ip-recvttl!!- \
SYSTEM:'export; sleep 1' |
grep SOCAT
waits for an incoming UDP packet on port 9999 and prints
the environment variables provided by socat. On BSD based
systems you have to replace ip-pktinfo with
ip-recvdstaddr,ip-recvif. Especially of interest is
SOCAT_IP_DSTADDR: it contains the target address of the
packet which may be a unicast, multicast, or broadcast
address.
echo -e "M-SEARCH * HTTP/1.1\nHOST: 239.255.255.250:1900\nMAN: \"ssdp:discover\"\nMX: 4\nST: \"ssdp:all\"\n" |
socat - \
UDP-DATAGRAM:239.255.255.250:1900,crlf
sends an SSDP (Simple Service Discovery Protocol) query to
the local network and collects and outputs the answers
received.
systemd-socket-activate -l 1077 --inetd socat ACCEPT:0,fork PIPE
systemd-socket-activate is a program for testing systemd
socket activation of daemons. With --inetd it waits for a
connection on the specified port. It does not accept the
connection but passes the listening file descriptor as FDs
0 and 1. Socat accepts the waiting connection and starts
data transfer.
Socat uses a logging mechanism that allows filtering messages by
severity. The severities provided are more or less compatible to
the appropriate syslog priority. With one or up to four
occurrences of the -d command line option, the lowest priority of
messages that are issued can be selected. Each message contains a
single uppercase character specifying the messages severity (one
of F, E, W, N, I, or D)
FATAL: Conditions that require unconditional and immediate program
termination.
ERROR: Conditions that prevent proper program processing. Usually
the program is terminated (see option -s).
WARNING:
Something did not function correctly or is in a state where
correct further processing cannot be guaranteed, but might
be possible.
NOTICE:
Interesting actions of the program, e.g. for supervising
socat in some kind of server mode.
INFO: Description of what the program does, and maybe why it
happens. Allows monitoring the lifecycles of file
descriptors.
DEBUG: Description of how the program works, all system or library
calls and their results.
Log messages can be written to stderr, to a file, or to syslog.
On exit, socat gives status 0 if it terminated due to EOF or
inactivity timeout, with a positive value on error, and with a
negative value on fatal error.
/usr/bin/socat
/usr/bin/filan
/usr/bin/procan
SIGUSR1:
Causes logging of current transfer statistics.
See also option --statistics
Input variables carry information from the environment to socat,
output variables are set by socat for use in executed scripts and
programs.
In the output variables beginning with "SOCAT" this prefix is
actually replaced by the upper case name of the executable or the
value of option -lp.
SOCAT_DEFAULT_LISTEN_IP (input)
(Values 4 or 6) Sets the IP version to be used for listen,
recv, and recvfrom addresses if no pf (protocol-family)
option is given. Is overridden by socat options -4 or -6.
SOCAT_PREFERRED_RESOLVE_IP (input)
(Values 0, 4, or 6) Sets the IP version to be used when
resolving target host names when version is not specified
by address type, option pf (protocol-family), or address
format. If name resolution does not return a matching
entry, the first result (with differing IP version) is
taken. With value 0, socat always selects the first record
and its IP version.
SOCAT_MAIN_WAIT (input)
Specifies the time (seconds) to sleep the main process on
begin of main\(). Useful for debugging.
SOCAT_TRANSFER_WAIT (input)
Specifies the time (seconds) to sleep the process after
opening addresses before entering the transfer loop. Useful
for debugging.
SOCAT_FORK_WAIT (input)
Specifies the time (seconds) to sleep the parent and child
processes after successful fork(). Useful for debugging.
SOCAT_VERSION (output)
Socat sets this variable to its version string, e.g.
"1.7.0.0" for released versions or e.g. "1.6.0.1+envvar"
for temporary versions; can be used in scripts invoked by
socat.
SOCAT_PID (output)
Socat sets this variable to its process id. In case of fork
address option, SOCAT_PID gets the child processes id.
Forking for exec, system, and SHELL does not change
SOCAT_PID.
SOCAT_PPID (output)
Socat sets this variable to its process id. In case of
fork, SOCAT_PPID keeps the pid of the master process.
SOCAT_PEERADDR (output)
With passive socket addresses (all LISTEN and RECVFROM
addresses), this variable is set to a string describing the
peers socket address. Port information is not included.
SOCAT_PEERPORT (output)
With appropriate passive socket addresses (TCP, UDP, and
SCTP - LISTEN and RECVFROM), this variable is set to a
string containing the number of the peer port.
SOCAT_SOCKADDR (output)
With all LISTEN addresses, this variable is set to a string
describing the local socket address. Port information is
not included example
SOCAT_SOCKPORT (output)
With TCP-LISTEN, UDP-LISTEN, and SCTP-LISTEN addresses,
this variable is set to the local port.
SOCAT_TIMESTAMP (output)
With all RECVFROM addresses where address option
so-timestamp is applied, socat sets this variable to the
resulting timestamp.
SOCAT_IP_OPTIONS (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvopts is applied, socat fills this variable with the
IP options of the received packet.
SOCAT_IP_DSTADDR (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvdstaddr (BSD) or ip-pktinfo (other platforms) is
applied, socat sets this variable to the destination
address of the received packet. This is particularly useful
to identify broadcast and multicast addressed packets.
SOCAT_IP_IF (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvif (BSD) or ip-pktinfo (other platforms) is applied,
socat sets this variable to the name of the interface where
the packet was received.
SOCAT_IP_LOCADDR (output)
With all IPv4 based RECVFROM addresses where address option
ip-pktinfo is applied, socat sets this variable to the
address of the interface where the packet was received.
SOCAT_IP_TOS (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvtos is applied, socat sets this variable to the TOS
(type of service) of the received packet.
SOCAT_IP_TTL (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvttl is applied, socat sets this variable to the TTL
(time to live) of the received packet.
SOCAT_IPV6_HOPLIMIT (output)
With all IPv6 based RECVFROM addresses where address option
ipv6-recvhoplimit is applied, socat sets this variable to
the hoplimit value of the received packet.
SOCAT_IPV6_DSTADDR (output)
With all IPv6 based RECVFROM addresses where address option
ipv6-recvpktinfo is applied, socat sets this variable to
the destination address of the received packet.
SOCAT_IPV6_TCLASS (output)
With all IPv6 based RECVFROM addresses where address option
ipv6-recvtclass is applied, socat sets this variable to the
transfer class of the received packet.
SOCAT_OPENSSL_X509_ISSUER (output)
Issuer field from peer certificate
SOCAT_OPENSSL_X509_SUBJECT (output)
Subject field from peer certificate
SOCAT_OPENSSL_X509_COMMONNAME (output)
commonName entries from peer certificates subject. Multiple
values are separated by " // ".
SOCAT_OPENSSL_X509_* (output)
all other entries from peer certificates subject
SOCAT_OPENSSL_X509V3_DNS (output)
DNS entries from peer certificates extensions -
subjectAltName field. Multiple values are separated by " //
".
HOSTNAME (input)
Is used to determine the hostname for logging (see -lh).
LOGNAME (input)
Is used as name for the socks client user name if no
socksuser is given.
With options su and su-d, LOGNAME is set to the given user
name.
USER (input)
Is used as name for the socks client user name if no
socksuser is given and LOGNAME is empty.
With options su and su-d, USER is set to the given user
name.
SHELL (output)
With options su and su-d, SHELL is set to the login shell
of the given user.
PATH (output)
Can be set with option path for exec, system, and SHELL
addresses.
HOME (output)
With options su and su-d, HOME is set to the home directory
of the given user.
The work of the following groups and organizations was invaluable
for this project:
The FSF (GNU, http://www.fsf.org/) project with their free and
portable development software and lots of other useful tools and
libraries.
The Linux developers community (http://www.linux.org/) for
providing a free, open source operating system.
The Open Group (http://www.unix-systems.org/) for making their
standard specifications available on the Internet for free.
This man page describes version 1.8.0 of socat.
Addresses cannot be nested, so a single socat process cannot,
e.g., drive ssl over socks.
Address option ftruncate without value uses default 1 instead of
0.
Verbose modes (-x and/or -v) display line termination characters
inconsistently when address options cr or crnl are used: They show
the data after conversion in either direction.
The data transfer blocksize setting (-b) is ignored with address
readline.
Send bug reports to <socat@dest-unreach.org>
nc(1), rinetd(8), openssl(1), stunnel(8), rlwrap(1), setsid(1)
Socat home page http://www.dest-unreach.org/socat/
Gerhard Rieger <rieger@dest-unreach.org> and contributors
This page is part of the socat (Multipurpose relay (SOcket CAT))
project. Information about the project can be found at
⟨http://www.dest-unreach.org/socat/⟩. It is not known how to
report bugs for this man page; if you know, please send a mail to
man-pages@man7.org. This page was obtained from the tarball
socat-1.8.0.3.tar.gz fetched from
⟨http://www.dest-unreach.org/socat/download/⟩ on 2025-08-11. If
you discover any rendering problems in this HTML version of the
page, or you believe there is a better or more up-to-date source
for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
socat(1)
Pages that refer to this page: systemd-socket-proxyd(8)
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