ncat(1) — Linux manual page


NCAT(1)                   Ncat Reference Guide                   NCAT(1)

NAME         top

       ncat - Concatenate and redirect sockets

SYNOPSIS         top

       ncat [OPTIONS...] [hostname] [port]

DESCRIPTION         top

       Ncat is a feature-packed networking utility which reads and
       writes data across networks from the command line. Ncat was
       written for the Nmap Project and is the culmination of the
       currently splintered family of Netcat incarnations. It is
       designed to be a reliable back-end tool to instantly provide
       network connectivity to other applications and users. Ncat will
       not only work with IPv4 and IPv6 but provides the user with a
       virtually limitless number of potential uses.

       Among Ncat's vast number of features there is the ability to
       chain Ncats together; redirection of TCP, UDP, and SCTP ports to
       other sites; SSL support; and proxy connections via SOCKS4,
       SOCKS5 or HTTP proxies (with optional proxy authentication as
       well). Some general principles apply to most applications and
       thus give you the capability of instantly adding networking
       support to software that would normally never support it.


           Ncat 7.94 ( )
           Usage: ncat [options] [hostname] [port]

           Options taking a time assume seconds. Append 'ms' for milliseconds,
           's' for seconds, 'm' for minutes, or 'h' for hours (e.g. 500ms).
             -4                         Use IPv4 only
             -6                         Use IPv6 only
             -U, --unixsock             Use Unix domain sockets only
                 --vsock                Use vsock sockets only
             -C, --crlf                 Use CRLF for EOL sequence
             -c, --sh-exec <command>    Executes the given command via /bin/sh
             -e, --exec <command>       Executes the given command
                 --lua-exec <filename>  Executes the given Lua script
             -g hop1[,hop2,...]         Loose source routing hop points (8 max)
             -G <n>                     Loose source routing hop pointer (4, 8, 12, ...)
             -m, --max-conns <n>        Maximum <n> simultaneous connections
             -h, --help                 Display this help screen
             -d, --delay <time>         Wait between read/writes
             -o, --output <filename>    Dump session data to a file
             -x, --hex-dump <filename>  Dump session data as hex to a file
             -i, --idle-timeout <time>  Idle read/write timeout
             -p, --source-port port     Specify source port to use
             -s, --source addr          Specify source address to use (doesn't affect -l)
             -l, --listen               Bind and listen for incoming connections
             -k, --keep-open            Accept multiple connections in listen mode
             -n, --nodns                Do not resolve hostnames via DNS
             -t, --telnet               Answer Telnet negotiations
             -u, --udp                  Use UDP instead of default TCP
                 --sctp                 Use SCTP instead of default TCP
             -v, --verbose              Set verbosity level (can be used several times)
             -w, --wait <time>          Connect timeout
             -z                         Zero-I/O mode, report connection status only
                 --append-output        Append rather than clobber specified output files
                 --send-only            Only send data, ignoring received; quit on EOF
                 --recv-only            Only receive data, never send anything
                 --no-shutdown          Continue half-duplex when receiving EOF on stdin
                 --allow                Allow only given hosts to connect to Ncat
                 --allowfile            A file of hosts allowed to connect to Ncat
                 --deny                 Deny given hosts from connecting to Ncat
                 --denyfile             A file of hosts denied from connecting to Ncat
                 --broker               Enable Ncat's connection brokering mode
                 --chat                 Start a simple Ncat chat server
                 --proxy <addr[:port]>  Specify address of host to proxy through
                 --proxy-type <type>    Specify proxy type ("http", "socks4", "socks5")
                 --proxy-auth <auth>    Authenticate with HTTP or SOCKS proxy server
                 --proxy-dns <type>     Specify where to resolve proxy destination
                 --ssl                  Connect or listen with SSL
                 --ssl-cert             Specify SSL certificate file (PEM) for listening
                 --ssl-key              Specify SSL private key (PEM) for listening
                 --ssl-verify           Verify trust and domain name of certificates
                 --ssl-trustfile        PEM file containing trusted SSL certificates
                 --ssl-ciphers          Cipherlist containing SSL ciphers to use
                 --ssl-servername       Request distinct server name (SNI)
                 --ssl-alpn             ALPN protocol list to use
                 --version              Display Ncat's version information and exit

           See the ncat(1) manpage for full options, descriptions and usage examples


       Ncat operates in one of two primary modes: connect mode and
       listen mode. Other modes, such as the HTTP proxy server, act as
       special cases of these two. In connect mode, Ncat works as a
       client. In listen mode it is a server.

       In connect mode, the hostname and port arguments tell what to
       connect to.  hostname is required, and may be a hostname or IP
       address. If port is supplied, it must be a decimal port number.
       If omitted, it defaults to 31337.

       In listen mode, hostname and port control the address the server
       will bind to. Both arguments are optional in listen mode. If
       hostname is omitted, it defaults to listening on all available
       addresses over IPv4 and IPv6. If port is omitted, it defaults to


       -4 (IPv4 only)
           Force the use of IPv4 only.

       -6 (IPv6 only)
           Force the use of IPv6 only.

       -U, --unixsock (Use Unix domain sockets)
           Use Unix domain sockets rather than network sockets. This
           option may be used on its own for stream sockets, or combined
           with --udp for datagram sockets. A description of -U mode is
           in the section called “UNIX DOMAIN SOCKETS”.

       -u, --udp (Use UDP)
           Use UDP for the connection (the default is TCP).

       --sctp (Use SCTP)
           Use SCTP for the connection (the default is TCP). SCTP
           support is implemented in TCP-compatible mode.

       --vsock (Use AF_VSOCK sockets)
           Use AF_VSOCK sockets rather than the default TCP sockets
           (Linux only). This option may be used on its own for stream
           sockets or combined with --udp for datagram sockets. A
           description of --vsock mode is in the section called
           “AF_VSOCK SOCKETS”.


       -g hop1[,hop2,...] (Loose source routing)
           Sets hops for IPv4 loose source routing. You can use -g once
           with a comma-separated list of hops, use -g multiple times
           with single hops to build the list, or combine the two. Hops
           can be given as IP addresses or hostnames.

       -G ptr (Set source routing pointer)
           Sets the IPv4 source route “pointer” for use with -g. The
           argument must be a multiple of 4 and no more than 28. Not all
           operating systems support setting this pointer to anything
           other than four.

       -p port, --source-port port (Specify source port)
           Set the port number for Ncat to bind to.

       -s host, --source host (Specify source address)
           Set the address for Ncat to bind to.


       See the section called “ACCESS CONTROL OPTIONS” for information
       on limiting the hosts that may connect to the listening Ncat

       -l, --listen (Listen for connections)
           Listen for connections rather than connecting to a remote

       -m numconns, --max-conns numconns (Specify maximum number of
           The maximum number of simultaneous connections accepted by an
           Ncat instance. 100 is the default (60 on Windows).

       -k, --keep-open (Accept multiple connections)
           Normally a listening server accepts only one connection and
           then quits when the connection is closed. This option makes
           it accept multiple simultaneous connections and wait for more
           connections after they have all been closed. It must be
           combined with --listen. In this mode there is no way for Ncat
           to know when its network input is finished, so it will keep
           running until interrupted. This also means that it will never
           close its output stream, so any program reading from Ncat and
           looking for end-of-file will also hang.

       --broker (Connection brokering)
           Allow multiple parties to connect to a centralised Ncat
           server and communicate with each other. Ncat can broker
           communication between systems that are behind a NAT or
           otherwise unable to directly connect. This option is used in
           conjunction with --listen, which causes the --listen port to
           have broker mode enabled.

       --chat (Ad-hoc “chat server”)
           The --chat option enables chat mode, intended for the
           exchange of text between several users. In chat mode,
           connection brokering is turned on. Ncat prefixes each message
           received with an ID before relaying it to the other
           connections. The ID is unique for each connected client. This
           helps distinguish who sent what. Additionally, non-printing
           characters such as control characters are escaped to keep
           them from doing damage to a terminal.

SSL OPTIONS         top

       --ssl (Use SSL)
           In connect mode, this option transparently negotiates an SSL
           session with an SSL server to securely encrypt the
           connection. This is particularly handy for talking to SSL
           enabled HTTP servers, etc.

           In server mode, this option listens for incoming SSL
           connections, rather than plain untunneled traffic.

           In UDP mode, this option enables Datagram TLS (DTLS).

       --ssl-verify (Verify server certificates)
           In client mode, --ssl-verify is like --ssl except that it
           also requires verification of the server certificate. Ncat
           comes with a default set of trusted certificates in the file
           ca-bundle.crt.  Some operating systems provide a default list
           of trusted certificates; these will also be used if
           available. Use --ssl-trustfile to give a custom list. Use -v
           one or more times to get details about verification failures.
           Ncat does not check for revoked certificates.

           This option has no effect in server mode.

       --ssl-cert certfile.pem (Specify SSL certificate)
           This option gives the location of a PEM-encoded certificate
           files used to authenticate the server (in listen mode) or the
           client (in connect mode). Use it in combination with

       --ssl-key keyfile.pem (Specify SSL private key)
           This option gives the location of the PEM-encoded private key
           file that goes with the certificate named with --ssl-cert.

       --ssl-trustfile cert.pem (List trusted certificates)
           This option sets a list of certificates that are trusted for
           purposes of certificate verification. It has no effect unless
           combined with --ssl-verify. The argument to this option is
           the name of a PEM file containing trusted certificates.
           Typically, the file will contain certificates of
           certification authorities, though it may also contain server
           certificates directly. When this option is used, Ncat does
           not use its default certificates.

       --ssl-ciphers cipherlist (Specify SSL ciphersuites)
           This option sets the list of ciphersuites that Ncat will use
           when connecting to servers or when accepting SSL connections
           from clients. The syntax is described in the OpenSSL
           ciphers(1) man page, and defaults to

       --ssl-servername name (Request distinct server name)
           In client mode, this option sets the TLS SNI (Server Name
           Indication) extension, which tells the server the name of the
           logical server Ncat is contacting. This is important when the
           target server hosts multiple virtual servers at a single
           underlying network address. If the option is not provided,
           the TLS SNI extension will be populated with the target
           server hostname.

       --ssl-alpn ALPN list (Specify ALPN protocol list)
           This option allows you to specify a comma-separated list of
           protocols to send via the Application-Layer Protocol
           Negotiation (ALPN) TLS extension. Not supported by all
           versions of OpenSSL.

PROXY OPTIONS         top

       --proxy host[:port] (Specify proxy address)
           Requests proxying through host:port, using the protocol
           specified by --proxy-type.

           If no port is specified, the proxy protocol's well-known port
           is used (1080 for SOCKS and 3128 for HTTP). When specifying
           an IPv6 HTTP proxy server using the IP address rather than
           the hostname, the square-bracket notation (for example
           [2001:db8::1]:8080) MUST be used to separate the port from
           the IPv6 address. If the proxy requires authentication, use

       --proxy-type proto (Specify proxy protocol)
           In connect mode, this option requests the protocol proto to
           connect through the proxy host specified by --proxy. In
           listen mode, this option has Ncat act as a proxy server using
           the specified protocol.

           The currently available protocols in connect mode are http
           (CONNECT), socks4 (SOCKSv4), and socks5 (SOCKSv5). The only
           server currently supported is http. If this option is not
           used, the default protocol is http.

       --proxy-auth user[:pass] (Specify proxy credentials)
           In connect mode, gives the credentials that will be used to
           connect to the proxy server. In listen mode, gives the
           credentials that will be required of connecting clients. For
           use with --proxy-type http or --proxy-type socks5, the form
           should be username:password. For --proxy-type socks4, it
           should be a username only.

           These credentials can be alternatively passed onto Ncat by
           setting environment variable NCAT_PROXY_AUTH, which reduces
           the risk of the credentials being captured in process logs.
           (Option --proxy-auth takes precedence.)

       --proxy-dns type (Specify where to resolve proxy destination)
           In connect mode, it provides control over whether proxy
           destination hostnames are resolved by the remote proxy server
           or locally, by Ncat itself. Possible values for type are:

           local - Hostnames are resolved locally on the Ncat host. Ncat
           exits with error if the hostname cannot be resolved.

           remote - Hostnames are passed directly onto the remote proxy
           server. This is the default behavior.

           both - Hostname resolution is first attempted on the Ncat
           host. Unresolvable hostnames are passed onto the remote proxy

           none - Hostname resolution is completely disabled. Only a
           literal IPv4 or IPv6 address can be used as the proxy

           Local hostname resolution generally respects IP version
           specified with options -4 or -6, except for SOCKS4, which is
           incompatible with IPv6.


       -e command, --exec command (Execute command)
           Execute the specified command after a connection has been
           established. The command must be specified as a full
           pathname. All input from the remote client will be sent to
           the application and responses sent back to the remote client
           over the socket, thus making your command-line application
           interactive over a socket. Combined with --keep-open, Ncat
           will handle multiple simultaneous connections to your
           specified port/application like inetd. Ncat will only accept
           a maximum, definable, number of simultaneous connections
           controlled by the -m option. By default this is set to 100
           (60 on Windows).

       -c command, --sh-exec command (Execute command via sh)
           Same as -e, except it tries to execute the command via
           /bin/sh. This means you don't have to specify the full path
           for the command, and shell facilities like environment
           variables are available.

       --lua-exec file (Execute a .lua script)
           Runs the specified file as a Lua script after a connection
           has been established, using a built-in interpreter. Both the
           script's standard input and the standard output are
           redirected to the connection data streams.

       All exec options add the following variables to the child's

           The IP address and port number of the remote host. In connect
           mode, it's the target's address; in listen mode, it's the
           client's address.

           The IP address and port number of the local end of the

           The protocol in use: one of TCP, UDP, and SCTP.


       --allow host[,host,...] (Allow connections)
           The list of hosts specified will be the only hosts allowed to
           connect to the Ncat process. All other connection attempts
           will be disconnected. In case of a conflict between --allow
           and --deny, --allow takes precedence. Host specifications
           follow the same syntax used by Nmap.

       --allowfile file (Allow connections from file)
           This has the same functionality as --allow, except that the
           allowed hosts are provided in a new-line delimited allow
           file, rather than directly on the command line.

       --deny host[,host,...] (Deny connections)
           Issue Ncat with a list of hosts that will not be allowed to
           connect to the listening Ncat process. Specified hosts will
           have their session silently terminated if they try to
           connect. In case of a conflict between --allow and --deny,
           --allow takes precedence. Host specifications follow the same
           syntax used by Nmap.

       --denyfile file (Deny connections from file)
           This is the same functionality as --deny, except that
           excluded hosts are provided in a new-line delimited deny
           file, rather than directly on the command line.

TIMING OPTIONS         top

       These options accept a time parameter. This is specified in
       seconds by default, though you can append ms, s, m, or h to the
       value to specify milliseconds, seconds, minutes, or hours.

       -d time, --delay time (Specify line delay)
           Set the delay interval for lines sent. This effectively
           limits the number of lines that Ncat will send in the
           specified period. This may be useful for low-bandwidth sites,
           or have other uses such as coping with annoying iptables
           --limit options.

       -i time, --idle-timeout time (Specify idle timeout)
           Set a fixed timeout for idle connections. If the idle timeout
           is reached, the connection is terminated.

       -w time, --wait time (Specify connect timeout)
           Set a fixed timeout for connection attempts.

OUTPUT OPTIONS         top

       -o file, --output file (Save session data)
           Dump session data to a file

       -x file, --hex-dump file (Save session data in hex)
           Dump session data in hex to a file.

       --append-output (Append output)
           Issue Ncat with --append-ouput along with -o and/or -x and it
           will append the resulted output rather than truncating the
           specified output files.

       -v, --verbose (Be verbose)
           Issue Ncat with -v and it will be verbose and display all
           kinds of useful connection based information. Use more than
           once (-vv, -vvv...) for greater verbosity.

MISC OPTIONS         top

       -C, --crlf (Use CRLF as EOL)
           This option tells Ncat to convert LF line endings to CRLF
           when taking input from standard input.  This is useful for
           talking to some stringent servers directly from a terminal in
           one of the many common plain-text protocols that use CRLF for

       -h, --help (Help screen)
           Displays a short help screen with common options and
           parameters, and then exits.

       --recv-only (Only receive data)
           If this option is passed, Ncat will only receive data and
           will not try to send anything.

       --send-only (Only send data)
           If this option is passed, then Ncat will only send data and
           will ignore anything received. This option also causes Ncat
           to close the network connection and terminate after EOF is
           received on standard input.

       --no-shutdown (Do not shutdown into half-duplex mode)
           If this option is passed, Ncat will not invoke shutdown on a
           socket after seeing EOF on stdin. This is provided for
           backward-compatibility with OpenBSD netcat, which exhibits
           this behavior when executed with its '-d' option.

       -n, --nodns (Do not resolve hostnames)
           Completely disable hostname resolution across all Ncat
           options, such as the destination, source address, source
           routing hops, and the proxy. All addresses must be specified
           numerically. (Note that resolution of proxy destinations is
           controlled separately via option --proxy-dns.)

       -t, --telnet (Answer Telnet negotiations)
           Handle DO/DONT WILL/WONT Telnet negotiations. This makes it
           possible to script Telnet sessions with Ncat.

       --version (Display version)
           Displays the Ncat version number and exits.


       The -U option (same as --unixsock) causes Ncat to use Unix domain
       sockets rather than network sockets. Unix domain sockets exist as
       an entry in the filesystem. You must give the name of a socket to
       connect to or to listen on. For example, to make a connection,

       ncat -U ~/unixsock

       To listen on a socket:

       ncat -l -U ~/unixsock

       Listen mode will create the socket if it doesn't exist. The
       socket will continue to exist after the program ends.

       Both stream and datagram domain sockets are supported. Use -U on
       its own for stream sockets, or combine it with --udp for datagram
       sockets. Datagram sockets require a source socket to connect
       from. By default, a source socket with a random filename will be
       created as needed, and deleted when the program ends. Use the
       --source with a path to use a source socket with a specific name.

AF_VSOCK SOCKETS         top

       The --vsock option causes Ncat to use AF_VSOCK sockets rather
       than network sockets. A CID must be given instead of a hostname
       or IP address. For example, to make a connection to the host,

       ncat --vsock 2 1234

       To listen on a socket:

       ncat -l --vsock 1234

       Both stream and datagram domain sockets are supported, but socket
       type availability depends on the hypervisor. Use --vsock on its
       own for stream sockets, or combine it with --udp for datagram

EXAMPLES         top

       Connect to on TCP port 8080.
           ncat 8080

       Listen for connections on TCP port 8080.
           ncat -l 8080

       Redirect TCP port 8080 on the local machine to host on port 80.
           ncat --sh-exec "ncat 80" -l 8080 --keep-open

       Bind to TCP port 8081 and attach /bin/bash for the world to
       access freely.
           ncat --exec "/bin/bash" -l 8081 --keep-open

       Bind a shell to TCP port 8081, limit access to hosts on a local
       network, and limit the maximum number of simultaneous connections
       to 3.
           ncat --exec "/bin/bash" --max-conns 3 --allow
           -l 8081 --keep-open

       Connect to smtphost:25 through a SOCKS4 server on port 1080.
           ncat --proxy socks4host --proxy-type socks4 --proxy-auth joe
           smtphost 25

       Connect to smtphost:25 through a SOCKS5 server on port 1080.
           ncat --proxy socks5host --proxy-type socks5 --proxy-auth
           joe:secret smtphost 25

       Create an HTTP proxy server on localhost port 8888.
           ncat -l --proxy-type http localhost 8888

       Send a file over TCP port 9899 from host2 (client) to host1
           HOST1$ ncat -l 9899 > outputfile

           HOST2$ ncat HOST1 9899 < inputfile

       Transfer in the other direction, turning Ncat into a “one file”
           HOST1$ ncat -l 9899 < inputfile

           HOST2$ ncat HOST1 9899 > outputfile

EXIT CODE         top

       The exit code reflects whether a connection was made and
       completed successfully. 0 means there was no error. 1 means there
       was a network error of some kind, for example “Connection
       refused” or “Connection reset”. 2 is reserved for all other
       errors, like an invalid option or a nonexistent file.

BUGS         top

       Like its authors, Ncat isn't perfect. But you can help make it
       better by sending bug reports or even writing patches. If Ncat
       doesn't behave the way you expect, first upgrade to the latest
       version available from . If the problem persists,
       do some research to determine whether it has already been
       discovered and addressed. Try Googling the error message or
       browsing the nmap-dev archives at .

       Read this full manual page as well. If nothing comes of this,
       mail a bug report to <>. Please include everything
       you have learned about the problem, as well as what version of
       Ncat you are running and what operating system version it is
       running on. Problem reports and Ncat usage questions sent to are far more likely to be answered than those sent
       to Fyodor directly.

       Code patches to fix bugs are even better than bug reports. Basic
       instructions for creating patch files with your changes are
       available at . Patches may be
       sent to nmap-dev (recommended) or to Fyodor directly.

AUTHORS         top

       •   Chris Gibson <>

       •   Gordon Lyon (Fyodor)<> ( )

       •   Kris Katterjohn <>

       •   Mixter <>

       The original Netcat was written by *Hobbit* <>.
       While Ncat isn't built on any code from the “traditional” Netcat
       (or any other implementation), Ncat is most definitely based on
       Netcat in spirit and functionality.

LEGAL NOTICES         top

   Ncat Copyright and Licensing
       Ncat is (C) 2005–2022 Nmap Software LLC. It is distributed as
       free and open source software under the same license terms as our
       Nmap software. Precise terms and further details are available
       from .

   Creative Commons License for this Ncat Guide
       This Ncat Reference Guide is (C) 2005–2022 Nmap Software LLC. It
       is hereby placed under version 3.0 of the Creative Commons
       Attribution License[1]. This allows you redistribute and modify
       the work as you desire, as long as you credit the original
       source. Alternatively, you may choose to treat this document as
       falling under the same license as Ncat itself (discussed

   Source Code Availability and Community Contributions
       Source is provided to this software because we believe users have
       a right to know exactly what a program is going to do before they
       run it. This also allows you to audit the software for security
       holes (none have been found so far).

       Source code also allows you to port Nmap (which includes Ncat) to
       new platforms, fix bugs, and add new features. You are highly
       encouraged to send your changes to <> for possible
       incorporation into the main distribution. By sending these
       changes to Fyodor or one of the Insecure.Org development mailing
       lists, it is assumed that you are offering the Nmap Project (Nmap
       Software LLC) the unlimited, non-exclusive right to reuse,
       modify, and relicense the code. Nmap will always be available
       open source, but this is important because the inability to
       relicense code has caused devastating problems for other Free
       Software projects (such as KDE and NASM). We also occasionally
       relicense the code to third parties as discussed in the Nmap man
       page. If you wish to specify special license conditions of your
       contributions, just say so when you send them.

   No Warranty
       This program is distributed in the hope that it will be useful,
       but WITHOUT ANY WARRANTY; without even the implied warranty of
       Public Source License for more details at ,
       or in the LICENSE file included with Nmap.

   Inappropriate Usage
       Ncat should never be installed with special privileges (e.g. suid
       root).  That would open up a major security vulnerability as
       other users on the system (or attackers) could use it for
       privilege escalation.

   Third-Party Software
       This product includes software developed by the Apache Software
       Foundation[2]. A modified version of the Libpcap portable packet
       capture library[3] is distributed along with Ncat. The Windows
       version of Ncat utilized the Libpcap-derived Npcap library[4]
       instead. Certain raw networking functions use the Libdnet[5]
       networking library, which was written by Dug Song.  A modified
       version is distributed with Ncat. Ncat can optionally link with
       the OpenSSL cryptography toolkit[6] for SSL version detection
       support. All of the third-party software described in this
       paragraph is freely redistributable under BSD-style software

NOTES         top

        1. Creative Commons Attribution License

        2. Apache Software Foundation

        3. Libpcap portable packet capture library

        4. Npcap library

        5. Libdnet

        6. OpenSSL cryptography toolkit

COLOPHON         top

       This page is part of the nmap (a network scanner) project.
       Information about the project can be found at ⟨⟩.
       If you have a bug report for this manual page, send it to  This page was obtained from the project's upstream
       Git mirror of the Subversion repository
       ⟨⟩ on 2023-12-22.  (At that time, the
       date of the most recent commit that was found in the repository
       was 2023-12-14.)  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

Ncat                           05/17/2023                        NCAT(1)