ip(7) — Linux manual page


IP(7)                     Linux Programmer's Manual                    IP(7)

NAME         top

       ip - Linux IPv4 protocol implementation

SYNOPSIS         top

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/ip.h> /* superset of previous */

       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
       raw_socket = socket(AF_INET, SOCK_RAW, protocol);

DESCRIPTION         top

       Linux implements the Internet Protocol, version 4, described in
       RFC 791 and RFC 1122.  ip contains a level 2 multicasting
       implementation conforming to RFC 1112.  It also contains an IP router
       including a packet filter.

       The programming interface is BSD-sockets compatible.  For more
       information on sockets, see socket(7).

       An IP socket is created using socket(2):

           socket(AF_INET, socket_type, protocol);

       Valid socket types include SOCK_STREAM to open a stream socket,
       SOCK_DGRAM to open a datagram socket, and SOCK_RAW to open a raw(7)
       socket to access the IP protocol directly.

       protocol is the IP protocol in the IP header to be received or sent.
       Valid values for protocol include:

       · 0 and IPPROTO_TCP for tcp(7) stream sockets;

       · 0 and IPPROTO_UDP for udp(7) datagram sockets;

       · IPPROTO_SCTP for sctp(7) stream sockets; and

       · IPPROTO_UDPLITE for udplite(7) datagram sockets.

       For SOCK_RAW you may specify a valid IANA IP protocol defined in
       RFC 1700 assigned numbers.

       When a process wants to receive new incoming packets or connections,
       it should bind a socket to a local interface address using bind(2).
       In this case, only one IP socket may be bound to any given local
       (address, port) pair.  When INADDR_ANY is specified in the bind call,
       the socket will be bound to all local interfaces.  When listen(2) is
       called on an unbound socket, the socket is automatically bound to a
       random free port with the local address set to INADDR_ANY.  When
       connect(2) is called on an unbound socket, the socket is
       automatically bound to a random free port or to a usable shared port
       with the local address set to INADDR_ANY.

       A TCP local socket address that has been bound is unavailable for
       some time after closing, unless the SO_REUSEADDR flag has been set.
       Care should be taken when using this flag as it makes TCP less

   Address format
       An IP socket address is defined as a combination of an IP interface
       address and a 16-bit port number.  The basic IP protocol does not
       supply port numbers, they are implemented by higher level protocols
       like udp(7) and tcp(7).  On raw sockets sin_port is set to the IP

           struct sockaddr_in {
               sa_family_t    sin_family; /* address family: AF_INET */
               in_port_t      sin_port;   /* port in network byte order */
               struct in_addr sin_addr;   /* internet address */

           /* Internet address. */
           struct in_addr {
               uint32_t       s_addr;     /* address in network byte order */

       sin_family is always set to AF_INET.  This is required; in Linux 2.2
       most networking functions return EINVAL when this setting is missing.
       sin_port contains the port in network byte order.  The port numbers
       below 1024 are called privileged ports (or sometimes: reserved
       ports).  Only a privileged process (on Linux: a process that has the
       CAP_NET_BIND_SERVICE capability in the user namespace governing its
       network namespace) may bind(2) to these sockets.  Note that the raw
       IPv4 protocol as such has no concept of a port, they are implemented
       only by higher protocols like tcp(7) and udp(7).

       sin_addr is the IP host address.  The s_addr member of struct in_addr
       contains the host interface address in network byte order.  in_addr
       should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK)
       using htonl(3) or set using the inet_aton(3), inet_addr(3),
       inet_makeaddr(3) library functions or directly with the name resolver
       (see gethostbyname(3)).

       IPv4 addresses are divided into unicast, broadcast, and multicast
       addresses.  Unicast addresses specify a single interface of a host,
       broadcast addresses specify all hosts on a network, and multicast
       addresses address all hosts in a multicast group.  Datagrams to
       broadcast addresses can be sent or received only when the SO_BROAD‐
       CAST socket flag is set.  In the current implementation, connection-
       oriented sockets are allowed to use only unicast addresses.

       Note that the address and the port are always stored in network byte
       order.  In particular, this means that you need to call htons(3) on
       the number that is assigned to a port.  All address/port manipulation
       functions in the standard library work in network byte order.

       There are several special addresses: INADDR_LOOPBACK (
       always refers to the local host via the loopback device; INADDR_ANY
       ( means any address for binding; INADDR_BROADCAST
       ( means any host and has the same effect on bind as
       INADDR_ANY for historical reasons.

   Socket options
       IP supports some protocol-specific socket options that can be set
       with setsockopt(2) and read with getsockopt(2).  The socket option
       level for IP is IPPROTO_IP.  A boolean integer flag is zero when it
       is false, otherwise true.

       When an invalid socket option is specified, getsockopt(2) and
       setsockopt(2) fail with the error ENOPROTOOPT.

       IP_ADD_MEMBERSHIP (since Linux 1.2)
              Join a multicast group.  Argument is an ip_mreqn structure.

           struct ip_mreqn {
               struct in_addr imr_multiaddr; /* IP multicast group
                                                address */
               struct in_addr imr_address;   /* IP address of local
                                                interface */
               int            imr_ifindex;   /* interface index */

       imr_multiaddr contains the address of the multicast group the appli‐
       cation wants to join or leave.  It must be a valid multicast address
       (or setsockopt(2) fails with the error EINVAL).  imr_address is the
       address of the local interface with which the system should join the
       multicast group; if it is equal to INADDR_ANY, an appropriate inter‐
       face is chosen by the system.  imr_ifindex is the interface index of
       the interface that should join/leave the imr_multiaddr group, or 0 to
       indicate any interface.

              The ip_mreqn structure is available only since Linux 2.2.  For
              compatibility, the old ip_mreq structure (present since Linux
              1.2) is still supported; it differs from ip_mreqn only by not
              including the imr_ifindex field.  (The kernel determines which
              structure is being passed based on the size passed in optlen.)

              IP_ADD_MEMBERSHIP is valid only for setsockopt(2).

       IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
              Join a multicast group and allow receiving data only from a
              specified source.  Argument is an ip_mreq_source structure.

           struct ip_mreq_source {
               struct in_addr imr_multiaddr;  /* IP multicast group
                                                 address */
               struct in_addr imr_interface;  /* IP address of local
                                                 interface */
               struct in_addr imr_sourceaddr; /* IP address of
                                                 multicast source */

       The ip_mreq_source structure is similar to ip_mreqn described under
       IP_ADD_MEMBERSHIP.  The imr_multiaddr field contains the address of
       the multicast group the application wants to join or leave.  The
       imr_interface field is the address of the local interface with which
       the system should join the multicast group.  Finally, the
       imr_sourceaddr field contains the address of the source the applica‐
       tion wants to receive data from.

              This option can be used multiple times to allow receiving data
              from more than one source.

       IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
              Inform the kernel to not reserve an ephemeral port when using
              bind(2) with a port number of 0.  The port will later be auto‐
              matically chosen at connect(2) time, in a way that allows
              sharing a source port as long as the 4-tuple is unique.

       IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
              Stop receiving multicast data from a specific source in a
              given group.  This is valid only after the application has
              subscribed to the multicast group using either IP_ADD_MEMBER‐

              Argument is an ip_mreq_source structure as described under

       IP_DROP_MEMBERSHIP (since Linux 1.2)
              Leave a multicast group.  Argument is an ip_mreqn or ip_mreq
              structure similar to IP_ADD_MEMBERSHIP.

       IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
              Leave a source-specific group—that is, stop receiving data
              from a given multicast group that come from a given source.
              If the application has subscribed to multiple sources within
              the same group, data from the remaining sources will still be
              delivered.  To stop receiving data from all sources at once,
              use IP_DROP_MEMBERSHIP.

              Argument is an ip_mreq_source structure as described under

       IP_FREEBIND (since Linux 2.4)
              If enabled, this boolean option allows binding to an IP
              address that is nonlocal or does not (yet) exist.  This per‐
              mits listening on a socket, without requiring the underlying
              network interface or the specified dynamic IP address to be up
              at the time that the application is trying to bind to it.
              This option is the per-socket equivalent of the ip_nonlo‐
              cal_bind /proc interface described below.

       IP_HDRINCL (since Linux 2.0)
              If enabled, the user supplies an IP header in front of the
              user data.  Valid only for SOCK_RAW sockets; see raw(7) for
              more information.  When this flag is enabled, the values set
              by IP_OPTIONS, IP_TTL, and IP_TOS are ignored.

       IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
              This option provides access to the advanced full-state filter‐
              ing API.  Argument is an ip_msfilter structure.

           struct ip_msfilter {
               struct in_addr imsf_multiaddr; /* IP multicast group
                                                 address */
               struct in_addr imsf_interface; /* IP address of local
                                                 interface */
               uint32_t       imsf_fmode;     /* Filter-mode */

               uint32_t       imsf_numsrc;    /* Number of sources in
                                                 the following array */
               struct in_addr imsf_slist[1];  /* Array of source
                                                 addresses */

       There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can be
       used to specify the filtering mode.  Additionally, the IP_MSFIL‐
       TER_SIZE(n) macro exists to determine how much memory is needed to
       store ip_msfilter structure with n sources in the source list.

              For the full description of multicast source filtering refer
              to RFC 3376.

       IP_MTU (since Linux 2.2)
              Retrieve the current known path MTU of the current socket.
              Returns an integer.

              IP_MTU is valid only for getsockopt(2) and can be employed
              only when the socket has been connected.

       IP_MTU_DISCOVER (since Linux 2.2)
              Set or receive the Path MTU Discovery setting for a socket.
              When enabled, Linux will perform Path MTU Discovery as defined
              in RFC 1191 on SOCK_STREAM sockets.  For non-SOCK_STREAM sock‐
              ets, IP_PMTUDISC_DO forces the don't-fragment flag to be set
              on all outgoing packets.  It is the user's responsibility to
              packetize the data in MTU-sized chunks and to do the retrans‐
              mits if necessary.  The kernel will reject (with EMSGSIZE)
              datagrams that are bigger than the known path MTU.  IP_PMTUD‐
              ISC_WANT will fragment a datagram if needed according to the
              path MTU, or will set the don't-fragment flag otherwise.

              The system-wide default can be toggled between IP_PMTUD‐
              ISC_WANT and IP_PMTUDISC_DONT by writing (respectively, zero
              and nonzero values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc

              Path MTU discovery value   Meaning
              IP_PMTUDISC_WANT           Use per-route settings.
              IP_PMTUDISC_DONT           Never do Path MTU Discovery.
              IP_PMTUDISC_DO             Always do Path MTU Discovery.
              IP_PMTUDISC_PROBE          Set DF but ignore Path MTU.

              When PMTU discovery is enabled, the kernel automatically keeps
              track of the path MTU per destination host.  When it is con‐
              nected to a specific peer with connect(2), the currently known
              path MTU can be retrieved conveniently using the IP_MTU socket
              option (e.g., after an EMSGSIZE error occurred).  The path MTU
              may change over time.  For connectionless sockets with many
              destinations, the new MTU for a given destination can also be
              accessed using the error queue (see IP_RECVERR).  A new error
              will be queued for every incoming MTU update.

              While MTU discovery is in progress, initial packets from data‐
              gram sockets may be dropped.  Applications using UDP should be
              aware of this and not take it into account for their packet
              retransmit strategy.

              To bootstrap the path MTU discovery process on unconnected
              sockets, it is possible to start with a big datagram size
              (headers up to 64 kilobytes long) and let it shrink by updates
              of the path MTU.

              To get an initial estimate of the path MTU, connect a datagram
              socket to the destination address using connect(2) and
              retrieve the MTU by calling getsockopt(2) with the IP_MTU

              It is possible to implement RFC 4821 MTU probing with
              SOCK_DGRAM or SOCK_RAW sockets by setting a value of IP_PMTUD‐
              ISC_PROBE (available since Linux 2.6.22).  This is also par‐
              ticularly useful for diagnostic tools such as tracepath(8)
              that wish to deliberately send probe packets larger than the
              observed Path MTU.

       IP_MULTICAST_ALL (since Linux 2.6.31)
              This option can be used to modify the delivery policy of mul‐
              ticast messages to sockets bound to the wildcard INADDR_ANY
              address.  The argument is a boolean integer (defaults to 1).
              If set to 1, the socket will receive messages from all the
              groups that have been joined globally on the whole system.
              Otherwise, it will deliver messages only from the groups that
              have been explicitly joined (for example via the IP_ADD_MEM‐
              BERSHIP option) on this particular socket.

       IP_MULTICAST_IF (since Linux 1.2)
              Set the local device for a multicast socket.  The argument for
              setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq
              structure similar to IP_ADD_MEMBERSHIP, or an in_addr struc‐
              ture.  (The kernel determines which structure is being passed
              based on the size passed in optlen.)  For getsockopt(2), the
              argument is an in_addr structure.

       IP_MULTICAST_LOOP (since Linux 1.2)
              Set or read a boolean integer argument that determines whether
              sent multicast packets should be looped back to the local

       IP_MULTICAST_TTL (since Linux 1.2)
              Set or read the time-to-live value of outgoing multicast pack‐
              ets for this socket.  It is very important for multicast pack‐
              ets to set the smallest TTL possible.  The default is 1 which
              means that multicast packets don't leave the local network
              unless the user program explicitly requests it.  Argument is
              an integer.

       IP_NODEFRAG (since Linux 2.6.36)
              If enabled (argument is nonzero), the reassembly of outgoing
              packets is disabled in the netfilter layer.  The argument is
              an integer.

              This option is valid only for SOCK_RAW sockets.

       IP_OPTIONS (since Linux 2.0)
              Set or get the IP options to be sent with every packet from
              this socket.  The arguments are a pointer to a memory buffer
              containing the options and the option length.  The
              setsockopt(2) call sets the IP options associated with a
              socket.  The maximum option size for IPv4 is 40 bytes.  See
              RFC 791 for the allowed options.  When the initial connection
              request packet for a SOCK_STREAM socket contains IP options,
              the IP options will be set automatically to the options from
              the initial packet with routing headers reversed.  Incoming
              packets are not allowed to change options after the connection
              is established.  The processing of all incoming source routing
              options is disabled by default and can be enabled by using the
              accept_source_route /proc interface.  Other options like time‐
              stamps are still handled.  For datagram sockets, IP options
              can be only set by the local user.  Calling getsockopt(2) with
              IP_OPTIONS puts the current IP options used for sending into
              the supplied buffer.

       IP_PKTINFO (since Linux 2.2)
              Pass an IP_PKTINFO ancillary message that contains a pktinfo
              structure that supplies some information about the incoming
              packet.  This only works for datagram oriented sockets.  The
              argument is a flag that tells the socket whether the IP_PKT‐
              INFO message should be passed or not.  The message itself can
              only be sent/retrieved as control message with a packet using
              recvmsg(2) or sendmsg(2).

                  struct in_pktinfo {
                      unsigned int   ipi_ifindex;  /* Interface index */
                      struct in_addr ipi_spec_dst; /* Local address */
                      struct in_addr ipi_addr;     /* Header Destination
                                                      address */

              ipi_ifindex is the unique index of the interface the packet
              was received on.  ipi_spec_dst is the local address of the
              packet and ipi_addr is the destination address in the packet
              header.  If IP_PKTINFO is passed to sendmsg(2) and
              ipi_spec_dst is not zero, then it is used as the local source
              address for the routing table lookup and for setting up IP
              source route options.  When ipi_ifindex is not zero, the pri‐
              mary local address of the interface specified by the index
              overwrites ipi_spec_dst for the routing table lookup.

       IP_RECVERR (since Linux 2.2)
              Enable extended reliable error message passing.  When enabled
              on a datagram socket, all generated errors will be queued in a
              per-socket error queue.  When the user receives an error from
              a socket operation, the errors can be received by calling
              recvmsg(2) with the MSG_ERRQUEUE flag set.  The
              sock_extended_err structure describing the error will be
              passed in an ancillary message with the type IP_RECVERR and
              the level IPPROTO_IP.  This is useful for reliable error han‐
              dling on unconnected sockets.  The received data portion of
              the error queue contains the error packet.

              The IP_RECVERR control message contains a sock_extended_err

                  #define SO_EE_ORIGIN_NONE    0
                  #define SO_EE_ORIGIN_LOCAL   1
                  #define SO_EE_ORIGIN_ICMP    2
                  #define SO_EE_ORIGIN_ICMP6   3

                  struct sock_extended_err {
                      uint32_t ee_errno;   /* error number */
                      uint8_t  ee_origin;  /* where the error originated */
                      uint8_t  ee_type;    /* type */
                      uint8_t  ee_code;    /* code */
                      uint8_t  ee_pad;
                      uint32_t ee_info;    /* additional information */
                      uint32_t ee_data;    /* other data */
                      /* More data may follow */

                  struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

              ee_errno contains the errno number of the queued error.
              ee_origin is the origin code of where the error originated.
              The other fields are protocol-specific.  The macro
              SO_EE_OFFENDER returns a pointer to the address of the network
              object where the error originated from given a pointer to the
              ancillary message.  If this address is not known, the sa_fam‐
              ily member of the sockaddr contains AF_UNSPEC and the other
              fields of the sockaddr are undefined.

              IP uses the sock_extended_err structure as follows: ee_origin
              is set to SO_EE_ORIGIN_ICMP for errors received as an ICMP
              packet, or SO_EE_ORIGIN_LOCAL for locally generated errors.
              Unknown values should be ignored.  ee_type and ee_code are set
              from the type and code fields of the ICMP header.  ee_info
              contains the discovered MTU for EMSGSIZE errors.  The message
              also contains the sockaddr_in of the node caused the error,
              which can be accessed with the SO_EE_OFFENDER macro.  The
              sin_family field of the SO_EE_OFFENDER address is AF_UNSPEC
              when the source was unknown.  When the error originated from
              the network, all IP options (IP_OPTIONS, IP_TTL, etc.) enabled
              on the socket and contained in the error packet are passed as
              control messages.  The payload of the packet causing the error
              is returned as normal payload.  Note that TCP has no error
              queue; MSG_ERRQUEUE is not permitted on SOCK_STREAM sockets.
              IP_RECVERR is valid for TCP, but all errors are returned by
              socket function return or SO_ERROR only.

              For raw sockets, IP_RECVERR enables passing of all received
              ICMP errors to the application, otherwise errors are only
              reported on connected sockets

              It sets or retrieves an integer boolean flag.  IP_RECVERR
              defaults to off.

       IP_RECVOPTS (since Linux 2.2)
              Pass all incoming IP options to the user in a IP_OPTIONS con‐
              trol message.  The routing header and other options are
              already filled in for the local host.  Not supported for
              SOCK_STREAM sockets.

       IP_RECVORIGDSTADDR (since Linux 2.6.29)
              This boolean option enables the IP_ORIGDSTADDR ancillary mes‐
              sage in recvmsg(2), in which the kernel returns the original
              destination address of the datagram being received.  The
              ancillary message contains a struct sockaddr_in.

       IP_RECVTOS (since Linux 2.2)
              If enabled, the IP_TOS ancillary message is passed with incom‐
              ing packets.  It contains a byte which specifies the Type of
              Service/Precedence field of the packet header.  Expects a
              boolean integer flag.

       IP_RECVTTL (since Linux 2.2)
              When this flag is set, pass a IP_TTL control message with the
              time-to-live field of the received packet as a 32 bit integer.
              Not supported for SOCK_STREAM sockets.

       IP_RETOPTS (since Linux 2.2)
              Identical to IP_RECVOPTS, but returns raw unprocessed options
              with timestamp and route record options not filled in for this

       IP_ROUTER_ALERT (since Linux 2.2)
              Pass all to-be forwarded packets with the IP Router Alert
              option set to this socket.  Valid only for raw sockets.  This
              is useful, for instance, for user-space RSVP daemons.  The
              tapped packets are not forwarded by the kernel; it is the
              user's responsibility to send them out again.  Socket binding
              is ignored, such packets are only filtered by protocol.
              Expects an integer flag.

       IP_TOS (since Linux 1.0)
              Set or receive the Type-Of-Service (TOS) field that is sent
              with every IP packet originating from this socket.  It is used
              to prioritize packets on the network.  TOS is a byte.  There
              are some standard TOS flags defined: IPTOS_LOWDELAY to mini‐
              mize delays for interactive traffic, IPTOS_THROUGHPUT to opti‐
              mize throughput, IPTOS_RELIABILITY to optimize for reliabil‐
              ity, IPTOS_MINCOST should be used for "filler data" where slow
              transmission doesn't matter.  At most one of these TOS values
              can be specified.  Other bits are invalid and shall be
              cleared.  Linux sends IPTOS_LOWDELAY datagrams first by
              default, but the exact behavior depends on the configured
              queueing discipline.  Some high-priority levels may require
              superuser privileges (the CAP_NET_ADMIN capability).

       IP_TRANSPARENT (since Linux 2.6.24)
              Setting this boolean option enables transparent proxying on
              this socket.  This socket option allows the calling applica‐
              tion to bind to a nonlocal IP address and operate both as a
              client and a server with the foreign address as the local end‐
              point.  NOTE: this requires that routing be set up in a way
              that packets going to the foreign address are routed through
              the TProxy box (i.e., the system hosting the application that
              employs the IP_TRANSPARENT socket option).  Enabling this
              socket option requires superuser privileges (the CAP_NET_ADMIN

              TProxy redirection with the iptables TPROXY target also
              requires that this option be set on the redirected socket.

       IP_TTL (since Linux 1.0)
              Set or retrieve the current time-to-live field that is used in
              every packet sent from this socket.

       IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
              Unblock previously blocked multicast source.  Returns EADDRNO‐
              TAVAIL when given source is not being blocked.

              Argument is an ip_mreq_source structure as described under

   /proc interfaces
       The IP protocol supports a set of /proc interfaces to configure some
       global parameters.  The parameters can be accessed by reading or
       writing files in the directory /proc/sys/net/ipv4/.  Interfaces
       described as Boolean take an integer value, with a nonzero value
       ("true") meaning that the corresponding option is enabled, and a zero
       value ("false") meaning that the option is disabled.

       ip_always_defrag (Boolean; since Linux 2.2.13)
              [New with kernel 2.2.13; in earlier kernel versions this fea‐
              ture was controlled at compile time by the CON‐
              FIG_IP_ALWAYS_DEFRAG option; this option is not present in
              2.4.x and later]

              When this boolean flag is enabled (not equal 0), incoming
              fragments (parts of IP packets that arose when some host
              between origin and destination decided that the packets were
              too large and cut them into pieces) will be reassembled
              (defragmented) before being processed, even if they are about
              to be forwarded.

              Enable only if running either a firewall that is the sole link
              to your network or a transparent proxy; never ever use it for
              a normal router or host.  Otherwise, fragmented communication
              can be disturbed if the fragments travel over different links.
              Defragmentation also has a large memory and CPU time cost.

              This is automagically turned on when masquerading or transpar‐
              ent proxying are configured.

       ip_autoconfig (since Linux 2.2 to 2.6.17)
              Not documented.

       ip_default_ttl (integer; default: 64; since Linux 2.2)
              Set the default time-to-live value of outgoing packets.  This
              can be changed per socket with the IP_TTL option.

       ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
              Enable dynamic socket address and masquerading entry rewriting
              on interface address change.  This is useful for dialup inter‐
              face with changing IP addresses.  0 means no rewriting, 1
              turns it on and 2 enables verbose mode.

       ip_forward (Boolean; default: disabled; since Linux 1.2)
              Enable IP forwarding with a boolean flag.  IP forwarding can
              be also set on a per-interface basis.

       ip_local_port_range (since Linux 2.2)
              This file contains two integers that define the default local
              port range allocated to sockets that are not explicitly bound
              to a port number—that is, the range used for ephemeral ports.
              An ephemeral port is allocated to a socket in the following

              *  the port number in a socket address is specified as 0 when
                 calling bind(2);

              *  listen(2) is called on a stream socket that was not previ‐
                 ously bound;

              *  connect(2) was called on a socket that was not previously

              *  sendto(2) is called on a datagram socket that was not pre‐
                 viously bound.

              Allocation of ephemeral ports starts with the first number in
              ip_local_port_range and ends with the second number.  If the
              range of ephemeral ports is exhausted, then the relevant sys‐
              tem call returns an error (but see BUGS).

              Note that the port range in ip_local_port_range should not
              conflict with the ports used by masquerading (although the
              case is handled).  Also, arbitrary choices may cause problems
              with some firewall packet filters that make assumptions about
              the local ports in use.  The first number should be at least
              greater than 1024, or better, greater than 4096, to avoid
              clashes with well known ports and to minimize firewall prob‐

       ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
              If enabled, don't do Path MTU Discovery for TCP sockets by
              default.  Path MTU discovery may fail if misconfigured fire‐
              walls (that drop all ICMP packets) or misconfigured interfaces
              (e.g., a point-to-point link where the both ends don't agree
              on the MTU) are on the path.  It is better to fix the broken
              routers on the path than to turn off Path MTU Discovery glob‐
              ally, because not doing it incurs a high cost to the network.

       ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
              If set, allows processes to bind(2) to nonlocal IP addresses,
              which can be quite useful, but may break some applications.

       ip6frag_time (integer; default: 30)
              Time in seconds to keep an IPv6 fragment in memory.

       ip6frag_secret_interval (integer; default: 600)
              Regeneration interval (in seconds) of the hash secret (or
              lifetime for the hash secret) for IPv6 fragments.

       ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
              If the amount of queued IP fragments reaches
              ipfrag_high_thresh, the queue is pruned down to
              ipfrag_low_thresh.  Contains an integer with the number of

              See arp(7).

       All ioctls described in socket(7) apply to ip.

       Ioctls to configure generic device parameters are described in

ERRORS         top

       EACCES The user tried to execute an operation without the necessary
              permissions.  These include: sending a packet to a broadcast
              address without having the SO_BROADCAST flag set; sending a
              packet via a prohibit route; modifying firewall settings
              without superuser privileges (the CAP_NET_ADMIN capability);
              binding to a privileged port without superuser privileges (the
              CAP_NET_BIND_SERVICE capability).

              Tried to bind to an address already in use.

              A nonexistent interface was requested or the requested source
              address was not local.

       EAGAIN Operation on a nonblocking socket would block.

              A connection operation on a nonblocking socket is already in

              A connection was closed during an accept(2).

              No valid routing table entry matches the destination address.
              This error can be caused by an ICMP message from a remote
              router or for the local routing table.

       EINVAL Invalid argument passed.  For send operations this can be
              caused by sending to a blackhole route.

              connect(2) was called on an already connected socket.

              Datagram is bigger than an MTU on the path and it cannot be

              Not enough free memory.  This often means that the memory
              allocation is limited by the socket buffer limits, not by the
              system memory, but this is not 100% consistent.

       ENOENT SIOCGSTAMP was called on a socket where no packet arrived.

       ENOPKG A kernel subsystem was not configured.

              Invalid socket option passed.

              The operation is defined only on a connected socket, but the
              socket wasn't connected.

       EPERM  User doesn't have permission to set high priority, change
              configuration, or send signals to the requested process or

       EPIPE  The connection was unexpectedly closed or shut down by the
              other end.

              The socket is not configured or an unknown socket type was

       Other errors may be generated by the overlaying protocols; see
       tcp(7), raw(7), udp(7), and socket(7).

NOTES         top

       IP_TRANSPARENT are Linux-specific.

       Be very careful with the SO_BROADCAST option - it is not privileged
       in Linux.  It is easy to overload the network with careless
       broadcasts.  For new application protocols it is better to use a
       multicast group instead of broadcasting.  Broadcasting is

       Some other BSD sockets implementations provide IP_RCVDSTADDR and
       IP_RECVIF socket options to get the destination address and the
       interface of received datagrams.  Linux has the more general
       IP_PKTINFO for the same task.

       Some BSD sockets implementations also provide an IP_RECVTTL option,
       but an ancillary message with type IP_RECVTTL is passed with the
       incoming packet.  This is different from the IP_TTL option used in

       Using the SOL_IP socket options level isn't portable; BSD-based
       stacks use the IPPROTO_IP level.

       INADDR_ANY ( and INADDR_BROADCAST ( are byte-
        This means htonl(3) has no effect on them.

       For compatibility with Linux 2.0, the obsolete socket(AF_INET,
       SOCK_PACKET, protocol) syntax is still supported to open a packet(7)
       socket.  This is deprecated and should be replaced by
       socket(AF_PACKET, SOCK_RAW, protocol) instead.  The main difference
       is the new sockaddr_ll address structure for generic link layer
       information instead of the old sockaddr_pkt.

BUGS         top

       There are too many inconsistent error values.

       The error used to diagnose exhaustion of the ephemeral port range
       differs across the various system calls (connect(2), bind(2),
       listen(2), sendto(2)) that can assign ephemeral ports.

       The ioctls to configure IP-specific interface options and ARP tables
       are not described.

       Receiving the original destination address with MSG_ERRQUEUE in
       msg_name by recvmsg(2) does not work in some 2.2 kernels.

SEE ALSO         top

       recvmsg(2), sendmsg(2), byteorder(3), capabilities(7), icmp(7),
       ipv6(7), netdevice(7), netlink(7), raw(7), socket(7), tcp(7), udp(7),

       The kernel source file Documentation/networking/ip-sysctl.txt.

       RFC 791 for the original IP specification.  RFC 1122 for the IPv4
       host requirements.  RFC 1812 for the IPv4 router requirements.

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-06-09                            IP(7)

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