ip

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



NAME
       ip - Linux IPv4 protocol implementation

SYNOPSIS
       #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
       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 are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM
       to open a udp(7) socket, or 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.  The only valid values for protocol are 0 and
       IPPROTO_TCP for TCP sockets, and 0 and IPPROTO_UDP for UDP 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 reliable.

   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
       protocol.

           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_BROADCAST
       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 (127.0.0.1) always
       refers to the local host via the loopback device; INADDR_ANY (0.0.0.0)
       means any address for binding; INADDR_BROADCAST (255.255.255.255) 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
       application 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
       interface 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_MEMBERSIP.  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 application
       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
              automatically 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_MEMBERSHIP or
              IP_ADD_SOURCE_MEMBERSHIP.

              Argument is an ip_mreq_source structure as described under
              IP_ADD_SOURCE_MEMBERSHIP.

       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_ADD_SOURCE_MEMBERSHIP.

       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 permits
              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_nonlocal_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 filtering
              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_MSFILTER_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
              sockets, 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 retransmits
              if necessary.  The kernel will reject (with EMSGSIZE) datagrams
              that are bigger than the known path MTU.  IP_PMTUDISC_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_PMTUDISC_WANT
              and IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
              values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.

              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
              connected 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
              datagram 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 option.

              It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
              or SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE
              (available since Linux 2.6.22).  This is also particularly
              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
              multicast 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_MEMBERSHIP 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 structure.
              (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
              sockets.

       IP_MULTICAST_TTL (since Linux 1.2)
              Set or read the time-to-live value of outgoing multicast packets
              for this socket.  It is very important for multicast packets 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 timestamps 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_PKTINFO
              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 primary 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 handling 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
              structure:

                  #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_family 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 control
              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 message
              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 incoming
              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
              hop.

       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 minimize
              delays for interactive traffic, IPTOS_THROUGHPUT to optimize
              throughput, IPTOS_RELIABILITY to optimize for reliability,
              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 application to
              bind to a nonlocal IP address and operate both as a client and a
              server with the foreign address as the local endpoint.  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 capability).

              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
              EADDRNOTAVAIL when given source is not being blocked.

              Argument is an ip_mreq_source structure as described under
              IP_ADD_SOURCE_MEMBERSHIP.

   /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 feature
              was controlled at compile time by the CONFIG_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 transparent
              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
              interface 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
              circumstances:

              *  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
                 previously bound;

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

              *  sendto(2) is called on a datagram socket that was not
                 previously 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 system
              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 problems.

       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 firewalls
              (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 globally, 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 bytes.

       neigh/*
              See arp(7).

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

       Ioctls to configure generic device parameters are described in
       netdevice(7).

ERRORS
       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).

       EADDRINUSE
              Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

       EALREADY
              A connection operation on a nonblocking socket is already in
              progress.

       ECONNABORTED
              A connection was closed during an accept(2).

       EHOSTUNREACH
              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.

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

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

       ENOBUFS, ENOMEM
              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.

       ENOPROTOOPT and EOPNOTSUPP
              Invalid socket option passed.

       ENOTCONN
              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
              group.

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

       ESOCKTNOSUPPORT
              The socket is not configured or an unknown socket type was
              requested.

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

NOTES
       IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER, IP_RECVORIGDSTADDR,
       IP_PKTINFO, IP_RECVERR, IP_ROUTER_ALERT, and 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 discouraged.

       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 Linux.

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

       INADDR_ANY (0.0.0.0) and INADDR_BROADCAST (255.255.255.255) are byte-
       order-neutral.
        This means htonl(3) has no effect on them.

   Compatibility
       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
       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
       recvmsg(2), sendmsg(2), byteorder(3), ipfw(4), capabilities(7),
       icmp(7), ipv6(7), netlink(7), raw(7), socket(7), tcp(7), udp(7), ip(8)

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

COLOPHON
       This page is part of release 5.04 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06                             IP(7)