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

       ip - Linux IPv4 protocol implementation

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

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

       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 by calling the socket(2) function as
       socket(PF_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

       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 an 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.

       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 */
             u_int16_t      sin_port;   /* port in network byte order */
             struct in_addr sin_addr;   /* internet address */

         /* Internet address. */
         struct in_addr {
             u_int32_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 reserved ports.  Only privileged processes (i.e.,
       those having the CAP_NET_BIND_SERVICE capability) may bind(2) to these
       sockets.  Note that the raw IPv4 protocol as such has no concept of a
       port, they are only implemented by higher protocols like tcp(7) and

       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_ANY) 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 only sent or received when the SO_BROADCAST
       socket flag is set.  In the current implementation connection oriented
       sockets are only allowed to use 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

       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.

              Sets 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
              sysctl.  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.

              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.

              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.

              When this flag is set pass a IP_TTL control message with the
              time to live field of the received packet as a byte. Not
              supported for SOCK_STREAM sockets.

              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

              Identical to IP_RECVOPTS but returns raw unprocessed options
              with timestamp and route record options not filled in for this

       IP_TOS 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 behaviour depends on the configured queueing discipline.
              Some high priority levels may require superuser privileges (the
              CAP_NET_ADMIN capability).  The priority can also be set in a
              protocol independent way by the (SOL_SOCKET, SO_PRIORITY) socket
              option (see socket(7)).

       IP_TTL Set or retrieve the current time to live field that is used in
              every packet sent from this socket.

              If enabled the user supplies an ip header in front of the user
              data. Only valid 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_RECVERR (defined in <linux/errqueue.h>)
              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 a 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

              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 {
                    u_int32_t ee_errno;   /* error number */
                    u_int8_t  ee_origin;  /* where the error originated */
                    u_int8_t  ee_type;    /* type */
                    u_int8_t  ee_code;    /* code */
                    u_int8_t  ee_pad;
                    u_int32_t ee_info;    /* additional information */
                    u_int32_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 illegal on SOCK_STREAM sockets.  Thus 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.

              Sets or receives the Path MTU Discovery setting for a socket.
              When enabled, Linux will perform Path MTU Discovery as defined
              in RFC 1191 on this socket. The don't fragment flag is set on
              all outgoing datagrams.  The system-wide default is controlled
              by the ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and
              disabled on all others. For non SOCK_STREAM sockets 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
              packets that are bigger than the known path MTU if this flag is
              set (with EMSGSIZE ).

              Path MTU discovery flags   Meaning
              IP_PMTUDISC_WANT           Use per-route settings.
              IP_PMTUDISC_DONT           Never do Path MTU Discovery.
              IP_PMTUDISC_DO             Always do Path MTU Discovery.

              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 a EMSGSIZE error occurred).  It may change
              over time.  For connectionless sockets with many destinations
              the new also 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

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

              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.

       IP_MTU Retrieve the current known path MTU of the current socket.  Only
              valid when the socket has been connected. Returns an integer.
              Only valid as a getsockopt(2).

              Pass all to-be forwarded packets with the IP Router Alert option
              set to this socket. Only valid 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 users responsibility
              to send them out again.  Socket binding is ignored, such packets
              are only filtered by protocol.  Expects an integer flag.

              Set or reads 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

              Sets or reads a boolean integer argument whether sent multicast
              packets should be looped back to the local sockets.

              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.  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.

              For compatibility, the old ip_mreq structure is still supported.
              It differs from ip_mreqn only by not including the imr_ifindex
              field. Only valid as a setsockopt(2).

              Leave a multicast group. Argument is an ip_mreqn or ip_mreq
              structure similar to IP_ADD_MEMBERSHIP.

              Set the local device for a multicast socket. Argument is an
              ip_mreqn or ip_addr structure similar to IP_ADD_MEMBERSHIP.

              When an invalid socket option is passed, ENOPROTOOPT is

       The IP protocol supports the sysctl interface to configure some global
       options.  The sysctls can be accessed by reading or writing the
       /proc/sys/net/ipv4/* files or using the sysctl(2) interface.  Variables
       described as Boolean take an integer value, with a non-zero value
       ("true") meaning that the corresponding option is enabled, and a zero
       value ("false") meaning that the option is disabled.

       ip_always_defrag (Boolean)
              [New with kernel 2.2.13; in earlier kernel version the feature
              was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG
              option; this file is not present in 2.4.x and later]

              When this boolean frag 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.

              Only enable if running either a firewall that is the sole link
              to your network or a transparent proxy; never ever turn on here
              for a normal router or host. Otherwise fragmented communication
              may me disturbed when the fragments would travel over different
              links. Defragmentation also has a large memory and CPU time

              This is automagically turned on when masquerading or transparent
              proxying are configured.

              Not documented.

       ip_default_ttl (integer; default: 64)
              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)
              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)
              Enable IP forwarding with a boolean flag.  IP forwarding can be
              also set on a per interface basis.

              Contains two integers that define the default local port range
              allocated to sockets.  Allocation starts with the first number
              and ends with the second number.  Note that these 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.  First number should be at least >1024,
              better >4096 to avoid clashes with well known ports and to
              minimize firewall problems.

       ip_no_pmtu_disc (Boolean; default: disabled)
              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)
              If set, allows processes to bind() to non-local 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.

              See arp(7).

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

       Ioctls to configure generic device parameters are described in

       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 SOL_IP socket options level isn't portable, BSD-based stacks use
       IPPROTO_IP level.

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

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

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

       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 reserved port without superuser privileges (the
              CAP_NET_BIND_SERVICE capability).

              Tried to bind to an address already in use.

              Invalid socket option passed.

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

              A non-existent interface was requested or the requested source
              address was not local.

       EAGAIN Operation on a non-blocking socket would block.

              The socket is not configured or an unknown socket type was

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

              An connection operation on a non-blocking socket is already in

              A connection was closed during an accept(2).

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

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

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

       ENODEV Network device not available or not capable of sending IP.

       ENOPKG A kernel subsystem was not configured.

              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.

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

       new options in Linux 2.2.  They are also all Linux specific and should
       not be used in programs intended to be portable.

       struct ip_mreqn is new in Linux 2.2.  Linux 2.0 only supported ip_mreq.

       The sysctls were introduced with Linux 2.2.

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

       There are too many inconsistent error values.

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

       Some versions of glibc forget to declare in_pktinfo.  Workaround
       currently is to copy it into your program from this man page.

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

       recvmsg(2), sendmsg(2), byteorder(3), ipfw(4), capabilities(7),
       netlink(7), raw(7), socket(7), tcp(7), udp(7)

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

Linux Man Page                    2001-06-19                             IP(7)