Provided by: manpages_2.17-1_all bug


       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).   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) or connect(2) are called on an
       unbound socket, it is automatically bound to a random  free  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  processes  with  effective
       user  ID  0 or 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 addr  member  of  struct  in_addr
       contains  the  host interface address in network order.  in_addr should
       be only accessed 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 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 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 an effective user ID of 0
              or 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_mreq 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.

       The ioctls to configure firewalling are documented in ipfw(4) from  the
       ipchains package.

       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 CAP_NET_ADMIN or effective user  ID  0.   Binding  to  a
              reserved  port  without  the  CAP_NET_BIND_SERVICE capability or
              effective user ID 0.

              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), 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.