Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Apropos / Subsearch:
optional field

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

       ip - Linux IPv4 protocol implementation

       #include <&lt;sys/socket.h>&gt;
       #include <&lt;netinet/in.h>&gt;
       #include <&lt;netinet/ip.h>&gt; /* 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  con-
       forming  to RFC 1112.  It also contains an IP router including a packet

       The programming interface is BSD sockets compatible.  For more informa-
       tion 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  num-

       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.

   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  sup-
       ply  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 */
               uint16_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 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  func-
       tions  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 func-
       tions 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

   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.

              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 pro-
              cessing 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 rout-
              ing 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  Ser-
              vice/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 sup-
              ported 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 trans-
              mission 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).   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 &lt;linux/errqueue.h&gt;)
              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

                  #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_ori-
              gin 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

              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  nor-
              mal  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.

              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 dis-
              abled 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 ).

              tab(:); c l l l.  Path  MTU  discovery  flags:Meaning  IP_PMTUD-
              ISC_WANT:Use per-route settings.  IP_PMTUDISC_DONT:Never do Path
              MTU Discovery.  IP_PMTUDISC_DO:Always  do  Path  MTU  Discovery.
              IP_PMTUDISC_PROBE:Set DF but ignore Path MTU.

              When  PMTU  discovery  is enabled the kernel automatically keeps
              track of the path MTU per destination host.   When  it  is  con-
              nected  to  a  specific peer with connect(2) the currently known
              path MTU can be retrieved conveniently using the  IP_MTU  socket
              option  (e.g.,  after 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 sock-
              ets 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.

              It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
              or SOCK_RAW sockets by setting  a  value  of  IP_PMTUDISC_PROBE.
              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_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  use-
              ful,  for  instance,  for  user  space RSVP daemons.  The tapped
              packets are not forwarded by the kernel, it is the users respon-
              sibility  to  send  them  out again.  Socket binding is ignored,
              such packets are only filtered by protocol.  Expects an  integer

              Set  or reads the time-to-live value of outgoing multicast pack-
              ets 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.

              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 multi-
              cast 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 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 frag is enabled (not equal 0) incoming frag-
              ments (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 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.

              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  inter-
              face  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, bet-
              ter >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(2) 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  life-
              time 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 netde-

       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.

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

       EAGAIN Operation on a non-blocking socket would block.

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

              A connection was closed during an accept(2).

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

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

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

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

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

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

       ENOPKG A kernel subsystem was not configured.

              Invalid socket option passed.

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

       EPERM  User  doesn't  have permission to set high priority, change con-
              figuration, or send signals to the requested process or group.

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

              The  socket  is  not  configured  or  an unknown socket type was

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

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

       IP_PMTUDISC_PROBE is new in Linux 2.6.22.

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

       The sysctls were introduced with Linux 2.2.

       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 inter-
       face 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.

       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 sock-
       addr_ll address structure for generic link layer information instead of
       the old sockaddr_pkt.

       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  cur-
       rently 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.

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2001-06-19                             IP(7)