ip (7)


       ip - Linux IPv4 protocol implementation


       #include <sys/socket.h>
       #include <netinet/in.h>

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


       Linux  implements the Internet Protocol, version 4, described in RFC791
       and RFC1122.  ip contains a level 2  multicasting  implementation  con-
       forming  to  RFC1112.  It also contains an IP router including a packet

       The programmer's interface is BSD sockets compatible.  For more  infor-
       mation 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 RFC1700  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 a
       unbound socket the socket 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 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 */
                  u_int16_t      sin_port;   /* port in network byte order */
                  struct in_addr  sin_addr;  /* internet address */

       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  gethostby-
       name(3)).   IPv4 addresses are divided into unicast, broadcast and mul-
       ticast 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  broad-
       cast  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 SOL_IP.  A boolean integer flag is zero when it is false,  other-
       wise 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 RFC791 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.

              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) then the outgoing packet
              will be sent over the interface specified  in  ipi_ifindex  with
              the destination address set to ipi_spec_dst

              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_RECVTTL 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 sockets.

              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 pri-
              oritize 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  through-
              put,  IPTOS_RELIABILITY  to optimize for reliability, IPTOS_MIN-
              COST should be used for "filler data"  where  slow  transmission
              doesn't  matter.   At most one of these TOS values can be speci-
              fied. 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  send  in

       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 SOL_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_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 val-
              ues 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

              Sets  or  receives  the Path MTU Discovery setting for a socket.
              When enabled, Linux will perform Path MTU Discovery  as  defined
              in RFC1191 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 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.

       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.
              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 a struct 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.

              Set the default time-to-live value of outgoing packets. This can
              be changed per socket with the IP_TTL option.

              Enable IP forwarding with a boolean flag. IP forwarding  can  be
              also set on a per interface basis.

              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.

              Not documented.
              >4096  to  avoid  clashes  with well known ports and to minimize
              firewall problems.

              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.

       ipfrag_high_thresh, ipfrag_low_thresh
              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.

              [New with Kernel 2.2.13; in earlier kernel version  the  feature
              was  controlled  at  compile time by the CONFIG_IP_ALWAYS_DEFRAG

              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 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.

              See arp(7).


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

       The  ioctls to configure firewalling are documented in ipfw(7) from the
       ipchains package.

       Ioctls to configure generic device parameters are described  in  netde-


       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

       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 with-
              out CAP_NET_ADMIN or effective user id 0.  Binding to a reserved
              port  without the CAP_NET_BIND_SERVICE capacibility 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  con-
              figuration, or send signals to the requested process or group,

              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.
       raw(7), udp(7) and socket(7).


       IP_ROUTER_ALERT  are 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_RAW, 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 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.


       This man page was written by Andi Kleen.


       sendmsg(2),  recvmsg(2), socket(7), netlink(7), tcp(7), udp(7), raw(7),

       RFC791 for the original IP specification.
       RFC1122 for the IPv4 host requirements.
       RFC1812 for the IPv4 router requirements.

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