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NAME    [Toc]    [Back]

       ip - Linux IPv4 protocol implementation

SYNOPSIS    [Toc]    [Back]

       #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);

DESCRIPTION    [Toc]    [Back]

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

       The programmer's 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 RFC1700	assigned  numbers.

       When a process wants to receive new incoming packets or connections, it
       should bind a socket to a local interface address using bind(2).   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.

ADDRESS FORMAT    [Toc]    [Back]

       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 */

	      /* 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  gethostby-
       name(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

SOCKET OPTIONS    [Toc]    [Back]

       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,	otherwise

       IP_OPTIONS    [Toc]    [Back]
	      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  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.

       IP_PKTINFO    [Toc]    [Back]
	      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) then the outgoing packet
	      will be sent over the interface specified  in  ipi_ifindex  with
	      the destination address set to ipi_spec_dst

       IP_RECVTOS    [Toc]    [Back]
	      If  enabled the IP_TOS ancillary message is passed with incoming
	      packets. It contains a byte which specifies  the	Type  of  Service/Precedence
  field  of the packet header.  Expects a boolean
	      integer flag.

       IP_RECVTTL    [Toc]    [Back]
	      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 supported
 for SOCK_STREAM sockets.

       IP_RECVOPTS    [Toc]    [Back]
	      Pass all incoming IP options to the user in a IP_OPTIONS control
	      message. The routing header and other options are already filled
	      in for the local host. Not supported for SOCK_STREAM sockets.

       IP_RETOPTS    [Toc]    [Back]
	      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_MIN-
	      COST 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  send  in
	      every packet send from this socket.

       IP_HDRINCL    [Toc]    [Back]
	      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 persocket
 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 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.

       IP_PMTU_DISCOVER    [Toc]    [Back]
	      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 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 64Kheaders
  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).

       IP_ROUTER_ALERT    [Toc]    [Back]
	      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.

       IP_MULTICAST_TTL    [Toc]    [Back]
	      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 integer.

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

       IP_ADD_MEMBERSHIP    [Toc]    [Back]
	      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	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).

       IP_DROP_MEMBERSHIP    [Toc]    [Back]
	      Leave a multicast group. Argument  is  an  ip_mreqn  or  ip_mreq
	      structure similar to IP_ADD_MEMBERSHIP.

       IP_MULTICAST_IF    [Toc]    [Back]
	      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

SYSCTLS    [Toc]    [Back]

       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.

	      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 arbitary 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.

	      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  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.

	      See arp(7).

IOCTLS    [Toc]    [Back]

       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-

NOTES    [Toc]    [Back]

       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.

ERRORS    [Toc]    [Back]

	      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.

       EMSGSIZE    [Toc]    [Back]
	      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 capacibility or effective
	      user id 0.

       EADDRINUSE    [Toc]    [Back]
	      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 group,

       EADDRNOTAVAIL    [Toc]    [Back]
	      A  non-existent  interface was requested or the requested source
	      address was not local.

       EAGAIN Operation on a non-blocking socket would block.

       ESOCKTNOSUPPORT    [Toc]    [Back]
	      The socket is not configured  or	an  unknown  socket  type  was

       EISCONN    [Toc]    [Back]
	      connect(2) was called on an already connected socket.

       EALREADY    [Toc]    [Back]
	      An  connection  operation on a non-blocking socket is already in

       ECONNABORTED    [Toc]    [Back]
	      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.

       EHOSTUNREACH    [Toc]    [Back]
	      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).

VERSIONS    [Toc]    [Back]

       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.

COMPATIBILITY    [Toc]    [Back]

       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

BUGS    [Toc]    [Back]

       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.

AUTHORS    [Toc]    [Back]

       This man page was written by Andi Kleen.

SEE ALSO    [Toc]    [Back]

       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)
[ Back ]
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