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

       ip - Linux IPv4 protocol implementation

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

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

       Linux implements the Internet Protocol, version 4, described in RFC 791
       and RFC 1122.  ip contains a level 2 multicasting  implementation  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.

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

	 /* Internet address. */
	 struct in_addr {
	     u_int32_t	    s_addr;	/* address in network byte order */

       sin_family  is  always  set to AF_INET.	This is required; in Linux 2.2
       most networking functions return EINVAL when this setting  is  missing.
       sin_port	 contains  the	port  in network byte order.  The port numbers
       below 1024 are called reserved ports.  Only privileged processes (i.e.,
       those  having the CAP_NET_BIND_SERVICE capability) may bind(2) to these
       sockets.	 Note that the raw IPv4 protocol as such has no concept	 of  a
       port,  they  are	 only  implemented by higher protocols like tcp(7) and

       sin_addr is the IP host address.	 The s_addr member of  struct  in_addr
       contains	 the  host  interface  address in network byte order.  in_addr
       should be assigned one of the INADDR_* values (e.g., INADDR_ANY) or set
       using  the  inet_aton(3),  inet_addr(3), inet_makeaddr(3) library 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

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

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

	      If  enabled  the user supplies an ip header in front of the user
	      data. Only valid for  SOCK_RAW  sockets.	See  raw(7)  for  more
	      information.  When  this	flag  is  enabled  the	values	set by
	      IP_OPTIONS, IP_TTL and IP_TOS are ignored.

       IP_RECVERR (defined in <linux/errqueue.h>)
	      Enable extended reliable error message passing.  When enabled on
	      a	 datagram socket all generated errors will be queued in a per-
	      socket error queue. When the  user  receives  an	error  from  a
	      socket   operation   the	errors	can  be	 received  by  calling
	      recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_extended_err
	      structure	 describing  the  error	 will be passed in a ancillary
	      message with the type IP_RECVERR and the level IPPROTO_IP.  This
	      is  useful  for  reliable error handling on unconnected sockets.
	      The received data portion of the error queue contains the	 error

	      The  IP_RECVERR  control	message	 contains  a sock_extended_err

		#define SO_EE_ORIGIN_NONE    0
		#define SO_EE_ORIGIN_LOCAL   1
		#define SO_EE_ORIGIN_ICMP    2
		#define SO_EE_ORIGIN_ICMP6   3

		struct sock_extended_err {
		    u_int32_t ee_errno;	  /* error number */
		    u_int8_t  ee_origin;  /* where the error originated */
		    u_int8_t  ee_type;	  /* type */
		    u_int8_t  ee_code;	  /* code */
		    u_int8_t  ee_pad;
		    u_int32_t ee_info;	  /* additional information */
		    u_int32_t ee_data;	  /* other data */
		    /* More data may follow */

		struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

	      ee_errno contains the errno number of the queued error.  ee_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
	      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 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.

	      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_addr structure similar to IP_ADD_MEMBERSHIP.

	      When   an	 invalid  socket  option  is  passed,  ENOPROTOOPT  is

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

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

	      When  this  boolean frag is enabled (not equal 0) incoming 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.

	      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() 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-

       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.

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

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

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

       EACCES The user tried to execute an  operation  without	the  necessary
	      permissions.   These  include:  sending  a packet to a broadcast
	      address without having the  SO_BROADCAST	flag  set;  sending  a
	      packet via a prohibit route; modifying firewall settings without
	      superuser privileges (the CAP_NET_ADMIN capability); binding  to
	      a	   reserved    port    without	 superuser   privileges	  (the
	      CAP_NET_BIND_SERVICE capability).

	      Tried to bind to an address already in use.

	      Invalid socket option passed.

       EPERM  User doesn't have permission to set high priority,  change  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.

       ENOPKG A kernel subsystem was not configured.

	      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.

       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 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.

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