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

NAME
       ip - Linux IPv4 protocol	implementation

SYNOPSIS
       #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
       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
       filter.

       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-
       bers.

       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
       An IP socket address is defined as a combination	of an IP interface ad-
       dress 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  be-
       low 1024	are called reserved ports.  Only processes with	effective user
       id 0 or the CAP_NET_BIND_SERVICE	capability may bind(2) to these	 sock-
       ets.  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 udp(7).

       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 (127.0.0.1)	always
       refers to the local host	via the	loopback device; INADDR_ANY  (0.0.0.0)
       means any address for binding; INADDR_BROADCAST (255.255.255.255) means
       any host	and has	the same effect	on bind	as INADDR_ANY  for  historical
       reasons.

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 SOL_IP.  A boolean integer flag is zero when it is false,	other-
       wise true.

       IP_OPTIONS
	      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 op-
	      tions.   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  op-
	      tions used for sending into the supplied buffer.

       IP_PKTINFO
	      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 ar-
	      gument 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
	      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.

       IP_RECVTTL
	      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.

       IP_RECVOPTS
	      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
	      Identical	 to  IP_RECVOPTS  but  returns raw unprocessed options
	      with timestamp and route record options not filled in  for  this
	      hop.

       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	behav-
	      iour  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
	      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 in-
	      formation. When this flag	is enabled the values  set  by	IP_OP-
	      TIONS, 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 SOL_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
	      structure:

	      #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 un-
	      defined.

	      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  ac-
	      cessed  with  the	 SO_EE_OFFENDER	macro. The sin_family field of
	      the SO_EE_OFFENDER address is AF_UNSPEC when the source was  un-
	      known.   When  the error originated from the network, all	IP op-
	      tions (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and  con-
	      tained  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 il-
	      legal 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 de-
	      faults to	off.

       IP_PMTU_DISCOVER
	      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 re-
	      sponsibility 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
	      strategy.

	      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
	      MTU.

	      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
	      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
	      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.

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

       IP_ADD_MEMBERSHIP
	      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).

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

       IP_MULTICAST_IF
	      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  re-
	      turned.

SYSCTLS
       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.

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

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

       ip_dynaddr
	      Enable  dynamic  socket address and masquerading entry rewriting
	      on interface address change. This	is useful for dialup interface
	      with changing IP addresses.  0 means no rewriting, 1 turns it on
	      and 2 enables verbose mode.

       ip_autoconfig
	      Not documented.

       ip_local_port_range
	      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.

       ip_no_pmtu_disc
	      If enabled, don't	do Path	MTU Discovery for TCP sockets  by  de-
	      fault.  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.

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

	      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) be-
	      fore 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
	      cost.

	      This is automagically turned on when masquerading	or transparent
	      proxying are configured.

       neigh/*
	      See arp(7).

IOCTLS
       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-
       vice(7).

NOTES
       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.

ERRORS
       ENOTCONN
	      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
	      Datagram is bigger than an MTU on	the  path  and	it  cannot  be
	      fragmented.

       EACCES The  user	 tried	to  execute an operation without the necessary
	      permissions.  These include: Sending a packet to a broadcast ad-
	      dress  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.

       EADDRINUSE
	      Tried to bind to an address already in use.

       ENOPROTOOPT and EOPNOTSUPP
	      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,

       EADDRNOTAVAIL
	      A	non-existent interface was requested or	the  requested	source
	      address was not local.

       EAGAIN Operation	on a non-blocking socket would block.

       ESOCKTNOSUPPORT
	      The  socket  is not configured or	an unknown socket type was re-
	      quested.

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

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

       ECONNABORTED
	      A	connection was closed during an	accept(2).

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

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

       EHOSTUNREACH
	      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.

       ENOBUFS,	ENOMEM
	      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).

VERSIONS
       IP_PKTINFO,   IP_MTU,   IP_PMTU_DISCOVER,  IP_PKTINFO,  IP_RECVERR  and
       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
       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
       sockaddr_pkt.

BUGS
       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.

AUTHORS
       This man	page was written by Andi Kleen.

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

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