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

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

DESCRIPTION
       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
       filter.

       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(AF_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-
       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).   In
       this case, 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) is called on an
       unbound socket, the socket is automatically bound to a random free port
       with the	local address set to INADDR_ANY.  When connect(2) is called on
       an unbound socket, the socket is	automatically bound to a  random  free
       port  or	 to  a	usable	shared	port with the local address set	to IN-
       ADDR_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 16-bit 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 */
	       in_port_t      sin_port;	  /* port in network byte order	*/
	       struct in_addr sin_addr;	  /* internet address */
	   };

	   /* Internet address.	*/
	   struct in_addr {
	       uint32_t	      s_addr;	  /* address in	network	byte order */
	   };

       sin_family  is  always  set to AF_INET.	This is	required; in Linux 2.2
       most networking functions return	EINVAL when this setting  is  missing.
       sin_port	contains the port in network byte order.  The port numbers be-
       low 1024	are called privileged ports (or	 sometimes:  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  pro-
       tocol  as  such	has no concept of a port, they are implemented only by
       higher protocols	like tcp(7) and	udp(7).

       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  ad-
       dresses.	  Unicast  addresses  specify  a  single  interface of a host,
       broadcast addresses specify all hosts on	a network  and	multicast  ad-
       dresses address all hosts in a multicast	group.	Datagrams to broadcast
       addresses can be	sent or	received only  when  the  SO_BROADCAST	socket
       flag  is	set.  In the current implementation, connection-oriented sock-
       ets are allowed to use only 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 (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  IPPROTO_IP.   A	boolean	integer	flag is	zero when it is	false,
       otherwise true.

       IP_ADD_MEMBERSHIP (since	Linux 1.2)
	      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 (or setsockopt(2) fails  with  the  error  EINVAL).
	      imr_address is the address of the	local interface	with which the
	      system should join the multicast group; if it is	equal  to  IN-
	      ADDR_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	inter-
	      face.

	      The ip_mreqn structure is	available only since Linux  2.2.   For
	      compatibility,  the  old	ip_mreq	structure (present since Linux
	      1.2) is still supported; it differs from ip_mreqn	 only  by  not
	      including	the imr_ifindex	field.	Only valid as a	setsockopt(2).

       IP_ADD_SOURCE_MEMBERSHIP	(since Linux 2.4.22 / 2.5.68)
	      Join  a  multicast  group	 and  allow receiving data only	from a
	      specified	source.	 Argument is an	ip_mreq_source structure.

		  struct ip_mreq_source	{
		      struct in_addr imr_multiaddr;  /*	IP multicast group
							address	*/
		      struct in_addr imr_interface;  /*	IP address of local
							interface */
		      struct in_addr imr_sourceaddr; /*	IP address of
							multicast source */
		  };

	      The ip_mreq_source structure is similar  to  ip_mreqn  described
	      under  IP_ADD_MEMBERSIP.	 The  imr_multiaddr field contains the
	      address of the multicast group the application wants to join  or
	      leave.   The imr_interface field is the address of the local in-
	      terface with which the system should join	the  multicast	group.
	      Finally,	the  imr_sourceaddr  field contains the	address	of the
	      source the application wants to receive data from.

	      This option can be used multiple times to	allow  receiving  data
	      from more	than one source.

       IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
	      Stop  receiving multicast	data from a specific source in a given
	      group.  This is valid only after the application has  subscribed
	      to   the	multicast  group  using	 either	 IP_ADD_MEMBERSHIP  or
	      IP_ADD_SOURCE_MEMBERSHIP.

	      Argument is  an  ip_mreq_source  structure  as  described	 under
	      IP_ADD_SOURCE_MEMBERSHIP.

       IP_DROP_MEMBERSHIP (since Linux 1.2)
	      Leave  a	multicast  group.   Argument is	an ip_mreqn or ip_mreq
	      structure	similar	to IP_ADD_MEMBERSHIP.

       IP_DROP_SOURCE_MEMBERSHIP (since	Linux 2.4.22 / 2.5.68)
	      Leave a source-specific group--that is, stop receiving data from
	      a	 given	multicast group	that come from a given source.	If the
	      application has subscribed to multiple sources within  the  same
	      group,  data from	the remaining sources will still be delivered.
	      To  stop	receiving  data	 from  all  sources   at   once,   use
	      IP_LEAVE_GROUP.

	      Argument	is  an	ip_mreq_source	structure  as  described under
	      IP_ADD_SOURCE_MEMBERSHIP.

       IP_FREEBIND (since Linux	2.4)
	      If enabled, this boolean option allows binding to	an IP  address
	      that  is nonlocal	or does	not (yet) exist.  This permits listen-
	      ing on a socket, without requiring the underlying	network	inter-
	      face  or	the  specified dynamic IP address to be	up at the time
	      that the application is trying to	bind to	it.   This  option  is
	      the  per-socket  equivalent of the ip_nonlocal_bind /proc	inter-
	      face described below.

       IP_HDRINCL (since Linux 2.0)
	      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_MSFILTER (since Linux	2.4.22 / 2.5.68)
	      This option provides access to the advanced full-state filtering
	      API.  Argument is	an ip_msfilter structure.

		  struct ip_msfilter {
		      struct in_addr imsf_multiaddr; /*	IP multicast group
							address	*/
		      struct in_addr imsf_interface; /*	IP address of local
							interface */
		      uint32_t	     imsf_fmode;     /*	Filter-mode */

		      uint32_t	     imsf_numsrc;    /*	Number of sources in
							the following array */
		      struct in_addr imsf_slist[1];  /*	Array of source
							addresses */
		  };

	      There are	two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can
	      be used to specify the filtering mode.  Additionally, the	IP_MS-
	      FILTER_SIZE(n) macro exists to  determine	 how  much  memory  is
	      needed  to  store	 ip_msfilter  structure	 with n	sources	in the
	      source list.

	      For the full description of multicast source filtering refer  to
	      RFC 3376.

       IP_MTU (since Linux 2.2)
	      Retrieve	the  current  known  path  MTU	of the current socket.
	      Valid only when the socket has been connected.  Returns an inte-
	      ger.  Only valid as a getsockopt(2).

       IP_MTU_DISCOVER (since Linux 2.2)
	      Set  or  receive	the  Path  MTU Discovery setting for a socket.
	      When enabled, Linux will perform Path MTU	Discovery  as  defined
	      in  RFC 1191  on SOCK_STREAM sockets.  For non-SOCK_STREAM sock-
	      ets, IP_PMTUDISC_DO forces the don't-fragment flag to be set  on
	      all outgoing packets.  It	is the user's responsibility to	packe-
	      tize the data in MTU-sized chunks	and to do the  retransmits  if
	      necessary.   The	kernel	will  reject (with EMSGSIZE) datagrams
	      that are bigger than the known path MTU.	IP_PMTUDISC_WANT  will
	      fragment a datagram if needed according to the path MTU, or will
	      set the don't-fragment flag otherwise.

	      The system-wide default can be toggled between  IP_PMTUDISC_WANT
	      and  IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
	      values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc	file.

	      Path MTU discovery value	 Meaning
	      IP_PMTUDISC_WANT		 Use per-route settings.
	      IP_PMTUDISC_DONT		 Never do Path MTU Discovery.
	      IP_PMTUDISC_DO		 Always	do Path	MTU Discovery.
	      IP_PMTUDISC_PROBE		 Set DF	but ignore Path	MTU.

	      When PMTU	discovery is enabled, the kernel  automatically	 keeps
	      track  of	 the  path  MTU	per destination	host.  When it is con-
	      nected to	a specific peer	with connect(2), the  currently	 known
	      path  MTU	 can be	retrieved conveniently using the IP_MTU	socket
	      option (e.g., after an EMSGSIZE error occurred).	The  path  MTU
	      may change over time.  For connectionless	sockets	with many des-
	      tinations, the new MTU for a given destination can also  be  ac-
	      cessed 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	 data-
	      gram  sockets  may be dropped.  Applications using UDP should be
	      aware of this and	not take it into account for their packet  re-
	      transmit 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.

	      It is possible to	implement RFC 4821 MTU probing with SOCK_DGRAM
	      or  SOCK_RAW  sockets  by	 setting  a value of IP_PMTUDISC_PROBE
	      (available since Linux 2.6.22).  This is also particularly  use-
	      ful  for	diagnostic tools such as tracepath(8) that wish	to de-
	      liberately send probe packets larger than	the observed Path MTU.

       IP_MULTICAST_ALL	(since Linux 2.6.31)
	      This option can be used to modify	the delivery policy of	multi-
	      cast  messages  to  sockets bound	to the wildcard	INADDR_ANY ad-
	      dress.  The argument is a	boolean	integer	(defaults to  1).   If
	      set  to  1, the socket will receive messages from	all the	groups
	      that have	been joined globally on	the whole system.   Otherwise,
	      it will deliver messages only from the groups that have been ex-
	      plicitly joined (for example via the  IP_ADD_MEMBERSHIP  option)
	      on this particular socket.

       IP_MULTICAST_IF (since Linux 1.2)
	      Set  the	local  device  for a multicast socket.	Argument is an
	      ip_mreqn or ip_mreq  (since  Linux  3.5)	structure  similar  to
	      IP_ADD_MEMBERSHIP.

	      When  an	invalid	 socket	 option	 is passed, ENOPROTOOPT	is re-
	      turned.

       IP_MULTICAST_LOOP (since	Linux 1.2)
	      Set or read a boolean integer argument that  determines  whether
	      sent  multicast packets should be	looped back to the local sock-
	      ets.

       IP_MULTICAST_TTL	(since Linux 1.2)
	      Set or read 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_NODEFRAG (since Linux	2.6.36)
	      If enabled (argument is nonzero),	 the  reassembly  of  outgoing
	      packets  is  disabled  in	 the  netfilter	layer.	This option is
	      valid only for SOCK_RAW sockets.	The argument is	an integer.

       IP_OPTIONS (since Linux 2.0)
	      Set or get the IP	options	to be sent with	every packet from this
	      socket.  The arguments are a pointer to a	memory buffer contain-
	      ing the options and the option length.  The  setsockopt(2)  call
	      sets  the	 IP options associated with a socket.  The maximum op-
	      tion size	for IPv4 is 40 bytes.  See RFC 791 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
	      /proc interface.	Other options like timestamps are  still  han-
	      dled.   For  datagram sockets, IP	options	can be only set	by the
	      local user.  Calling getsockopt(2) with IP_OPTIONS puts the cur-
	      rent IP options used for sending into the	supplied buffer.

       IP_PKTINFO (since Linux 2.2)
	      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) 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.

       IP_RECVERR (since Linux 2.2)
	      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_ex-
	      tended_err structure describing the error	will be	passed	in  an
	      ancillary	 message  with	the  type IP_RECVERR and the level IP-
	      PROTO_IP.	 This is useful	for reliable error handling on	uncon-
	      nected  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 {
		      uint32_t ee_errno;   /* error number */
		      uint8_t  ee_origin;  /* where the	error originated */
		      uint8_t  ee_type;	   /* type */
		      uint8_t  ee_code;	   /* code */
		      uint8_t  ee_pad;
		      uint32_t ee_info;	   /* additional information */
		      uint32_t ee_data;	   /* other data */
		      /* More data may follow */
		  };

		  struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

	      ee_errno contains	the errno number of the	queued error.  ee_ori-
	      gin is the origin	code of	where the error	originated.  The other
	      fields  are protocol-specific.  The macro	SO_EE_OFFENDER returns
	      a	pointer	to the address of the network object where  the	 error
	      originated  from	given  a pointer to the	ancillary message.  If
	      this address is not known, the sa_family member of the  sockaddr
	      contains	AF_UNSPEC and the other	fields of the sockaddr are 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
	      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  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 not
	      permitted	on SOCK_STREAM sockets.	 IP_RECVERR is valid for  TCP,
	      but  all errors are returned by socket function return or	SO_ER-
	      ROR 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_RECVOPTS (since Linux	2.2)
	      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_RECVORIGDSTADDR (since Linux 2.6.29)
	      This boolean option enables the IP_ORIGDSTADDR ancillary message
	      in recvmsg(2), in	which the kernel returns the original destina-
	      tion address of the datagram being received.  The	ancillary mes-
	      sage contains a struct sockaddr_in.

       IP_RECVTOS (since Linux 2.2)
	      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 (since Linux 2.2)
	      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.

       IP_RETOPTS (since Linux 2.2)
	      Identical	to IP_RECVOPTS,	but returns  raw  unprocessed  options
	      with  timestamp  and route record	options	not filled in for this
	      hop.

       IP_ROUTER_ALERT (since Linux 2.2)
	      Pass all to-be forwarded packets with the	IP Router Alert	option
	      set  to  this socket.  Only valid	for raw	sockets.  This is use-
	      ful, for instance, for  user-space  RSVP	daemons.   The	tapped
	      packets  are  not	 forwarded by the kernel; it is	the user's re-
	      sponsibility to send them	out again.  Socket binding is ignored,
	      such  packets are	only filtered by protocol.  Expects an integer
	      flag.

       IP_TOS (since Linux 1.0)
	      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 de-
	      lays  for	 interactive  traffic,	IPTOS_THROUGHPUT  to  optimize
	      throughput,  IPTOS_RELIABILITY  to optimize for reliability, IP-
	      TOS_MINCOST should be used for "filler data" where  slow	trans-
	      mission  doesn't matter.	At most	one of these TOS values	can be
	      specified.  Other	bits are invalid and shall be cleared.	 Linux
	      sends  IPTOS_LOWDELAY  datagrams first by	default, but the exact
	      behavior depends on the configured  queueing  discipline.	  Some
	      high  priority  levels  may  require  superuser  privileges (the
	      CAP_NET_ADMIN capability).  The priority can also	be  set	 in  a
	      protocol independent way by the (SOL_SOCKET, SO_PRIORITY)	socket
	      option (see socket(7)).

       IP_TRANSPARENT (since Linux 2.6.24)
	      Setting this boolean option enables transparent proxying on this
	      socket.	This  socket  option allows the	calling	application to
	      bind to a	nonlocal IP address and	operate	both as	a client and a
	      server  with  the	 foreign address as the	local endpoint.	 NOTE:
	      this requires that routing be set	up in a	way that packets going
	      to  the foreign address are routed through the TProxy box	(i.e.,
	      the system hosting the application that employs the IP_TRANSPAR-
	      ENT  socket option).  Enabling this socket option	requires supe-
	      ruser privileges (the CAP_NET_ADMIN capability).

	      TProxy redirection with the iptables TPROXY target also requires
	      that this	option be set on the redirected	socket.

       IP_TTL (since Linux 1.0)
	      Set  or  retrieve	the current time-to-live field that is used in
	      every packet sent	from this socket.

       IP_UNBLOCK_SOURCE (since	Linux 2.4.22 / 2.5.68)
	      Unblock previously blocked multicast source.   Returns  EADDRNO-
	      TAVAIL when given	source is not being blocked.

	      Argument	is  an	ip_mreq_source	structure  as  described under
	      IP_ADD_SOURCE_MEMBERSHIP.

   /proc interfaces
       The IP protocol supports	a set of /proc interfaces  to  configure  some
       global  parameters.  The	parameters can be accessed by reading or writ-
       ing files in the	directory /proc/sys/net/ipv4/.	 Interfaces  described
       as Boolean take an integer value, with a	nonzero	value ("true") meaning
       that the	corresponding option is	enabled, and a	zero  value  ("false")
       meaning that the	option is disabled.

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

	      When  this boolean flag 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 use it	for  a
	      normal  router or	host.  Otherwise, fragmented communication can
	      be disturbed if the fragments travel over	different links.   De-
	      fragmentation also has a large memory and	CPU time cost.

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

       ip_autoconfig (since Linux 2.2 to 2.6.17)
	      Not documented.

       ip_default_ttl (integer;	default: 64; since Linux 2.2)
	      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; since Linux 2.0.31)
	      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; since Linux 1.2)
	      Enable IP	forwarding with	a boolean flag.	 IP forwarding can  be
	      also set on a per-interface basis.

       ip_local_port_range (since Linux	2.2)
	      This  file  contains  two	integers that define the default local
	      port range allocated to sockets that are not explicitly bound to
	      a	 port number--that is, the range used for ephemeral ports.  An
	      ephemeral	port is	allocated to a socket in the following circum-
	      stances:

	      *	 the  port  number  in a socket	address	is specified as	0 when
		 calling bind(2);

	      *	 listen(2) is called on	a stream socket	that  was  not	previ-
		 ously bound;

	      *	 connect(2)  was  called  on  a	socket that was	not previously
		 bound;

	      *	 sendto(2) is called on	a datagram socket that was not	previ-
		 ously bound.

	      Allocation  of  ephemeral	 ports starts with the first number in
	      ip_local_port_range and ends with	the  second  number.   If  the
	      range  of	ephemeral ports	is exhausted, then the relevant	system
	      call returns an error (but see BUGS).

	      Note that	the port range in ip_local_port_range should not  con-
	      flict  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.  The first number should be	at least greater  than
	      1024,  or	 better, greater than 4096, to avoid clashes with well
	      known ports and to minimize firewall problems.

       ip_no_pmtu_disc (Boolean; default: disabled; since Linux	2.2)
	      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.

       ip_nonlocal_bind	(Boolean; default: disabled; since Linux 2.4)
	      If set, allows processes to bind(2) to  nonlocal	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 in-
	      teger with the number of bytes.

       neigh/*
	      See arp(7).

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

       Ioctls to configure generic device parameters are described  in	netde-
       vice(7).

ERRORS
       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	without	 supe-
	      ruser  privileges	 (the  CAP_NET_ADMIN capability); binding to a
	      privileged    port    without    superuser    privileges	  (the
	      CAP_NET_BIND_SERVICE capability).

       EADDRINUSE
	      Tried to bind to an address already in use.

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

       EAGAIN Operation	on a nonblocking socket	would block.

       EALREADY
	      An connection operation on a nonblocking socket  is  already  in
	      progress.

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

       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.

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

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

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

       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.

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

       ENOPKG A	kernel subsystem was not configured.

       ENOPROTOOPT and EOPNOTSUPP
	      Invalid socket option passed.

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

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

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

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

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

NOTES
       IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER,  IP_RECVORIGDSTADDR,
       IP_PKTINFO,  IP_RECVERR,	IP_ROUTER_ALERT, and IP_TRANSPARENT are	Linux-
       specific.

       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 in-
       stead 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.

   Compatibility
       For   compatibility   with  Linux  2.0,	the  obsolete  socket(AF_INET,
       SOCK_PACKET, protocol) syntax is	still supported	to  open  a  packet(7)
       socket.	This is	deprecated and should be replaced by socket(AF_PACKET,
       SOCK_RAW, protocol) instead.  The main  difference  is  the  new	 sock-
       addr_ll address structure for generic link layer	information instead of
       the old sockaddr_pkt.

BUGS
       There are too many inconsistent error values.

       The error used to diagnose exhaustion of	the ephemeral port range  dif-
       fers  across  the various system	calls (connect(2), bind(2), listen(2),
       sendto(2)) that can assign ephemeral ports.

       The ioctls to configure IP-specific interface options  and  ARP	tables
       are not described.

       Receiving   the	original  destination  address	with  MSG_ERRQUEUE  in
       msg_name	by recvmsg(2) does not work in some 2.2	kernels.

SEE ALSO
       recvmsg(2),   sendmsg(2),   byteorder(3),   ipfw(4),   capabilities(7),
       icmp(7),	ipv6(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.

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-05-10				 IP(7)

NAME | SYNOPSIS | DESCRIPTION | ERRORS | NOTES | BUGS | SEE ALSO | COLOPHON

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