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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 using socket(2):

	   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 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).   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
       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
       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 */
	       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
       below 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
       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 sent  or  received	 only  when  the  SO_BROADCAST
       socket flag is set.  In the current implementation, connection-oriented
       sockets 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.

       When an invalid socket option is specified, getsockopt(2) and  setsock-
       opt(2) fail with the error ENOPROTOOPT.

       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
	      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	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.	(The kernel  determines	 which
	      structure is being passed based on the size passed in optlen.)

	      IP_ADD_MEMBERSHIP is valid only for 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
	      interface 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.   Valid  only  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_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_MSFILTER_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.
	      Returns an integer.

	      IP_MTU is valid only for getsockopt(2) and can be employed  only
	      when the socket has been connected.

       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
	      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	 data-
	      gram  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.

	      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
	      deliberately 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
	      address.	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
	      explicitly 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.  The	 argument  for
	      setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq struc-
	      ture similar to  IP_ADD_MEMBERSHIP,  or  an  in_addr  structure.
	      (The  kernel determines which structure is being passed based on
	      the size passed in optlen.)  For getsockopt(2), the argument  is
	      an in_addr structure.

       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.  The argument is  an
	      integer.

	      This option is valid only for SOCK_RAW sockets.

       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
	      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
	      /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
	      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.

       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_extended_err	 structure describing the error will be passed
	      in an 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 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
	      undefined.

	      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 nor-
	      mal 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_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_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.  Valid only 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
	      responsibility  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
	      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
	      behavior	depends	 on  the configured queueing discipline.  Some
	      high-priority  levels  may  require  superuser  privileges  (the
	      CAP_NET_ADMIN capability).

       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)
	      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 use it for a
	      normal router or host.  Otherwise, fragmented communication  can
	      be  disturbed  if	 the  fragments	 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.

       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
	      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; 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
	      integer 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
	      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	   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
	      requested.

       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
       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.

   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 4.04 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				  2015-05-07				 IP(7)