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

       ip - Linux IPv4 protocol implementation

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

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

       Linux implements the Internet Protocol, version 4, described in RFC 791
       and RFC 1122.  ip contains a level 2 multicasting  implementation  con‐
       forming	to RFC 1112.  It also contains an IP router including a packet

       The programming interface is BSD-sockets compatible.  For more informa‐
       tion on sockets, see socket(7).

       An   IP	socket	is  created  by	 calling  the  socket(2)  function  as
       socket(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‐

       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

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

   Socket options
       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
	      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‐

	      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
	      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

	      Argument is  an  ip_mreq_source  structure  as  described	 under

       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

	      Argument	is  an	ip_mreq_source	structure  as  described under

       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
	      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.
	      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
	      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

	      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

       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.   Argument	is  an
	      ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.

	      When   an	 invalid  socket  option  is  passed,  ENOPROTOOPT  is

       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‐

       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
	      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

		  #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

	      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

       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

       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
	      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).  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.
	      Enabling this socket option requires superuser  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

   /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)
	      Contains two integers that define the default local  port	 range
	      allocated	 to  sockets.  Allocation starts with the first number
	      and ends with the second number.	Note  that  these  should  not
	      conflict	with the ports used by masquerading (although the case
	      is handled).  Also arbitrary choices  may	 cause	problems  with
	      some  firewall  packet  filters  that make assumptions about the
	      local ports in use.  First number should	be  at	least  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.

	      See arp(7).

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

       Ioctls to configure generic device parameters are described  in	netde‐

       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).

	      Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

	      An connection operation on a nonblocking socket  is  already  in

	      A connection was closed during an accept(2).

	      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.

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

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

	      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.

	      Invalid socket option passed.

	      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

	      The socket is not configured  or	an  unknown  socket  type  was

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


       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.

       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.

       There are too many inconsistent error values.

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

       Some  versions  of glibc forget to declare in_pktinfo.  Workaround cur‐
       rently is to copy it into your program from this man page.

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

       recvmsg(2),   sendmsg(2),   byteorder(3),   ipfw(4),   capabilities(7),
       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.

       This  page  is  part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-04-16				 IP(7)

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