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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 an 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	 only  implemented  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	 only  sent  or received when the SO_BROADCAST
       socket flag is set.  In the current implementation, connection-oriented
       sockets are only allowed to use unicast addresses.

       Note  that  the	address and the port are always stored in network byte
       order.  In particular, this means that you need to call htons(3) on the
       number that is assigned to a port.  All address/port manipulation func‐
       tions in the standard library work in network byte order.

       There are several special addresses: INADDR_LOOPBACK ( always
       refers  to the local host via the loopback device; INADDR_ANY (
       means any address for binding; INADDR_BROADCAST ( means
       any  host  and has the same effect on bind as INADDR_ANY for historical

   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 is available only since Linux 2.2.  For compatibil‐
	      ity, 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 2.5.68)
	      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 */

	      ip_mreq_source  structure	 is  similar  to ip_mreqn described at
	      IP_ADD_MEMBERSIP.	 imr_multiaddr contains	 the  address  of  the
	      multicast	  group	 the  application  wants  to  join  or	leave.
	      imr_interface is the address of the local interface  with	 which
	      the   system   should   join   the   multicast  group.   Finally
	      imr_sourceaddr field contains address of the source the applica‐
	      tion 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 2.5.68)
	      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  at

       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 2.5.68)
	      Leave  a source-specific group, i.e., 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   at

       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 2.5.68)
	      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.   Additionaly,  IP_MSFIL‐
	      TER_SIZE(n)  macro exists to determine how much memory is needed
	      to store ip_msfilter structure with  n  sources  in  the	source

	      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.  Only
	      valid  when  the socket has been connected.  Returns an integer.
	      Only valid as a getsockopt(2).

       IP_MTU_DISCOVER (since Linux 2.2)
	      Sets or receives the Path MTU Discovery setting  for  a  socket.
	      When  enabled,  Linux will perform Path MTU Discovery as defined
	      in RFC 1191 on this socket.  The don't-fragment flag is  set  on
	      all  outgoing  datagrams.	 The system-wide default is controlled
	      by the /proc/sys/net/ipv4/ip_no_pmtu_disc file  for  SOCK_STREAM
	      sockets,	and disabled on all others.  For non-SOCK_STREAM sock‐
	      ets, it is the user's responsibility to packetize	 the  data  in
	      MTU  sized  chunks  and to do the retransmits if necessary.  The
	      kernel will reject packets that are bigger than the  known  path
	      MTU if this flag is set (with EMSGSIZE).

	      Path MTU discovery flags	 Meaning
	      IP_PMTUDISC_WANT		 Use per-route settings.
	      IP_PMTUDISC_DONT		 Never do Path MTU Discovery.
	      IP_PMTUDISC_DO		 Always do Path MTU Discovery.
	      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 a EMSGSIZE error occurred).	 It may change
	      over time.  For connectionless sockets with  many	 destinations,
	      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)
	      Sets the policy for multicast delivery to the  socket.  Argument
	      is a boolean integer that enables or disables multicast delivery
	      from all	groups.	  If  not  set,	 delivery  to  the  socket  is
	      restricted  to  data  from those multicast groups that have been
	      explicitly subscribed to via a multicast join operation for this
	      socket.  The  default  is	 1  which means that a socket which is
	      bound to the wildcard address (INADDR_ANY) will  receive	multi‐
	      cast  packets  from  all	groups that have been subscribed to on
	      this system.

       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)
	      Sets or reads 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_OPTIONS (since Linux 2.0)
	      Sets or get the IP options to be sent  with  every  packet  from
	      this  socket.   The  arguments  are a pointer to a memory buffer
	      containing the options and the option length.  The setsockopt(2)
	      call  sets the IP options associated with a socket.  The maximum
	      option size for IPv4 is 40 bytes.	 See RFC 791 for  the  allowed
	      options.	 When  the  initial  connection	 request  packet for a
	      SOCK_STREAM socket contains IP options, the IP options  will  be
	      set  automatically  to  the options from the initial packet with
	      routing headers reversed.	 Incoming packets are not  allowed  to
	      change  options  after  the connection is established.  The pro‐
	      cessing of all incoming source routing options  is  disabled  by
	      default  and  can	 be  enabled  by using the accept_source_route
	      /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 2.5.68)
	      Unblock  previously  blocked  multicast source. Returns EADDRNO‐
	      TAVAIL when given source is not being blocked.

	      Argument	is  an	ip_mreq_source	structure  as	described   at

   /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 non-zero value ("true")	 mean‐
       ing  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 frag is enabled (not equal 0), incoming	 frag‐
	      ments  (parts  of	 IP  packets that arose when some host between
	      origin and destination decided that the packets were  too	 large
	      and  cut	them  into  pieces) will be reassembled (defragmented)
	      before being processed, even if they are about to be forwarded.

	      Only enable if running either a firewall that is the  sole  link
	      to  your network or a transparent proxy; never ever 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 non-local	IP  addresses,
	      which can be quite useful, but may break some applications.

       ip6frag_time (integer; default 30)
	      Time in seconds to keep an IPv6 fragment in memory.

       ip6frag_secret_interval (integer; default 600)
	      Regeneration  interval (in seconds) of the hash secret (or life‐
	      time for the hash secret) for IPv6 fragments.

       ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
	      If the amount of queued IP fragments reaches ipfrag_high_thresh,
	      the  queue  is  pruned  down  to ipfrag_low_thresh.  Contains an
	      integer with the number of bytes.

	      See arp(7).

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

       Ioctls to configure generic device parameters are described  in	netde‐

       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 non-blocking socket would block.

	      An connection operation on a non-blocking 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  only	defined 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).

       specific and should not be used in programs intended  to	 be  portable.
       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),
       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 3.22 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				  2009-02-28				 IP(7)

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