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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>

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

       Linux  implements  the  Internet Protocol, version 4, described in RFC791 and RFC1122.  ip
       contains a level 2 multicasting implementation conforming to RFC1112.  It also contains an
       IP router including a packet filter.

       The  programmer's  interface  is BSD sockets compatible.  For more information on sockets,
       see socket(7).

       An IP socket is created by calling the socket(2) function as socket(PF_INET,  socket_type,
       protocol).  Valid socket types are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM to open
       a udp(7) socket, or SOCK_RAW to open a raw(7) socket to access the IP  protocol	directly.
       protocol  is the IP protocol in the IP header to be received or sent.  The only valid val-
       ues for protocol are 0 and IPPROTO_TCP for TCP sockets and 0 and IPPROTO_UDP for UDP sock-
       ets.   For  SOCK_RAW  you may specify a valid IANA IP protocol defined in RFC1700 assigned

       When a process wants to receive new incoming packets or	connections,  it  should  bind	a
       socket to a local interface address using bind(2).  Only one IP socket may be bound to any
       given local (address, port) pair.  When INADDR_ANY is  specified  in  the  bind	call  the
       socket will be bound to all local interfaces. When listen(2) or connect(2) are called on a
       unbound socket the socket is automatically bound to a random  free  port  with  the  local
       address set to INADDR_ANY.

       A TCP local socket address that has been bound is unavailable for some time after closing,
       unless the SO_REUSEADDR flag has been set.  Care should be taken when using this  flag  as
       it makes TCP less reliable.

       An  IP  socket  address	is defined as a combination of an IP interface address and a port
       number. The basic IP protocol does not supply port numbers, they are implemented by higher
       level  protocols  like udp(7) and tcp(7).  On raw sockets sin_port is set to the IP proto-

	      struct sockaddr_in {
		  sa_family_t	 sin_family; /* address family: AF_INET */
		  u_int16_t	 sin_port;   /* port in network byte order */
		  struct in_addr  sin_addr;  /* internet address */

	      /* Internet address. */
	      struct in_addr {
		  u_int32_t	 s_addr;     /* address in network byte order */

       sin_family is always set to AF_INET.  This is required; in Linux 2.2 most networking func-
       tions  return  EINVAL when this setting is missing.  sin_port contains the port in network
       byte order. The port numbers below 1024 are called reserved ports.   Only  processes  with
       effective  user	id 0 or the CAP_NET_BIND_SERVICE capability may bind(2) to these sockets.
       Note that the raw IPv4 protocol as such has no concept of a port,  they	are  only  imple-
       mented by higher protocols like tcp(7) and udp(7).

       sin_addr  is  the  IP  host  address.  The addr member of struct in_addr contains the host
       interface  address  in  network	order.	 in_addr  should  be  only  accessed  using   the
       inet_aton(3),  inet_addr(3),  inet_makeaddr(3) library functions or directly with the name
       resolver (see 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 broadcast addresses can be only sent or received when the
       SO_BROADCAST socket flag is set.  In the current implementation connection oriented  sock-
       ets are only allowed to use unicast addresses.

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

       There  are  several  special  addresses:  INADDR_LOOPBACK ( 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 reasons.

       IP supports some protocol specific socket options that can be set with  setsockopt(2)  and
       read  with  getsockopt(2).   The  socket option level for IP is SOL_IP.	A boolean integer
       flag is zero when it is false, otherwise true.

	      Sets or get the IP options to be sent with every	packet	from  this  socket.   The
	      arguments  are  a  pointer to a memory buffer containing the options and the option
	      length.  The setsockopt(2) call sets the IP options associated with a socket.   The
	      maximum  option size for IPv4 is 40 bytes. See RFC791 for the allowed 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 processing of all incoming source routing
	      options is disabled by default and can be enabled by using the  accept_source_route
	      sysctl.  Other options like timestamps are still handled.  For datagram sockets, IP
	      options can be only set by the local user.  Calling getsockopt(2)  with  IP_OPTIONS
	      puts the current IP options used for sending into the supplied buffer.

	      Pass  an	IP_PKTINFO  ancillary message that contains a pktinfo structure that sup-
	      plies 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_PKT-
	      INFO message should be passed or not. The message itself can only be sent/retrieved
	      as control message with a packet using recvmsg(2) or sendmsg(2).

	      struct in_pktinfo {
		  unsigned int	 ipi_ifindex;  /* Interface index */
		  struct in_addr ipi_spec_dst; /* Local address */
		  struct in_addr ipi_addr;     /* Header Destination address */

	      ipi_ifindex  is  the  unique  index  of  the  interface the packet was received on.
	      ipi_spec_dst is the local address of the packet and  ipi_addr  is  the  destination
	      address  in the packet header.  If IP_PKTINFO is passed to sendmsg(2) then the out-
	      going packet will be sent over the interface specified in ipi_ifindex with the des-
	      tination address set to ipi_spec_dst

	      If  enabled  the	IP_TOS ancillary message is passed with incoming packets. It con-
	      tains a byte which specifies the Type of Service/Precedence  field  of  the  packet
	      header.  Expects a boolean integer flag.

	      When this flag is set pass a IP_RECVTTL control message with the time to live field
	      of the received packet as a byte. Not supported for SOCK_STREAM sockets.

	      Pass all incoming IP options to the user in a IP_OPTIONS control message. The rout-
	      ing  header  and	other  options are already filled in for the local host. Not sup-
	      ported for SOCK_STREAM sockets.

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

       IP_TOS Set  or  receive	the Type-Of-Service (TOS) field that is sent with every IP packet
	      originating from this socket. It is used to 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 transmission 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 behaviour depends  on  the
	      configured queueing discipline.  Some high priority levels may require an effective
	      user id of 0 or the CAP_NET_ADMIN capability.  The priority can also be  set  in	a
	      protocol	independent  way  by  the  ( SOL_SOCKET, SO_PRIORITY ) socket option (see
	      socket(7) ).

       IP_TTL Set or retrieve the current time to live field that is send in  every  packet  send
	      from this socket.

	      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_RECVERR (defined in <linux/errqueue.h>)
	      Enable  extended reliable error message passing.	When enabled on a datagram socket
	      all generated errors will be queued in a per-socket  error  queue.  When	the  user
	      receives	an  error  from  a socket operation the errors can be received by calling
	      recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_extended_err structure describ-
	      ing  the	error  will be passed in a ancillary message with the type IP_RECVERR and
	      the level SOL_IP.  This is useful for reliable error handling on unconnected  sock-
	      ets.  The received data portion of the error queue contains the error packet.

	      The IP_RECVERR control message contains a sock_extended_err structure:

	      #define SO_EE_ORIGIN_NONE       0
	      #define SO_EE_ORIGIN_LOCAL      1
	      #define SO_EE_ORIGIN_ICMP       2
	      #define SO_EE_ORIGIN_ICMP6      3

	      struct sock_extended_err {
		  u_int32_t	  ee_errno;   /* error number */
		  u_int8_t	  ee_origin;  /* where the error originated */
		  u_int8_t	  ee_type;    /* type */
		  u_int8_t	  ee_code;    /* code */
		  u_int8_t	  ee_pad;
		  u_int32_t	  ee_info;    /* additional information */
		  u_int32_t	  ee_data;    /* other data */
		  /* More data may follow */

	      struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

	      ee_errno	contains  the  errno number of the queued error.  ee_origin 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_ORI-
	      GIN_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_fam-
	      ily 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 normal pay-
	      load.  Note that TCP has no error queue; MSG_ERRQUEUE  is  illegal  on  SOCK_STREAM
	      sockets.	Thus all errors are returned by socket function return or SO_ERROR only.

	      For  raw	sockets,  IP_RECVERR  enables  passing of all received ICMP errors to the
	      application, otherwise errors are only reported on connected sockets

	      It sets or retrieves an integer boolean flag.  IP_RECVERR defaults to off.

	      Sets or receives the Path MTU Discovery setting for a socket. When  enabled,  Linux
	      will  perform  Path  MTU	Discovery as defined in RFC1191 on this socket. The don't
	      fragment flag is set on all outgoing datagrams.  The system-wide	default  is  con-
	      trolled  by the ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and disabled on all
	      others. For non SOCK_STREAM sockets it is the user's  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.

	      When PMTU discovery is enabled the kernel automatically keeps track of the path MTU
	      per destination host.  When it is connected 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  con-
	      nectionless sockets with many destinations the new also MTU for a given destination
	      can also be accessed using the error queue (see IP_RECVERR).  A new error  will  be
	      queued for every incoming MTU update.

	      While  MTU  discovery  is  in progress initial packets from datagram sockets may be
	      dropped.	Applications using UDP should be aware of  this  and  not  take  it  into
	      account for their packet retransmit strategy.

	      To  bootstrap  the path MTU discovery process on unconnected sockets 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 desti-
	      nation address using connect(2) and retrieve the MTU by calling getsockopt(2)  with
	      the IP_MTU option.

       IP_MTU Retrieve	the  current  known  path MTU of the current socket.  Only valid when the
	      socket has been connected. Returns an integer. Only valid as a getsockopt(2).

	      Pass all to-be forwarded packets with the  IP  Router  Alert  option  set  to  this
	      socket.  Only  valid  for raw sockets. This is useful, for instance, for user space
	      RSVP daemons. The tapped packets are not forwarded by the kernel, it is  the  users
	      responsibility  to send them out again. Socket binding is ignored, such packets are
	      only filtered by protocol.  Expects an integer flag.

	      Set or reads 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.

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

	      Join a multicast group. Argument is a struct ip_mreqn structure.

	      struct ip_mreqn {
		  struct in_addr imr_multiaddr; /* IP multicast group address */
		  struct in_addr imr_address;	/* IP address of local interface */
		  int		 imr_ifindex;	/* interface index */

	      imr_multiaddr contains the address of the multicast group the application wants  to
	      join  or	leave.	It must be a valid multicast address.  imr_address is the address
	      of the local interface with which the system should join the multicast group; if it
	      is  equal  to  INADDR_ANY  an  appropriate  interface  is  chosen  by  the  system.
	      imr_ifindex is the interface index of the  interface  that  should  join/leave  the
	      imr_multiaddr group, or 0 to indicate any interface.

	      For  compatibility,  the	old ip_mreq structure is still supported. It differs from
	      ip_mreqn only by not including the imr_ifindex field.  Only  valid  as  a  setsock-

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

	      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 returned.

       The  IP	protocol  supports  the  sysctl  interface  to configure some global options. The
       sysctls can be accessed by reading or writing the /proc/sys/net/ipv4/* files or using  the
       sysctl(2) interface.

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

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

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

	      Not documented.

	      Contains	two  integers that define the default local port range allocated to sock-
	      ets. Allocation starts with the first number and ends with the second number.  Note
	      that  these  should  not conflict with the ports used by masquerading (although the
	      case is handled). Also arbitary choices  may  cause  problems  with  some  firewall
	      packet  filters  that  make assumptions about the local ports in use.  First number
	      should be at least >1024, better >4096 to avoid clashes with well known  ports  and
	      to minimize firewall problems.

	      If  enabled,  don't do Path MTU Discovery for TCP sockets by default. Path MTU dis-
	      covery may fail if misconfigured firewalls (that drop all ICMP packets) or  miscon-
	      figured  interfaces (e.g., a point-to-point link where the both ends don't agree on
	      the MTU) are on the path. It is better to fix the broken routers on the  path  than
	      to turn off Path MTU Discovery globally, because not doing it incurs a high cost to
	      the network.

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

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

	      When this boolean frag is enabled (not equal 0) incoming	fragments  (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 turn on here for a normal router or  host.	Otherwise
	      fragmented communication may me disturbed when the fragments would travel over dif-
	      ferent links. Defragmentation also has a large memory and CPU time cost.

	      This is automagically turned on when masquerading or transparent proxying are  con-

	      See arp(7).

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

       The ioctls to configure firewalling are documented in ipfw(7) from the ipchains package.

       Ioctls to configure generic device parameters are described in netdevice(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 bet-
       ter 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 interface of received datagrams. Linux has the more
       general IP_PKTINFO for the same task.

	      The  operation  is  only	defined on a connected socket, but the socket wasn't con-

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

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

       EACCES The  user  tried	to execute an operation without the necessary permissions.  These
	      include: Sending a packet to a broadcast address without	having	the  SO_BROADCAST
	      flag  set.   Sending  a  packet  via a prohibit route.  Modifying firewall settings
	      without CAP_NET_ADMIN or effective user id 0.  Binding to a reserved  port  without
	      the CAP_NET_BIND_SERVICE capacibility or effective user id 0.

	      Tried to bind to an address already in use.

	      Invalid socket option passed.

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

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

       EAGAIN Operation on a non-blocking socket would block.

	      The socket is not configured or an unknown socket type was requested.

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

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

	      A connection was closed during an accept(2).

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

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

	      No  valid  routing  table entry matches the destination address.	This error can be
	      caused by a ICMP message from a remote router or for the local routing table.

       ENODEV Network device not available or not capable of sending IP.

       ENOPKG A kernel subsystem was not configured.

	      Not enough free memory.  This often means that the memory allocation is limited  by
	      the  socket  buffer  limits, not by the system memory, but this is not 100% consis-

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

       options in Linux 2.2.  They are also all Linux specific and should not be used in programs
       intended to be portable.

       struct ip_mreqn is new in Linux 2.2.  Linux 2.0 only supported ip_mreq.

       The sysctls were introduced with Linux 2.2.

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

       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 currently 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.

       This man page was written by Andi Kleen.

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

       RFC791 for the original IP specification.
       RFC1122 for the IPv4 host requirements.
       RFC1812 for the IPv4 router requirements.

Linux Man Page				    2001-06-19					    IP(7)
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