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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
       conforming  to  RFC 1112.   It  also  contains an IP router including a
       packet filter.

       The  programming  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(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).   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 an
       unbound socket, it 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.

   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
       supply  port  numbers,  they  are implemented by higher level protocols
       like udp(7) and tcp(7).  On raw sockets  sin_port  is  set  to  the  IP
       protocol.

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

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

       sin_family  is  always  set to AF_INET.  This is required; in Linux 2.2
       most networking functions return EINVAL when this setting  is  missing.
       sin_port  contains  the  port  in network byte order.  The port numbers
       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
       protocol 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
       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
       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
       functions in the standard library work in network byte order.

       There are several special addresses: INADDR_LOOPBACK (127.0.0.1) always
       refers to the local host via the loopback device; INADDR_ANY  (0.0.0.0)
       means any address for binding; INADDR_BROADCAST (255.255.255.255) means
       any host and has the same effect on bind as INADDR_ANY  for  historical
       reasons.

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

       IP_ADD_MEMBERSHIP (since Linux 1.2)
              Join a multicast group.  Argument is an ip_mreqn structure.

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

              imr_multiaddr contains the address of the  multicast  group  the
              application  wants  to  join  or  leave.   It  must  be  a valid
              multicast  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
              interface.

              The  ip_mreqn  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_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_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_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)
              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 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
              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.
              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
              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  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
              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 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
              useful  for  diagnostic  tools such as tracepath(8) that wish to
              deliberately send probe packets larger than  the  observed  Path
              MTU.

       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
              returned.

       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
              sockets.

       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 only
              valid 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
              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
              processing 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
              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.

       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
              routing 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_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_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
              normal payload.  Note that TCP has no error queue;  MSG_ERRQUEUE
              is  not  permitted  on SOCK_STREAM sockets.  IP_RECVERR is valid
              for TCP, but all errors are returned by socket  function  return
              or SO_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_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
              Service/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
              supported for SOCK_STREAM sockets.

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

       IP_ROUTER_ALERT (since Linux 2.2)
              Pass all to-be forwarded packets with the IP Router Alert option
              set  to  this  socket.   Only  valid  for  raw sockets.  This is
              useful, 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
              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 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_TTL (since Linux 1.0)
              Set or retrieve the current time-to-live field that is  used  in
              every packet sent from this socket.

   /proc interfaces
       The  IP  protocol  supports a set of /proc interfaces to configure some
       global parameters.  The  parameters  can  be  accessed  by  reading  or
       writing   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
              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 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
              interface 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
              lifetime 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
       netdevice(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
              allocation 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  only  defined on a connected socket, but the
              socket wasn't connected.

       EPERM  User doesn't  have  permission  to  set  high  priority,  change
              configuration,  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_MTU, IP_MTU_DISCOVER, IP_PKTINFO, IP_RECVERR and IP_ROUTER_ALERT are
       Linux-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
       interface 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
       sockaddr_ll  address  structure  for  generic  link  layer  information
       instead of the old sockaddr_pkt.

BUGS

       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.

SEE ALSO

       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.

COLOPHON

       This  page  is  part of release 3.27 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/.