ip


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

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

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

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

       An   IP  socket  is  created  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-
       bers.

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

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

   Address format
       An IP socket address is defined as a combination  of  an  IP  interface
       address  and a 16-bit port number.  The basic IP protocol does not sup-
       ply port numbers, they are implemented by higher level  protocols  like
       udp(7) and tcp(7).  On raw sockets sin_port is set to the IP protocol.

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

       capability) may bind(2) to these sockets.  Note that the raw IPv4  pro-
       tocol  as  such  has no concept of a port, they are implemented only by
       higher protocols like tcp(7) and udp(7).

       sin_addr is the IP host address.  The s_addr member of  struct  in_addr
       contains  the  host  interface  address in network byte order.  in_addr
       should be assigned one of the INADDR_* values (e.g., INADDR_ANY) or set
       using  the  inet_aton(3),  inet_addr(3), inet_makeaddr(3) library func-
       tions or directly with the name resolver (see gethostbyname(3)).

       IPv4 addresses  are  divided  into  unicast,  broadcast  and  multicast
       addresses.   Unicast  addresses  specify  a single interface of a host,
       broadcast addresses specify  all  hosts  on  a  network  and  multicast
       addresses  address all hosts in a multicast group.  Datagrams to broad-
       cast addresses can be sent  or  received  only  when  the  SO_BROADCAST
       socket flag is set.  In the current implementation, connection-oriented
       sockets are allowed to use only unicast addresses.

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

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

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

       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-
                      struct in_addr imr_multiaddr;  /* IP multicast group
                                                        address */
                      struct in_addr imr_interface;  /* IP address of local
                                                        interface */
                      struct in_addr imr_sourceaddr; /* IP address of
                                                        multicast source */
                  };

              The ip_mreq_source structure is similar  to  ip_mreqn  described
              under  IP_ADD_MEMBERSIP.   The  imr_multiaddr field contains the
              address of the multicast group the application wants to join  or
              leave.   The  imr_interface  field  is  the address of the local
              interface with which the system should join the multicast group.
              Finally,  the  imr_sourceaddr  field contains the address of the
              source the application wants to receive data from.

              This option can be used multiple times to allow  receiving  data
              from more than one source.

       IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
              Stop  receiving multicast data from a specific source in a given
              group.  This is valid only after the application has  subscribed
              to   the  multicast  group  using  either  IP_ADD_MEMBERSHIP  or
              IP_ADD_SOURCE_MEMBERSHIP.

              Argument is  an  ip_mreq_source  structure  as  described  under
              IP_ADD_SOURCE_MEMBERSHIP.

       IP_DROP_MEMBERSHIP (since Linux 1.2)
              Leave  a  multicast  group.   Argument is an ip_mreqn or ip_mreq
              structure similar to IP_ADD_MEMBERSHIP.

       IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
              Leave a source-specific group--that is, stop receiving data from
              a  given  multicast group that come from a given source.  If the
              application has subscribed to multiple sources within  the  same
              group,  data from the remaining sources will still be delivered.
              To  stop  receiving  data  from  all  sources   at   once,   use
              IP_LEAVE_GROUP.

              Argument  is  an  ip_mreq_source  structure  as  described under
              IP_ADD_SOURCE_MEMBERSHIP.

       IP_FREEBIND (since Linux 2.4)
              If enabled, this boolean option allows binding to an IP  address
              that  is nonlocal or does not (yet) exist.  This permits listen-
              ing on a socket, without requiring the underlying network inter-
              face  or  the  specified dynamic IP address to be up at the time
              that the application is trying to bind to it.   This  option  is
              the  per-socket  equivalent of the ip_nonlocal_bind /proc inter-
              face described below.

       IP_HDRINCL (since Linux 2.0)
              If enabled, the user supplies an IP header in front of the  user
                                                        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.
              socket  to the destination address using connect(2) and retrieve
              the MTU by calling getsockopt(2) with the IP_MTU option.

              It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
              or  SOCK_RAW  sockets  by  setting  a value of IP_PMTUDISC_PROBE
              (available since Linux 2.6.22).  This is also particularly  use-
              ful  for  diagnostic  tools  such  as  tracepath(8) that wish to
              deliberately send probe packets larger than  the  observed  Path
              MTU.

       IP_MULTICAST_ALL (since Linux 2.6.31)
              This  option can be used to modify the delivery policy of multi-
              cast messages  to  sockets  bound  to  the  wildcard  INADDR_ANY
              address.  The argument is a boolean integer (defaults to 1).  If
              set to 1, the socket will receive messages from all  the  groups
              that  have been joined globally on the whole system.  Otherwise,
              it will deliver messages only from the  groups  that  have  been
              explicitly joined (for example via the IP_ADD_MEMBERSHIP option)
              on this particular socket.

       IP_MULTICAST_IF (since Linux 1.2)
              Set the local device for a multicast  socket.   Argument  is  an
              ip_mreqn  or  ip_mreq  (since  Linux  3.5)  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  sock-
              ets.

       IP_MULTICAST_TTL (since Linux 1.2)
              Set or read the time-to-live value of outgoing multicast packets
              for this socket.  It is very important for multicast packets  to
              set  the  smallest  TTL  possible.  The default is 1 which means
              that multicast packets don't leave the local network unless  the
              user program explicitly requests it.  Argument is an integer.

       IP_NODEFRAG (since Linux 2.6.36)
              If  enabled  (argument  is  nonzero), the reassembly of outgoing
              packets is disabled in the  netfilter  layer.   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
              packet.  This only works for  datagram  oriented  sockets.   The
              argument  is a flag that tells the socket whether the IP_PKTINFO
              message should be passed or not.  The message itself can only be
              sent/retrieved as control message with a packet using recvmsg(2)
              or sendmsg(2).

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

              ipi_ifindex is the unique index of the interface the packet  was
              received  on.   ipi_spec_dst  is the local address of the packet
              and ipi_addr is the destination address in  the  packet  header.
              If  IP_PKTINFO  is  passed to sendmsg(2) and ipi_spec_dst is not
              zero, then it is used as the local source address for the  rout-
              ing  table  lookup  and  for setting up IP source route options.
              When ipi_ifindex is not zero, the primary local address  of  the
              interface specified by the index overwrites ipi_spec_dst for the
              routing table lookup.

       IP_RECVERR (since Linux 2.2)
              Enable extended reliable error message passing.  When enabled on
              a datagram socket, all generated errors will be queued in a per-
              socket error queue.  When the user  receives  an  error  from  a
              socket   operation,  the  errors  can  be  received  by  calling
              recvmsg(2)   with    the    MSG_ERRQUEUE    flag    set.     The
              sock_extended_err  structure describing the error will be passed
              in an ancillary message with the type IP_RECVERR and  the  level
              IPPROTO_IP.   This  is  useful  for  reliable  error handling on
              unconnected sockets.  The received data  portion  of  the  error
              queue contains the error packet.

              The  IP_RECVERR  control  message  contains  a sock_extended_err
              structure:

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

                  struct sock_extended_err {
                      uint32_t ee_errno;   /* error number */
                      uint8_t  ee_origin;  /* where the error originated */
                      uint8_t  ee_type;    /* type */
                      uint8_t  ee_code;    /* code */
                      uint8_t  ee_pad;
                      uint32_t ee_info;    /* additional information */
                      uint32_t ee_data;    /* other data */
                      /* More data may follow */
                  };

              set to SO_EE_ORIGIN_ICMP for errors received as an ICMP  packet,
              or  SO_EE_ORIGIN_LOCAL  for  locally  generated errors.  Unknown
              values should be ignored.  ee_type and ee_code are set from  the
              type  and  code fields of the ICMP header.  ee_info contains the
              discovered MTU for EMSGSIZE errors.  The message  also  contains
              the  sockaddr_in  of  the  node  caused  the error, which can be
              accessed with the SO_EE_OFFENDER macro.  The sin_family field of
              the  SO_EE_OFFENDER  address  is  AF_UNSPEC  when the source was
              unknown.  When the error originated from  the  network,  all  IP
              options  (IP_OPTIONS,  IP_TTL,  etc.)  enabled on the socket and
              contained in the error packet are passed  as  control  messages.
              The  payload of the packet causing the error is returned as nor-
              mal payload.  Note that TCP has no error queue; MSG_ERRQUEUE  is
              not  permitted  on SOCK_STREAM sockets.  IP_RECVERR is valid for
              TCP, but all errors are returned by socket  function  return  or
              SO_ERROR only.

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

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

       IP_RECVOPTS (since Linux 2.2)
              Pass all incoming IP options to the user in a IP_OPTIONS control
              message.   The  routing  header  and  other  options are already
              filled in for the local host.   Not  supported  for  SOCK_STREAM
              sockets.

       IP_RECVORIGDSTADDR (since Linux 2.6.29)
              This boolean option enables the IP_ORIGDSTADDR ancillary message
              in recvmsg(2), in which the kernel returns the original destina-
              tion address of the datagram being received.  The ancillary mes-
              sage contains a struct sockaddr_in.

       IP_RECVTOS (since Linux 2.2)
              If enabled the IP_TOS ancillary message is passed with  incoming
              packets.   It  contains  a byte which specifies the Type of Ser-
              vice/Precedence field of the packet header.  Expects  a  boolean
              integer flag.

       IP_RECVTTL (since Linux 2.2)
              When  this  flag  is set, pass a IP_TTL control message with the
              time to live field of the received packet as a byte.   Not  sup-
              ported for SOCK_STREAM sockets.

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

       IP_ROUTER_ALERT (since Linux 2.2)
              Pass all to-be forwarded packets with the IP Router Alert option
              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
              IP_ADD_SOURCE_MEMBERSHIP.

   /proc interfaces
       The IP protocol supports a set of /proc interfaces  to  configure  some
       global  parameters.  The parameters can be accessed by reading or writ-
       ing files in the directory /proc/sys/net/ipv4/.   Interfaces  described
       as Boolean take an integer value, with a nonzero value ("true") meaning
       that the corresponding option is enabled, and a  zero  value  ("false")
       meaning that the option is disabled.

       ip_always_defrag (Boolean; since Linux 2.2.13)
              [New with kernel 2.2.13; in earlier kernel versions this feature
              was controlled at compile time  by  the  CONFIG_IP_ALWAYS_DEFRAG
              option; this option is not present in 2.4.x and later]

              When  this boolean flag is enabled (not equal 0), incoming frag-
              ments (parts of IP packets that arose  when  some  host  between
              origin  and  destination decided that the packets were too large
              and cut them into pieces)  will  be  reassembled  (defragmented)
              before being processed, even if they are about to be forwarded.

       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.

       neigh/*
              See arp(7).

              a    privileged   port   without   superuser   privileges   (the
              CAP_NET_BIND_SERVICE capability).

       EADDRINUSE
              Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

       EALREADY
              An  connection  operation  on a nonblocking socket is already in
              progress.

       ECONNABORTED
              A connection was closed during an accept(2).

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

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

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

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

       ENOBUFS, ENOMEM
              Not enough free memory.  This often means that the memory  allo-
              cation is limited by the socket buffer limits, not by the system
              memory, but this is not 100% consistent.

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

       ENOPKG A kernel subsystem was not configured.

       ENOPROTOOPT and EOPNOTSUPP
              Invalid socket option passed.

       ENOTCONN
              The operation is defined only on a  connected  socket,  but  the
              socket wasn't connected.

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

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

       Be  very careful with the SO_BROADCAST option - it is not privileged in
       Linux.  It is easy to overload the network  with  careless  broadcasts.
       For  new  application  protocols  it is better to use a multicast group
       instead of broadcasting.  Broadcasting is discouraged.

       Some  other  BSD  sockets  implementations  provide  IP_RCVDSTADDR  and
       IP_RECVIF  socket options to get the destination address and the inter-
       face of received datagrams.  Linux has the more general IP_PKTINFO  for
       the same task.

       Some BSD sockets implementations also provide an IP_RECVTTL option, but
       an ancillary message with type IP_RECVTTL is passed with  the  incoming
       packet.  This is different from the IP_TTL option used in Linux.

       Using  SOL_IP socket options level isn't portable, BSD-based stacks use
       IPPROTO_IP level.

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

BUGS
       There are too many inconsistent error values.

       The  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.

SEE ALSO
       recvmsg(2),   sendmsg(2),   byteorder(3),   ipfw(4),   capabilities(7),
       icmp(7), ipv6(7), netlink(7), raw(7), socket(7), tcp(7), udp(7)

       RFC 791  for the original IP specification.  RFC 1122 for the IPv4 host
       requirements.  RFC 1812 for the IPv4 router requirements.

COLOPHON
       This page is part of release 3.54 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                             2013-09-17                             IP(7)
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