Man Linux: Main Page and Category List


       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
       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

       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

           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 ( 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
              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

              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

              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

       IP_MULTICAST_IF (since Linux 1.2)
              Set the local device for a multicast  socket.   Argument  is  an
              ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.

              When   an  invalid  socket  option  is  passed,  ENOPROTOOPT  is

       IP_MULTICAST_LOOP (since Linux 1.2)
              Set or read a boolean integer argument that  determines  whether
              sent  multicast  packets  should  be  looped  back  to the local

       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)
              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

                  #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

       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

       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.

              See arp(7).

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

       Ioctls  to  configure  generic  device  parameters  are  described   in


       EACCES The  user  tried  to  execute an operation without the necessary
              permissions.  These include: sending a  packet  to  a  broadcast
              address  without  having  the  SO_BROADCAST  flag set; sending a
              packet via a prohibit route; modifying firewall settings without
              superuser  privileges (the CAP_NET_ADMIN capability); binding to
              a   privileged   port   without   superuser   privileges    (the
              CAP_NET_BIND_SERVICE capability).

              Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

              An connection operation on a nonblocking socket  is  already  in

              A connection was closed during an accept(2).

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

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

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

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

              Not  enough  free  memory.   This  often  means  that the memory
              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.

              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
              configuration,  or  send  signals  to  the  requested process or

       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).


       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.

       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.


       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.


       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.


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