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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(PF_INET, SOCK_STREAM, 0);
       udp_socket = socket(PF_INET, SOCK_DGRAM, 0);
       raw_socket = socket(PF_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(PF_INET,  socket_type,  protocol).   Valid  socket   types   are
       SOCK_STREAM  to  open  a  tcp(7)  socket,  SOCK_DGRAM  to open a udp(7)
       socket, or SOCK_RAW to open a raw(7) socket to access the  IP  protocol
       directly.   protocol is the IP protocol in the IP header to be received
       or sent.  The only valid 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 */
               uint16_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 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_OPTIONS
              Sets  or  get  the  IP options to be sent with every packet from
              this socket.  The arguments are a pointer  to  a  memory  buffer
              containing the options and the option length.  The setsockopt(2)
              call sets the IP options associated with a socket.  The  maximum
              option  size  for IPv4 is 40 bytes.  See RFC 791 for the allowed
              options.  When the  initial  connection  request  packet  for  a
              SOCK_STREAM  socket  contains IP options, the IP options will be
              set automatically to the options from the  initial  packet  with
              routing  headers  reversed.  Incoming packets are not allowed to
              change  options  after  the  connection  is  established.    The
              processing of all incoming source routing options is disabled by
              default and can be  enabled  by  using  the  accept_source_route
              sysctl.   Other  options like timestamps are still handled.  For
              datagram sockets, IP options can be only set by the local  user.
              Calling  getsockopt(2)  with  IP_OPTIONS  puts  the  current  IP
              options used for sending into the supplied buffer.

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

       IP_TOS Set or receive the Type-Of-Service (TOS) field that is sent with
              every  IP  packet  originating  from this socket.  It is used to
              prioritize packets on the network.  TOS is a  byte.   There  are
              some  standard  TOS  flags  defined:  IPTOS_LOWDELAY to minimize
              delays for interactive  traffic,  IPTOS_THROUGHPUT  to  optimize
              throughput,   IPTOS_RELIABILITY  to  optimize  for  reliability,
              IPTOS_MINCOST should  be  used  for  "filler  data"  where  slow
              transmission  doesn’t  matter.   At most one of these TOS values
              can be specified.  Other bits are invalid and shall be  cleared.
              Linux  sends  IPTOS_LOWDELAY datagrams first by default, but the
              exact 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 Set or retrieve the current time to live field that is  used  in
              every packet sent from this socket.

       IP_HDRINCL
              If  enabled  the user supplies an IP header in front of the user
              data.  Only valid for SOCK_RAW sockets.   See  raw(7)  for  more
              information.   When  this  flag  is  enabled  the  values set by
              IP_OPTIONS, IP_TTL and IP_TOS are ignored.

       IP_RECVERR (defined in <linux/errqueue.h>)
              Enable extended reliable error message passing.  When enabled on
              a  datagram socket all generated errors will be queued in a per-
              socket error queue.  When the user  receives  an  error  from  a
              socket   operation   the  errors  can  be  received  by  calling
              recvmsg(2)   with    the    MSG_ERRQUEUE    flag    set.     The
              sock_extended_err  structure 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 illegal 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_MTU_DISCOVER
              Sets or receives the Path MTU Discovery setting  for  a  socket.
              When  enabled,  Linux will perform Path MTU Discovery as defined
              in RFC 1191 on this socket.  The don’t fragment flag is  set  on
              all  outgoing  datagrams.  The system-wide default is controlled
              by the  ip_no_pmtu_disc  sysctl  for  SOCK_STREAM  sockets,  and
              disabled  on  all others.  For non-SOCK_STREAM sockets it is the
              user’s responsibility to packetize the data in MTU sized  chunks
              and  to do the retransmits if necessary.  The kernel will reject
              packets that are bigger than the known path MTU if this flag  is
              set (with EMSGSIZE ).

              Path MTU discovery flags   Meaning
              IP_PMTUDISC_WANT           Use per-route settings.
              IP_PMTUDISC_DONT           Never do Path MTU Discovery.
              IP_PMTUDISC_DO             Always do Path MTU Discovery.
              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  also  MTU for a given destination can also be accessed
              using the error queue (see IP_RECVERR).  A  new  error  will  be
              queued for every incoming MTU update.

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

              To bootstrap the  path  MTU  discovery  process  on  unconnected
              sockets  it is possible to start with a big datagram size (up to
              64K-headers bytes long) and let it shrink by updates of the path
              MTU.

              To  get  an  initial estimate of the path MTU connect a datagram
              socket to the 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.
              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_MTU Retrieve the current known path MTU of the current socket.  Only
              valid when the socket has been connected.  Returns  an  integer.
              Only valid as a getsockopt(2).

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

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

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

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

              For compatibility, the old ip_mreq structure is still supported.
              It  differs  from ip_mreqn only by not including the imr_ifindex
              field.  Only valid as a setsockopt(2).

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

       IP_MULTICAST_IF
              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.

   Sysctls
       The  IP protocol supports the sysctl interface to configure some global
       options.  The sysctls  can  be  accessed  by  reading  or  writing  the
       /proc/sys/net/ipv4/* files or using the sysctl(2) interface.  Variables
       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)
              [New with kernel 2.2.13; in earlier kernel versions this feature
              was  controlled  at  compile time by the CONFIG_IP_ALWAYS_DEFRAG
              option; this option is not present in 2.4.x and later]

              When this  boolean  frag  is  enabled  (not  equal  0)  incoming
              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
              Not documented.

       ip_default_ttl (integer; default: 64)
              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)
              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)
              Enable IP forwarding with a boolean flag.  IP forwarding can  be
              also set on a per interface basis.

       ip_local_port_range
              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 >1024,
              better >4096 to avoid clashes  with  well  known  ports  and  to
              minimize firewall problems.

       ip_no_pmtu_disc (Boolean; default: disabled)
              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)
              If  set,  allows processes to bind(2) to non-local IP addresses,
              which can be quite useful, but may break some applications.

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

       ip6frag_secret_interval (integer; default 600)
              Regeneration interval  (in  seconds)  of  the  hash  secret  (or
              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    reserved    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 non-blocking socket would block.

       EALREADY
              An  connection  operation on a non-blocking 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).

VERSIONS

       IP_MTU, IP_MTU_DISCOVER, IP_PKTINFO, IP_RECVERR and IP_ROUTER_ALERT are
       new  options in Linux 2.2.  They are also all Linux-specific and should
       not be used in programs intended to be portable.

       IP_PMTUDISC_PROBE is new in Linux 2.6.22.

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

       The sysctls were introduced with Linux 2.2.

NOTES

       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(PF_INET,
       SOCK_PACKET,  protocol)  syntax  is still supported to open a packet(7)
       socket.  This is deprecated and should be replaced by socket(PF_PACKET,
       SOCK_RAW,   protocol)   instead.    The  main  difference  is  the  new
       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

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.