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       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

       An IP socket is created by calling the socket(2) function as socket(AF_INET,  socket_type,
       protocol).  Valid socket types are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM to open
       a udp(7) socket, or SOCK_RAW to open a raw(7) socket to access the IP  protocol  directly.
       protocol  is  the  IP  protocol  in  the IP header to be received or sent.  The only valid
       values for protocol are 0 and IPPROTO_TCP for TCP sockets, and 0 and IPPROTO_UDP  for  UDP
       sockets.   For  SOCK_RAW  you  may  specify  a  valid IANA IP protocol defined in RFC 1700
       assigned numbers.

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

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

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

           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

       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 reasons.

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

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

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

              imr_multiaddr contains the address of the multicast group the application wants  to
              join  or  leave.  It must be a valid multicast address (or setsockopt(2) fails with
              the error EINVAL).  imr_address is the address of the local  interface  with  which
              the  system  should  join  the  multicast  group;  if  it is equal to INADDR_ANY an
              appropriate interface is chosen by the system.  imr_ifindex is the interface  index
              of  the  interface that should join/leave the imr_multiaddr group, or 0 to indicate
              any interface.

              The ip_mreqn structure 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

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

       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 listening on a socket, without requiring the
              underlying network interface 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 interface described below.

       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 SOCK_STREAM sockets.  For
              non-SOCK_STREAM sockets, IP_PMTUDISC_DO forces the don't-fragment flag to be set on
              all outgoing packets.  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
              (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 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  an  EMSGSIZE  error occurred).  The path MTU may change over
              time.  For connectionless sockets with many destinations, the new MTU for  a  given
              destination  can  also  be  accessed using the error queue (see IP_RECVERR).  A new
              error will be queued for every incoming MTU update.

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

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

              To get an initial estimate of the path  MTU,  connect  a  datagram  socket  to  the
              destination  address using connect(2) and retrieve the MTU by calling getsockopt(2)
              with the IP_MTU option.

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

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

              When an invalid socket option is passed, ENOPROTOOPT is returned.

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

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

       IP_NODEFRAG (since Linux 2.6.36)
              If enabled (argument is nonzero), the reassembly of outgoing packets is disabled in
              the netfilter layer.  This option is only valid for SOCK_RAW sockets.  The argument
              is an integer.

       IP_OPTIONS (since Linux 2.0)
              Set or get the IP options to be sent with  every  packet  from  this  socket.   The
              arguments  are  a  pointer to a memory buffer containing the options and the option
              length.  The setsockopt(2) call sets the IP options associated with a socket.   The
              maximum  option  size  for  IPv4 is 40 bytes.  See RFC 791 for the allowed options.
              When the initial connection request packet for a  SOCK_STREAM  socket  contains  IP
              options,  the  IP options will be set automatically to the options from the initial
              packet with routing headers reversed.  Incoming packets are not allowed  to  change
              options after the connection is established.  The processing of all incoming source
              routing  options  is  disabled  by  default  and  can  be  enabled  by  using   the
              accept_source_route  /proc  interface.   Other  options  like  timestamps are still
              handled.  For datagram sockets, IP options can be  only  set  by  the  local  user.
              Calling  getsockopt(2) with IP_OPTIONS puts the current IP options used for sending
              into the supplied buffer.

       IP_PKTINFO (since Linux 2.2)
              Pass an IP_PKTINFO  ancillary  message  that  contains  a  pktinfo  structure  that
              supplies  some information about the incoming packet.  This only works for datagram
              oriented sockets.  The argument is  a  flag  that  tells  the  socket  whether  the
              IP_PKTINFO  message  should  be  passed  or  not.   The  message itself can only be
              sent/retrieved as control message with a packet using recvmsg(2) or sendmsg(2).

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

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

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

              The IP_RECVERR control message contains a sock_extended_err structure:

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

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

                  struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

              ee_errno  contains  the  errno number of the queued error.  ee_origin is the origin
              code of where the error originated.  The other fields are  protocol-specific.   The
              macro  SO_EE_OFFENDER  returns a pointer to the address of the network object where
              the error originated from given a  pointer  to  the  ancillary  message.   If  this
              address  is  not known, the sa_family member of the sockaddr contains AF_UNSPEC and
              the other fields of the sockaddr are undefined.

              IP  uses  the  sock_extended_err  structure  as  follows:  ee_origin  is   set   to
              SO_EE_ORIGIN_ICMP  for errors received as an ICMP packet, or SO_EE_ORIGIN_LOCAL for
              locally generated errors.  Unknown values should be ignored.  ee_type  and  ee_code
              are  set  from  the  type and code fields of the ICMP header.  ee_info contains the
              discovered MTU for EMSGSIZE errors.  The message also contains the  sockaddr_in  of
              the  node  caused  the  error, which can be accessed with the SO_EE_OFFENDER macro.
              The sin_family field of the SO_EE_OFFENDER address is AF_UNSPEC when the source was
              unknown.   When  the error originated from the network, all IP options (IP_OPTIONS,
              IP_TTL, etc.) enabled on the socket and contained in the error packet are passed as
              control  messages.   The  payload  of  the  packet causing the error is returned as
              normal payload.  Note that TCP has no error queue; MSG_ERRQUEUE is not permitted on
              SOCK_STREAM  sockets.   IP_RECVERR is valid for TCP, but all errors are returned by
              socket function return or SO_ERROR only.

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

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

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

       IP_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 destination address  of  the  datagram  being
              received.  The ancillary message 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 Service/Precedence field of the  packet
              header.  Expects a boolean integer flag.

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

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

       IP_ROUTER_ALERT (since Linux 2.2)
              Pass  all  to-be  forwarded  packets  with  the  IP Router Alert option set to this
              socket.  Only valid for raw sockets.  This is useful, for instance, for  user-space
              RSVP daemons.  The tapped packets are not forwarded by the kernel; it is the user's
              responsibility to send them out again.  Socket binding is ignored, such packets are
              only filtered by protocol.  Expects an integer flag.

       IP_TOS (since Linux 1.0)
              Set  or  receive  the Type-Of-Service (TOS) field that is sent with every IP packet
              originating from this socket.  It is used to prioritize  packets  on  the  network.
              TOS  is  a  byte.   There  are  some  standard TOS flags defined: IPTOS_LOWDELAY to
              minimize delays for interactive traffic, IPTOS_THROUGHPUT to  optimize  throughput,
              IPTOS_RELIABILITY  to  optimize  for  reliability, IPTOS_MINCOST should be used for
              "filler data" where slow transmission doesn't matter.  At most  one  of  these  TOS
              values can be specified.  Other bits are invalid and shall be cleared.  Linux sends
              IPTOS_LOWDELAY datagrams first by default, but the exact behavior  depends  on  the
              configured  queueing  discipline.   Some high priority levels may require superuser
              privileges (the CAP_NET_ADMIN capability).  The priority  can  also  be  set  in  a
              protocol  independent  way  by  the  (SOL_SOCKET,  SO_PRIORITY)  socket option (see

       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.

   /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

       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 netdevice(7).


       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

       EAGAIN Operation on a nonblocking socket would block.

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

              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 fragmented.

              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%

       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

       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.

              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


       IP_ROUTER_ALERT, and IP_TRANSPARENT are Linux-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 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


       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.


       This page is part of release 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at