ip

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

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

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

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

       An IP socket is created using socket(2):

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

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

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

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

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

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

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

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

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

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

       When an invalid socket option is specified, getsockopt(2) and  setsock-
       opt(2) fail with the error ENOPROTOOPT.

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

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

              imr_multiaddr  contains  the  address of the multicast group the
              application wants to join or leave.  It must be a  valid  multi-
              cast  address  (or  setsockopt(2)  fails with the error EINVAL).
              imr_address is the address of the local interface with which the
              system  should  join  the  multicast  group;  if  it is equal to

       IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
              Join  a  multicast  group  and  allow receiving data only from a
              specified source.  Argument is an ip_mreq_source structure.

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

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

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

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

              Argument is  an  ip_mreq_source  structure  as  described  under
              IP_ADD_SOURCE_MEMBERSHIP.

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

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

              Argument  is  an  ip_mreq_source  structure  as  described under
              IP_ADD_SOURCE_MEMBERSHIP.

       IP_FREEBIND (since Linux 2.4)
              If enabled, this boolean option allows binding to an IP  address
              that  is nonlocal or does not (yet) exist.  This permits listen-
              ing on a socket, without requiring the underlying network inter-
              face  or  the  specified dynamic IP address to be up at the time
              API.  Argument is an ip_msfilter structure.

                  struct ip_msfilter {
                      struct in_addr imsf_multiaddr; /* IP multicast group
                                                        address */
                      struct in_addr imsf_interface; /* IP address of local
                                                        interface */
                      uint32_t       imsf_fmode;     /* Filter-mode */

                      uint32_t       imsf_numsrc;    /* Number of sources in
                                                        the following array */
                      struct in_addr imsf_slist[1];  /* Array of source
                                                        addresses */
                  };

              There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can
              be  used  to  specify  the  filtering  mode.   Additionally, the
              IP_MSFILTER_SIZE(n) macro exists to determine how much memory is
              needed  to  store  ip_msfilter  structure  with n sources in the
              source list.

              For the full description of multicast source filtering refer  to
              RFC 3376.

       IP_MTU (since Linux 2.2)
              Retrieve  the  current  known  path  MTU  of the current socket.
              Returns an integer.

              IP_MTU is valid only for getsockopt(2) and can be employed  only
              when the socket has been connected.

       IP_MTU_DISCOVER (since Linux 2.2)
              Set  or  receive  the  Path  MTU Discovery setting for a socket.
              When enabled, Linux will perform Path MTU Discovery  as  defined
              in  RFC 1191  on SOCK_STREAM sockets.  For non-SOCK_STREAM sock-
              ets, IP_PMTUDISC_DO forces the don't-fragment flag to be set  on
              all outgoing packets.  It is the user's responsibility to packe-
              tize the data in MTU-sized chunks and to do the  retransmits  if
              necessary.   The  kernel  will  reject (with EMSGSIZE) datagrams
              that are bigger than the known path MTU.  IP_PMTUDISC_WANT  will
              fragment a datagram if needed according to the path MTU, or will
              set the don't-fragment flag otherwise.

              The system-wide default can be toggled between  IP_PMTUDISC_WANT
              and  IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
              values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.

              Path MTU discovery value   Meaning
              IP_PMTUDISC_WANT           Use per-route settings.
              IP_PMTUDISC_DONT           Never do Path MTU Discovery.
              IP_PMTUDISC_DO             Always do Path MTU Discovery.
              IP_PMTUDISC_PROBE          Set DF but ignore Path MTU.

              When PMTU discovery is enabled, the kernel  automatically  keeps
              retransmit strategy.

              To bootstrap the path MTU discovery process on unconnected sock-
              ets, 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  use-
              ful  for  diagnostic  tools  such  as  tracepath(8) that wish to
              deliberately send probe packets larger than  the  observed  Path
              MTU.

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

       IP_MULTICAST_IF (since Linux 1.2)
              Set the local device for a multicast socket.  The  argument  for
              setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq struc-
              ture similar to  IP_ADD_MEMBERSHIP,  or  an  in_addr  structure.
              (The  kernel determines which structure is being passed based on
              the size passed in optlen.)  For getsockopt(2), the argument  is
              an in_addr structure.

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

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

       IP_NODEFRAG (since Linux 2.6.36)
              If  enabled  (argument  is  nonzero), the reassembly of outgoing
              packets is disabled in the netfilter layer.  The argument is  an
              integer.

              This option is valid only for SOCK_RAW sockets.
              cessing  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 han-
              dled.  For datagram sockets, IP options can be only set  by  the
              local user.  Calling getsockopt(2) with IP_OPTIONS puts the cur-
              rent 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  rout-
              ing  table  lookup  and  for setting up IP source route options.
              When ipi_ifindex is not zero, the primary local address  of  the
              interface specified by the index overwrites ipi_spec_dst for the
              routing table lookup.

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

              The  IP_RECVERR  control  message  contains  a sock_extended_err
              structure:

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

                  struct sock_extended_err {

              ee_errno contains the errno number of the queued error.  ee_ori-
              gin 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 nor-
              mal payload.  Note that TCP has no error queue; MSG_ERRQUEUE  is
              not  permitted  on SOCK_STREAM sockets.  IP_RECVERR is valid for
              TCP, but all errors are returned by socket  function  return  or
              SO_ERROR only.

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

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

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

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

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

       IP_RECVTTL (since Linux 2.2)
              When  this  flag  is set, pass a IP_TTL control message with the
              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 trans-
              mission doesn't matter.  At most one of these TOS values can  be
              specified.   Other bits are invalid and shall be cleared.  Linux
              sends IPTOS_LOWDELAY datagrams first by default, but  the  exact
              behavior  depends  on  the configured queueing discipline.  Some
              high-priority  levels  may  require  superuser  privileges  (the
              CAP_NET_ADMIN capability).

       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  (i.e.,
              the system hosting the application that employs the IP_TRANSPAR-
              ENT socket option).  Enabling this socket option requires  supe-
              ruser privileges (the CAP_NET_ADMIN capability).

              TProxy redirection with the iptables TPROXY target also requires
              that this option be set on the redirected socket.

       IP_TTL (since Linux 1.0)
              Set or retrieve the current time-to-live field that is  used  in
              every packet sent from this socket.

       IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
              Unblock  previously  blocked multicast source.  Returns EADDRNO-
              TAVAIL when given source is not being blocked.

              Argument is  an  ip_mreq_source  structure  as  described  under
              IP_ADD_SOURCE_MEMBERSHIP.

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

       ip_always_defrag (Boolean; since Linux 2.2.13)
              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 inter-
              face with changing IP addresses.  0 means no rewriting, 1  turns
              it on and 2 enables verbose mode.

       ip_forward (Boolean; default: disabled; since Linux 1.2)
              Enable  IP forwarding with a boolean flag.  IP forwarding can be
              also set on a per-interface basis.

       ip_local_port_range (since Linux 2.2)
              This file contains two integers that define  the  default  local
              port range allocated to sockets that are not explicitly bound to
              a port number--that is, the range used for ephemeral ports.   An
              ephemeral port is allocated to a socket in the following circum-
              stances:

              *  the port number in a socket address is specified  as  0  when
                 calling bind(2);

              *  listen(2)  is  called  on a stream socket that was not previ-
                 ously bound;

              *  connect(2) was called on a socket  that  was  not  previously
                 bound;

              *  sendto(2)  is called on a datagram socket that was not previ-
                 ously bound.

              Allocation of ephemeral ports starts with the  first  number  in
              ip_local_port_range  and  ends  with  the second number.  If the
              range of ephemeral ports is exhausted, then the relevant  system
              call returns an error (but see BUGS).

              Note  that the port range in ip_local_port_range should not con-
              flict 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.  The first number should be at least greater than
              1024, or better, greater than 4096, to avoid clashes  with  well
              If  set,  allows  processes to bind(2) to nonlocal IP addresses,
              which can be quite useful, but may break some applications.

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

       ip6frag_secret_interval (integer; default: 600)
              Regeneration interval (in seconds) of the hash secret (or  life-
              time for the hash secret) for IPv6 fragments.

       ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
              If the amount of queued IP fragments reaches ipfrag_high_thresh,
              the queue is pruned  down  to  ipfrag_low_thresh.   Contains  an
              integer with the number of bytes.

       neigh/*
              See arp(7).

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

       Ioctls  to  configure generic device parameters are described in netde-
       vice(7).

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

       EADDRINUSE
              Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

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

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

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

              cation is limited by the socket buffer limits, not by the system
              memory, but this is not 100% consistent.

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

       ENOPKG A kernel subsystem was not configured.

       ENOPROTOOPT and EOPNOTSUPP
              Invalid socket option passed.

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

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

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

       ESOCKTNOSUPPORT
              The  socket  is  not  configured  or  an unknown socket type was
              requested.

       Other errors may be generated by the overlaying protocols; see  tcp(7),
       raw(7), udp(7), and socket(7).

NOTES
       IP_FREEBIND,  IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER, IP_RECVORIGDSTADDR,
       IP_PKTINFO, IP_RECVERR, 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 inter-
       face of received datagrams.  Linux has the more general IP_PKTINFO  for
       the same task.

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

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

   Compatibility
       For  compatibility  with  Linux  2.0,  the   obsolete   socket(AF_INET,
       SOCK_PACKET,  protocol)  syntax  is still supported to open a packet(7)
       socket.  This is deprecated and should be replaced by socket(AF_PACKET,
       SOCK_RAW,  protocol)  instead.   The  main  difference is the new sock-

       Receiving  the  original  destination  address  with  MSG_ERRQUEUE   in
       msg_name by recvmsg(2) does not work in some 2.2 kernels.

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

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

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-05-07                             IP(7)
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