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IP(7)                      Linux Programmer's Manual                     IP(7)



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(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  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 sock-
       ets, see socket(7).

       An IP socket is created  by  calling  the  socket(2)  function  as  socket(AF_INET,
       socket_type,  protocol).   Valid  socket  types  are  SOCK_STREAM  to open a tcp(7)
       socket, SOCK_DGRAM to open a udp(7) socket, or SOCK_RAW to open a raw(7) socket  to
       access  the  IP protocol directly.  protocol is the IP protocol in the IP header to
       be received or sent.  The only valid values for protocol are 0 and IPPROTO_TCP  for
       TCP sockets, and 0 and IPPROTO_UDP for UDP sockets.  For SOCK_RAW you may specify a
       valid IANA IP protocol defined in RFC 1700 assigned 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 an 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 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 */
               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  network-
       ing  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 imple-
       mented 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 gethost-
       byname(3)).

       IPv4 addresses are divided into unicast, broadcast and multicast  addresses.   Uni-
       cast  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-ori-
       ented 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 setsock-
       opt(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  setsock-
              opt(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 is available only since Linux 2.2.  For compatibility, the old
              ip_mreq structure (present since Linux 1.2) is still supported.  It  differs
              from  ip_mreqn only by not including the imr_ifindex field.  Only valid as a
              setsockopt(2).

       IP_ADD_SOURCE_MEMBERSHIP (since Linux 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 */
                  };

              ip_mreq_source  structure is similar to ip_mreqn described at IP_ADD_MEMBER-
              SIP.  imr_multiaddr contains the address of the multicast group the applica-
              tion  wants  to  join  or  leave.  imr_interface is the address of the local
              interface with which the system should join the  multicast  group.   Finally
              imr_sourceaddr field contains 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 2.5.68)
              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 at IP_ADD_SOURCE_MEM-
              BERSHIP.

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

       IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.5.68)
              Leave a source-specific group, i.e., stop receiving data from a given multi-
              cast group that come from a given source).   If  the  application  has  sub-
              scribed  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 at IP_ADD_SOURCE_MEM-
              BERSHIP.

       IP_FREEBIND (since Linux 2.4)
              If enabled, this boolean option allows binding to an IP address that is non-
              local  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_MSFILTER (since Linux 2.5.68)
              This option provides access to the advanced full-state filtering API.  Argu-
              ment 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.  Additionaly, 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.  Only valid  when
              the  socket  has  been connected.  Returns an integer.  Only valid as a get-
              sockopt(2).

       IP_MTU_DISCOVER (since Linux 2.2)
              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 /proc/sys/net/ipv4/ip_no_pmtu_disc file for
              SOCK_STREAM sockets, and disabled on all others.  For non-SOCK_STREAM  sock-
              ets,  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 pack-
              ets  that are bigger than the known path MTU if this flag is set (with EMSG-
              SIZE).

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

              When PMTU discovery is enabled, the kernel automatically keeps track of  the
              path MTU per destination host.  When it is connected to a specific peer with
              connect(2), the currently known path MTU can be retrieved conveniently using
              the  IP_MTU  socket  option (e.g., after a EMSGSIZE error occurred).  It may
              change over time.  For connectionless sockets with  many  destinations,  the
              new  MTU  for a given destination can also be accessed using the error queue
              (see IP_RECVERR).  A new error will be queued for every incoming MTU update.

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

              To  bootstrap  the  path MTU discovery process on unconnected sockets, it is
              possible to start with a big datagram size (up to  64K-headers  bytes  long)
              and let it shrink by updates of the path 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  get-
              sockopt(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 tra-
              cepath(8) that wish to deliberately  send  probe  packets  larger  than  the
              observed Path MTU.

       IP_MULTICAST_ALL (since Linux 2.6.31)
              Sets  the policy for multicast delivery to the socket. Argument is a boolean
              integer that enables or disables multicast delivery from all groups.  If not
              set,  delivery  to  the  socket  is  restricted to data from those multicast
              groups that have been explicitly subscribed to via a multicast  join  opera-
              tion  for  this  socket. The default is 1 which means that a socket which is
              bound to the wildcard address (INADDR_ANY) will  receive  multicast  packets
              from all groups that have been subscribed to on this system.

       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)
              Sets or reads a boolean integer argument that determines whether sent multi-
              cast 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_OPTIONS (since Linux 2.0)
              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 automati-
              cally  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 dis-
              abled 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  getsock-
              opt(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 destina-
              tion  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 ancil-
              lary  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 ancil-
              lary  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  unde-
              fined.

              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_ORI-
              GIN_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 mes-
              sage 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  con-
              trol  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  per-
              mitted  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 sock-
              addr_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 sock-
              ets.

       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 ker-
              nel; 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 inde-
              pendent  way by the (SOL_SOCKET, SO_PRIORITY) socket option (see socket(7)).

       IP_TRANSPARENT (since Linux 2.6.24)
              Setting this boolean option enables transparent  proxying  on  this  socket.
              This  socket  option allows the calling application to bind to a nonlocal IP
              address and operate both as a client and a server with the  foreign  address
              as  the local endpoint.  NOTE: this requires that routing be set up in a way
              that packets going to the foreign address are routed through the TProxy box.
              Enabling this socket option requires superuser privileges (the CAP_NET_ADMIN
              capability).

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

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

       IP_UNBLOCK_SOURCE (since Linux 2.5.68)
              Unblock previously blocked  multicast  source.  Returns  EADDRNOTAVAIL  when
              given source is not being blocked.

              Argument  is  an ip_mreq_source structure as described at IP_ADD_SOURCE_MEM-
              BERSHIP.

   /proc interfaces
       The IP protocol supports a set of /proc interfaces to configure some global parame-
       ters.   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
       non-zero  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  con-
              trolled  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  net-
              work  or a transparent proxy; never ever use it for a normal router or host.
              Otherwise fragmented communication can be disturbed if the fragments  travel
              over  different links.  Defragmentation also has a large memory and CPU time
              cost.

              This is automagically turned on when masquerading  or  transparent  proxying
              are configured.

       ip_autoconfig (since Linux 2.2 to 2.6.17)
              Not documented.

       ip_default_ttl (integer; default: 64; since Linux 2.2)
              Set the default time-to-live value of outgoing packets.  This can be changed
              per socket with the IP_TTL option.

       ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
              Enable dynamic socket address and masquerading entry rewriting on  interface
              address  change.   This  is  useful  for  dialup  interface with changing IP
              addresses.  0 means no rewriting, 1 turns it on and 2 enables verbose  mode.

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

       ip_local_port_range (since Linux 2.2)
              Contains two integers that define the default local port range allocated  to
              sockets.   Allocation  starts with the first number and ends with the second
              number.  Note that these should not conflict with the  ports  used  by  mas-
              querading  (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  pack-
              ets) 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  bro-
              ken  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 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 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 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 rout-
              ing 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 lim-
              ited 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).

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

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

       Some  BSD  sockets implementations also provide an IP_RECVTTL option, but an ancil-
       lary message with type IP_RECVTTL is passed with the incoming packet.  This is dif-
       ferent 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,  proto-
       col)  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 dif-
       ference 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
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-02-28                             IP(7)

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