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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 sockets, see socket(7).

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

       When a process wants to receive new incoming packets or connections, it should bind a socket to a local  inter-
       face  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_REUSE-
       ADDR 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 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 gethost-
       byname(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_BROAD-
       CAST socket flag is set.  In the current implementation, connection-oriented sockets are only  allowed  to  use
       unicast addresses.

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

       There  are several special addresses: INADDR_LOOPBACK (127.0.0.1) always refers to the local host via the loop-
       back 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 getsock-
       opt(2).  The socket option level for IP is IPPROTO_IP.  A boolean integer flag is zero when it is false, other-
       wise 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  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_MEMBERSIP.   imr_multiaddr  contains
              the address of the multicast group the application 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_MEM-
              BERSHIP.

              Argument is an ip_mreq_source structure as described at 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 2.5.68)
              Leave  a  source-specific group, i.e., 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 at 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  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.  Argument is an ip_msfilter struc-
              ture.

                  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_msfil-
              ter 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  con-
              nected.  Returns an integer.  Only valid as a getsockopt(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  data-
              grams.   The  system-wide  default  is  controlled  by  the  /proc/sys/net/ipv4/ip_no_pmtu_disc file for
              SOCK_STREAM sockets, and disabled on all others.  For non-SOCK_STREAM sockets, it is the user's  respon-
              sibility  to  packetize the data in MTU sized chunks and to do the retransmits if necessary.  The kernel
              will reject packets that are bigger than the known path MTU if this flag is set (with EMSGSIZE).

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

              When PMTU discovery is enabled, the kernel automatically keeps track of the  path  MTU  per  destination
              host.   When  it  is  connected  to a specific peer with connect(2), the currently known path MTU can be
              retrieved conveniently using the IP_MTU socket option (e.g., after a EMSGSIZE error occurred).   It  may
              change  over  time.  For connectionless sockets with many destinations, the new MTU for a given destina-
              tion 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_ALL (since Linux 2.6.31)
              Sets the policy for multicast delivery to the socket. Argument is a boolean integer that enables or dis-
              ables 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 operation 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 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_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 automatically to the options from the initial  packet  with  routing
              headers  reversed.   Incoming  packets  are not allowed to change options after the connection is estab-
              lished.  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 pri-
              mary 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 con-
              tains  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 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 spec-
              ifies 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 for-
              warded 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  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 redi-
              rected 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_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 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 controlled at compile time  by  the
              CONFIG_IP_ALWAYS_DEFRAG option; this option is not present in 2.4.x and later]

              When this boolean frag is enabled (not equal 0), incoming fragments (parts of IP packets that arose when
              some host between origin and destination decided that the packets were  too  large  and  cut  them  into
              pieces)  will  be  reassembled  (defragmented) before being processed, even if they are about to be for-
              warded.

              Only enable if running either a firewall that is the sole link to your network or a  transparent  proxy;
              never  ever  use it for a normal router or host.  Otherwise fragmented communication can be disturbed if
              the fragments travel over different links.  Defragmentation also has a large memory and CPU time cost.

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

       ip_autoconfig (since Linux 2.2 to 2.6.17)
              Not documented.

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

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

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

       ip_local_port_range (since Linux 2.2)
              Contains  two integers that define the default local port range allocated to sockets.  Allocation starts
              with the first number and ends with the second number.  Note that these should  not  conflict  with  the
              ports  used  by  masquerading (although the case is handled).  Also arbitrary choices may cause problems
              with some firewall packet filters that make assumptions about the local  ports  in  use.   First  number
              should  be  at  least  greater than 1024, or better, greater than 4096, to avoid clashes with well known
              ports and to minimize firewall problems.

       ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
              If enabled, don't do Path MTU Discovery for TCP sockets by default.  Path MTU discovery may fail if mis-
              configured  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 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 frag-
              ments.

       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  mes-
              sage from a remote router or for the local routing table.

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

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

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

       ENOBUFS, ENOMEM
              Not  enough  free  memory.   This often means that the memory allocation is limited by the socket buffer
              limits, not by the system memory, but this is not 100% consistent.

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

       ENOPKG A kernel subsystem was not configured.

       ENOPROTOOPT and EOPNOTSUPP
              Invalid socket option passed.

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

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

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

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

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

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 broadcast-
       ing.  Broadcasting is discouraged.

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

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

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

   Compatibility
       For compatibility with Linux 2.0, the obsolete socket(AF_INET, SOCK_PACKET, protocol) syntax is still supported
       to open a packet(7) socket.  This is deprecated and should be replaced by socket(AF_PACKET, SOCK_RAW, protocol)
       instead.  The main difference is the new sockaddr_ll address  structure  for  generic  link  layer  information
       instead of the old sockaddr_pkt.

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 informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-02-28                             IP(7)