ip(7)



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  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 IN-
       ADDR_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 ad-
       dress  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 be-
       low 1024 are called privileged ports (or  sometimes:  reserved  ports).
       Only   a   privileged  process  (on  Linux:  a  process  that  has  the
       CAP_NET_BIND_SERVICE capability in the  user  namespace  governing  its
       network  namespace)  may  bind(2)  to these sockets.  Note that the raw
       IPv4 protocol 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_LOOPBACK)
       using  htonl(3)  or   set   using   the   inet_aton(3),   inet_addr(3),
       inet_makeaddr(3)  library  functions or directly with the name resolver
       (see gethostbyname(3)).

       IPv4 addresses are divided into unicast, broadcast, and  multicast  ad-
       dresses.   Unicast  addresses  specify  a  single  interface of a host,
       broadcast addresses specify all hosts on a network, and  multicast  ad-
       dresses address all hosts in a multicast group.  Datagrams to broadcast
       addresses can be sent or received only  when  the  SO_BROADCAST  socket
       flag  is set.  In the current implementation, connection-oriented sock-
       ets 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 applica-
       tion wants to join or leave.  It must be a valid multicast address  (or
       setsockopt(2) fails with the error EINVAL).  imr_address is the address
       of the local interface with which the system should join the  multicast
       group; if it is equal to INADDR_ANY, an appropriate interface is chosen
       by the system.  imr_ifindex is the interface  index  of  the  interface
       that  should  join/leave  the imr_multiaddr group, or 0 to indicate any
       interface.

              The ip_mreqn structure is available only since Linux  2.2.   For
              compatibility,  the  old  ip_mreq structure (present since Linux
              1.2) is still supported; it differs from ip_mreqn  only  by  not
              including  the  imr_ifindex field.  (The kernel determines which
              structure is being passed based on the size passed in optlen.)

              IP_ADD_MEMBERSHIP is valid only for setsockopt(2).

       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_inter-
       face 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_BIND_ADDRESS_NO_PORT (since Linux 4.2)
              Inform the kernel to not reserve an ephemeral  port  when  using
              bind(2)  with  a port number of 0.  The port will later be auto-
              matically chosen at connect(2) time, in a way that allows  shar-
              ing a source port as long as the 4-tuple is unique.

       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_DROP_MEMBERSHIP.

              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
              that the application is trying to bind to it.   This  option  is
              the  per-socket  equivalent of the ip_nonlocal_bind /proc inter-
              face described below.

       IP_HDRINCL (since Linux 2.0)
              If enabled, the user supplies an IP header in front of the  user
              data.   Valid only for SOCK_RAW sockets; see raw(7) for more in-
              formation.  When this flag is enabled, the values set by  IP_OP-
              TIONS, IP_TTL, and IP_TOS are ignored.

       IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
              This option provides access to the advanced full-state filtering
              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_MSFIL-
       TER_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.   Re-
              turns 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
              track of the path MTU per destination host.   When  it  is  con-
              nected  to  a specific peer with connect(2), the currently known
              path MTU can be retrieved conveniently using the  IP_MTU  socket
              option  (e.g.,  after an EMSGSIZE error occurred).  The path MTU
              may change over time.  For connectionless sockets with many des-
              tinations,  the  new MTU for a given destination can also be ac-
              cessed 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 data-
              gram sockets may be dropped.  Applications using UDP  should  be
              aware  of this and not take it into account for their packet re-
              transmit strategy.

              To bootstrap the path MTU discovery process on unconnected sock-
              ets,  it  is possible to start with a big datagram size (headers
              up to 64 kilobytes 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  de-
              liberately 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  ad-
              dress.   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 ex-
              plicitly  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.

       IP_OPTIONS (since Linux 2.0)
              Set or get the IP options to be sent with every packet from this
              socket.  The arguments are a pointer to a memory buffer contain-
              ing  the  options and the option length.  The setsockopt(2) call
              sets the IP options associated with a socket.  The  maximum  op-
              tion size for IPv4 is 40 bytes.  See RFC 791 for the allowed op-
              tions.   When  the  initial  connection  request  packet  for  a
              SOCK_STREAM  socket  contains IP options, the IP options will be
              set automatically to the options from the  initial  packet  with
              routing  headers  reversed.  Incoming packets are not allowed to
              change options after the connection is  established.   The  pro-
              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 ar-
              gument  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_ex-
              tended_err  structure  describing the error will be passed in an
              ancillary message with the type IP_RECVERR  and  the  level  IP-
              PROTO_IP.   This is useful for reliable error handling on uncon-
              nected 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_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  un-
              defined.

              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 ac-
              cessed with the SO_EE_OFFENDER macro.  The sin_family  field  of
              the  SO_EE_OFFENDER address is AF_UNSPEC when the source was un-
              known.  When the error originated from the network, all  IP  op-
              tions  (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and con-
              tained in the error packet are passed as control messages.   The
              payload  of  the  packet causing the error is returned as normal
              payload.  Note that TCP has no error queue; MSG_ERRQUEUE is  not
              permitted  on SOCK_STREAM sockets.  IP_RECVERR is valid for TCP,
              but all errors are returned by socket function return or  SO_ER-
              ROR 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 de-
              faults 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
              time-to-live field of the received packet as a byte.   Not  sup-
              ported 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.  Valid only for raw sockets.  This  is  use-
              ful,  for  instance,  for  user-space  RSVP daemons.  The tapped
              packets are not forwarded by the kernel; it is  the  user's  re-
              sponsibility 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  de-
              lays  for  interactive  traffic,  IPTOS_THROUGHPUT  to  optimize
              throughput, IPTOS_RELIABILITY to optimize for  reliability,  IP-
              TOS_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)
              [New with kernel 2.2.13; in earlier kernel versions this feature
              was  controlled  at  compile time by the CONFIG_IP_ALWAYS_DEFRAG
              option; this option is not present in 2.4.x and later]

              When this boolean flag is enabled (not equal 0), incoming  frag-
              ments  (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) be-
              fore being processed, even if they are about to be forwarded.

              Enable only 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.  De-
              fragmentation 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
              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 de-
              fault.  Path MTU discovery may fail if  misconfigured  firewalls
              (that  drop all ICMP packets) or misconfigured interfaces (e.g.,
              a point-to-point link where the both ends  don't  agree  on  the
              MTU) are on the path.  It is better to fix the broken routers on
              the path than to turn off Path MTU Discovery  globally,  because
              not doing it incurs a high cost to the network.

       ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
              If  set,  allows  processes to bind(2) to nonlocal IP addresses,
              which can be quite useful, but may break some applications.

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

       ip6frag_secret_interval (integer; default: 600)
              Regeneration interval (in seconds) of the hash secret (or  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  in-
              teger 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 ad-
              dress without having the SO_BROADCAST flag set; sending a packet
              via  a prohibit route; modifying firewall settings without supe-
              ruser 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
              A  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 an ICMP message from a remote router
              or for the local routing table.

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

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

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

       ENOBUFS, ENOMEM
              Not enough free memory.  This often means that the memory  allo-
              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 re-
              quested.

       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 in-
       stead 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  the SOL_IP socket options level isn't portable; BSD-based stacks
       use the IPPROTO_IP level.

       INADDR_ANY (0.0.0.0) and INADDR_BROADCAST (255.255.255.255)  are  byte-
       order-neutral.
        This means htonl(3) has no effect on them.

   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-
       addr_ll address structure for generic link layer information instead of
       the old sockaddr_pkt.

BUGS
       There are too many inconsistent error values.

       The error used to diagnose exhaustion of the ephemeral port range  dif-
       fers  across  the various system calls (connect(2), bind(2), listen(2),
       sendto(2)) that can assign ephemeral ports.

       The ioctls to configure IP-specific interface options  and  ARP  tables
       are not described.

       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), ip(8)

       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 4.16 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2018-02-02                             IP(7)

Man(1) output converted with man2html
list of all man pages