ip

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
       INADDR_ANY.

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

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

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

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

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

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

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

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

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

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

       imr_multiaddr  contains the address of the multicast group the 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
              information.  When this flag  is  enabled,  the  values  set  by
              IP_OPTIONS, 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.
              Returns an integer.

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

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

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

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

              When  PMTU  discovery is enabled, the kernel automatically keeps
              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
              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 data-
              gram 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 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
              deliberately  send  probe  packets larger than the observed Path
              MTU.

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

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

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

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

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

              This option is valid only for SOCK_RAW sockets.

       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
              option size for IPv4 is 40 bytes.  See RFC 791 for  the  allowed
              options.   When  the  initial  connection  request  packet for a
              SOCK_STREAM socket contains IP options, the IP options  will  be
              set  automatically  to  the options from the initial packet with
              routing headers reversed.  Incoming packets are not  allowed  to
              change  options  after  the connection is established.  The 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
              argument is a flag that tells the socket whether the  IP_PKTINFO
              message should be passed or not.  The message itself can only be
              sent/retrieved as control message with a packet using recvmsg(2)
              or sendmsg(2).

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

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

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

              The IP_RECVERR  control  message  contains  a  sock_extended_err
              structure:

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

                  struct sock_extended_err {
                      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
              undefined.

              IP uses the sock_extended_err structure as follows: ee_origin is
              set  to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet,
              or SO_EE_ORIGIN_LOCAL for  locally  generated  errors.   Unknown
              values  should be ignored.  ee_type and ee_code are set from the
              type and code fields of the ICMP header.  ee_info  contains  the
              discovered  MTU  for EMSGSIZE errors.  The message also contains
              the sockaddr_in of the node  caused  the  error,  which  can  be
              accessed with the SO_EE_OFFENDER macro.  The sin_family field of
              the SO_EE_OFFENDER address is  AF_UNSPEC  when  the  source  was
              unknown.   When  the  error  originated from the network, all IP
              options (IP_OPTIONS, IP_TTL, etc.) enabled  on  the  socket  and
              contained  in  the  error packet are passed as control messages.
              The payload of the packet causing the error is returned as  nor-
              mal  payload.  Note that TCP has no error queue; MSG_ERRQUEUE is
              not permitted on SOCK_STREAM sockets.  IP_RECVERR is  valid  for
              TCP,  but  all  errors are returned by socket function return or
              SO_ERROR only.

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

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

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

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

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

       IP_RECVTTL (since Linux 2.2)
              When this flag is set, pass a IP_TTL control  message  with  the
              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
              responsibility to  send  them  out  again.   Socket  binding  is
              ignored, such packets are only filtered by protocol.  Expects an
              integer flag.

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

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

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

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

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

              Argument  is  an  ip_mreq_source  structure  as  described under
              IP_ADD_SOURCE_MEMBERSHIP.

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

       ip_always_defrag (Boolean; since Linux 2.2.13)
              [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)
              before 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.
              Defragmentation also has a large memory and CPU time cost.

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

       ip_autoconfig (since Linux 2.2 to 2.6.17)
              Not documented.

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

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

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

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

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

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

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

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

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

              Note that the port range in ip_local_port_range should not  con-
              flict  with the ports used by masquerading (although the case is
              handled).  Also, arbitrary choices may cause problems with  some
              firewall  packet  filters  that make assumptions about the local
              ports in use.  The first number should be at least greater  than
              1024,  or  better, greater than 4096, to avoid clashes with well
              known ports and to minimize firewall problems.

       ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
              If enabled, don't do Path  MTU  Discovery  for  TCP  sockets  by
              default.  Path MTU discovery may fail if misconfigured firewalls
              (that drop all ICMP packets) or misconfigured interfaces  (e.g.,
              a  point-to-point  link  where  the both ends don't agree on the
              MTU) are on the path.  It is better to fix the broken routers on
              the  path  than to turn off Path MTU Discovery globally, because
              not doing it incurs a high cost to the network.

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

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

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

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

       neigh/*
              See arp(7).

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

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

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

       EADDRINUSE
              Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

       EALREADY
              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
              requested.

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

NOTES
       IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER,  IP_RECVORIGDSTADDR,
       IP_PKTINFO,  IP_RECVERR, IP_ROUTER_ALERT, and IP_TRANSPARENT are Linux-
       specific.

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

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

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

       Using 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 4.15 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)
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