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

       ip - Linux IPv4 protocol implementation

       #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);

       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

       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 val‐
       ues 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 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

           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 func‐
       tions  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 implemented only by higher protocols like tcp(7) and udp(7).

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

       Note  that  the address and the port are always stored in network byte order.  In particu‐
       lar, 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

       There are several special addresses: INADDR_LOOPBACK  (  always  refers  to  the
       local  host  via  the loopback device; INADDR_ANY ( means any address for binding;
       INADDR_BROADCAST ( 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.

       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 appro‐
              priate  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

              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.  Only valid as a setsock‐

       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_MEMBER‐
              SIP.  The imr_multiaddr field contains the  address  of  the  multicast  group  the
              application  wants to join or leave.  The imr_interface field is the address of the
              local interface with which the system should join the  multicast  group.   Finally,
              the  imr_sourceaddr  field contains the address of the source the application wants
              to receive data from.

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

       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

              Argument  is  an  ip_mreq_source structure as described under IP_ADD_SOURCE_MEMBER‐

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

       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 deliv‐
              ered.  To stop receiving data from all sources at once, use IP_LEAVE_GROUP.

              Argument is an ip_mreq_source structure as  described  under  IP_ADD_SOURCE_MEMBER‐

       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 2.4.22 / 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_MSFILTER_SIZE(n) macro exists to deter‐
              mine 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.  Valid only when the
              socket has been connected.  Returns an integer.  Only valid as a getsockopt(2).

       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 sockets, IP_PMTUDISC_DO forces the don't-fragment flag to be set on
              all  outgoing  packets.   It  is the user's responsibility to packetize the data in
              MTU-sized chunks and to do the retransmits if necessary.  The  kernel  will  reject
              (with  EMSGSIZE)  datagrams  that  are  bigger  than the known path MTU.  IP_PMTUD‐
              ISC_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_PMTUD‐
              ISC_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 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 an EMSGSIZE error occurred).  The  path  MTU  may  change  over
              time.   For  connectionless sockets with many destinations, the new MTU for a given
              destination can also be accessed using the error queue  (see  IP_RECVERR).   A  new
              error will be queued for every incoming MTU update.

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

              To  bootstrap the path MTU discovery process on unconnected sockets, it is possible
              to start with a big datagram size (up to 64K-headers bytes long) and let it  shrink
              by updates of the path MTU.

              To get an initial estimate of the path MTU, connect a datagram socket to the desti‐
              nation 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 sock‐
              ets 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)
              This option can be used to modify the delivery  policy  of  multicast  messages  to
              sockets  bound to the wildcard INADDR_ANY address.  The argument is a boolean inte‐
              ger (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.  Argument is an ip_mreqn or ip_mreq
              (since Linux 3.5) structure similar to IP_ADD_MEMBERSHIP.

              When an invalid socket option is passed, ENOPROTOOPT is returned.

       IP_MULTICAST_LOOP (since Linux 1.2)
              Set or read a boolean integer argument that determines whether sent multicast pack‐
              ets 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_NODEFRAG (since Linux 2.6.36)
              If enabled (argument is nonzero), the reassembly of outgoing packets is disabled in
              the netfilter layer.  This option is valid only for SOCK_RAW sockets.  The argument
              is an integer.

       IP_OPTIONS (since Linux 2.0)
              Set or get the IP options to be sent with every packet from this socket.  The argu‐
              ments  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 established.  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 han‐
              dled.  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  sup‐
              plies  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_PKT‐
              INFO   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 over‐
              writes 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_ORI‐
              GIN_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 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 con‐
              tains a byte which specifies the Type of Service/Precedence  field  of  the  packet
              header.  Expects a boolean integer flag.

       IP_RECVTTL (since Linux 2.2)
              When this flag is set, pass a IP_TTL control message with the time to live field of
              the received packet as a byte.  Not supported for SOCK_STREAM 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 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 mini‐
              mize 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  pro‐
              tocol   independent  way  by  the  (SOL_SOCKET,  SO_PRIORITY)  socket  option  (see

       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  end‐
              point.   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_TRANSPARENT socket option).  Enabling this socket
              option requires superuser privileges (the CAP_NET_ADMIN capability).

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

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

       IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 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 under IP_ADD_SOURCE_MEMBER‐

   /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
       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 fragments  (parts  of  IP
              packets  that  arose when some host between origin and destination decided that the
              packets were too large and cut them into pieces) will be reassembled (defragmented)
              before being processed, even if they are about to be forwarded.

              Only enable if running either a firewall that is the sole link to your network or a
              transparent proxy; never ever use it for a normal router or host.  Otherwise, frag‐
              mented 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  con‐

       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)
              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

              *  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 previously bound;

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

              *  sendto(2) is called on a datagram socket that was not previously 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 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.  The first number should be at least greater than 1024, or bet‐
              ter, 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 dis‐
              covery may fail if misconfigured firewalls (that drop all ICMP packets) or  miscon‐
              figured  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 lifetime for the hash
              secret) for IPv6 fragments.

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

              See arp(7).

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

       Ioctls to configure generic device parameters are described in netdevice(7).

       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 with‐
              out superuser privileges (the CAP_NET_ADMIN capability); binding  to  a  privileged
              port without superuser privileges (the CAP_NET_BIND_SERVICE capability).

              Tried to bind to an address already in use.

              A  nonexistent  interface  was  requested  or  the requested source address was not

       EAGAIN Operation on a nonblocking socket would block.

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

              A connection was closed during an accept(2).

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

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

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

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

              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% consis‐

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

       ENOPKG A kernel subsystem was not configured.

              Invalid socket option passed.

              The operation is defined only on a connected socket, but  the  socket  wasn't  con‐

       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.

              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

       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 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  mes‐
       sage  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.

       For compatibility with Linux 2.0, the obsolete socket(AF_INET, SOCK_PACKET, protocol) syn‐
       tax  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 sock‐

       There are too many inconsistent error values.

       The error used to diagnose exhaustion of the ephemeral port range differs across the vari‐
       ous  system  calls  (connect(2),  bind(2), listen(2), sendto(2)) that can assign ephemeral

       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.

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

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

       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-05-10                                      IP(7)

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