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       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
       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 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
       functions  return  EINVAL  when  this  setting  is missing.  sin_port contains the port in
       network byte order.   The  port  numbers  below  1024  are  called  privileged  ports  (or
       sometimes:   reserved   ports).    Only  privileged  processes  (i.e.,  those  having  the
       CAP_NET_BIND_SERVICE capability) may bind(2) to these sockets.  Note  that  the  raw  IPv4
       protocol  as  such has no concept of a port, they are 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
       particular,  this means that you need to call htons(3) on the number that is assigned to a
       port.  All address/port manipulation functions in the standard  library  work  in  network
       byte order.

       There  are  several  special  addresses:  INADDR_LOOPBACK ( 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
              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.   Only  valid   as   a

       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_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_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
              delivered.  To stop receiving data from all sources at once, use IP_LEAVE_GROUP.

              Argument     is     an     ip_mreq_source     structure    as    described    under

       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
              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.   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_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 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
              destination  address using connect(2) and retrieve the MTU by calling getsockopt(2)
              with the IP_MTU option.

              It is possible to implement RFC  4821  MTU  probing  with  SOCK_DGRAM  or  SOCK_RAW
              sockets  by  setting  a  value of IP_PMTUDISC_PROBE (available since Linux 2.6.22).
              This is also particularly useful for diagnostic tools  such  as  tracepath(8)  that
              wish to deliberately send probe packets larger than the observed Path MTU.

       IP_MULTICAST_ALL (since Linux 2.6.31)
              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
              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.  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
              packets should be looped back to the local sockets.

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

       IP_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
              arguments are a pointer to a memory buffer containing the options  and  the  option
              length.   The setsockopt(2) call sets the IP options associated with a socket.  The
              maximum option size for IPv4 is 40 bytes.  See RFC 791  for  the  allowed  options.
              When  the  initial  connection  request packet for a SOCK_STREAM socket contains IP
              options, the IP options will be set automatically to the options from  the  initial
              packet  with  routing headers reversed.  Incoming packets are not allowed to change
              options after the connection is 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
              handled.   For  datagram  sockets,  IP  options  can be only set by the local user.
              Calling getsockopt(2) with IP_OPTIONS puts the current IP options used for  sending
              into the supplied buffer.

       IP_PKTINFO (since Linux 2.2)
              Pass  an  IP_PKTINFO  ancillary  message  that  contains  a  pktinfo structure that
              supplies some information about the incoming packet.  This only works for  datagram
              oriented  sockets.   The  argument  is  a  flag  that  tells the socket whether the
              IP_PKTINFO message should be passed  or  not.   The  message  itself  can  only  be
              sent/retrieved as control message with a packet using recvmsg(2) or sendmsg(2).

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

              ipi_ifindex  is  the  unique  index  of  the  interface the packet was received on.
              ipi_spec_dst is the local address of the packet and  ipi_addr  is  the  destination
              address  in  the  packet  header.   If  IP_PKTINFO  is  passed  to  sendmsg(2)  and
              ipi_spec_dst is not zero, then it is used as  the  local  source  address  for  the
              routing  table lookup and for setting up IP source route options.  When ipi_ifindex
              is not zero, the 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_origin  is  the  origin
              code  of  where the error originated.  The other fields are protocol-specific.  The
              macro SO_EE_OFFENDER returns a pointer to the address of the network  object  where
              the  error  originated  from  given  a  pointer  to the ancillary message.  If this
              address is not known, the sa_family member of the sockaddr contains  AF_UNSPEC  and
              the other fields of the sockaddr are undefined.

              IP   uses   the  sock_extended_err  structure  as  follows:  ee_origin  is  set  to
              SO_EE_ORIGIN_ICMP for errors received as an ICMP packet, or SO_EE_ORIGIN_LOCAL  for
              locally  generated  errors.  Unknown values should be ignored.  ee_type and ee_code
              are set from the type and code fields of the ICMP  header.   ee_info  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
              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_ERROR only.

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

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

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

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

       IP_RECVTOS (since Linux 2.2)
              If enabled the IP_TOS ancillary  message  is  passed  with  incoming  packets.   It
              contains  a byte which 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
              minimize  delays  for interactive traffic, IPTOS_THROUGHPUT to optimize throughput,
              IPTOS_RELIABILITY to optimize for reliability, IPTOS_MINCOST  should  be  used  for
              "filler  data"  where  slow  transmission doesn't matter.  At most one of these TOS
              values can be specified.  Other bits are invalid and shall be cleared.  Linux sends
              IPTOS_LOWDELAY  datagrams  first  by default, but the exact behavior depends on the
              configured queueing discipline.  Some high priority levels  may  require  superuser
              privileges  (the  CAP_NET_ADMIN  capability).   The  priority  can also be set in a
              protocol 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
              endpoint.  NOTE: this requires that routing be set up in a way that  packets  going
              to  the  foreign  address  are routed through the TProxy box.  Enabling this socket
              option requires superuser privileges (the CAP_NET_ADMIN capability).

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

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

       IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 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

   /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
              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

       ip_autoconfig (since Linux 2.2 to 2.6.17)
              Not documented.

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

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

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

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

       ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
              If  enabled,  don't  do  Path  MTU  Discovery for TCP sockets by default.  Path MTU
              discovery may fail if 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 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
              without  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%

       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

       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
       message  with  type IP_RECVTTL is passed with the incoming packet.  This is different from
       the IP_TTL option used in Linux.

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

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


       There are too many inconsistent error values.

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

       Some versions of glibc forget to declare in_pktinfo.  Workaround currently is to  copy  it
       into your program from this man page.

       Receiving  the  original  destination  address with MSG_ERRQUEUE in msg_name by recvmsg(2)
       does not work in some 2.2 kernels.


       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.


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       project,     and    information    about    reporting    bugs,    can    be    found    at