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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 using socket(2):

           socket(AF_INET, socket_type, protocol);

       Valid socket types include SOCK_STREAM to open a  stream  socket,  SOCK_DGRAM  to  open  a
       datagram socket, and 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.  Valid values for
       protocol include:

       •  0 and IPPROTO_TCP for tcp(7) stream sockets;

       •  0 and IPPROTO_UDP for udp(7) datagram sockets;

       •  IPPROTO_SCTP for sctp(7) stream sockets; and

       •  IPPROTO_UDPLITE for udplite(7) datagram sockets.

       For SOCK_RAW you may specify a  valid  IANA  IP  protocol  defined  in  RFC 1700  assigned

       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 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

       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.

       When an invalid socket option is specified, getsockopt(2) and setsockopt(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 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.  (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 / Linux 2.5.68)
              Join  a  multicast  group  and  allow  receiving data only from a specified source.
              Argument is an ip_mreq_source structure.

           struct ip_mreq_source {
               struct in_addr imr_multiaddr;  /* IP multicast group
                                                 address */
               struct in_addr imr_interface;  /* IP address of local
                                                 interface */
               struct in_addr imr_sourceaddr; /* IP address of
                                                 multicast source */

       The ip_mreq_source structure is similar to  ip_mreqn  described  under  IP_ADD_MEMBERSHIP.
       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_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 automatically chosen at connect(2) time, in  a
              way that allows sharing 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

              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

              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.   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_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.  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 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 (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  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.  The argument for setsockopt(2) is an
              ip_mreqn or (since Linux 3.5) ip_mreq structure 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

       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.  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 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  set  only  by  the  local  user.
              Calling  getsockopt(2) with IP_OPTIONS puts the current IP options used for sending
              into the supplied buffer.

       IP_PASSSEC (since Linux 2.6.17)
              If labeled IPSEC or NetLabel is configured on the sending and receiving hosts, this
              option enables receiving of the security context of the peer socket in an ancillary
              message of type SCM_SECURITY retrieved using recvmsg(2).  This option is  supported
              only  for  UDP  sockets;  for  TCP  or  SCTP  sockets,  see  the description of the
              SO_PEERSEC option below.

              The value given as an argument to setsockopt(2)  and  returned  as  the  result  of
              getsockopt(2) is an integer boolean flag.

              The  security context returned in the SCM_SECURITY ancillary message is of the same
              format as the one described under the SO_PEERSEC option below.

              Note: the reuse of the SCM_SECURITY message type for the IP_PASSSEC  socket  option
              was  likely  a  mistake,  since  other  IP control messages use their own numbering
              scheme in the IP namespace and often use the socket option  value  as  the  message
              type.   There  is  no conflict currently since the IP option with the same value as
              SCM_SECURITY is IP_HDRINCL and this is never used for a control message type.

       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 works only 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  be
              sent/retrieved only as  a  control  message  with  a  packet  using  recvmsg(2)  or

                  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 reported only 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 32 bit integer.  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.   Valid only 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
              filtered only 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).

       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_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

       SO_PEERSEC (since Linux 2.6.17)
              If labeled IPSEC or NetLabel is configured on both the sending and receiving hosts,
              this read-only socket option returns  the  security  context  of  the  peer  socket
              connected  to  this  socket.   By  default,  this  will be the same as the security
              context of the process that created the peer socket unless overridden by the policy
              or by a process with the required permissions.

              The  argument  to getsockopt(2) is a pointer to a buffer of the specified length in
              bytes into which the security context string will be copied.  If the buffer  length
              is  less than the length of the security context string, then getsockopt(2) returns
              -1, sets errno to ERANGE, and returns the required length via optlen.   The  caller
              should  allocate at least NAME_MAX bytes for the buffer initially, although this is
              not guaranteed to be sufficient.  Resizing the buffer to the  returned  length  and
              retrying may be necessary.

              The  security  context  string  may  include  a  terminating  null character in the
              returned length, but is not guaranteed to do so: a security context "foo" might  be
              represented  as either {'f','o','o'} of length 3 or {'f','o','o','\0'} of length 4,
              which are considered to be interchangeable.  The  string  is  printable,  does  not
              contain  non-terminating  null  characters,  and  is in an unspecified encoding (in
              particular, it is not guaranteed to be ASCII or UTF-8).

              The use of this option for sockets in the AF_INET address family is supported since
              Linux 2.6.17 for TCP sockets, and since Linux 4.17 for SCTP sockets.

              For  SELinux,  NetLabel conveys only the MLS portion of the security context of the
              peer across the wire, defaulting the rest of the security  context  to  the  values
              defined  in  the policy for the netmsg initial security identifier (SID).  However,
              NetLabel can be configured to pass full security contexts over  loopback.   Labeled
              IPSEC  always  passes  full  security contexts as part of establishing the security
              association (SA) and looks them up based on the association for each packet.

   /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 Linux 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
              Linux 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.

              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

       ip_autoconfig (since Linux 2.2 to Linux 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
              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.

              A 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 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.

              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



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

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

       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 error used to diagnose exhaustion of the  ephemeral  port  range  differs  across  the
       various 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 Linux 2.2 kernels.


       recvmsg(2),  sendmsg(2),  byteorder(3),  capabilities(7),  icmp(7), ipv6(7), netdevice(7),
       netlink(7), raw(7), socket(7), tcp(7), udp(7), ip(8)

       The kernel source file Documentation/networking/ip-sysctl.txt.

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