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NAME

       ip - Linux IPv4 protocol implementation

SYNOPSIS

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/ip.h> /* superset of previous */

       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
       raw_socket = socket(AF_INET, SOCK_RAW, protocol);

DESCRIPTION

       Linux  implements  the  Internet  Protocol,  version 4, described in RFC 791 and RFC 1122.  ip contains a
       level 2 multicasting implementation 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 socket(7).

       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 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 protocol.

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

           /* Internet address. */
           struct in_addr {
               uint32_t       s_addr;     /* address in network byte order */
           };

       sin_family  is  always  set  to AF_INET.  This is required; in Linux 2.2 most networking functions return
       EINVAL when this setting is missing.  sin_port contains the port in network byte order.  The port numbers
       below  1024  are  called  privileged ports (or sometimes: reserved ports).  Only a privileged process (on
       Linux: a process that has the CAP_NET_BIND_SERVICE capability in the user namespace governing its network
       namespace)  may  bind(2)  to  these sockets.  Note that the raw IPv4 protocol as such has no concept of a
       port, they are implemented only by higher protocols like tcp(7) and udp(7).

       sin_addr is the IP host address.  The s_addr member of struct in_addr contains the host interface address
       in  network  byte  order.   in_addr should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK)
       using htonl(3) or set  using  the  inet_aton(3),  inet_addr(3),  inet_makeaddr(3)  library  functions  or
       directly with the name resolver (see gethostbyname(3)).

       IPv4 addresses are divided into unicast, broadcast, and multicast addresses.  Unicast addresses specify a
       single interface of a host, broadcast addresses specify all hosts on a network, and  multicast  addresses
       address  all  hosts  in a multicast group.  Datagrams to 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 addresses.

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

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

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

       When  an  invalid  socket  option  is  specified,  getsockopt(2)  and  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 / 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 source.

       IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
              Inform the kernel to not reserve an ephemeral port when using bind(2) with a  port  number  of  0.
              The  port  will  later  be 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  IP_ADD_MEMBERSHIP  or
              IP_ADD_SOURCE_MEMBERSHIP.

              Argument is an ip_mreq_source structure as described under IP_ADD_SOURCE_MEMBERSHIP.

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

       IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
              Leave  a source-specific group—that is, stop receiving data from a given multicast group that come
              from a given source.  If the application has subscribed to multiple sources within the same group,
              data  from the remaining sources will still be delivered.  To stop receiving data from all sources
              at once, use IP_DROP_MEMBERSHIP.

              Argument is an ip_mreq_source structure as described under IP_ADD_SOURCE_MEMBERSHIP.

       IP_FREEBIND (since Linux 2.4)
              If enabled, this boolean option allows binding to an IP address that is nonlocal or does not (yet)
              exist.   This permits 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 structure.

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

       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 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.

              Enable  only  if  running either a firewall that is the sole link to your network or a transparent
              proxy; never ever use it for a normal router or host.  Otherwise, fragmented communication can  be
              disturbed  if  the fragments travel over different links.  Defragmentation also has a large memory
              and CPU time cost.

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

       ip_autoconfig (since Linux 2.2 to 2.6.17)
              Not documented.

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

       ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
              Enable  dynamic socket address and masquerading entry rewriting on interface address change.  This
              is useful for dialup 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 circumstances:

              *  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.

       neigh/*
              See arp(7).

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

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

ERRORS

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

       EADDRINUSE
              Tried to bind to an address already in use.

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

       EAGAIN Operation on a nonblocking socket would block.

       EALREADY
              A connection operation on a nonblocking socket is already in progress.

       ECONNABORTED
              A connection was closed during an accept(2).

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

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

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

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

       ENOBUFS, ENOMEM
              Not  enough  free  memory.   This  often means that the memory allocation is limited by the socket
              buffer limits, not by the system memory, but this is not 100% consistent.

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

       ENOPKG A kernel subsystem was not configured.

       ENOPROTOOPT and EOPNOTSUPP
              Invalid socket option passed.

       ENOTCONN
              The operation is defined only on a connected socket, but the socket wasn't connected.

       EPERM  User doesn't have permission to set high priority, change configuration, or send  signals  to  the
              requested process or group.

       EPIPE  The connection was unexpectedly closed or shut down by the other end.

       ESOCKTNOSUPPORT
              The socket is not configured or an unknown socket type was requested.

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

NOTES

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

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

       Some other BSD sockets implementations provide IP_RCVDSTADDR and IP_RECVIF  socket  options  to  get  the
       destination  address  and the 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 level.

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

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

BUGS

       There are too many inconsistent error values.

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

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

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

SEE ALSO

       recvmsg(2),  sendmsg(2),  byteorder(3),  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.

COLOPHON

       This page is part of release 5.10 of  the  Linux  man-pages  project.   A  description  of  the  project,
       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.