ip

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



NAME
       ip - Linux IPv4 protocol implementation

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

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

DESCRIPTION
       Linux implements the Internet Protocol, version 4, described in RFC 791
       and RFC 1122.  ip contains a level 2 multicasting implementation
       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 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                              IP(7)