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INET(4)                  BSD Kernel Interfaces Manual                  INET(4)

     inet -- Internet protocol family

     #include <&lt;sys/types.h>&gt;
     #include <&lt;netinet/in.h>&gt;

     The Internet protocol family is a collection of protocols layered atop
     the Internet Protocol (IP) transport layer, and utilizing the Internet
     address format.  The Internet family provides protocol support for the
     SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW inter-
     face provides access to the IP protocol.

     Internet addresses are four byte quantities, stored in network standard
     format (on little endian machines, such as the alpha, amd64, i386 and
     ia64 these are word and byte reversed).  The include file <netinet/in.h>
     defines this address as a discriminated union.

     Sockets bound to the Internet protocol family utilize the following
     addressing structure,

           struct sockaddr_in {
                   uint8_t         sin_len;
                   sa_family_t     sin_family;
                   in_port_t       sin_port;
                   struct in_addr  sin_addr;
                   char            sin_zero[8];

     Sockets may be created with the local address INADDR_ANY to affect
     ``wildcard'' matching on incoming messages.  The address in a connect(2)
     or sendto(2) call may be given as INADDR_ANY to mean ``this host''.  The
     distinguished address INADDR_BROADCAST is allowed as a shorthand for the
     broadcast address on the primary network if the first network configured
     supports broadcast.

     The Internet protocol family is comprised of the IP network protocol,
     Internet Control Message Protocol (ICMP), Internet Group Management Pro-
     tocol (IGMP), Transmission Control Protocol (TCP), and User Datagram Pro-
     tocol (UDP).  TCP is used to support the SOCK_STREAM abstraction while
     UDP is used to support the SOCK_DGRAM abstraction.  A raw interface to IP
     is available by creating an Internet socket of type SOCK_RAW.  The ICMP
     message protocol is accessible from a raw socket.

     The 32-bit Internet address contains both network and host parts.  How-
     ever, direct examination of addresses is discouraged.  For those programs
     which absolutely need to break addresses into their component parts, the
     following ioctl(2) commands are provided for a datagram socket in the
     Internet domain; they have the same form as the SIOCIFADDR command (see

     SIOCSIFNETMASK  Set interface network mask.  The network mask defines the
                     network part of the address; if it contains more of the
                     address than the address type would indicate, then sub-
                     nets are in use.

     SIOCGIFNETMASK  Get interface network mask.

   MIB Variables
     A number of variables are implemented in the net.inet branch of the
     sysctl(3) MIB.  In addition to the variables supported by the transport
     protocols (for which the respective manual pages may be consulted), the
     following general variables are defined:

     IPCTL_FORWARDING      (ip.forwarding) Boolean: enable/disable forwarding
                           of IP packets.  Defaults to off.

     IPCTL_FASTFORWARDING  (ip.fastforwarding) Boolean: enable/disable the use
                           of fast IP forwarding code.  Defaults to off.  When
                           fast IP forwarding is enabled, IP packets are for-
                           warded directly to the appropriate network inter-
                           face with direct processing to completion, which
                           greatly improves the throughput.  All packets for
                           local IP addresses, non-unicast, or with IP options
                           are handled by the normal IP input processing path.
                           All features of the normal (slow) IP forwarding
                           path are supported including firewall (through
                           pfil(9) hooks) checking, except ipsec(4) tunnel
                           brokering.  The IP fastforwarding path does not
                           generate ICMP redirect or source quench messages.

     IPCTL_SENDREDIRECTS   (ip.redirect) Boolean: enable/disable sending of
                           ICMP redirects in response to IP packets for which
                           a better, and for the sender directly reachable,
                           route and next hop is known.  Defaults to on.

     IPCTL_DEFTTL          (ip.ttl) Integer: default time-to-live (``TTL'') to
                           use for outgoing IP packets.

                           (ip.accept_sourceroute) Boolean: enable/disable
                           accepting of source-routed IP packets (default

     IPCTL_SOURCEROUTE     (ip.sourceroute) Boolean: enable/disable forwarding
                           of source-routed IP packets (default false).

     IPCTL_RTEXPIRE        (ip.rtexpire) Integer: lifetime in seconds of pro-
                           tocol-cloned IP routes after the last reference
                           drops (default one hour).  This value varies dynam-
                           ically as described above.

     IPCTL_RTMINEXPIRE     (ip.rtminexpire) Integer: minimum value of ip.rtex-
                           pire (default ten seconds).  This value has no
                           effect on user modifications, but restricts the
                           dynamic adaptation described above.

     IPCTL_RTMAXCACHE      (ip.rtmaxcache) Integer: trigger level of cached,
                           unreferenced, protocol-cloned routes which initi-
                           ates dynamic adaptation (default 128).

     ip.process_options    Integer: control IP options processing.  By setting
                           this variable to 0, all IP options in the incoming
                           packets will be ignored, and the packets will be
                           passed unmodified.  By setting to 1, IP options in
                           the incoming packets will be processed accordingly.
                           By setting to 2, an ICMP ``prohibited by filter''
                           message will be sent back in respose to incoming
                           packets with IP options.  Default is 1.  This
                           sysctl(8) variable affects packets destined for a
                           local host as well as packets forwarded to some
                           other host.

     ip.random_id          Boolean: control IP IDs generation behaviour.  Set-
                           ting this sysctl(8) to non-zero causes the ID field
                           in IP packets to be randomized instead of incre-
                           mented by 1 with each packet generated.  This
                           closes a minor information leak which allows remote
                           observers to determine the rate of packet genera-
                           tion on the machine by watching the counter.  In
                           the same time, on high-speed links, it can decrease
                           the ID reuse cycle greatly.  Default is 0 (sequen-
                           tial IP IDs).  IPv6 flow IDs and fragment IDs are
                           always random.

     ioctl(2), socket(2), sysctl(3), icmp(4), intro(4), ip(4), ipfirewall(4),
     route(4), tcp(4), ttcp(4), udp(4), pfil(9)

     "An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.

     "An Advanced 4.3 BSD Interprocess Communication Tutorial", PS1, 8.

     The Internet protocol support is subject to change as the Internet proto-
     cols develop.  Users should not depend on details of the current imple-
     mentation, but rather the services exported.

     The inet protocol interface appeared in 4.2BSD.  The ``protocol cloning''
     code appeared in FreeBSD 2.1.

BSD                            October 23, 2004                            BSD