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NAME

     inet -- Internet protocol family

SYNOPSIS

     #include <sys/types.h>
     #include <netinet/in.h>

DESCRIPTION

     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
     interface provides access to the IP protocol.

ADDRESSING

     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.

PROTOCOLS

     The Internet protocol family is comprised of the IP network protocol,
     Internet Control Message Protocol (ICMP), Internet Group Management
     Protocol (IGMP), Transmission Control Protocol (TCP), and User Datagram
     Protocol (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.
     However, 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 intro(4)).

     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
                     subnets 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
                           forwarded directly to the appropriate network
                           interface 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.

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

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

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

     IPCTL_RTMINEXPIRE     (ip.rtminexpire) Integer: minimum value of
                           ip.rtexpire (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
                           initiates 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 response 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.
                           Setting this sysctl(8) to non-zero causes the ID
                           field in IP packets to be randomized instead of
                           incremented by 1 with each packet generated.  This
                           closes a minor information leak which allows remote
                           observers to determine the rate of packet
                           generation 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
                           (sequential IP IDs).  IPv6 flow IDs and fragment
                           IDs are always random.

     ip.maxfragpackets     Integer: maximum number of fragmented packets the
                           host will accept and hold in the reassembling queue
                           simultaneously.  0 means that the host will not
                           accept any fragmented packets.  -1 means that the
                           host will accept as many fragmented packets as it
                           receives.

     ip.maxfragsperpacket  Integer: maximum number of fragments the host will
                           accept and hold in the reassembling queue for a
                           packet.  0 means that the host will not accept any
                           fragmented packets.

SEE ALSO

     ioctl(2), socket(2), sysctl(3), icmp(4), intro(4), ip(4), ipfirewall(4),
     route(4), tcp(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.

CAVEATS

     The Internet protocol support is subject to change as the Internet
     protocols develop.  Users should not depend on details of the current
     implementation, but rather the services exported.

HISTORY

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