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

     inet — Internet protocol family

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

     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 dis‐
     tinguished 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 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.  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.

     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

     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.

     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.

     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				 April 9, 2005				   BSD

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