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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 ad-
     dressing 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, In-
     ternet Control Message Protocol (ICMP), Internet Group Management Proto-
     col (IGMP), Transmission Control Protocol (TCP), and User Datagram	Proto-
     col (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 mes-
     sage protocol is accessible from a	raw socket.

     The inet address on an interface consist of the address itself, the net-
     mask, either broadcast address in case of a broadcast interface or	peers
     address in	case of	point-to-point interface.  The following ioctl(2) com-
     mands are provided	for a datagram socket in the Internet domain:

	   SIOCAIFADDR	   Add address to an interface.	 The command requires
			   struct in_aliasreq as argument.
	   SIOCDIFADDR	   Delete address from an interface.  The command re-
			   quires struct ifreq as argument.
	   SIOCGIFNETMASK  Return address information from interface. The re-
			   turned value	is in struct ifreq.  This way of ad-
			   dress information retrieval is obsoleted, a pre-
			   ferred way is to use	getifaddrs(3) API.

   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 ac-
			   cepting 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 ef-
			   fect	on user	modifications, but restricts the dy-
			   namic 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 ac-
			   cept	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), getifaddrs(3), 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 inet protocol interface appeared in 4.2BSD.  The "protocol cloning"
     code appeared in FreeBSD 2.1.

     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.

BSD			       January 26, 2012				   BSD


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