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.\"     From: @(#)inet.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD: src/share/man/man4/inet.4,v 1.11.2.6 2001/12/17 11:30:12 ru Exp $
.\"
.Dd March 22, 2015
.Dt INET 4
.Os
.Sh NAME
.Nm inet
.Nd Internet protocol family
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.Sh DESCRIPTION
The Internet protocol family is a collection of protocols
layered atop the
.Em Internet Protocol
.Pq Tn IP
transport layer, and utilizing the Internet address format.
The Internet family provides protocol support for the
.Dv SOCK_STREAM , SOCK_DGRAM ,
and
.Dv SOCK_RAW
socket types; the
.Dv SOCK_RAW
interface provides access to the
.Tn IP
protocol.
.Sh ADDRESSING
Internet addresses are four byte quantities, stored in
network standard format (on the
.Tn VAX
these are word and byte reversed).
The include file
.In netinet/in.h
defines this address
as a discriminated union.
.Pp
Sockets bound to the Internet protocol family utilize
the following addressing structure,
.Bd -literal -offset indent
struct sockaddr_in {
	u_char		sin_len;
	sa_family_t	sin_family;
	u_short		sin_port;
	struct in_addr	sin_addr;
	char		sin_zero[8];
};
.Ed
.Pp
Sockets may be created with the local address
.Dv INADDR_ANY
to affect
.Dq wildcard
matching on incoming messages.
The address in a
.Xr connect 2
or
.Xr sendto 2
call may be given as
.Dv INADDR_ANY
to mean
.Dq this host .
The distinguished address
.Dv INADDR_BROADCAST
is allowed as a shorthand for the broadcast address on the primary
network if the first network configured supports broadcast.
.Sh PROTOCOLS
The Internet protocol family is comprised of
the
.Tn IP
network protocol, Internet Control
Message Protocol
.Pq Tn ICMP ,
Internet Group Management Protocol
.Pq Tn IGMP ,
Transmission Control
Protocol
.Pq Tn TCP ,
and User Datagram Protocol
.Pq Tn UDP .
.Tn TCP
is used to support the
.Dv SOCK_STREAM
abstraction while
.Tn UDP
is used to support the
.Dv SOCK_DGRAM
abstraction.
A raw interface to
.Tn IP
is available
by creating an Internet socket of type
.Dv SOCK_RAW .
The
.Tn ICMP
message protocol is accessible from a raw socket.
.Pp
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
.Xr ioctl 2
commands are provided for a datagram socket in the Internet domain;
they have the same form as the
.Dv SIOCSIFADDR
and
.Dv SIOCGIFADDR
commands (see
.Xr intro 4 ) .
.Bl -tag -width ".Dv SIOCSIFNETMASK"
.It Dv 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.
.It Dv SIOCGIFNETMASK
Get interface network mask.
.El
.Sh ROUTING
The current implementation of Internet protocols includes some routing-table
adaptations to provide enhanced caching of certain end-to-end
information necessary for Transaction TCP and Path MTU Discovery.
The following changes are the most significant:
.Bl -enum
.It
All IP routes, except those with the
.Dv RTF_CLONING
flag and those to multicast destinations, have the
.Dv RTF_PRCLONING
flag forcibly enabled (they are thus said to be
.Dq "protocol cloning" ) .
.It
When the last reference to an IP route is dropped, the route is
examined to determine if it was created by cloning such a route.
If this is the case, the
.Dv RTF_PROTO3
flag is turned on, and the expiration timer is initialized to go off in
.Va net.inet.ip.rtexpire
seconds.
If such a route is re-referenced, the flag and expiration timer are reset.
.It
A kernel timeout runs once every ten minutes, or sooner if there are
soon-to-expire routes in the kernel routing table, and deletes the
expired routes.
.El
.Pp
A dynamic process is in place to modify the value of
.Va net.inet.ip.rtexpire
if the number of cached routes grows too large.
If after an expiration run there are still more than
.Va net.inet.ip.rtmaxcache
unreferenced routes remaining, the rtexpire
value is multiplied by \(34, and any routes which have longer
expiration times have those times adjusted.
This process is damped somewhat by specification of a minimum rtexpire value
.Va ( net.inet.ip.rtminexpire ) ,
and by restricting the reduction to once in a ten-minute period.
.Pp
If some external process deletes the original route from which a
protocol-cloned route was generated, the ``child route'' is deleted.
(This is actually a generic mechanism in the routing code support for
protocol-requested cloning.)
.Pp
No attempt is made to manage routes which were not created by protocol
cloning; these are assumed to be static, under the management of an
external routing process, or under the management of a link layer
(e.g.,
.Tn ARP
for Ethernets).
.Pp
Only certain types of network activity will result in the cloning of a
route using this mechanism.
Specifically, those protocols (such as
.Tn TCP
and
.Tn UDP )
which themselves cache a long-lasting reference to route for a destination
will trigger the mechanism; whereas raw
.Tn IP
packets, whether locally-generated or forwarded, will not.
.Ss MIB Variables
A number of variables are implemented in the net.inet branch of the
.Xr 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:
.Bl -tag -width IPCTL_FASTFORWARDING
.It Dv IPCTL_FORWARDING
.Pq ip.forwarding
Boolean: enable/disable forwarding of IP packets.
Defaults to off.
.It Dv IPCTL_FASTFORWARDING
.Pq ip.fastforwarding
Boolean: enable/disable the use of fast IP forwarding code.
Defaults to off.
When fast forwarding is enabled, IP packets are forwarded directly to
the appropriate network interface with a minimal validity checking, which
greatly improves the throughput.
On the other hand, they bypass the
standard procedures, such as IP option processing and
.Xr ipfirewall 4
checking.
It is not guaranteed that every packet will be fast-forwarded.
.It Dv IPCTL_SENDREDIRECTS
.Pq ip.redirect
Boolean: enable/disable sending of ICMP redirects in response to
unforwardable
.Tn IP
packets.
Defaults to on.
.It Dv IPCTL_DEFTTL
.Pq ip.ttl
Integer: default time-to-live
.Pq Dq TTL
to use for outgoing
.Tn IP
packets.
.It Dv IPCTL_SOURCEROUTE
.Pq ip.sourceroute
Boolean: enable/disable forwarding of source-routed IP packets (default false).
.It Dv IPCTL_RTEXPIRE
.Pq ip.rtexpire
Integer: lifetime in seconds of protocol-cloned
.Tn IP
routes after the last reference drops (default one hour).
This value varies dynamically as described above.
.It Dv IPCTL_RTMINEXPIRE
.Pq 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.
.It Dv IPCTL_RTMAXCACHE
.Pq ip.rtmaxcache
Integer: trigger level of cached, unreferenced, protocol-cloned routes
which initiates dynamic adaptation (default 128).
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr icmp 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr tcp 4 ,
.Xr udp 4
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re
.Sh 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.
.Sh HISTORY
The
.Nm
protocol interface appeared in
.Bx 4.2 .
The
.Dq protocol cloning
code appeared in
.Fx 2.1 .