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.\"     From: @(#)inet.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd October 23, 2004
.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 little endian machines, such as the
.Tn alpha ,
.Tn amd64 ,
.Tn i386
and
.Tn ia64
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 {
	uint8_t		sin_len;
	sa_family_t	sin_family;
	in_port_t	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 SIOCIFADDR
command (see
.Xr intro 4 ) .
.Pp
.Bl -tag -width 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
.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
.Tn fast IP forwarding
code.
Defaults to off.
When
.Tn 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
.Xr pfil 9
hooks) checking, except
.Xr IPSEC 4
tunnel brokering.
The
.Tn IP fastforwarding
path does not generate ICMP redirect or source quench messages.
.It Dv IPCTL_SENDREDIRECTS
.Pq ip.redirect
Boolean: enable/disable sending of ICMP redirects in response to
.Tn IP
packets for which a better, and for the sender directly reachable, route
and next hop is known.
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_ACCEPTSOURCEROUTE
.Pq ip.accept_sourceroute
Boolean: enable/disable accepting of source-routed IP packets (default false).
.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).
.It Va 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
.Tn ICMP
.Dq "prohibited by filter"
message will be sent back in respose to incoming packets with IP options.
Default is 1.
This
.Xr sysctl 8
variable affects packets destined for a local host as well as packets
forwarded to some other host.
.It Va ip.random_id
Boolean: control IP IDs generation behaviour.
Setting this
.Xr 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.
.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 route 4 ,
.Xr tcp 4 ,
.Xr ttcp 4 ,
.Xr udp 4 ,
.Xr pfil 9
.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 CAVEAT
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 .