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.\"     From: @(#)inet.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd Feb 4, 2016
.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
and
.Tn i386
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
.Nm
address on an interface consist of the address itself, the
netmask, either broadcast address in case of a broadcast
interface or peers address in case of point-to-point interface.
The following
.Xr ioctl 2
commands are provided for a datagram socket in the Internet domain:
.Pp
.Bl -tag -width ".Dv SIOCGIFBRDADDR" -offset indent -compact
.It Dv SIOCAIFADDR
Add address to an interface.
The command requires
.Ft struct in_aliasreq
as argument.
.It Dv SIOCDIFADDR
Delete address from an interface.
The command requires
.Ft struct ifreq
as argument.
.It Dv SIOCGIFADDR
.It Dv SIOCGIFBRDADDR
.It Dv SIOCGIFDSTADDR
.It Dv SIOCGIFNETMASK
Return address information from interface.
The returned value is in
.Ft struct ifreq .
This way of address information retrieval is obsoleted, a
preferred way is to use
.Xr getifaddrs 3
API.
.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_ACCEPTSOURCEROUTE
.It Dv IPCTL_FORWARDING
.Pq ip.forwarding
Boolean: enable/disable forwarding of IP packets.
Defaults to off.
.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 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 response 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.rfc6864
Boolean: control IP IDs generation behaviour.
True value enables RFC6864 support, which specifies that IP ID field of
.Em atomic
datagrams can be set to any value.
The
.Fx implementation sets it to zero.
Enabled by default.
.It Va ip.random_id
Boolean: control IP IDs generation behaviour.
Setting this
.Xr sysctl 8
to 1 causes the ID field in
.Em non-atomic
IP datagrams (or all IP datagrams, if
.Va ip.rfc6864
is disabled) 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.
At 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.
.It Va 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.
.It Va 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.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr getifaddrs 3 ,
.Xr sysctl 3 ,
.Xr icmp 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr route 4 ,
.Xr tcp 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 HISTORY
The
.Nm
protocol interface appeared in
.Bx 4.2 .
The
.Dq protocol cloning
code appeared in
.Fx 2.1 .
.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.