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[thirdparty/man-pages.git] / man7 / ip.7

'\" t
.\" Don't change the line above. it tells man that tbl is needed.
.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
.\" $Id: ip.7,v 1.19 2000/12/20 18:10:31 ak Exp $
.TH IP  7 2009-02-28 "Linux" "Linux Programmer's Manual"
.SH NAME
ip \- Linux IPv4 protocol implementation
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.\" .B #include <net/netinet.h> -- does not exist anymore
.\" .B #include <linux/errqueue.h> -- never include <linux/foo.h>
.B #include <netinet/in.h>
.br
.B #include <netinet/ip.h>        \fR/* superset of previous */
.sp
.IB tcp_socket " = socket(AF_INET, SOCK_STREAM, 0);"
.br
.IB udp_socket " = socket(AF_INET, SOCK_DGRAM, 0);"
.br
.IB raw_socket " = socket(AF_INET, SOCK_RAW, " protocol ");"
.SH DESCRIPTION
Linux implements the Internet Protocol, version 4,
described in RFC\ 791 and RFC\ 1122.
.B ip
contains a level 2 multicasting implementation conforming to RFC\ 1112.
It also contains an IP router including a packet filter.
.\" FIXME has someone verified that 2.1 is really 1812 compliant?
.PP
The programming interface is BSD-sockets compatible.
For more information on sockets, see
.BR socket (7).
.PP
An IP socket is created by calling the
.BR socket (2)
function as
.BR "socket(AF_INET, socket_type, protocol)" .
Valid socket types are
.B SOCK_STREAM
to open a
.BR tcp (7)
socket,
.B SOCK_DGRAM
to open a
.BR udp (7)
socket, or
.B SOCK_RAW
to open a
.BR raw (7)
socket to access the IP protocol directly.
.I protocol
is the IP protocol in the IP header to be received or sent.
The only valid values for
.I protocol
are 0 and
.B IPPROTO_TCP
for TCP sockets, and 0 and
.B IPPROTO_UDP
for UDP sockets.
For
.B SOCK_RAW
you may specify a valid IANA IP protocol defined in
RFC\ 1700 assigned numbers.
.PP
.\" FIXME ip current does an autobind in listen, but I'm not sure
.\" if that should be documented.
When a process wants to receive new incoming packets or connections, it
should bind a socket to a local interface address using
.BR bind (2).
Only one IP socket may be bound to any given local (address, port) pair.
When
.B INADDR_ANY
is specified in the bind call, the socket will be bound to
.I all
local interfaces.
When
.BR listen (2)
or
.BR connect (2)
are called on an unbound socket, it is automatically bound to a
random free port with the local address set to
.BR INADDR_ANY .

A TCP local socket address that has been bound is unavailable for
some time after closing, unless the
.B SO_REUSEADDR
flag has been set.
Care should be taken when using this flag as it makes TCP less reliable.
.SS Address Format
An IP socket address is defined as a combination of an IP interface
address and a 16-bit port number.
The basic IP protocol does not supply port numbers, they
are implemented by higher level protocols like
.BR udp (7)
and
.BR tcp (7).
On raw sockets
.I sin_port
is set to the IP protocol.
.PP
.in +4n
.nf
struct sockaddr_in {
    sa_family_t    sin_family; /* address family: AF_INET */
    in_port_t      sin_port;   /* port in network byte order */
    struct in_addr sin_addr;   /* internet address */
};

/* Internet address. */
struct in_addr {
    uint32_t       s_addr;     /* address in network byte order */
};
.fi
.in
.PP
.I sin_family
is always set to
.BR AF_INET .
This is required; in Linux 2.2 most networking functions return
.B EINVAL
when this setting is missing.
.I sin_port
contains the port in network byte order.
The port numbers below 1024 are called
.IR "privileged ports"
(or sometimes:
.IR "reserved ports" ).
Only privileged processes (i.e., those having the
.B CAP_NET_BIND_SERVICE
capability) may
.BR bind (2)
to these sockets.
Note that the raw IPv4 protocol as such has no concept of a
port, they are only implemented by higher protocols like
.BR tcp (7)
and
.BR udp (7).
.PP
.I sin_addr
is the IP host address.
The
.I s_addr
member of
.I struct in_addr
contains the host interface address in network byte order.
.I in_addr
should be assigned one of the INADDR_* values (e.g.,
.BR INADDR_ANY )
or set using the
.BR inet_aton (3),
.BR inet_addr (3),
.BR inet_makeaddr (3)
library functions or directly with the name resolver (see
.BR gethostbyname (3)).

IPv4 addresses are divided into unicast, broadcast
and multicast addresses.
Unicast addresses specify a single interface of a host,
broadcast addresses specify all hosts on a network and multicast
addresses address all hosts in a multicast group.
Datagrams to broadcast addresses can be only sent or received when the
.B SO_BROADCAST
socket flag is set.
In the current implementation, connection-oriented sockets are only allowed
to use unicast addresses.
.\" Leave a loophole for XTP @)

Note that the address and the port are always stored in
network byte order.
In particular, this means that you need to call
.BR htons (3)
on the number that is assigned to a port.
All address/port manipulation
functions in the standard library work in network byte order.

There are several special addresses:
.B INADDR_LOOPBACK
(127.0.0.1)
always refers to the local host via the loopback device;
.B INADDR_ANY
(0.0.0.0)
means any address for binding;
.B INADDR_BROADCAST
(255.255.255.255)
means any host and has the same effect on bind as
.B INADDR_ANY
for historical reasons.
.SS Socket Options
IP supports some protocol-specific socket options that can be set with
.BR setsockopt (2)
and read with
.BR getsockopt (2).
The socket option level for IP is
.BR IPPROTO_IP .
.\" or SOL_IP on Linux
A boolean integer flag is zero when it is false, otherwise true.
.\"
.\" FIXME Document IP_FREEBIND
.\" Boolean
.\" Since Linux 2.4.0-test10
.\"
.TP