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.\"     @(#)ip.4	6.5 (Berkeley) 3/28/91
.\"
.Dd March 28, 1991
.Dt IP 4
.Os BSD 4.2
.Sh NAME
.Nm ip
.Nd Internet Protocol
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Fd #include <netinet/in.h>
.Ft int
.Fn socket AF_INET SOCK_RAW proto
.Sh DESCRIPTION
.Tn IP 
is the transport layer protocol used
by the Internet protocol family.
Options may be set at the
.Tn IP
level
when using higher-level protocols that are based on
.Tn IP
(such as
.Tn TCP
and
.Tn UDP ) .
It may also be accessed
through a
.Dq raw socket
when developing new protocols, or
special purpose applications.
.Pp
A single generic option is supported at the
.Tn IP
level,
.Dv IP_OPTIONS ,
that may be used to provide
.Tn IP
options to be transmitted in the
.Tn IP
header of each outgoing packet.
Options are set with
.Xr setsockopt 2
and examined with
.Xr getsockopt 2 .
The format of
.Tn IP
options to be sent is that specified by the
.Tn IP protocol
specification, with one exception:
the list of addresses for Source Route options must include the first-hop
gateway at the beginning of the list of gateways.
The first-hop gateway address will be extracted from the option list
and the size adjusted accordingly before use.
.Tn IP
options may be used with any socket type in the Internet family.
.Pp
Raw
.Tn IP
sockets are connectionless,
and are normally used with the
.Xr sendto
and
.Xr recvfrom
calls, though the
.Xr connect 2
call may also be used to fix the destination for future
packets (in which case the 
.Xr read 2
or
.Xr recv 2
and 
.Xr write 2
or
.Xr send 2
system calls may be used).
.Pp
If
.Fa proto
is 0, the default protocol
.Dv IPPROTO_RAW
is used for outgoing
packets, and only incoming packets destined for that protocol
are received.
If
.Fa proto
is non-zero, that protocol number will be used on outgoing packets
and to filter incoming packets.
.Pp
Outgoing packets automatically have an
.Tn IP
header prepended to
them (based on the destination address and the protocol
number the socket is created with).
Incoming packets are received with
.Tn IP
header and options intact.
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one, or when trying to send a datagram with the destination
address specified and the socket is already connected;
.It Bq Er ENOTCONN
when trying to send a datagram, but
no destination address is specified, and the socket hasn't been
connected;
.It Bq Er ENOBUFS
when the system runs out of memory for
an internal data structure;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a 
socket with a network address for which no network interface
exists.
.El
.Pp
The following errors specific to
.Tn IP
may occur when setting or getting
.Tn IP
options:
.Bl -tag -width EADDRNOTAVAILxx
.It Bq Er EINVAL
An unknown socket option name was given.
.It Bq Er EINVAL
The IP option field was improperly formed;
an option field was shorter than the minimum value
or longer than the option buffer provided.
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr send 2 ,
.Xr recv 2 ,
.Xr intro 4 ,
.Xr icmp 4 ,
.Xr inet 4
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.2 .