.\"	$OpenBSD: icmp.4,v 1.9 2003/06/02 23:30:12 millert Exp $
.\"	$NetBSD: icmp.4,v 1.3 1994/11/30 16:22:14 jtc Exp $
.\"
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.\"     @(#)icmp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 5, 1993
.Dt ICMP 4
.Os
.Sh NAME
.Nm icmp
.Nd Internet Control Message 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 ICMP
is the error and control message protocol used
by
.Tn IP
and the Internet protocol family.
It may be accessed through a
.Dq raw socket
for network monitoring
and diagnostic functions.
The
.Fa proto
parameter to the socket call to create an
.Tn ICMP
socket
is obtained from
.Xr getprotobyname 3 .
.Tn ICMP
sockets are connectionless,
and are normally used with the
.Xr sendto 2
and
.Xr recvfrom 2
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
Outgoing packets automatically have an
.Tn IP
header prepended to
them (based on the destination address).
Incoming packets are received with the
.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
.Sh SEE ALSO
.Xr recv 2 ,
.Xr send 2 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr netintro 4
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.3 .