man-pages/man7/icmp.7

.\" 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,
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.\" $Id: icmp.7,v 1.6 2000/08/14 08:03:45 ak Exp $
.TH ICMP 7 1999-04-27 "Linux" "Linux Programmer's Manual"
.SH NAME
icmp, IPPROTO_ICMP \- Linux IPv4 ICMP kernel module.
.SH DESCRIPTION
This kernel protocol module implements the Internet Control
Message Protocol defined in RFC\ 792.
It is used to signal error conditions and for diagnosis.
The user doesn't interact directly with this module;
instead it communicates with the other protocols in the kernel
and these pass the ICMP errors to the application layers.
The kernel ICMP module also answers ICMP requests.
.PP
A user protocol may receive ICMP packets for all local sockets by opening
a raw socket with the protocol
.BR IPPROTO_ICMP .
See
.BR raw (7)
for more information.
The types of ICMP packets passed to the socket can be filtered using the
.B ICMP_FILTER
socket option.
ICMP packets are always processed by the kernel too, even
when passed to a user socket.
.LP
Linux limits the rate of ICMP error packets to each destination.
.B ICMP_REDIRECT
and
.B ICMP_DEST_UNREACH
are also limited by the destination route of the incoming packets.
.SS Sysctls
ICMP supports a sysctl interface to configure some global IP parameters.
The sysctls can be accessed by reading or writing the
.I /proc/sys/net/ipv4/*
files or with the
.BR sysctl (2)
interface.
Most of these sysctls are rate limitations for specific ICMP types.
Linux 2.2 uses a token bucket filter to limit ICMPs.
.\" FIXME better description needed
The value is the timeout in jiffies until the token bucket filter is
cleared after a burst.
A jiffy is a system dependent unit, usually 10ms on i386 and
about 1ms on alpha and ia64.
.TP
.I icmp_destunreach_rate
Maximum rate to send ICMP Destination Unreachable packets.
This limits the rate at which packets are sent to any individual
route or destination.
The limit does not affect sending of
.B ICMP_FRAG_NEEDED
packets needed for path MTU discovery.
.TP
.I icmp_echo_ignore_all
If this value is non-zero, Linux will ignore all
.B ICMP_ECHO
requests.
.TP
.I icmp_echo_ignore_broadcasts
If this value is non-zero, Linux will ignore all
.B ICMP_ECHO
packets sent to broadcast addresses.
.TP
.I icmp_echoreply_rate
Maximum rate for sending
.B ICMP_ECHOREPLY
packets in response to
.B ICMP_ECHOREQUEST
packets.
.TP
.I icmp_paramprob_rate
Maximum rate for sending
.B ICMP_PARAMETERPROB
packets.
These packets are sent when a packet arrives with an invalid IP header.
.TP
.I icmp_timeexceed_rate
Maximum rate for sending
.B ICMP_TIME_EXCEEDED
packets.
These packets are
sent to prevent loops when a packet has crossed too many hops.
.SH VERSIONS
Support for the
.B ICMP_ADDRESS
request was removed in 2.2.
.PP
Support for
.B ICMP_SOURCE_QUENCH
was removed in Linux 2.2.
.SH NOTES
As many other implementations don't support
.B IPPROTO_ICMP
raw sockets, this feature
should not be relied on in portable programs.
.\" not really true ATM
.\" .PP
.\" Linux ICMP should be compliant to RFC 1122.
.PP
.B ICMP_REDIRECT
packets are not sent when Linux is not acting as a router.
They are also only accepted from the old gateway defined in the
routing table and the redirect routes are expired after some time.
.PP
The 64-bit timestamp returned by
.B ICMP_TIMESTAMP
is in milliseconds since January 1, 1970.
.PP
Linux ICMP internally uses a raw socket to send ICMPs.
This raw socket may appear in
.BR netstat (8)
output with a zero inode.
.SH "SEE ALSO"
.BR ip (7)
.PP
RFC\ 792 for a description of the ICMP protocol.