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[thirdparty/man-pages.git] / 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,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
.\" $Id: icmp.7,v 1.6 2000/08/14 08:03:45 ak Exp $
.TH ICMP 7 2012-05-10 "Linux" "Linux Programmer's Manual"
.SH NAME
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 /proc interfaces
ICMP supports a set of
.I /proc
interfaces to configure some global IP parameters.
The parameters can be accessed by reading or writing files in the directory
.IR /proc/sys/net/ipv4/ .
Most of these parameters 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
.IR icmp_destunreach_rate " (Linux 2.2 to 2.4.9)"
.\" Precisely: from 2.1.102
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
.IR icmp_echo_ignore_all " (since Linux 2.2)"
.\" Precisely: 2.1.68
If this value is nonzero, Linux will ignore all
.B ICMP_ECHO
requests.
.TP
.IR icmp_echo_ignore_broadcasts " (since Linux 2.2)"
.\" Precisely: from 2.1.68
If this value is nonzero, Linux will ignore all
.B ICMP_ECHO
packets sent to broadcast addresses.
.TP
.IR icmp_echoreply_rate " (Linux 2.2 to 2.4.9)"
.\" Precisely: from 2.1.102
Maximum rate for sending
.B ICMP_ECHOREPLY
packets in response to
.B ICMP_ECHOREQUEST
packets.
.TP
.IR icmp_errors_use_inbound_ifaddr " (Boolean; default: disabled; since Linux 2.6.12)"
.\" The following taken from 2.6.28-rc4 Documentation/networking/ip-sysctl.txt
If disabled, ICMP error messages are sent with the primary address of
the exiting interface.

If enabled, the message will be sent with the primary address of
the interface that received the packet that caused the ICMP error.
This is the behavior that many network administrators will expect from
a router.
And it can make debugging complicated network layouts much easier.

Note that if no primary address exists for the interface selected,
then the primary address of the first non-loopback interface that
has one will be used regardless of this setting.
.TP