mirror of https://github.com/mkerrisk/man-pages
117 lines
3.7 KiB
Groff
117 lines
3.7 KiB
Groff
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.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
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.\" Permission is granted to distribute possibly modified copies
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.\" of this page provided the header is included verbatim,
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.\" and in case of nontrivial modification author and date
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.\" of the modification is added to the header.
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.\" $Id: icmp.7,v 1.6 2000/08/14 08:03:45 ak Exp $
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.TH ICMP 7 1999-04-27 "Linux Man Page" "Linux Programmer's Manual"
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.SH NAME
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icmp, IPPROTO_ICMP \- Linux IPv4 ICMP kernel module.
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.SH DESCRIPTION
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This kernel protocol module implements the Internet Control Message Protocol
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defined in RFC792. It is used to signal error conditions and for diagnosis.
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The user doesn't interact directly with this module; instead it communicates
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with the other protocols in the kernel and these pass the ICMP
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errors to the application layers. The kernel ICMP module also
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answers ICMP requests.
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.PP
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A user protocol may receive ICMP packets for all local sockets by opening
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a raw socket with the protocol
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.BR IPPROTO_ICMP .
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See
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.BR raw (7)
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for more information.
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The types of ICMP packets passed to the socket can be filtered using the
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.B ICMP_FILTER
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socket option. ICMP packets are always processed by the kernel too, even
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when passed to a user socket.
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.LP
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Linux limits the rate of ICMP error packets to each destination.
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.B ICMP_REDIRECT
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and
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.B ICMP_DEST_UNREACH
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are also limited by the destination route of the incoming packets.
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.SH SYSCTLS
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ICMP supports a sysctl interface to configure some global IP parameters.
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The sysctls can be accessed by reading or writing the
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.B /proc/sys/net/ipv4/*
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files or with the
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.BR sysctl (2)
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interface. Most of these sysctls are rate limitations for specific ICMP types.
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Linux 2.2 uses a token bucket filter to limit ICMPs.
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.\" XXX better description needed
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The value is the timeout in jiffies until the token bucket filter is cleared
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after a burst. A jiffy is a system dependent unit, usually 10ms on x86 and
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about 1ms on alpha and IA64.
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.TP
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.B icmp_destunreach_rate
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Maximum rate to send ICMP Destination Unreachable packets. This limits the
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rate at which packets are sent to any individual route or destination.
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The limit does not affect sending of
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.B ICMP_FRAG_NEEDED
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packets needed for path MTU discovery.
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.TP
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.B icmp_echo_ignore_all
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If this value is non-zero, Linux will ignore all
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.B ICMP_ECHO
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requests.
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.TP
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.B icmp_echo_ignore_broadcasts
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If this value is non-zero, Linux will ignore all
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.B ICMP_ECHO
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packets sent to broadcast addresses.
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.TP
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.B icmp_echoreply_rate
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Maximum rate for sending
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.B ICMP_ECHOREPLY
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packets in response to
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.B ICMP_ECHOREQUEST
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packets.
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.TP
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.B icmp_paramprob_rate
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Maximum rate for sending
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.B ICMP_PARAMETERPROB
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packets.
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These packets are sent when a packet arrives with an invalid IP header.
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.TP
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.B icmp_timeexceed_rate
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Maximum rate for sending
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.B ICMP_TIME_EXCEEDED
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packets. These packets are
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sent to prevent loops when a packet has crossed too many hops.
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.SH NOTES
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As many other implementations don't support
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.B IPPROTO_ICMP
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raw sockets, this feature
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should not be relied on in portable programs.
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.\" not really true ATM
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.\" .PP
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.\" Linux ICMP should be compliant to RFC1122.
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.PP
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.B ICMP_REDIRECT
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packets are not sent when Linux is not acting as a router.
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They are also only accepted from the old gateway defined in the routing table and
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the redirect routes are expired after some time.
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.PP
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The 64-bit timestamp returned by
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.B ICMP_TIMESTAMP
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is in milliseconds since January 1, 1970.
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.PP
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Linux ICMP internally uses a raw socket to send ICMPs. This raw socket
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may appear in
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.BR netstat (8)
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output with a zero inode.
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.PP
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.SH VERSIONS
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Support for the
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.B ICMP_ADDRESS
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request was removed in 2.2.
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.PP
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Support for
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.B ICMP_SOURCE_QUENCH
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was removed in Linux 2.2.
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.SH "SEE ALSO"
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.BR ip (7)
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.PP
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RFC792 for a description of the ICMP protocol.
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