man7/icmp.7

   1 .\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
   2 .\" Permission is granted to distribute possibly modified copies
   3 .\" of this page provided the header is included verbatim,
   4 .\" and in case of nontrivial modification author and date
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   6 .\" $Id: icmp.7,v 1.6 2000/08/14 08:03:45 ak Exp $
   7 .TH ICMP 7 1999-04-27 "Linux" "Linux Programmer's Manual"
   8 .SH NAME
   9 icmp, IPPROTO_ICMP \- Linux IPv4 ICMP kernel module.
  10 .SH DESCRIPTION
  11 This kernel protocol module implements the Internet Control
  12 Message Protocol defined in RFC\ 792.
  13 It is used to signal error conditions and for diagnosis.
  14 The user doesn't interact directly with this module;
  15 instead it communicates with the other protocols in the kernel
  16 and these pass the ICMP errors to the application layers.
  17 The kernel ICMP module also answers ICMP requests.
  18 .PP
  19 A user protocol may receive ICMP packets for all local sockets by opening
  20 a raw socket with the protocol
  21 .BR IPPROTO_ICMP .
  22 See
  23 .BR raw (7)
  24 for more information.
  25 The types of ICMP packets passed to the socket can be filtered using the
  26 .B ICMP_FILTER
  27 socket option.
  28 ICMP packets are always processed by the kernel too, even
  29 when passed to a user socket.
  30 .LP
  31 Linux limits the rate of ICMP error packets to each destination.
  32 .B ICMP_REDIRECT
  33 and
  34 .B ICMP_DEST_UNREACH
  35 are also limited by the destination route of the incoming packets.
  36 .SS Sysctls
  37 ICMP supports a sysctl interface to configure some global IP parameters.
  38 The sysctls can be accessed by reading or writing the
  39 .I /proc/sys/net/ipv4/*
  40 files or with the
  41 .BR sysctl (2)
  42 interface.
  43 Most of these sysctls are rate limitations for specific ICMP types.
  44 Linux 2.2 uses a token bucket filter to limit ICMPs.
  45 .\" FIXME better description needed
  46 The value is the timeout in jiffies until the token bucket filter is
  47 cleared after a burst.
  48 A jiffy is a system dependent unit, usually 10ms on x86 and
  49 about 1ms on alpha and ia64.
  50 .TP
  51 .B icmp_destunreach_rate
  52 Maximum rate to send ICMP Destination Unreachable packets.
  53 This limits the rate at which packets are sent to any individual
  54 route or destination.
  55 The limit does not affect sending of
  56 .B ICMP_FRAG_NEEDED
  57 packets needed for path MTU discovery.
  58 .TP
  59 .B icmp_echo_ignore_all
  60 If this value is non-zero, Linux will ignore all
  61 .B ICMP_ECHO
  62 requests.
  63 .TP
  64 .B icmp_echo_ignore_broadcasts
  65 If this value is non-zero, Linux will ignore all
  66 .B ICMP_ECHO
  67 packets sent to broadcast addresses.
  68 .TP
  69 .B icmp_echoreply_rate
  70 Maximum rate for sending
  71 .B ICMP_ECHOREPLY
  72 packets in response to
  73 .B ICMP_ECHOREQUEST
  74 packets.
  75 .TP
  76 .B icmp_paramprob_rate
  77 Maximum rate for sending
  78 .B ICMP_PARAMETERPROB
  79 packets.
  80 These packets are sent when a packet arrives with an invalid IP header.
  81 .TP
  82 .B icmp_timeexceed_rate
  83 Maximum rate for sending
  84 .B ICMP_TIME_EXCEEDED
  85 packets.
  86 These packets are
  87 sent to prevent loops when a packet has crossed too many hops.
  88 .SH VERSIONS
  89 Support for the
  90 .B ICMP_ADDRESS
  91 request was removed in 2.2.
  92 .PP
  93 Support for
  94 .B ICMP_SOURCE_QUENCH
  95 was removed in Linux 2.2.
  96 .SH NOTES
  97 As many other implementations don't support
  98 .B IPPROTO_ICMP
  99 raw sockets, this feature
 100 should not be relied on in portable programs.
 101 .\" not really true ATM
 102 .\" .PP
 103 .\" Linux ICMP should be compliant to RFC 1122.
 104 .PP
 105 .B ICMP_REDIRECT
 106 packets are not sent when Linux is not acting as a router.
 107 They are also only accepted from the old gateway defined in the
 108 routing table and the redirect routes are expired after some time.
 109 .PP
 110 The 64-bit timestamp returned by
 111 .B ICMP_TIMESTAMP
 112 is in milliseconds since January 1, 1970.
 113 .PP
 114 Linux ICMP internally uses a raw socket to send ICMPs.
 115 This raw socket may appear in
 116 .BR netstat (8)
 117 output with a zero inode.
 118 .SH "SEE ALSO"
 119 .BR ip (7)
 120 .PP
 121 RFC\ 792 for a description of the ICMP protocol.