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
   5 .\" of the modification is added to the header.
   6 .\" $Id: icmp.7,v 1.6 2000/08/14 08:03:45 ak Exp $
   7 .TH ICMP 7 2008-11-20 "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 /proc interfaces
  37 ICMP supports a set of
  38 .I /proc
  39 interfaces to configure some global IP parameters.
  40 The parameters can be accessed by reading or writing files in the directory
  41 .IR /proc/sys/net/ipv4/ .
  42 Most of these parameters are rate limitations for specific ICMP types.
  43 Linux 2.2 uses a token bucket filter to limit ICMPs.
  44 .\" FIXME better description needed
  45 The value is the timeout in jiffies until the token bucket filter is
  46 cleared after a burst.
  47 A jiffy is a system dependent unit, usually 10ms on i386 and
  48 about 1ms on alpha and ia64.
  49 .TP
  50 .I icmp_destunreach_rate
  51 Maximum rate to send ICMP Destination Unreachable packets.
  52 This limits the rate at which packets are sent to any individual
  53 route or destination.
  54 The limit does not affect sending of
  55 .B ICMP_FRAG_NEEDED
  56 packets needed for path MTU discovery.
  57 .TP
  58 .I icmp_echo_ignore_all
  59 If this value is non-zero, Linux will ignore all
  60 .B ICMP_ECHO
  61 requests.
  62 .TP
  63 .I icmp_echo_ignore_broadcasts
  64 If this value is non-zero, Linux will ignore all
  65 .B ICMP_ECHO
  66 packets sent to broadcast addresses.
  67 .TP
  68 .I icmp_echoreply_rate
  69 Maximum rate for sending
  70 .B ICMP_ECHOREPLY
  71 packets in response to
  72 .B ICMP_ECHOREQUEST
  73 packets.
  74 .TP
  75 .I icmp_paramprob_rate
  76 Maximum rate for sending
  77 .B ICMP_PARAMETERPROB
  78 packets.
  79 These packets are sent when a packet arrives with an invalid IP header.
  80 .TP
  81 .I icmp_timeexceed_rate
  82 Maximum rate for sending
  83 .B ICMP_TIME_EXCEEDED
  84 packets.
  85 These packets are
  86 sent to prevent loops when a packet has crossed too many hops.
  87 .SH VERSIONS
  88 Support for the
  89 .B ICMP_ADDRESS
  90 request was removed in 2.2.
  91 .PP
  92 Support for
  93 .B ICMP_SOURCE_QUENCH
  94 was removed in Linux 2.2.
  95 .SH NOTES
  96 As many other implementations don't support
  97 .B IPPROTO_ICMP
  98 raw sockets, this feature
  99 should not be relied on in portable programs.
 100 .\" not really true ATM
 101 .\" .PP
 102 .\" Linux ICMP should be compliant to RFC 1122.
 103 .PP
 104 .B ICMP_REDIRECT
 105 packets are not sent when Linux is not acting as a router.
 106 They are also only accepted from the old gateway defined in the
 107 routing table and the redirect routes are expired after some time.
 108 .PP
 109 The 64-bit timestamp returned by
 110 .B ICMP_TIMESTAMP
 111 is in milliseconds since January 1, 1970.
 112 .PP
 113 Linux ICMP internally uses a raw socket to send ICMPs.
 114 This raw socket may appear in
 115 .BR netstat (8)
 116 output with a zero inode.
 117 .SH "SEE ALSO"
 118 .BR ip (7)
 119 .PP
 120 RFC\ 792 for a description of the ICMP protocol.