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 2008-11-24 "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 .IR icmp_destunreach_rate " (Linux 2.2 to 2.4.10)"
  51 .\" Precisely: from 2.1.102
  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 .IR icmp_echo_ignore_all " (since Linux 2.2)"
  60 .\" Precisely: 2.1.68
  61 If this value is non-zero, Linux will ignore all
  62 .B ICMP_ECHO
  63 requests.
  64 .TP
  65 .IR icmp_echo_ignore_broadcasts " (since Linux 2.2)"
  66 .\" Precisely: from 2.1.68
  67 If this value is non-zero, Linux will ignore all
  68 .B ICMP_ECHO
  69 packets sent to broadcast addresses.
  70 .TP
  71 .IR icmp_echoreply_rate " (Linux 2.2 to 2.4.10)"
  72 .\" Precisely: from 2.1.102
  73 Maximum rate for sending
  74 .B ICMP_ECHOREPLY
  75 packets in response to
  76 .B ICMP_ECHOREQUEST
  77 packets.
  78 .TP
  79 .IR icmp_paramprob_rate " (Linux 2.2 to 2.4.10)"
  80 .\" Precisely: from 2.1.102
  81 Maximum rate for sending
  82 .B ICMP_PARAMETERPROB
  83 packets.
  84 These packets are sent when a packet arrives with an invalid IP header.
  85 .TP
  86 .IR icmp_ratelimit " (integer; default: 1000; since Linux 2.4.10)"
  87 .\" The following taken from 2.6.28-rc4 Documentation/networking/ip-sysctl.txt
  88 Limit the maximum rates for sending ICMP packets whose type matches
  89 .IR icmp_ratemask
  90 (see below) to specific targets.
  91 0 to disable any limiting,
  92 otherwise the minimum space between responses in milliseconds.
  93 .TP
  94 .IR icmp_ratemask " (integer; default: see below; since Linux 2.4.10)"
  95 .\" The following taken from 2.6.28-rc4 Documentation/networking/ip-sysctl.txt
  96 Mask made of ICMP types for which rates are being limited.
  97
  98 Significant bits: IHGFEDCBA9876543210
  99 .br
 100 Default mask:     0000001100000011000 (0x1818)
 101
 102 Bit definitions (see the kernel source file
 103 .IR include/linux/icmp.h ):
 104
 105 .in +4n
 106 .nf
 107 0 Echo Reply
 108 3 Destination Unreachable *
 109 4 Source Quench *
 110 5 Redirect
 111 8 Echo Request
 112 B Time Exceeded *
 113 C Parameter Problem *
 114 D Timestamp Request
 115 E Timestamp Reply
 116 F Info Request
 117 G Info Reply
 118 H Address Mask Request
 119 I Address Mask Reply
 120 .fi
 121 .in
 122
 123 The bits marked with an asterisk are rate limited by default
 124 (see the default mask above).
 125 .TP
 126 .IR icmp_timeexceed_rate " (Linux 2.2 to 2.4.10)"
 127 Maximum rate for sending
 128 .B ICMP_TIME_EXCEEDED
 129 packets.
 130 These packets are
 131 sent to prevent loops when a packet has crossed too many hops.
 132 .SH VERSIONS
 133 Support for the
 134 .B ICMP_ADDRESS
 135 request was removed in 2.2.
 136 .PP
 137 Support for
 138 .B ICMP_SOURCE_QUENCH
 139 was removed in Linux 2.2.
 140 .SH NOTES
 141 As many other implementations don't support
 142 .B IPPROTO_ICMP
 143 raw sockets, this feature
 144 should not be relied on in portable programs.
 145 .\" not really true ATM
 146 .\" .PP
 147 .\" Linux ICMP should be compliant to RFC 1122.
 148 .PP
 149 .B ICMP_REDIRECT
 150 packets are not sent when Linux is not acting as a router.
 151 They are also only accepted from the old gateway defined in the
 152 routing table and the redirect routes are expired after some time.
 153 .PP
 154 The 64-bit timestamp returned by
 155 .B ICMP_TIMESTAMP
 156 is in milliseconds since January 1, 1970.
 157 .PP
 158 Linux ICMP internally uses a raw socket to send ICMPs.
 159 This raw socket may appear in
 160 .BR netstat (8)
 161 output with a zero inode.
 162 .SH "SEE ALSO"
 163 .BR ip (7)
 164 .PP
 165 RFC\ 792 for a description of the ICMP protocol.