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,
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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 Man Page" "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 Message Protocol
  12 defined in RFC792. It is used to signal error conditions and for diagnosis.
  13 The user doesn't interact directly with this module; instead it communicates
  14 with the other protocols in the kernel and these pass the ICMP
  15 errors to the application layers. The kernel ICMP module also
  16 answers ICMP requests.
  17 .PP
  18 A user protocol may receive ICMP packets for all local sockets by opening
  19 a raw socket with the protocol
  20 .BR IPPROTO_ICMP .
  21 See
  22 .BR raw (7)
  23 for more information.
  24 The types of ICMP packets passed to the socket can be filtered using the
  25 .B ICMP_FILTER
  26 socket option. ICMP packets are always processed by the kernel too, even
  27 when passed to a user socket.
  28 .LP
  29 Linux limits the rate of ICMP error packets to each destination.
  30 .B ICMP_REDIRECT
  31 and
  32 .B ICMP_DEST_UNREACH
  33 are also limited by the destination route of the incoming packets.
  34 .SH SYSCTLS
  35 ICMP supports a sysctl interface to configure some global IP parameters.
  36 The sysctls can be accessed by reading or writing the
  37 .B /proc/sys/net/ipv4/*
  38 files or with the
  39 .BR sysctl (2)
  40 interface. Most of these sysctls are rate limitations for specific ICMP types.
  41 Linux 2.2 uses a token bucket filter to limit ICMPs.
  42 .\" XXX  better description needed
  43 The value is the timeout in jiffies until the token bucket filter is cleared
  44 after a burst. A jiffy is a system dependent unit, usually 10ms on x86 and
  45 about 1ms on alpha and IA64.
  46 .TP
  47 .B icmp_destunreach_rate
  48 Maximum rate to send ICMP Destination Unreachable packets.  This limits the
  49 rate at which packets are sent to any individual route or destination.
  50 The limit does not affect sending of
  51 .B ICMP_FRAG_NEEDED
  52 packets needed for path MTU discovery.
  53 .TP
  54 .B icmp_echo_ignore_all
  55 If this value is non-zero, Linux will ignore all
  56 .B ICMP_ECHO
  57 requests.
  58 .TP
  59 .B icmp_echo_ignore_broadcasts
  60 If this value is non-zero, Linux will ignore all
  61 .B ICMP_ECHO
  62 packets sent to broadcast addresses.
  63 .TP
  64 .B icmp_echoreply_rate
  65 Maximum rate for sending
  66 .B ICMP_ECHOREPLY
  67 packets in response to
  68 .B ICMP_ECHOREQUEST
  69 packets.
  70 .TP
  71 .B icmp_paramprob_rate
  72 Maximum rate for sending
  73 .B ICMP_PARAMETERPROB
  74 packets.
  75 These packets are sent when a packet arrives with an invalid IP header.
  76 .TP
  77 .B icmp_timeexceed_rate
  78 Maximum rate for sending
  79 .B ICMP_TIME_EXCEEDED
  80 packets.  These packets are
  81 sent to prevent loops when a packet has crossed too many hops.
  82 .SH NOTES
  83 As many other implementations don't support
  84 .B IPPROTO_ICMP
  85 raw sockets, this feature
  86 should not be relied on in portable programs.
  87 .\" not really true ATM
  88 .\" .PP
  89 .\" Linux ICMP should be compliant to RFC1122.
  90 .PP
  91 .B ICMP_REDIRECT
  92 packets are not sent when Linux is not acting as a router.
  93 They are also only accepted from the old gateway defined in the routing table and
  94 the redirect routes are expired after some time.
  95 .PP
  96 The 64-bit timestamp returned by
  97 .B ICMP_TIMESTAMP
  98 is in milliseconds since January 1, 1970.
  99 .PP
 100 Linux ICMP internally uses a raw socket to send ICMPs. This raw socket
 101 may appear in
 102 .BR netstat (8)
 103 output with a zero inode.
 104 .PP
 105 .SH VERSIONS
 106 Support for the
 107 .B ICMP_ADDRESS
 108 request was removed in 2.2.
 109 .PP
 110 Support for
 111 .B ICMP_SOURCE_QUENCH
 112 was removed in Linux 2.2.
 113 .SH "SEE ALSO"
 114 .BR ip (7)
 115 .PP
 116 RFC792 for a description of the ICMP protocol.