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ICMP(7) 			    Linux Programmer's Manual				  ICMP(7)

NAME
       icmp - Linux IPv4 ICMP kernel module.

DESCRIPTION
       This  kernel  protocol  module implements the Internet Control Message Protocol defined in
       RFC 792.  It is used to signal error conditions	and  for  diagnosis.   The  user  doesn't
       interact  directly  with  this module; instead it communicates with the other protocols in
       the kernel and these pass the ICMP errors to the application layers.  The kernel ICMP mod-
       ule also answers ICMP requests.

       A  user	protocol  may  receive ICMP packets for all local sockets by opening a raw socket
       with the protocol IPPROTO_ICMP.	See raw(7) for more information.  The types of ICMP pack-
       ets  passed to the socket can be filtered using the ICMP_FILTER socket option.  ICMP pack-
       ets are always processed by the kernel too, even when passed to a user socket.

       Linux limits the rate of ICMP  error  packets  to  each	destination.   ICMP_REDIRECT  and
       ICMP_DEST_UNREACH are also limited by the destination route of the incoming packets.

   /proc interfaces
       ICMP  supports  a  set  of  /proc  interfaces to configure some global IP parameters.  The
       parameters  can	be  accessed   by   reading   or   writing   files   in   the	directory
       /proc/sys/net/ipv4/.   Most  of	these  parameters  are rate limitations for specific ICMP
       types.  Linux 2.2 uses a token bucket filter to limit ICMPs.  The value is the timeout  in
       jiffies	until  the  token  bucket  filter  is cleared after a burst.  A jiffy is a system
       dependent unit, usually 10ms on i386 and about 1ms on alpha and ia64.

       icmp_destunreach_rate (Linux 2.2 to 2.4.9)
	      Maximum rate to send ICMP Destination Unreachable packets.  This limits the rate at
	      which  packets are sent to any individual route or destination.  The limit does not
	      affect sending of ICMP_FRAG_NEEDED packets needed for path MTU discovery.

       icmp_echo_ignore_all (since Linux 2.2)
	      If this value is nonzero, Linux will ignore all ICMP_ECHO requests.

       icmp_echo_ignore_broadcasts (since Linux 2.2)
	      If this value is nonzero, Linux will ignore all ICMP_ECHO packets sent to broadcast
	      addresses.

       icmp_echoreply_rate (Linux 2.2 to 2.4.9)
	      Maximum  rate  for  sending  ICMP_ECHOREPLY packets in response to ICMP_ECHOREQUEST
	      packets.

       icmp_errors_use_inbound_ifaddr (Boolean; default: disabled; since Linux 2.6.12)
	      If disabled, ICMP error messages are sent with the primary address of  the  exiting
	      interface.

	      If enabled, the message will be sent with the primary address of the interface that
	      received the packet that caused the ICMP error.  This is	the  behavior  that  many
	      network  administrators  will expect from a router.  And it can make debugging com-
	      plicated network layouts much easier.

	      Note that if no primary address exists for the interface selected, then the primary
	      address of the first non-loopback interface that has one will be used regardless of
	      this setting.

       icmp_ignore_bogus_error_responses (Boolean; default: disabled; since Linux 2.2)
	      Some routers violate RFC1122 by sending bogus responses to broadcast frames.   Such
	      violations are normally logged via a kernel warning.  If this parameter is enabled,
	      the kernel will not give such warnings, which will avoid log file clutter.

       icmp_paramprob_rate (Linux 2.2 to 2.4.9)
	      Maximum rate for sending ICMP_PARAMETERPROB packets.  These packets are sent when a
	      packet arrives with an invalid IP header.

       icmp_ratelimit (integer; default: 1000; since Linux 2.4.10)
	      Limit  the  maximum rates for sending ICMP packets whose type matches icmp_ratemask
	      (see below) to specific targets.	0 to disable any limiting, otherwise the  minimum
	      space between responses in milliseconds.

       icmp_ratemask (integer; default: see below; since Linux 2.4.10)
	      Mask made of ICMP types for which rates are being limited.

	      Significant bits: IHGFEDCBA9876543210
	      Default mask:	0000001100000011000 (0x1818)

	      Bit definitions (see the Linux kernel source file include/linux/icmp.h):

		   0 Echo Reply
		   3 Destination Unreachable *
		   4 Source Quench *
		   5 Redirect
		   8 Echo Request
		   B Time Exceeded *
		   C Parameter Problem *
		   D Timestamp Request
		   E Timestamp Reply
		   F Info Request
		   G Info Reply
		   H Address Mask Request
		   I Address Mask Reply

       The bits marked with an asterisk are rate limited by default (see the default mask above).

       icmp_timeexceed_rate (Linux 2.2 to 2.4.9)
	      Maximum  rate  for  sending  ICMP_TIME_EXCEEDED packets.	These packets are sent to
	      prevent loops when a packet has crossed too many hops.

VERSIONS
       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

NOTES
       As many other implementations don't support IPPROTO_ICMP raw sockets, this feature  should
       not be relied on in portable programs.

       ICMP_REDIRECT  packets  are  not sent when Linux is not acting as a router.  They are also
       accepted only from the old gateway defined in the routing table and  the  redirect  routes
       are expired after some time.

       The  64-bit  timestamp  returned  by  ICMP_TIMESTAMP  is  in milliseconds since the Epoch,
       1970-01-01 00:00:00 +0000 (UTC).

       Linux ICMP internally uses a raw socket to send ICMPs.  This raw socket may appear in net-
       stat(8) output with a zero inode.

SEE ALSO
       ip(7)

       RFC 792 for a description of the ICMP protocol.

COLOPHON
       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2012-05-10					  ICMP(7)
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