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
       module also answers ICMP requests.

       A user protocol may receive ICMP packets for all local sockets by open-
       ing  a  raw socket with the protocol IPPROTO_ICMP.  See raw(7) for more
       information.  The types of ICMP packets passed to  the  socket  can  be
       filtered	 using the ICMP_FILTER socket option.  ICMP packets 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 pack-
	      ets 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  compli-
	      cated 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.

       ping_group_range (two integers; default: see below; since Linux 2.6.39)
	      Range of the group IDs (minimum and maximum  group  IDs,	inclu-
	      sive) that are allowed to create ICMP Echo sockets.  The default
	      is "1 0", which means no group is allowed to  create  ICMP  Echo
	      sockets.

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 netstat(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 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2015-04-19			       ICMP(7)