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NAME    [Toc]    [Back]

       icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.

DESCRIPTION    [Toc]    [Back]

       This  kernel  protocol  module  implements the Internet Control Message
       Protocol defined in RFC792. 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 opening
  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.

SYSCTLS    [Toc]    [Back]

       ICMP supports a sysctl interface to configure some  global  IP  parameters.
	The  sysctls  can  be  accessed  by  reading  or  writing  the
       /proc/sys/net/ipv4/* files or with the  sysctl(2)  interface.  Most  of
       these  sysctls 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 x86 and about 1ms on  alpha
       and IA64.

	      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.

	      If this value is	non-zero,  Linux  will	ignore	all  ICMP_ECHO

	      If this value is non-zero, Linux will ignore all ICMP_ECHO packets
 sent to broadcast addresses.

	      Maximum rate for sending ICMP_ECHOREPLY packets in  response  to
	      ICMP_ECHOREQUEST packets.

	      Maximum  rate  for  sending  ICMP_PARAMETERPROB  packets.  These
	      packets are sent when  a	packet	arrives  with  an  invalid  IP

	      Maximum  rate  for  sending  ICMP_TIME_EXCEEDED  packets.  These
	      packets are sent to prevent loops when a packet has crossed  too
	      many hops.

NOTES    [Toc]    [Back]

       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 only accepted 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 January 1, 1970.

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

VERSIONS    [Toc]    [Back]

       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

SEE ALSO    [Toc]    [Back]


       RFC792 for a description of the ICMP protocol.

Linux Man Page			  1999-04-27			       ICMP(7)
[ Back ]
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