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

       icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.

       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  inter-
       act  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 pack-
       ets 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.

       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 requests.

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

	      Maximum  rate  for  sending  ICMP_ECHOREPLY packets in response to ICMP_ECHOREQUEST

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

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

       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 net-
       stat(8) output with a zero inode.

       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.


       RFC792 for a description of the ICMP protocol.

Linux Man Page				    1999-04-27					  ICMP(7)
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