icmp

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



NAME
       icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.

DESCRIPTION
       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  destina-
       tion route of the incoming packets.

SYSCTLS
       ICMP  supports  a sysctl interface to configure some global IP parame-
       ters.   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.

       icmp_destunreach_rate
	      Maximum rate to  send  ICMP  Destination	Unreachable  packets.
	      This  limits the rate at which packets are sent to any individ-
	      ual route or destination.	 The limit does not affect sending of
	      ICMP_FRAG_NEEDED packets needed for path MTU discovery.

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

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

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

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

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

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 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
       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

SEE ALSO
       ip(7)

       RFC792 for a description of the ICMP protocol.



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