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       This  kernel  protocol module implements the Internet Con­
       trol 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

       Linux limits the rate of ICMP error packets to each desti­
       nation.  ICMP_REDIRECT and ICMP_DEST_UNREACH are also lim­
       ited 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  limi­
       tations  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 broadcast addresses.

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

       ICMP_REDIRECT packets are not sent when Linux is not  act­
       ing 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 mil­
       liseconds 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.


       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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