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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 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.

       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 i386 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 pack-
              ets 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.

       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

       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.


       RFC 792 for a description of the ICMP protocol.

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             1999-04-27                           ICMP(7)