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NAME

       icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.

DESCRIPTION

       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 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.

   /proc interfaces
       ICMP  supports  a  set  of  /proc  interfaces to configure some global IP parameters.  The
       parameters  can  be  accessed   by   reading   or   writing   files   in   the   directory
       /proc/sys/net/ipv4/.   Most  of  these  parameters  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.

       icmp_destunreach_rate (Linux 2.2 to 2.4.9)
              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.

       icmp_echo_ignore_all (since Linux 2.2)
              If this value is nonzero, Linux will ignore all ICMP_ECHO requests.

       icmp_echo_ignore_broadcasts (since Linux 2.2)
              If this value is nonzero, Linux will ignore all ICMP_ECHO packets sent to broadcast
              addresses.

       icmp_echoreply_rate (Linux 2.2 to 2.4.9)
              Maximum  rate  for  sending  ICMP_ECHOREPLY packets in response to ICMP_ECHOREQUEST
              packets.

       icmp_errors_use_inbound_ifaddr (Boolean; default: disabled; since Linux 2.6.12)
              If disabled, ICMP error messages are sent with the primary address of  the  exiting
              interface.

              If enabled, the message will be sent with the primary address of the interface that
              received the packet that caused the ICMP error.  This is  the  behavior  that  many
              network  administrators  will  expect  from  a  router.   And it can make debugging
              complicated network layouts much easier.

              Note that if no primary address exists for the interface selected, then the primary
              address of the first non-loopback interface that has one will be used regardless of
              this setting.

       icmp_ignore_bogus_error_responses (Boolean; default: disabled; since Linux 2.2)
              Some routers violate RFC1122 by sending bogus responses to broadcast frames.   Such
              violations are normally logged via a kernel warning.  If this parameter is enabled,
              the kernel will not give such warnings, which will avoid log file clutter.

       icmp_paramprob_rate (Linux 2.2 to 2.4.9)
              Maximum rate for sending ICMP_PARAMETERPROB packets.  These packets are sent when a
              packet arrives with an invalid IP header.

       icmp_ratelimit (integer; default: 1000; since Linux 2.4.10)
              Limit  the  maximum rates for sending ICMP packets whose type matches icmp_ratemask
              (see below) to specific targets.  0 to disable any limiting, otherwise the  minimum
              space between responses in milliseconds.

       icmp_ratemask (integer; default: see below; since Linux 2.4.10)
              Mask made of ICMP types for which rates are being limited.

              Significant bits: IHGFEDCBA9876543210
              Default mask:     0000001100000011000 (0x1818)

              Bit definitions (see the kernel source file include/linux/icmp.h):

                  0 Echo Reply
                  3 Destination Unreachable *
                  4 Source Quench *
                  5 Redirect
                  8 Echo Request
                  B Time Exceeded *
                  C Parameter Problem *
                  D Timestamp Request
                  E Timestamp Reply
                  F Info Request
                  G Info Reply
                  H Address Mask Request
                  I Address Mask Reply

              The  bits marked with an asterisk are rate limited by default (see the default mask
              above).

       icmp_timeexceed_rate (Linux 2.2 to 2.4.9)
              Maximum rate for sending ICMP_TIME_EXCEEDED packets.  These  packets  are  sent  to
              prevent loops when a packet has crossed too many hops.

VERSIONS

       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

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  the  Epoch,
       1970-01-01 00:00:00 +0000 (UTC).

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

SEE ALSO

       ip(7)

       RFC 792 for a description of the ICMP protocol.

COLOPHON

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