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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  non-zero,  Linux  will ignore all ICMP_ECHO
              requests.

       icmp_echo_ignore_broadcasts (since Linux 2.2)
              If this value is  non-zero,  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 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.

SEE ALSO

       ip(7)

       RFC 792 for a description of the ICMP protocol.

COLOPHON

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