icmp(7)

NAME

   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   Linux   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.

   ping_group_range (two integers; default: see below; since Linux 2.6.39)
          Range  of  the  group  IDs  (minimum  and  maximum  group   IDs,
          inclusive)  that  are  allowed to create ICMP Echo sockets.  The
          default is "1 0", which means no group is allowed to create ICMP
          Echo sockets.

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 accepted only 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

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.

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