ICMP(7)                   (2017-11-26)                    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 pro-
          cessed by the kernel too, even when passed to a user socket.

          Linux limits the rate of ICMP error packets to each destina-
          tion.  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 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
          i386 and about 1ms on alpha and ia64.

               Maximum rate to send ICMP Destination Unreachable pack-
               ets.  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 nonzero, Linux will ignore all
               ICMP_ECHO requests.

               If this value is nonzero, Linux will ignore all
               ICMP_ECHO packets sent to broadcast addresses.

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

     Linux 2.6.12)

               htm

          htmlmanrefstarticmp_ignore_bogus_error_responses(Boolean;default:disabled;sinceLinux2.2)    .}f Some routers violate RFC1122 by sending bogus
               responses to broadcast frames.  Such violations are
               normally logged via a kernel warning.  If this parame-
               ter is enabled, the kernel will not give such warnings,
               which will avoid log file clutter.

               Maximum rate for sending ICMP_PARAMETERPROB packets.
               These packets are sent when a packet arrives with an
               invalid IP header.

               htmlmanrefendicmp_ratelimit(integer;d
                  htmlmanrefstarticmp_ratemaskhtmlmanrefendicmp_ratemask

          htmlmanrefstarticmp_ratemask(integer;default:seebelow;sinceLinux2.4.10)
               .}f 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):
                      l l.  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

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                      Reply

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

               Maximum rate for sending ICMP_TIME_EXCEEDED packets.
               These packets are sent to prevent loops when a packet
               has crossed too many hops.

     2.6.39)

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