ICMP(7)                   Linux Programmer's Manual                  ICMP(7)

NAME         top

       icmp - Linux IPv4 ICMP kernel module.

DESCRIPTION         top

       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

       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

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

       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

       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

       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

                   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
              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         top

       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

NOTES         top

       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

       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         top


       RFC 792 for a description of the ICMP protocol.

COLOPHON         top

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       latest version of this page, can be found at

Linux                            2015-04-19                          ICMP(7)