icmp(7) — Linux manual page


icmp(7)             Miscellaneous Information 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

       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 Linux 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

       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 Linux 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

              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 Linux 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
                   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 Linux 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 Linux 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

       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         top

       ip(7), rdisc(8)

       RFC 792 for a description of the ICMP protocol.

Linux man-pages (unreleased)   2024-05-02                        icmp(7)

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