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icmp(7) Miscellaneous Information Manual icmp(7)
icmp - Linux IPv4 ICMP kernel module.
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 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
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 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 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 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
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 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
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.
Support for the ICMP_ADDRESS request was removed in Linux 2.2.
Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.
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.
ip(7), rdisc(8)
RFC 792 for a description of the ICMP protocol.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
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part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 icmp(7)
Pages that refer to this page: pmdanetcheck(1), ip(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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