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icmp(7) Miscellaneous Information Manual 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 parame‐
ters 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 af‐
fect 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 pack‐
ets.

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 in‐
terface.

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 net‐
work 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 vi‐
olations 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 pre‐
vent 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 Linux 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 ac‐
cepted only from the old gateway defined in the routing table and the redirect routes are ex‐
pired 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 net‐
stat(8) output with a zero inode.

man > icmp(7)