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 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 in‐
dividual 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.

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 pro‐
grams.

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.