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.
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.