Name
icmp, IPPROTO_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.
Sysctls
ICMP supports a sysctl interface to configure some
global IP parameters. The sysctls can be accessed by
reading or writing the /proc/sys/net/ipv4/* files or with the
sysctl(2) interface. Most
of these sysctls 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 x86 and about 1ms on alpha
and IA64.
- icmp_destunreach_rate
-
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
-
If this value is non-zero, Linux will ignore all
ICMP_ECHO requests.
- icmp_echo_ignore_broadcasts
-
If this value is non-zero, Linux will ignore all
ICMP_ECHO packets sent
to broadcast addresses.
- icmp_echoreply_rate
-
Maximum rate for sending ICMP_ECHOREPLY packets in response
to ICMP_ECHOREQUEST
packets.
- icmp_paramprob_rate
-
Maximum rate for sending ICMP_PARAMETERPROB packets. These
packets are sent when a packet arrives with an
invalid IP header.
- icmp_timeexceed_rate
-
Maximum rate for sending ICMP_TIME_EXCEEDED packets. These
packets are sent to prevent loops when a packet has
crossed too many hops.
VERSIONS
Support for the ICMP_ADDRESS
request was removed in 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
only accepted 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
January 1, 1970.
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
ip(7)
RFC 792 for a description of the ICMP protocol.
This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
Permission is granted to distribute possibly modified copies
of this page provided the header is included verbatim,
and in case of nontrivial modification author and date
of the modification is added to the header.
$Id: icmp.7,v 1.6 2000/08/14 08:03:45 ak Exp $
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