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ICMP(7) Linux Programmer's Manual ICMP(7)
icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.
This kernel protocol module implements the Internet Control Message Protocol defined in
RFC792. It is used to signal error conditions and for diagnosis. The user doesn't inter-
act 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 pack-
ets 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 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.
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.
If this value is non-zero, Linux will ignore all ICMP_ECHO requests.
If this value is non-zero, Linux will ignore all ICMP_ECHO packets sent to broad-
Maximum rate for sending ICMP_ECHOREPLY packets in response to ICMP_ECHOREQUEST
Maximum rate for sending ICMP_PARAMETERPROB packets. These packets are sent when a
packet arrives with an invalid IP header.
Maximum rate for sending ICMP_TIME_EXCEEDED packets. These packets are sent to
prevent loops when a packet has crossed too many hops.
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 net-
stat(8) output with a zero inode.
Support for the ICMP_ADDRESS request was removed in 2.2.
Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.
RFC792 for a description of the ICMP protocol.
Linux Man Page 1999-04-27 ICMP(7)
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