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 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.
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 broadcast addresses.
Maximum rate for sending ICMP_ECHOREPLY packets in response to ICMP_ECHOREQUEST packets.
Maximum rate for sending ICMP_PARAMETERPROB packets. These packets are sent when a packet arrives with an invalid IP header.
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.
Support for the ICMP_ADDRESS request was removed in 2.2.
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