ICMP(7) 						     Linux Programmer's Manual							   ICMP(7)

NAME

       icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.

DESCRIPTION

       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.

       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.

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.

VERSIONS

       Support for the ICMP_ADDRESS request was removed in 2.2.

       Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.

SEE ALSO

       ip(7)

       RFC792 for a description of the ICMP protocol.

Linux Man Page							    1999-04-27								   ICMP(7)