icmp(n) Tnm Tcl Extension icmp(n)
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NAME
icmp - Send ICMP messages to hosts on the Internet.
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DESCRIPTION
The icmp command allows to send Internet Control Messages (ICMP) (RFC 792, RFC 1122) to a list of hosts on the Internet. ICMP messages can
be used to check whether hosts are reachable. The icmp command can also be used to trace routes to Internet hosts by sending UDP datagrams
to unused UDP ports and interpreting the ICMP responses (Van Jacobsen and Steve Deering algorithm).
ICMP COMMAND
The icmp command always accepts a list of target hosts. ICMP messages are send to the targets in a round-robin fashion which allows to
check entire IP address ranges or a list of core routers efficiently. The user of the icmp command should be careful not to flood a network
with ICMP requests.
icmp [options]
Invoking the icmp command with options but without any command arguments allows to retrieve and change the default values. See the
description of supported options below. Default values are bound to a Tcl interpreter which allows to have multiple Tcl interpreter
with different defaults.
icmp [options] echo hosts
The icmp echo command can be used to test the reachability of IP devices by sending ICMP echo requests to the hosts. The command
returns a list of host / round trip time pairs. The round trip time is returned in milliseconds. A negative round trip time indi-
cates that a host did not respond in the timeout interval.
icmp [options] mask hosts
The icmp mask command sends ICMP mask requests and returns a list of host / netmask pairs. The mask will be set to 0.0.0.0 if a host
does not respond to ICMP mask requests.
icmp [options] timestamp hosts
The icmp timestamp command retrieves timestamps using the ICMP timestamp request. This command returns an estimate of the time dif-
ference between local time and the time on the hosts given in hosts. The command returns a list of host / time offset pairs. The
time offset is returned in milliseconds. An empty time offset indicates that a host did not respond in the timeout interval.
icmp [options] ttl num hosts
The icmp ttl command, which may not be supported on every operating system, sends a UDP packet to an unused port number with the
time to live field of the IP header set to num. This command returns a list of host / round trip time pairs, where the host is the
IP device being num hops away. The round trip time is returned in milliseconds. A negative round trip time indicates that a host did
not respond in the timeout interval.
icmp [options] trace num hosts
The icmp trace command works similar to the icmp ttl command but it always returns the destination address even when the responding
host chooses another interface with a different IP address to send the reply. This can be used to trace a route to a host since the
command returns the host that discards the packet if it does not reach the destination.
ICMP OPTIONS
The following options control how ICMP requests are send and how the icmp command deals with lost ICMP packets.
-timeout time
The -timeout option defines the time the icmp command will wait for a response. The time is defined in seconds with a default of 5
seconds.
-retries number
The -retries option defines how many times a request is retransmitted during the timeout interval. The default number of retries is
2.
-delay time
The -delay option defines the minimum delay between two outgoing ICMP packets. The delay time is defined in milliseconds with a
default delay of 0 milliseconds. Delays are useful to reduce the load on intermediate devices like gateways or bridges.
-size number
The -size option allows to control the size of ICMP packets. The number defines the size of ICMP packets in bytes with a default
size of 64 bytes. Sizes smaller than 64 bytes are silently rounded to 64 bytes.
BUGS
The icmp command requires the setuid root program ntping(8) because sending icmp packets requires access to raw sockets on most operating
systems.
SEE ALSO
scotty(1), ntping(8), Tnm(n), Tcl(n)
AUTHORS
Juergen Schoenwaelder <schoenw@cs.utwente.nl>
Tnm icmp(n)