4.13 Checking the Time
4.13.1 Description
A quick way to check and verify the time on remote machines over
the network.
Example
Flavors: AT&T, BSD
Shells: All
Syntax:
telnet host [port]
The time of any machine can be checked simply by issuing a Telnet
session to the port 13. This port is known as the
daytime port. It works like this:
% telnet rocket 13
Trying 209.10.11.102
Connected to rocket.
Escape character is '^]'.
Sun Oct 18 11:45:02 1998
Connection closed by foreign host.
For this to work, a couple of things need to be set up on the
remote machine you are monitoring the time on. The /etc/services file needs
to have a TCP and UDP entry made for daytime at port 13.
# vi /etc/services
daytime 13/tcp
daytime 13/udp
The /etc/inetd.conf file needs to be edited so that the daemon
wakes up and a connection can be established to port 13.
# vi /etc/inetd.conf
daytime stream tcp nowait root internal
daytime dgram udp wait root internal
When /etc/inetd.conf is edited, don't forget to get the process
ID of inetd and restart the daemon.
# ps -e | grep inetd
199 ? 0:00 inetd
# kill -HUP 199
Now when Telnet is initiated from a remote system you should see
something similar to
% telnet xinu 13
Trying 209.10.11.104
Connected to xinu.
Escape character is '^]'.
Sun Oct 18 11:52:13 1998
Connection closed by foreign host.
There are a couple of ways to expand this to a script and check
the time of multiple machines in your environment. The quick and dirty way is
to write a script called timecheck:
% vi timecheck
telnet rocket 13
telnet xinu 13
telnet ugu 13
Line 1: Telnet to the first machine and get the time.
Line 2: Telnet to the second machine and get the time.
Line 3: Telnet to the third machine and get the time.
And so on...
The
result is a bit messy looking with the output having the following appearance:
% ./timecheck
Trying 209.10.11.102...
Connected to rocket.
Escape character is '^]'.
Sun Oct 18 14:07:08 1998
Connection closed by foreign host.
Trying 209.10.11.104...
Connected to xinu.
Escape character is '^]'.
Sun Oct 18 14:07:08 1998
Connection closed by foreign host.
Trying 209.10.11.105...
Connected to ugu.
Escape character is '^]'.
Sun Oct 18 14:07:10 1998
Connection closed by foreign host.
To clean up the output, the script could be rewritten to get rid
of the extra output and print the time information that you are interested in.
% vi timecheck2
rocket=`telnet rocket 13 | tail -1`
xinu=`telnet xinu 13 | tail -1`
ugu=`telnet ugu 13 | tail -1`
echo
echo "rocket: $rocket"
echo "xinu: $xinu"
echo "ugu: $ugu"
Lines 1-3: Get the time for the remote host and store it into
special variables.
Lines
4-6: Display the hostname and the current time of the system from the variables
that the times were put into.
% ./timecheck2
Connection closed by foreign host.
Connection closed by foreign host.
Connection closed by foreign host.
Connection closed by foreign host.
rocket: Sun Oct 18 14:20:11 1998
xinu: Sun Oct 18 14:20:11 1998
ugu: Sun Oct 18 14:20:12 1998
Due to the way that Telnet and the telnetd daemon handle closing
a connection to a foreign host, the message "Connection closed by foreign
host" cannot be extracted from the tail. This is why the Telnet commands
were executed, passed into a variable, and echoed out later.
Reason
There is more than one reason why a system clock can be set incorrectly.
These range from a system staying up for long periods of time without rebooting
to replacing hardware in a system. The time should be checked regularly.
Real World Experience
Programmers and developers are strictly dependent on the system's
internal clock being correct for developing applications. It has been shown
that even if systems are locked to one designated time server, the systems that
are slaves to the time server can eventually provide false dates. When it is
so critical to have the system clock be right, the extra step should be well
worth it.
Other Resources
Man pages:
inetd, services,
telnet
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