telnet command with custom timeout duration

Question

I'm trying to execute telnet command and getting the out put on shell script. But when the port not open or the ip not exist. The command take too much time to give the response. Will it be possible to limit the maximum try time on telnet?

telnet host port

What OS/distribution and which version of telnet (there are many variants in the world) are you using? Are you allowed to use other programs than telnet, for example netcat? — yaegashi, Aug 21 '15 at 10:04
Oracle enterprise linux. And this is for some big company. They won't easily allow to install new tools. Need to use some thing in the server already. — sugunan, Aug 21 '15 at 10:06
Related and maybe useful: http://stackoverflow.com/q/1128011/4937930 — yaegashi, Aug 21 '15 at 10:24
nc not available in the server and they wont allow to install it also. — sugunan, Aug 21 '15 at 10:29
`nmap` available on server ? Or just write a simple `perl` script, e.g. http://www.perlmonks.org/?node_id=571066 — steve, Aug 21 '15 at 10:37
Please read answers other than the accepted one in the link above. — yaegashi, Aug 21 '15 at 10:39

score 16 · Answer 1 · edited Apr 14 '18 at 14:49

16

This will run for no more than 2 secounds:

[]# echo quit | timeout --signal=9 2 telnet [SERVER] [PORT]

edited Apr 14 '18 at 14:49

Jeff Schaller

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answered Apr 14 '18 at 13:36

user286146

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score 9 · Answer 2 · answered Jan 09 '20 at 05:32

9

You can consider the neater command netcat aka nc as discussed here

echo 'QUIT' | nc -w SECONDS YOUR_HOST PORT; echo $?

answered Jan 09 '20 at 05:32

Nam G VU

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This is a great one, thanks for this. I hate when can't quit `telnet` with even a `Ctrl + ]` nor `Ctrl + 5`... – antivirtel Jan 19 '22 at 18:06

score 6 · Answer 3 · edited Apr 16 '19 at 15:42

6

Do you necessary have to run telnet? I think timeout(1) will help you.

Alternative #1: Use timeout, which is part of GNU coreutils. This won't confirm if you were able to establish a connection or not.

TIMEOUT=10 timeout $TIMEOUT telnet <host> <port>

Alternative #2: Use Bash/Perl/Python to solve the same problem, with more or less granularity.

A few examples:

Bash: TIMEOUT=10; telnet <host> <port> 2>&1 >/dev/null & WPID=$!; sleep $TIMEOUT && kill $! & KPID=$!; wait $WPID

Perl: perl -e "alarm 10; exec @ARGV" "telnet my.server.com 23"

Python: python -c 'import telnetlib;HOST="my.server.com"; PORT=23; TIMEOUT=10; PATTERN="Login: "; t = telnetlib.Telnet (HOST, PORT); t_read = t.read_until(PATTERN, TIMEOUT); print(repr(t_read))'

edited Apr 16 '19 at 15:42

Rui F Ribeiro

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answered Jun 21 '17 at 16:29

William Sandin

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as always the Perl based cmd is the shortest one ;o) Thanks ! – Yordan Georgiev Mar 02 '22 at 09:51
You have some extra stuff in the BASH that I don't see the use for, for example KPID is not used – Mike Q Jan 31 '23 at 17:36

score 3 · Answer 4 · answered Feb 05 '17 at 22:24

If scripting, you can run the telnet in background, and then if it's still running after a delay you can kill it. Example:

{ printf "HEAD /\n\n"; sleep 1; } | telnet hostname port & sleep 2 && kill %1 2>/dev/null && echo "timeout"

This waits 2 seconds before killing the backgrounded telnet.

score 0 · Answer 5 · answered Jan 31 '23 at 17:41

Check if Host is reachable or port is reachable/refused or host is up

NOTE: this script is just providing visual feedback.

Example using BASH v4+ where we read the hosts from a list and use the port from that list. In this example I am just testing the port, and if it can reach the host etc.

#!/bin/bash
declare -a remote_hosts=("10.10.236.221:8080" "10.10.236.222:22")
for each in ${remote_hosts[@]} ;do

        ip=$(echo $each | cut -d ':' -f1)
        port=$(echo $each | cut -d ':' -f2)
        echo "Connecting ${ip}:${port} .."
        telnet $ip $port & WPID=$!
        sleep 3

        if ps -p $WPID > /dev/null ;then
            echo "FAILED TO REACH ($WPID is running)"
            kill $! 2>&1 >/dev/null
            wait $WPID 2>&1 >/dev/null
        fi
done

telnet command with custom timeout duration

5 Answers5