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

Answer 1

This will run for no more than 2 secounds:

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

Answer 2

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

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

Answer 3

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))'

Answer 4

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.

Answer 5

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