Check connection with timeout, script telnet web server in script

[bsddaemon]

Hi guys, Im having trouble finding the way to:

Check the connection if it has been established with timeout: ​

1. Create the connection
2. Check if it has been established
3. If yes, then do something
4. If no, for timeout = 5 seconds, exit with error code.

Netcat sounds like a perfect tools for this task:

nc -w 5 domain.com port

But the timeout switch takes no effect. I googled and many others reported the same symptom

Script to telnet web server​

Please see the attachment, I cant post here, for some reasons the forum complained the code with the "bad request" error

A similar script I wrote before was working, but unfortunately I forgot to keep copy of it. This time, it doesnt work. I must have syntax errors somewhere.

 

[DutchDaemon]

You could try net/tcping from ports.

# tcping -t 5 www.google.com 80
www.google.com port 80 open.
# echo $?
0

# tcping -t 5 www.google.com 90
www.google.com port 90 user timeout.
# echo $?
2

Easy to use in scripts:

if ( tcping -q -t 5 www.google.com 80 ); then echo alive; else echo dead; fi
alive

if ( tcping -q -t 5 www.google.com 90 ); then echo alive; else echo dead; fi
dead

Don't use 80 as a check port when you're behind a proxy server

 

[bsddaemon]

Thanks, but the script will be run in web server, so I cant install any 3rd party application.

 

[DutchDaemon]

When you say that the timeout switch has no effect, what do you mean? That you never get your prompt back unless you ^C? Try adding the -o switch. Oh, there's also the -z flag to consider.

 

[bsddaemon]

Hi DutchDaemon,

My web server running Linux, there is no -o switch for netcat

Let me elaborate a bit more in details. In my case, it takes about 2 seconds to establish a new connection (a deamon with listening port)

I run this command in background mode:

$ command to create new connection &

Then I run netcat. Instead of keeping trying to connect for 5 seconds, netcat quits immediately if it fails at the first attempt:

nc -zw 5 hostname port

Thats why I said the timeout feature takes no effect.

 

[bsddaemon]

I came up with this function:

repeatCMD () {
    count="0"

    $@
    while [ $? -ne 0 ]; do
        sleep 1
        echo " ===> I try again..."
        count="$(($count + 1))"
        if [ $count -eq 5 ]; then
            echo " ===> ERR: Max number of retry. Quitting..."
            exit 127
        fi
        $@
    done
}

Now I can run the command:

repeatCMD nc -zw 5 hostname port

It works (doesnt exactly do the same job as timeout, though)

But Im sure you guys have a more elegant solution. Seriously, 12 line of code for the timeout switch doesnt sound right.

And I havent recalled what I have done with the telnet script :S