Checking Connection without Telnet

Some of the minimal Linux distributions have no telnet client utility or similar utilities such as nc,ncat unless you install it. Most of time we need to do troubleshooting to check connection if  server/service is accessible. Do not worry–You still have mechanism inside the Linux kernel without installing above utilities. Take a look at below examples and change it accordingly for your case. Checking TCP connection:

root@debian2:# echo > /dev/tcp/8.8.8.8/53 && echo "PORT IS OPEN" || echo "PORT IS NOT OPEN"
PORT IS OPEN
root@debian2:~# echo > /dev/tcp/google.com/80 && echo "PORT IS OPEN" || echo "PORT IS NOT OPEN"
PORT IS OPEN
root@debian2:~# echo > /dev/tcp/google.com/443 && echo "PORT IS OPEN" || echo "PORT IS NOT OPEN"
PORT IS OPEN
#I have to send signal SIGINT.
root@debian2:~# echo > /dev/tcp/google.com/123 && echo "PORT IS OPEN" || echo "PORT IS NOT OPEN"
^C-su: connect: Network is unreachable

Checking UDP Connection:

root@debian2:~# echo > /dev/udp/0.pool.ntp.org/123 && echo "PORT IS OPEN" || echo "PORT IS NOT OPEN"
PORT IS OPEN

Note As UDP connnectionless protocol, even port is closed or unreachable it will print “PORT IS OPEN”. Best way to run tool tcpdump in the target machine if really packet received.