Using telnet to Test Network Connectivity

An easy way to tell if a remote server is listening on a specific TCP port is to use the telnet command. By default, telnet will try to connect on TCP port 23, but you can specify other TCP ports by typing them in after the target IP address. HTTP uses TCP port 80, HTTPS uses port 443.

Here is an example of testing server 192.168.1.102 on the TCP port 22 reserved for SSH:

     [root@bigboy tmp]# telnet 192.168.1.102 22

When using telnet TRoubleshooting, here are some useful guidelines to follow that will help isolate the source of the problem:

• Test connectivity from the remote PC or server.

• Test connectivity on the server itself. Try making the connection to the loopback address as well as the NIC IP address. If the server is running a firewall package, such as the Linux iptables software, all loopback connectivity could be allowed, but connectivity to desired TCP ports on the NIC interface may be blocked. Further discussion of the Linux iptables package is covered in a later section.

• Test connectivity from another server on the same network as the target server. This helps eliminate the influence of any firewalls protecting the entire network from outside.