Learn how to troubleshoot TCP connectivity with telnet. Find if firewall rules are blocking your connections and breaking your applications.

1. Test TCP connectivity with
telnet
Live Demo

2. Telnet
Telnet is a protocol used on the Internet or local area networks to provide
a bidirectional interactive text-oriented communication facility using a
virtual terminal connection.
Source Wikipedia, Telnet
● Uses the connection oriented TCP protocol
● Replaced by SSH for security reasons

3. Client Server Applications
Client Server

4. Firewalls
Client ServerFirewall
X

5. Test firewalls with telnet
> telnet <destination_host> [destionation_port]
<destination_host> = FQDN or IP
[destination_port] = TCP/IP port

6. Successful telnet connection: no firewall rules
> telnet test.netbeez.net 20018
Trying 52.83.122.233
Connected to test.netbeez.net.
Escape character is '^]'.
_

7. Failed telnet connection: firewall rules (reject)
> telnet test.netbeez.net 20018
Trying 52.83.122.233...
telnet: connect to address 52.83.122.233: Connection refused
telnet: Unable to connect to remote host
> _

8. Failed telnet connection: firewall rules (drop)
> telnet test.netbeez.net 20018
Trying 52.83.122.233...
_