How to Test and Debug Linux Network Ports with Telnet, Netcat, and Netstat

Introduction

When debugging Linux network issues, it is common to need to verify whether a specific port is reachable.

Telnet command

Usage: telnet serverIP port. The server IP and port must be separated by a space.

If the port is closed, telnet reports a connection refusal.

ggd@ubuntu:~$ telnet 192.168.70.1 20
Trying 192.168.70.1...
telnet: Unable to connect to remote host: Connection refused
ggd@ubuntu:~$

If the port is open, telnet connects successfully.

ggd@ubuntu:~$ telnet 192.168.70.1 8080
Trying 192.168.70.1...
Connected to 192.168.70.1.
Escape character is '^]'.

After a successful connection you can send and receive data through the terminal.

Exiting telnet

Two methods: press Ctrl+ ]+ then Ctrl+ D, or press Ctrl+ ]+ followed by the command quit .

Netcat (nc) command

Usage: nc -v serverIP port. The -v flag enables verbose output.

When the port is closed:

ggd@ubuntu:~$ nc -v 192.168.70.1 20
nc: connect to 192.168.70.1 port 20 (tcp) failed: Connection refused
ggd@ubuntu:~$

When the port is open:

ggd@ubuntu:~$ nc -v 192.168.70.1 8080
Connection to 192.168.70.1 8080 port [tcp/http-alt] succeeded!

nc also allows sending and receiving data after a successful connection.

Exiting nc

Press Ctrl+D or Ctrl+C to terminate the command.

Limitations on embedded systems

On some development boards the busybox version may lack full telnet or nc options, resulting in missing parameters or limited output.

Verifying with netstat

If telnet or nc only shows a blocked state, use netstat -atpn to view actual connection details.

Locate the tested IP and port in the Foreign Address column and check that the State is ESTABLISHED , which indicates the port is functional.

netstat is a useful tool for monitoring TCP/IP connections, showing routing tables, active connections, and socket states. Common flags include:

-a: display all sockets

-t: show TCP connections

-p: display process information

-n: show numeric addresses without DNS lookup

Important TCP states include LISTENING, SYN‑SENT, SYN‑RECEIVED, ESTABLISHED, FIN‑WAIT‑1, FIN‑WAIT‑2, CLOSE‑WAIT, CLOSING, LAST‑ACK, TIME‑WAIT, and CLOSED.