Using telnet to troubleshoot connectivity issues on a port

Introduction

Telnet is a command line utility that can be used to test the connectivity of a specific port.

Procedure

Telnet is provided by most operating systems and can be installed using your operating systems package manager:

Ubuntu

apt install telnet

CentOS 7

yum install telnet

CentOS 8 or AlmaLinux

dnf install telnet

Telnet accepts an IP or host along with a port. If the connection is blocked, you will get either a connection refused or a timeout.

[root@host ~]# telnet example.tld 25
telnet: connect to address 192.168.1.1: Connection refused

[root@host ~]# telnet host.example.tld 25
telnet example.tld 25
telnet: connect to address 10.0.0.1: Connection timed out

If the connection is successful, telnet will say connected and potentially return output in the below example, port 25 was used, so the SMTP banner returns:

[root@host ~]# telnet host.example.tld 25
Connected to host.example.tld.
Escape character is '^]'.
220-host.example.tld ESMTP Exim 4.95 #2 Wed, 11 Jan 2023 12:40:54 +0000
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.

Telnet will use the system's primary IP address by default. You can use the -b flag to test telnet connections from a specific IP address on your server. This is useful if you are sending mail from a different IP address:

[root@host ~]# telnet -b 10.0.0.1 host.example.tld 25
Connected to host.example.tld.
Escape character is '^]'.
220-host.example.tld ESMTP Exim 4.95 #2 Wed, 11 Jan 2023 12:40:54 +0000
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.

To exit a telnet session, use the CTRL+] keyboard combination, hit enter, then type close and hit enter again:

[root@host ~]# telnet host.example.tld 25 
Connected to host.example.tld.
Escape character is '^]'.
220-host.example.tld ESMTP Exim 4.95 #2 Wed, 11 Jan 2023 12:40:54 +0000
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
^]

telnet> close
Connection closed.