How to test whether a port is opened outbound in my firewall.

Introduction

Testing whether your server can connect outbound on a port can be very important for troubleshooting a wide range of issues.  Including whether SMTP is functional or whether or not your host can connect to MySQL remotely.

Procedure

You can use telnet to attempt to connect to a port on a remote host.  You will see "Connected to host" if it is successful.

[root@184-94-197-2 ~]# telnet portquiz.net 25
Trying 52.47.209.216...
Connected to portquiz.net.
Escape character is '^]'.o

You can also use nmap to test connectivity for multiple ports at once.

[root@184-94-197-2 ~]# nmap portquiz.net -p 25,26,465,567,80,443

Starting Nmap 6.40 ( http://nmap.org ) at 2021-04-13 12:32 CDT
Nmap scan report for portquiz.net (52.47.209.216)
Host is up (0.11s latency).
rDNS record for 52.47.209.216: ec2-52-47-209-216.eu-west-3.compute.amazonaws.com
PORT STATE SERVICE
25/tcp open smtp
26/tcp open rsftp
80/tcp open http
443/tcp open https
465/tcp closed smtps
567/tcp open unknown

portquiz.net is a domain that exists for port testing and will have all basic ports open for inbound connections.

http://portquiz.net/

For information on telnet and nmap, please see the following.

https://linux.die.net/man/1/nmap

https://linux.die.net/man/1/telnet