How to check blocked port of server.

Way 1: 

Using telnet command :

Syntax : telnet <Host_name>  <port_no>

telnet test.openspecimen.org 80

Use the above command from outside the server machine. If port 80 is open. "Connected to test.openspecimen.org." message will be displayed on the terminal.

Way 2 :

Listing down iptables rules. Below is the command to list iptables rules :

sudo iptables -L

Check if there is any rule set for port 80, in the same row if under 'Target' column DROP option is present then port 80 is blocked.

Way 3 :

The below commands will display open ports of the server.

Operating systemcommand
Ubuntusudo ufw status
CentOS / RHELsudo firewall-cmd --list-all