How do you know which firewall I am using Linux?

If your firewall uses the built-in kernel firewall, then sudo iptables –n -L will list all the iptables contents. If there is no firewall the output will be mostly empty. Your VPS may have ufw already installed, so try ufw status .

How do I check if firewall is blocking Linux?

If you have access to the system and you want to check whether it’s blocked or open, you can use netstat -tuplen | grep 25 to see if the service is on and is listening to the IP address or not. You can also try to use iptables -nL | grep <port number> to see if there is any rule set by your firewall.