I just noticed that my server is being blocked for rsync from a firewall outside of my server, so I can't rsync to any target. Now, I would also like to know what are all the ports that are being blocked by that firewall.
Is there any way to use nmap to do that? I know I can use nmap to scan the opened ports in a specific target, but what I want is to know what ports are closed in my server to send packets out.
No, you can not nmap to scan one computer from the same computer.
By definition, packets won't travel, and packets traveling is the whole base of the internet.
You need a router, printer, light-bulb or external computer that could run some commands and use it to look back to that computer.
I believe that you can send packets going out (to the outside) from any port to see which ports are being blocked in the outgoing direction without any security issue. But even to do that, you need some external address to send your packets to.
Trying to scan from outside to such firewalled computer may easily be seen as an attack and you may get banned or blocked even more than you are now. To actually perform such scan from the outside the best is to inform and ask for permission from the network managers in such system.
