Add ufw commands

src/linux.md

@@ -285,3 +285,20 @@ scp -r .\path\to\directory\* <user>@<server>:/path/to/remove/directory
 # Start a root shell. `exit` when done.
 sudo -s
 ```
+
+## Firewall (ufw)
+```bash
+# Show status of firewall. If active, also lists rules.
+sudo ufw status
+# Show rules even when ufw is inactive.
+sudo ufw show added
+
+# Allow by service.
+sudo ufw allow ssh
+# Allows 80 and 443.
+sudo ufw allow 'Nginx Full'
+
+# Allow port-number.
+sudo ufw allow <port-number>
+sudo ufw allow 8080
+```