Add dev server firewall configuration for EL distros (#6772)

* Add dev server firewall configuration for EL distros

* Fix typo in previous

* Indent the firewall block in install docs

docs/installation/3-netbox.md

@@ -267,6 +267,13 @@ Starting development server at http://0.0.0.0:8000/
 Quit the server with CONTROL-C.
 ```
 
+!!! note
+    By default RHEL based distros will likely block your testing attempts with firewalld. The development server port can be opened with `firewall-cmd` (add `--permanent` if you want the rule to survive server restarts):
+
+    ```no-highlight
+    firewall-cmd --zone=public --add-port=8000/tcp
+    ```
+
 Next, connect to the name or IP of the server (as defined in `ALLOWED_HOSTS`) on port 8000; for example, <http://127.0.0.1:8000/>. You should be greeted with the NetBox home page.
 
 !!! danger