add note regarding firewall port

admin-guide/custom_repositories.md

@@ -37,6 +37,9 @@ kinit <username>@D.PSI.CH
 kinit
 ```
 
+```{note}
+In case you are in a firewall protected zone, make sure that there is a rule to access lx-fs on TCP port 2049!
+```
 
 
 ## Overview