Add section on outgoing IP/firewall to Troubleshooting

mkdocs/docs/HPC/troubleshooting.md

@@ -291,6 +291,27 @@ ssh -vvv {{ userid }}@{{ loginnode }}
 and include the output of that command in the message.
 {% endif %}
 
+{% if site == gent %}
+## Issues reaching servers from HPC infrastructure
+
+If you have to reach license servers from {{ hpcinfra }} systems or you 
+have to directly load some database here, then it might not work
+(you will get errors like `Network connection timed out` or `Network connection refused`).
+
+Our firewall rules are quite strict, we only allow outging ports 22 (SSH protocol),
+ 80 (HTTP protocol), and 443 (HTTPS protcol), so if your download or license server 
+requires other ports, then we should make a modification in our firewall settings.
+For this, please contact us via <{{ hpcinfo }}>, and send the destination IP and ports. 
+(We only open our firewall for static IP addresses).
+
+It is possible that the other end also has firewall, or that the license server restricts 
+the incoming IP addresses. In this case you need the outgoing IP address of our systems,
+which is either of:
+
+- `157.193.240.251` (hostname `nathpca001.ugent.be`), or
+- `157.193.241.251` (hostname `nathpcb001.ugent.be`)
+{% endif %}
+
 ## Security warning about invalid host key
 
 If you get a warning that looks like the one below, it is possible that