I could SSH externally into VM, but suddenly stopped working, but worked again temporarily on router reboot. Why does this happen?

Question

Please be gentle; I'm completely new to network management.

Background: I am running an Ubuntu Desktop 20.x VM on a Synology DS916. Some pertinent details are as follows:

• The VM has a static private IP address, say 192.168.0.123
• The router, which has public IP address PUBLIC.IP, is forwarding port 2200 to the VM's port 22, and port 8800 to the VM's port 80 using TCP protocol. To the best of my knowledge, there is no clashing/duplicate private IPs or port forwarding clashes, including any clients using DHCP leases
• ssh is definitely enabled on the VM as per sudo systemctl status ssh with no error messages
• ufw is not enabled

The issue: After setting the VM up, I was able to SSH in perfectly fine into the VM externally, using ssh -p 2200 [email protected]. However, about a day later, I can't SSH in any more, with an Operation timed out message when I tried. Restarting the VM and ssh service on the VM did not help.

HOWEVER, rebooting my router fixed it, in that I could SSH in again ... but after a few hours, it stopped working again. Here are some other findings:

• While I can't access the VM externally, I was still able to SSH in from the local network with ssh [email protected]
• I can't find any info on the router's config pages suggesting that it was blocking any traffic (it is set to "allow all traffic")
• Router is a Netgear V7610
• During the time I was able to SSH in externally, I also hosted a web page on 192.168.0.123:80, which I was able to access externally using PUBLIC.IP:8800. When the SSH stopped working, the webpage was also unable to be accessed externally any more.

My question: Given that the problem temporarily fixed itself when I rebooted my router, I suspect that the router is the one causing the issue? It seems strange that traffic was OK for a few hours but would suddenly stop. I have absolutely no clue why and would appreciate any insight anyone else might have in to this issue.

Answer 1

I would comment on the post above rather posting a solution but do not have enough reputation...

On the router reserve the MAC address against the IP address you statically assigned against the VM (Your VM software should tell you what the MAC address is - usually a virtual one). I speculate another device is being allocated that IP via DHCP and therefore causing an internal IP conflict.

Another way around this is to allocate an internal IP out of your DHCP scope so for example... if your DHCP server is to use addresses between 192.168.1.100 and 192.168.1.200 use IP address 192.168.1.50 and port forward against that IP address instead.