I am running a quake 3 engine based game server (Wolfenstein: Enemy Territory) on Ubuntu 16.04 and I've ran into a fairly puzzling issue. A week ago players started having issues when connecting to the server. Basically the server tries to send a snapshot of the game state on client connect but this fails hundreds of times before succeeding slowing the average connection init time from ~few seconds to minutes.
These problems seem to be linked to only a single port. Moving the server to another port (default 27960 --> 27961) fixes the issue. It seems that some players are able to connect without issues but most of the players (from various countries, e.g. Finland, Spain, UK etc., not limited to a single ISP or country) it takes minutes.
I haven't touched any configurations related to firewall/network interface for months. I use fail2ban but it's using the default configuration (based on this https://plusbryan.com/my-first-5-minutes-on-a-server-or-essential-security-for-linux-servers). I tried disabling fail2ban but it has no effect.
I was wondering if there are any possible causes that could somehow cause packet loss on a single port?
