I recently played around with a new Synology NAS and got the crazy idea to run Plex on the system, but from a Docker container (there may be a post about that soon). The setup process should have been simple. Most of the environment settings and port publishing were configured automatically. I just had to add a volume mapping to my media share. Except the Plex container would not start.
Checking the logs for Docker showed that Plex could not bind to port UDP 1900 because it was already in use.determined the problem was caused by the container trying to bind to port UDP 1900. So what was port 1900 used for, and what was using it on the Synology NAS? The first stop in the hunt was the support site for Plex. Their documentation listed port 1900 being used for the built-in “DLNA Server.” DLNA was used as a way to share media between devices. An early example was streaming videos or music from a personal computer to a PlayStation 3. Based on other research I had done in the past, I knew DLNA relied on the Universal Plug and Play, a set of network protocols designed to help network devices find each other on a network.
Now that I knew I was looking for something UPnP related, it was time to dig into the Synology NAS and find the service using port 1900. The first stop was the CLI and the netstat command. There I discovered port 1900 was being used by “MINISSDPD.” SSDP (Simple Service Discovery Protocol) was a protocol used by UPnP to handle the discovery. With the name of the process known, I visited the Synology NAS’s Task Manager to find what service was using minissdpd. The answer: Windows network discovery.
Jumping over to the Synology Control Panel, I thought the solution to all of this was to disable WS-Discovery under the File Services > SMB section. I was wrong. The answer was to disable SSDP under the File Services > Advanced section. With SSDP disabled, I started the Plex docker container and imported my media.