Custom domains on the Wikimedia Cloud VPS web proxy

Published by Taavi Väänänen on November 1, 2024.

The shared web proxy used on Wikimedia Cloud VPS now has technical support for using arbitrary domains (and not just wmcloud.org subdomains) in proxy names. I think this is a good example of how software slowly evolves over time as new requirements emerge, with each new addition building on top of the previous ones.

According to the edit history on Wikitech, the web proxy service has its origins in 2012, although the current idea where you create a proxy and map it to a specific instance and port was only introduced a year later. (Before that, it just directly mapped the subdomain to the VPS instance with the same name).

There were some smaller changes in the coming years like the migration to acme-chief for TLS certificate management, but the overall logic stayed very similar until 2020 when the wmcloud.org domain was introduced. That was implemented by adding a config option listing all possible domains, so future domain additions would be as simple as adding the new domain to that list in the configuration.

Then the changes start becoming more frequent:

• In 2022, for my Terraform support project, a bunch of logic, including the list of supported backend domains was moved from the frontend code to the backend. This also made it possible to dynamically change which projects can use which domains suffixes for their proxies.
• Then, early this year, I added support for zones restricted to a single project, because we wanted to use the proxy for the *.svc.toolforge.org Toolforge infrastructure domains instead of coming up with a new system for that use case. This also added suport for using different TLS certificates for different domains so that we would not have to have a single giant certificate with all the names.
• Finally, the last step was to add two new features to the proxy system: support for adding a proxy at the apex of a domain, as well as support for domains that are not managed in Designate (the Cloud VPS/OpenStack auth DNS service). In addition, we needed a bit of config to ensure http-01 challenges get routed to the acme-chief instance.