Dependency Proxy requests fail with "unsupported protocol scheme" error

Description

Dependency proxy requests are failing with an unsupported protocol scheme error, which shows up in the Workhorse logs:

SendURL: Do request: Get <Dependency Proxy URL>: unsupported protocol scheme ""

This causes issues when pulling Docker images from the dependency proxy.

Impacted offerings:

GitLab Self-Managed

Impacted versions:

13.6.0 and up

Solution

This error can have multiple causes, and the solution depends on the root cause. Troubleshooting steps might include:

Run the Dependency Proxy migration process (sudo gitlab-rake "gitlab:dependency_proxy:migrate") to regenerate the dependency proxy cache.
If your container registry is behind a reverse proxy on a separate node, set the registry['token_realm'] configuration option for the registry.

Cause

This error is most commonly caused by dependency proxy cache corruption. The dependency proxy cache can become corrupted when moving from local to object storage without running the dependency proxy migration process.

Otherwise, the unsupported protocol scheme error can be caused by an incorrect configuration when using the container registry on a separate node, or rarely network issues.