110 Known issue: HTTP 403 error in "bundle install --verbose" output

When running bundle install --verbose on Bundler versions 1.12+, you might see errors of the form

CopyHTTP 403 Forbidden https://username:password@gems.railslts.com/versions

These errors are not fatal, Rails LTS should still be installed correctly. This has been tested with all Bundler versions 1.12.x and 1.13.x.

Background:

Since version 1.12, Bundler includes a performance optimization called "compact gem index". Our gem server does not support this.

When trying to access this new index our web server will return a 403. Bundler will log this to its output, but then fall back to using the old style index and proceed as usual.