external_url setting doesn't work

The document mentions this setting:

external_url "http://gitlab.example.com"

Is this the correct syntax? Or is an = sign required in between?

When I add the setting as described in the doc, it doesn't seem to have any effect. I had to add a frontend nginx server to rewrite all response redirect requests. And embedded content such as emoticons and attachments don't work, because they have URLs not matching the external_url setting.