Recently I was working on a Rails application on Heroku living behind a reverse proxy. This application serves requests coming to a specific folder on the target domain. For it to correctly generate full URLs, you have to somehow tell this app the hostname you want it to use. In Rails, you can configure a hostname in the environment config file, but that’s a static value, which has to be maintained and changed per environment. Also it does not work well if you want to access the application from multiple domains.
It’s much better to be able to set something up on the proxy itself. For this reason the X-Forwarded-Host HTTP header exists. Rails–being a good web citizen–supports it out of the box.
Before I learned about this header, I even implemented my own middleware to deal with this issue and a custom header. I was able to dump that extra code once I stumbled at this header.