[Rails] My Host

I need to get some absolute redirects working -- basically I set callback hooks to external services, but I want these hooks to use my configured CNAME's.  I'm trying to find a way so that my app doesn't have to know about these CNAME's.  So I'm looking at the request in the controller to try and get the base URL where I'm running.

To test, I'm using localtunnel.  I have found what I'm looking for in 3 places, and I'm wondering if one is better than another or if one is more reliably set than another.

request.env["SERVER_NAME"]

request.host

request.env["HTTP_HOST"]

All three of these have the value I'm looking for (e.g. "3eym.localtunnel.com") .  Is there any difference between them?  Should I favor one over another?

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