In our setup, I wanted to add authentication to a few dozen sub domains, but use a single oauth2proxy instance. Github Oauth makes this kind of gross, the callback must point to the same subdomain you're trying to authenticate. But it does allow something like /oauth2/callback/route.to.this.instead

In the end, to achieve what I wanted (a single oauth2proxy for multiple internal services) I had to - fork oauth2proxy and make a few small changes to the redirect-url implementation - create a small service with takes oauth.acme.co/oauth2/callback/subdomain.acme.co and redirects to subdomain.acme.co to comply with GitHub' oauth requirements - created a small reverse proxy in Go which does something similar to nginx_auth_request. I had a few specific reasons to do this (like proxying websockets and supporting JWT directly) https://gist.github.com/groob/ea563ea1f3092449cd75eeb78213cd...

I hope that someone ends up writing a k8s ingress controller specific to this use case.