Backport #27655 by @wolfogre
When `webhook.PROXY_URL` has been set, the old code will check if the
proxy host is in `ALLOWED_HOST_LIST` or reject requests through the
proxy. It requires users to add the proxy host to `ALLOWED_HOST_LIST`.
However, it actually allows all requests to any port on the host, when
the proxy host is probably an internal address.
But things may be even worse. `ALLOWED_HOST_LIST` doesn't really work
when requests are sent to the allowed proxy, and the proxy could forward
them to any hosts.
This PR fixes it by:
- If the proxy has been set, always allow connectioins to the host and
port.
- Check `ALLOWED_HOST_LIST` before forwarding.
Co-authored-by: Jason Song <i@wolfogre.com>
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