Ability to overwrite Host Header and use $s in values

[ryan-dyer-sp]

Is your enhancement request related to a problem? Please describe.
Yes. We have a vendor that we work with which expects us to setup reverse proxy for a specific path of our website to their server. They expect the Host header to align with $host, not with the Host of the proxy server we are directing requests to.

We have tried using a requestHeaderModifier as part of the HTTPRoute that we have setup for this proxy, but there are two issues. One is that the schema does not allow setting of the Host header at all. The second is that we cannot use $ within the values to be set.

We are being asked by the vendor to ultimately end up with a nginx configuration of RequestHeader set Host "$host"; . If there is another way to achieve this, then that works too.

What would you like to be added:
Ability to overwrite the Host header and use $s within the values instead of only standard strings

Why this is needed:
See above

Additional context
Add any other context or screenshots about the enhancement request here.

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[sjberman]

Hi @ryan-dyer-sp, I'm curious if this comment/map of how we set that header can help in any way? $gw_api_compliant_host is what we set the Host header to.

Set $gw_api_compliant_host variable to the value of $http_host unless $http_host is empty, then set it to the value
of $host. We prefer $http_host because it contains the original value of the host header, which is required by the
Gateway API. However, in an HTTP/1.0 request, it's possible that $http_host can be empty. In this case, we will use
the value of $host. See http://nginx.org/en/docs/http/ngx_http_core_module.html#var_host.

map $http_host $gw_api_compliant_host {
'' $host;
default $http_host;
}

[sjberman]

I'm going digging to try to remember why the Host header restriction was added, and I'll get back to you on that (though hopefully my previous comment helps where you don't actually need to set it?).

We do have an issue to support variables in the header modifier already: #2040