The value of variable `$http_host` will not fallback to the value of `:authority` pseudo-header when the client not providing the request header `Host`

[n0099@…]

This ticket was originally posted at github.com/nginx-quic/nginx-quic/issues/3

*plz replace example with a testing domain that is backed by nginx-quic to reproduce this backward compatibility break*

server {
        server_name example;
        listen 443 http3 reuseport;
        add_header Alt-Svc 'h3=":$server_port"; ma=86400';
        add_header X-HTTP-Host "value=$http_host";
        add_header X-Host "value=$host";
}

$ curl --http1.1 -sIw '%{stderr}using %{http_version}\n' example | grep -i host
using 1.1
X-HTTP-Host: value=example
X-Host: value=example
$ curl --http2 -sIw '%{stderr}using %{http_version}\n' example | grep -i host
x-http-host: value=example
x-host: value=example
using 2
$ docker run -it --rm ymuski/curl-http3 bash -c "curl --http3 -sIw '%{stderr}using %{http_version}\n' example | grep -i host"
x-http-host: value=
x-host: value=example
using 3

Note curl --http3 is sending the Host header:

$ docker run -it --rm ymuski/curl-http3 curl --http3 -vso /dev/null example
...
* using HTTP/3
* h2h3 [:method: GET]
* h2h3 [:path: /]
* h2h3 [:scheme: https]
* h2h3 [:authority: example]
* h2h3 [user-agent: curl/7.88.1-DEV]
* h2h3 [accept: */*]
* Using HTTP/3 Stream ID: 0 (easy handle 0x55a0d0982900)
> GET / HTTP/3
> Host: example
> user-agent: curl/7.88.1-DEV
> accept: */*
>

[Maxim Dounin]

Avoid using $http_host, it returns the value of the Host header, and not the :authority pseudo-header (or the authority component from the request line in HTTP/1.x). Use $host instead, it represents the host value nginx is actually using from the request.

Duplicate of #2281.