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$host

$host is in this order of precedence (from high to low):

  • Host name from the request line (rarely used)

    GET http://example.org/test/

  • Host request header field

  • The server_name (in Nginx config) matching a request, even if server_name is wildcard (*.example.org)

Host name from the request line​

When open URL http://example.org/test/ ...

Most browser send the request like this

GET /test/ HTTP/1.1
Host: example.org

Most browser doesn't send the request like this

GET http://example.org/test/ HTTP/1.1

Validation​

Nginx testing config​

server {
    listen       80;
    server_name  *.example.org;

    location / {
        default_type "text/plain";
        return 200 "[host] = $host";
    }
}

When all exist ...​

$host = host name from the request line

curl http://127.0.0.1 -v \
  --request-target http://request.example.org/test/ \
  --path-as-is \
  -H "Host: host.example.org"

This command will

  • Connect to 127.0.0.1
  • Send request path as GET http://request.example.org/test/ HTTP/1.1
  • Set Host header to Host: host.example.org
*   Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
> GET http://request.example.org/test/ HTTP/1.1
> Host: host.example.org
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.23.1
< Date: Fri, 21 Oct 2022 02:00:56 GMT
< Content-Type: text/plain
< Content-Length: 28
< Connection: keep-alive
<
* Connection #0 to host 127.0.0.1 left intact
[host] = request.example.org

When only Host header exist ...​

$host = Host header

curl http://127.0.0.1/test/ -v \
  -H "Host: host.example.org"
*   Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
> GET /test/ HTTP/1.1
> Host: host.example.org
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.23.1
< Date: Fri, 21 Oct 2022 02:01:37 GMT
< Content-Type: text/plain
< Content-Length: 25
< Connection: keep-alive
<
* Connection #0 to host 127.0.0.1 left intact
[host] = host.example.org

When none exist ...​

$host = server_name (in Nginx config)

# HTTP 1.1 must have Host header, so use HTTP 1.0
curl http://127.0.0.1/test/ -v -H "Host:" -0
*   Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 80 (#0)
> GET /test/ HTTP/1.0
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx/1.23.1
< Date: Fri, 21 Oct 2022 02:02:20 GMT
< Content-Type: text/plain
< Content-Length: 22
< Connection: close
<
* Closing connection 0
[host] = *.example.org

Ref: ngx_http_core_module