Nginx use subdomain as query parameter and rewrite URL

Question

I'm trying to use subdomains as query parameter on the domain itself. An example would be the following:

I want nginx to take ab.example.com and call example.com?key=ab, now the backend will return a specific config, which should be used for the subdomain "ab".

Afterwards the user should see the content (logo branding to be precise) of example.com?key=ab but in the client's URL field the ab.example.com should persist.

And all further requests should show for example ab.example.com/login instead of example.com/login.

I hope that what I have said is sufficiently understandable. I have tried various examples from the internet and tried to find some hints.

The nginx file looks like:

server {
    listen 80;
    listen [::]:80;

    server_name www.example.com *.example.com;
    return 301 https://$server_name$request_uri;
}

server {
    listen 443 ssl;
    server_name www.example.com *.example.com;
    ssl_certificate /path/to/certs/ssl.crt;
    ssl_certificate_key /path/to/keys/ssl.key;

    root /var/www/example_site;

    # Add index.php to the list if you are using PHP
    index index.html index.htm index.nginx-debian.html;

    location / {
        try_files $uri $uri/ /index.html =404;
        error_page 404 =200;
    }
}

I have already tried to map, but it redirects to a wrong domain:

map $host $subdomain {
    ~^(?<sub>.+)\.example\.com$ $sub;
}

And tried adding a static if statement in the server block, too:

if ($host = "ab.example.com") {
    rewrite . ?key=ab;
}

An additional server block did not help either:

server {
    listen 80;
    listen [::]:80;
    server_name www.ab.example.come ab.example.com;
    rewrite ^ https://example.com/?key=ab permanent;
}

Does anyone see what I am doing wrong or what part of the documentation I should read again?

Answer 1

You just need to do it inside your own server_name directive. You can assign a variable in a regexp directly there. If you need a different behavior for www. subdomain just remove *.example.com from the block and add this one in another file:

server {
    listen       80;
    server_name  ~^(?<subdomain>.+)\.example\.com$; 
    return       301 http://example.com$request_uri?key=$subdomain;
}

Note that I didn't use rewrite, which you shouldn't need. Using return performs better. 301 stands for the kind of redirect. And then, you use your server_name assigned variable to redirect where you need.