Nginx use subdomain as query parameter and rewrite URL - nginx

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.