Photo by José Martín Ramírez Carrasco on Unsplash

Even if you don't use B2B SaaS, you might have seen urls like https://theleaddeveloper.slack.com, where you can have your own subdomain on slack.com for your Slack channel.

While playing around with some nginx settings, I noticed that this could actually be used for dynamic subdomains per tenant.

Long story short, here's the solution (which can certainly be optimized):

# sample shows only relevant part of the config, the rest is omitted for the sake of brevity

server {
    listen 443 ssl;

    server_name *.tempuri.org;

     location / { 
        set $tenant ""; 
  
        if ($host ~* "^(.+)\.tempuri.org$") {
          set $tenant $1; 
        }

        resolver 127.0.0.11;
        proxy_pass http://app/tenant?slug=$tenant;
      }   
}

So what's going on there?

Given our domain is tempuri.org, we want tenants to be available via tenantname.tempuri.org, hence our nginx server is listening on *.tempuri.org.

Next we're using a little regular expression to pick up the name of the subdomain (which equals our tenant name/slug) the user entered.

After that we set the DNS resolver to 127.0.0.11 (yes: eleven) which is the internal Docker DNS resolver. That's required to make sure the host name app used for the proxy_pass directive can be resolved. app is the name of the container hosting the actual application which e.g. shows a branded login screen or third party identity providers per tenant.