Nginx TLS SNI routing, based on subdomain pattern

Nginx can be configured to route to a backend, based on the server's domain name, which is included in the SSL/TLS handshake (Server Name Indication, SNI).
This works for http upstream servers, but also for other protocols, that can be secured with TLS.

prerequisites

at least nginx 1.15.9 to use variables in ssl_certificate and ssl_certificate_key.

check nginx -V for the following:

...
TLS SNI support enabled
...
--with-stream_ssl_module 
--with-stream_ssl_preread_module

It works well with the nginx:1.15.9-alpine docker image.

non terminating, TLS pass through

Pass the TLS stream to an upstream server, based on the domain name from TLS SNI field. This does not terminate TLS.
The upstream server can serve HTTPS or other TLS secured TCP responses.

stream {  

  map $ssl_preread_server_name $targetBackend {
    ab.mydomain.com  upstream1.example.com:443;
    xy.mydomain.com  upstream2.example.com:443;
  }   
 
  server {
    listen 443; 
        
    proxy_connect_timeout 1s;
    proxy_timeout 3s;
    resolver 1.1.1.1;
    
    proxy_pass $targetBackend;       
    ssl_preread on;
  }
}

terminating TLS, forward TCP

Terminate TLS and forward the plain TCP to the upstream server.

stream {  

  map $ssl_server_name $targetBackend {
    ab.mydomain.com  upstream1.example.com:443;
    xy.mydomain.com  upstream2.example.com:443;
  }

  map $ssl_server_name $targetCert {
    ab.mydomain.com /certs/server-cert1.pem;
    xy.mydomain.com /certs/server-cert2.pem;
  }

  map $ssl_server_name $targetCertKey {
    ab.mydomain.com /certs/server-key1.pem;
    xy.mydomain.com /certs/server-key2.pem;
  }
  
  server {
    listen 443 ssl; 
    ssl_protocols       TLSv1.2;
    ssl_certificate     $targetCert;
    ssl_certificate_key $targetCertKey;
        
    proxy_connect_timeout 1s;
    proxy_timeout 3s;
    resolver 1.1.1.1;
      
    proxy_pass $targetBackend;
  } 
}

Choose upstream based on domain pattern

The domain name can be matched by a regex pattern, and extracted to variables. See regex_names.
This can be used to choose a backend/upstream based on the pattern of a (sub)domain. This is inspired by robszumski/k8s-service-proxy.

The following configuration extracts a subdomain into variables and uses them to create the upstream server name.

stream {  

  map $ssl_preread_server_name $targetBackend {
    ~^(?<app>.+)-(?<namespace>.+).mydomain.com$ $app-public.$namespace.example.com:8080;
  }
  ...
}

Your Nginx should be reachable over the wildcard subdomain *.mydomain.com.
A request to shop-staging.mydomain.com will be forwarded to shop-public.staging.example.com:8080.

K8s service exposing by pattern

In Kubernetes, you can use this to expose all services with a specific name pattern.
This configuration exposes all service which names end with -public.
A request to shop-staging-9999.mydomain.com will be forwarded to shop-public in the namespace staging on port 9999.
You will also need to update the resolver, see below.

stream {  

  map $ssl_preread_server_name $targetBackend {
    ~^(?<service>.+)-(?<namespace>.+)-(?<port>.+).mydomain.com$ $service-public.$namespace.svc.cluster.local:$port;
  }
  
  server {
    ...
    resolver kube-dns.kube-system.svc.cluster.local;
    ...
  }
}