How to configure nginx to auto redirect to the main hostname?

Question

Each nginx config can act for a wide range of domains but I want to auto-redirect requests to the first domain name (the official one).

server {
    server_name a.example.com b.example.com;
}

I want that if someone enters b.example.com/some, to go directly to a.example.com/some

Answer 1

This is pretty much the same thing as the GOOD example for http://wiki.nginx.org/Pitfalls#Server_Name. That is, you should use two servers:

server {
  server_name b.example.com;
  return 301 $scheme://a.example.com$request_uri;

  # For pre-0.8.42 installations:
  # rewrite ^ $scheme://a.example.com$request_uri? permanent;
}

server {
  server_name a.example.com;
  # Do stuff
}

Answer 2

To do this in a single server block, you can use an if and the $server_name variable:

    server_name primary.tld secondary.tld;
    if ($host != $server_name) {
        rewrite ^ $scheme://$server_name permanent;
    }

Or, to keep any query parameters:

    server_name primary.tld secondary.tld;
    if ($host != $server_name) {
        rewrite ^/(.*) $scheme://$server_name/$1 permanent;
    }

Here, $server_name refers to primary server name, which is the first name in the server_name directive, while $host refers to the hostname given in the HTTP request.

Note that the if statement in nginx configuration does not always do what you would expect and its use is discouraged by some. See also https://www.nginx.com/resources/wiki/start/topics/depth/ifisevil/

This answer was inspired by this answer to another question which uses a similar aproach.