server_name a.com www.a.com www.b.com b.com;

        root   /var/www/a.com/web/;

        if ($http_host = "a.com") {
            rewrite ^ $scheme://www.a.com$request_uri? permanent;
        }
        if ($http_host = "b.com") {
            rewrite ^ $scheme://www.b.com$request_uri? permanent;
        }