a2enmod rewrite proxy proxy_wstunnel

<VirtualHost subdomain.mydomain.com:80>
    Servername subdomain.mydomain.com

    RewriteEngine On
    RewriteCond %{REQUEST_URI}  ^/socket.io            [NC]
    RewriteCond %{QUERY_STRING} transport=websocket    [NC]
    RewriteRule /(.*)           ws://localhost:3001/$1 [P,L]

    ProxyPass        /socket.io http://localhost:3001/socket.io
    ProxyPassReverse /socket.io http://localhost:3001/socket.io
</VirtualHost>

server.listen('3001', function(){
  console.log('Socket.io server listening on *:3001');
});


io.listen(server).on('connection', function(client) {
    // Rest of the code
}

var socket = io.connect('http://subdomain.mydomain.com/', {query: {extra_info : extra_info}});

GET http://subdomain.mydomain.com:8100/socket.io/?extra_info=fe4567&EIO=3&transport=polling&t=1416987210079-31 net::ERR_CONNECTION_TIMED_OUT