Change the host header based on cookie

I want to replace www with beta in the $http_host if a cookie SHOW_BETA is set to true in the request.

I've put this in my host block but not sure how to do the actual replacement.

map $http_host $cookie_SHOW_BETA {
    // replace www with beta here if true.   <---- Question: what to put in here
    default $http_host;
}

also if I want to use if where I can easily split $http_host I cannot use and to check the cookie's value.

Your usage of map has issues:

• The "from" value for the map should be first, and the variable to set should be second
• Using $http_host as a variable to set might not work

I don't think replace is even possible in map.

The best you could try is to add a prefix:

map $cookie_show_beta $http_host {
    true beta.$http_host;
    default $http_host;
}