Controlling Nginx proxy target using a cookie?

Similar to this answer. Nginx's idiomatic approach to this kind of problems is via map.

Basically, you define a map in http section

map $cookie_proxy_override $my_upstream {
  default default-server-or-upstream;
  ~^(?P<name>[\w-]+) $name;
}

Then you simply use $my_upstream in location section(s):

location /original-request {
  proxy_pass http://$my_upstream$uri;
}

Nginx evaluates map variables lazily, only once (per request) and when you are using them.

Eventually my solution boils down to this:

server {
    ...
    set $upstream "default-server-or-upstream";
    if ($http_cookie ~ "proxy_override=([\w-]+)") {
        set $upstream $1;                                   
    }

    location /original-request {
        proxy_pass http://$upstream/original-application
    }
}

The test is done in the server scope for each request (before the actual redirect is resolved) and is just used to set a variable - this is apparently a supported usage of Nginx "rewrite" module. It also tests the entire $http_cookie like @Rikih suggested, but includes the name of the cookie to make sure I don't match random stuff that people might be throwing at me.

Then in the location scope where I want to do the redirect, I use the variable name that either contains the default upstream configuration or was overwritten by the cookie.

have you try $http_cookie ? http://wiki.nginx.org/HttpRewriteModule

if ($http_cookie ~* "proxy-target-A" ) { foo; }