Proxy domain rewrite using proxy_cookie_domain

[nitin]

I want to rewrite domains in the cookie when NGIX is acting as reverse
proxy. I see that NGIX support this using proxy_cookie_domain module.

But I am unable to find out where does it keep the original domain which is
being replaced? In my opinion NGIX would need the original domain to find
out where to send the cookie when it comes back to NIGX in next request. 

Let's says NGIX domain is: external.com
backend server #1 sets cookie domain as: server1.com 
backend server #2 sets cookie domain as: server2.com 

Both these domains are replaced by NGIX to NGIX's domain so both cookies'
domain is now external.com

when the request comes to NGIX, both these cookies will be sent by browser
to NGIX, now how does NGIX decide which cookies to be sent to the backend
server? it needs to keep the original domain mapping to find this how, does
it keep somewhere? 

please help in answering this question.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,264499,264499#msg-264499