How to rewrite the domain part of Set-Cookie in a nginx reverse proxy?

Question

I have a simple nginx reverse proxy:

server {
  server_name external.domain.com;
  location / {
    proxy_pass http://backend.int/;
  }
}

The problem is that Set-Cookie response headers contain ;Domain=backend.int, because the backend does not know it is being reverse proxied.

How can I make nginx rewrite the content of the Set-Cookie response headers, replacing ;Domain=backend.int with ;Domain=external.domain.com?

Passing the Host header unchanged is not an option in this case.

Apache httpd has had this feature for a while, see ProxyPassReverseCookieDomain, but I cannot seem to find a way to do the same in nginx.

Answer 1

This question came up in the nginx mailing list [1]. There's no way to directly do this in nginx. You have to resort to using the ngx_lua module (>=v0.3.1).

The user "agentzh" has an example of what this would look like inlined in the config file:

server_name external.domain.com;

location / { 
    proxy_pass http://backend.int/;

    header_filter_by_lua ' 
        local cookies = ngx.header.set_cookie 
        if not cookies then return end 
        local newcookies = {} 
        for i, val in ipairs(cookies) do 
            local newval = string.gsub(val, "([dD]omain)=[%w_-\\\\.]+", 
                      "%1=external.domain.com") 
            table.insert(newcookies, newval) 
        end 
        ngx.header.set_cookie = newcookies 
    '; 
} 

[1] http://nginx.2469901.n2.nabble.com/Rewriting-the-domain-part-of-Set-Cookie-in-a-proxy-pass-td6453554.html