How to configure nginx to handle cookies?

Question

Nginx

Existing config

server {
        error_page 404 = $document_root/404.html;
        set $cats $cookie_q;

        if ($cats = 0) {
                set $cats $args_q;
        }
        if ($cats = 0) {
                return 404;
        }

        listen 80;
        server_name site.ru www.site.ru;
        root /path/to/site/public_html;
        rewrite ^([^.\?]*[^/])$ $1/ permanent;

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        location = /wp-login.php {
                deny all;
                return 404;
        }

        location ~* \.(git|htaccess|htpasswd)$ {
                deny all;
                return 404;
        }

        location = /favicon.ico {
                log_not_found off;
                access_log off;
        }

        location = /robots.txt {
                allow all;
                log_not_found off;
                access_log off;
        }

        location ~ /\.ht {
                deny all;
                return 404;
        }

        location ~* \.(gif|jpeg|jpg|txt|png|tif|tiff|ico|jng|bmp|doc|pdf|rtf|xls|xlsx|ppt|rar|rpm|swf|zip|bin|deb|cur|ttf|woff|woff2|svg|xml|json|eot)$ {
                expires 180m;
        }

        location ~* \.(css|js)$ {
                expires 180m;
        }

        location / {
                fastcgi_intercept_errors on;
                try_files $uri $uri/index.php /index.php 404;
                fastcgi_pass unix:/var/run/php/php7.2-fpm.sock;
                fastcgi_index index.php;
                include fastcgi_params;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }

At the moment it returns to any request:
siter.u/404/404/404/404/404/404/404/404/404/404/40...
in the cookie or get parameter q the website address is written.
I need the following:
if the q cookie is set, take the address from it and proxy the request there.
if there is no cookie, check the get parameter q to take the address from it and proxy the request there.
if there is no cookie or get parameter, issue 404.
How to implement it?

Answer 1

It is done standardly via map (written in the http section!):

map "$arg_q:$cookie_q" $my_upstream {
        default 404;
        "~^:(?<cookie>.+)$" $cookie;
        "~^(?<get>.+):$" $get;
    }

Then in the desired server you can use it like this:

server {
    if ($my_upstream = 404) {
        return 404;
    }
    ...
    proxy_pass http://$my_upstream;

There is a good description here, see the " Upstream Selection " section . Just for this case.
I draw your attention to the fact that in the original TOR the option "there is both a cookie and a get parameter" is not considered. What then? In the config I provided, it will be 404.