Hi @jonirrings
Your welcome ☺️.
I personally work with nginx and have little experience working with caddy.
But I think your config is not complete.
In reverse_proxy you need to pass the real ip to the service (X-Real-IP).
Like this :

http://s.o0x0o.cn, https://s.o0x0o.cn {
    reverse_proxy /backend/* 127.0.0.1:18080 {
        header_up X-Real-IP {remote}
    }
}

nginx :

server {
    	server_name s.o0x0o.cn;
    	
    	location / {
    		proxy_redirect          off;
        	proxy_pass_header       Server;
        	proxy_set_header        X-Real-IP $remote_addr;
        	proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
        	proxy_set_header        X-Scheme $scheme;
        	proxy_set_header        Host $http_host;
        	proxy_set_header        X-NginX-Proxy true;
        	proxy_pass              http://127.0.0.1:18080/;
    	}
    }

thanks for your quick response.
I read your great code, and found trust_addr_proxy is checking X-Real-IP header.

I setup caddy as your suggestion, but got 127.0.0.1.

Then I run your code locally behind a nginx and a caddy.
guess what I found.

Nginx will set X-Real-IP as is;
Caddy will set X-Real-IP as X-Real-Ip.🤣

Caddy canonicalizes header fields Link

After add serveral rows code to check X-Real-Ip, librespeed-rs return the expected address.

any solution for this situation?

Edited
PS: caddy reverse could use header_up X-Real-IP {remote_host} for ip address only.

How interesting and strange 😂.
Characteristic solution.
Can you fix it in a PR ?
i want contribution to increase