DNS-over-HTTP support, making TLS optional

[lch361]

Motivation

There are many situations where DNSCrypt-proxy can run under the reverse TLS proxy. In that case, built-in DNSCrypt-proxy TLS server becomes redundant.

Solution

If the certificate and key are empty strings, instead of erroring out, start a server without TLS. Just pure HTTP.

Example

You can configure nginx like this:

http {
	ssl_certificate /etc/letsencrypt/live/whatever.net/fullchain.pem;
	ssl_certificate_key /etc/letsencrypt/live/whatever.net/privkey.pem;
	server {
		listen 443 ssl;
		root /srv/www;
		location /dns-query {
  			proxy_pass http://localhost:36153/;
		}
	}
}

And dnscrypt-proxy.toml like this:

[local_doh]
listen_addresses = ['[::1]:36153', '127.0.0.1:36153']
path = '/'

To get TLS from Nginx instead of dnscrypt-proxy.

Trivia

Tested this patch on my server, everything works just as I intended. Hopefully this will make a fine addition to this project :)