For me, an important use of the requests_doh library is skipping DNS requests entirely. That includes that I want to be able to avoid the DNS request to resolve the IP of the DNS resolver itself.

Right now, it's possible with some tricks:

import httpx
import requests
import requests_doh.adapter
import requests_doh.resolver

requests_doh.resolver.add_dns_provider("cloudflare_ip",
                                       'https://104.16.249.249/dns-query', switch=True)
requests_doh.resolver.set_resolver_session(httpx.Client(verify=False,
                                                        headers={"Host": "cloudflare-dns.com"}))

doh = requests_doh.adapter.DNSOverHTTPSAdapter(provider="cloudflare_ip")

session = requests.Session()
session.mount('https://', doh)
session.mount('http://', doh)

session.get('https://example.com')

There are two changes that I would like to propose:

• It would be nice if you could connect to a custom server, without the need to add them to the dictionary in the requests_doh.resolver module first, so something like DNSOverHTTPSAdapter(provider_url='https://104.16.249.249/dns-query', provider_host='cloudflare-dns.com').
• If a DNS resolver is added by IP like above, we could add a way to verify the SSL certificate against the hostname. The easiest way to do that might be to use requests.Session for the resolver session, instead of httpx.Client. Then it would be possible to add the domain, IP combination of the resolver to the cache, and re-use all existing logic.

What are your thoughts? I can work on a pull request if you're interested in the described changes.