Incorrectly strips trailing dot from absolute hostnames, causing TooManyRedirects

requests appears to be incorrectly stripping the trailing dot on absolute hostnames (i.e. explicitly marked as in the root DNS namespace) in URLs when constructing the Host: request header, like pyropus.ca.. This causes redirect loops when the site chooses the absolute hostname as canonical and redirects requests without the trailing dot.

The example site redirects requests with a host header of Host: pyropus.ca to the same URL with the host name changed to the canonical absolute version:

HTTP/1.1 301 Moved Permanently
[...]
Location: https://pyropus.ca./

requests then strips the dot and resends the request, resulting in a redirect loop.

The trailing dot needs to be stripped from the SNI header for https requests (required by TLS/etc spec) but it should not be stripped from the Host: header value.

When fixing the SNI trailing dot issue, curl had this bug because they changed it to also affect the Host: header value, but they've fixed it by reverting that part of the change:
https://github.com/curl/curl/issues/8290

Many other common user agents that I've tested handle this properly:

• all GUI browers, to my knowledge, preserve the absolute domain name in the Host: header - I've tested Firefox, Chromium, Vivaldi, Konqueror, and Safari
• wget also preserves the absolute domain name on redirect URLs
• curl is now fixed and handles it

A few less common ones have the same buggy behaviour, or did -- I haven't re-checked:

• lynx, links, and elinks handle it like curl, erroring out on a redirect loop

Expected Result

Successful 200 response with content.

Actual Result

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../test/.venv/lib/python3.9/site-packages/requests/api.py", line 75, in get
    return request('get', url, params=params, **kwargs)
  File ".../test/.venv/lib/python3.9/site-packages/requests/api.py", line 61, in request
    return session.request(method=method, url=url, **kwargs)
  File ".../test/.venv/lib/python3.9/site-packages/requests/sessions.py", line 529, in request
    resp = self.send(prep, **send_kwargs)
  File ".../test/.venv/lib/python3.9/site-packages/requests/sessions.py", line 667, in send
    history = [resp for resp in gen]
  File ".../test/.venv/lib/python3.9/site-packages/requests/sessions.py", line 667, in <listcomp>
    history = [resp for resp in gen]
  File ".../test/.venv/lib/python3.9/site-packages/requests/sessions.py", line 166, in resolve_redirects
    raise TooManyRedirects('Exceeded {} redirects.'.format(self.max_redirects), response=resp)
requests.exceptions.TooManyRedirects: Exceeded 30 redirects.

Reproduction Steps

import requests

requests.get("https://pyropus.ca./")

System Information

$ python -m requests.help

{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "2.0.12"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.3"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.9.2"
  },
  "platform": {
    "release": "5.17.5",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.27.1"
  },
  "system_ssl": {
    "version": "101010bf"
  },
  "urllib3": {
    "version": "1.26.9"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}

Edit: added a couple words to the summary to clarify.

Start with requests/api.py and requests/sessions.py, especially the request and redirect paths shown in the traceback, then follow how urllib3 constructs the TLS SNI and Host header. Reproduce the issue with requests.get("https://pyropus.ca./") and verify that the Host header preserves the trailing dot while the request completes without a redirect loop.