PYTHON-3800 Add lower() to node when validate hosts of srv records

[qkrwjdan]

Background

When using mongodb+srv DNS Seed List Connection String, I had no problems with the other drivers, but only when using the pymongo driver.

The error is as follows

python3 main.py        
Traceback (most recent call last):  
  File "/Users/user/Workspace/tmp/pymongo-test/main.py", line 11, in <module>  
    client = pymongo.MongoClient(uri)  
             ^^^^^^^^^^^^^^^^^^^^^^^^  
  File "/Users/user/Workspace/tmp/pymongo-test/venv/lib/python3.11/site-packages/pymongo/mongo_client.py", line 748, in __init__  
    res = uri_parser.parse_uri(  
          ^^^^^^^^^^^^^^^^^^^^^  
  File "/Users/user/Workspace/tmp/pymongo-test/venv/lib/python3.11/site-packages/pymongo/uri_parser.py", line 539, in parse_uri  
    nodes = dns_resolver.get_hosts()  
            ^^^^^^^^^^^^^^^^^^^^^^^^  
  File "/Users/user/Workspace/tmp/pymongo-test/venv/lib/python3.11/site-packages/pymongo/srv_resolver.py", line 121, in get_hosts  
    _, nodes = self._get_srv_response_and_hosts(True)  
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
  File "/Users/user/Workspace/tmp/pymongo-test/venv/lib/python3.11/site-packages/pymongo/srv_resolver.py", line 115, in _get_srv_response_and_hosts  
    raise ConfigurationError(f"Invalid SRV host: {node[0]}")  
pymongo.errors.ConfigurationError: Invalid SRV host: mongodb-r.jeongmu-rs.RegionOne.mg.db.***.com

It was weird that it only happened with the python driver.

Cause

First, the uri in the connection string that I was using contained uppercase letters, and the SRV Records also contained uppercase letters.
For example,

• connection string : mongodb+srv://username:password@mongodb-jeongmu-rs.RegionOne.mg.db.***.com
• SRV records :

nslookup -type=SRV _mongodb._tcp.mongodb-jeongmu-rs.RegionOne.mg.db.***.com
Server:         10.28.4.20
Address:        10.28.4.20#53

_mongodb._tcp.mongodb-jeongmu-rs.RegionOne.mg.db.***.com service = 1 1 27017 mongodb-r.jeongmu-rs.RegionOne.mg.db.***.com.
_mongodb._tcp.mongodb-jeongmu-rs.RegionOne.mg.db.***.com service = 1 1 27017 mongodb-r.jeongmu-rs.RegionOne.mg.db.***.com.
_mongodb._tcp.mongodb-jeongmu-rs.RegionOne.mg.db.***.com service = 1 1 27017 mongodb-r.jeongmu-rs.RegionOne.mg.db.***.com.

If you look at line 144 of uri_parser.py, inside the parse_host function, the hostname is converted to lowercase via lower(), but the imported SRV records still contain uppercase letters.

# Normalize hostname to lowercase, since DNS is case-insensitive:
# http://tools.ietf.org/html/rfc4343
# This prevents useless rediscovery if "foo.com" is in the seed list but
# "FOO.com" is in the hello response.
return host.lower(), port

Therefore, when comparing the fqdn and node at _get_srv_response_and_hosts, the fqdn has been changed to lowercase and the node still contains uppercase letters.
It cause pymongo.errors.ConfigurationError: Invalid SRV host.

[blink1073]

LGTM, thank you!