fix hostname accepting values longer than 253 chars when they contain dots

Fixes #413

The hostname validator was only checking individual label lengths via
regex but had no overall length limit. Hostnames with dots that had
each label under 63 chars but total length over 253 would slip through.

Added a 253 character check on the host segment both with and without
a port, per RFC 1123. Added a test case for this.

Author: karthiksai109

src/validators/hostname.py

@@ -114,13 +114,18 @@ def hostname(
         return False
 
     if may_have_port and (host_seg := _port_validator(value)):
+        if len(host_seg) > 253:
+            return False
         return (
             (_simple_hostname_regex().match(host_seg) if maybe_simple else False)
             or domain(host_seg, consider_tld=consider_tld, rfc_1034=rfc_1034, rfc_2782=rfc_2782)
             or (False if skip_ipv4_addr else ipv4(host_seg, cidr=False, private=private))
             or (False if skip_ipv6_addr else ipv6(host_seg, cidr=False))
         )
 
+    if len(value) > 253:
+        return False
+
     return (
         (_simple_hostname_regex().match(value) if maybe_simple else False)
         or domain(value, consider_tld=consider_tld, rfc_1034=rfc_1034, rfc_2782=rfc_2782)

tests/test_hostname.py

@@ -40,6 +40,16 @@ def test_returns_true_on_valid_hostname(value: str, rfc_1034: bool, rfc_2782: bo
 @pytest.mark.parametrize(
     ("value", "rfc_1034", "rfc_2782"),
     [
+        # bad (hostname exceeding 253 chars)
+        (
+            "kld8MXQh6YalMqKRbfs895gMjW5T4p2EwToPoCSThPHHbXgmXc."
+            "kld8MXQh6YalMqKRbfs895gMjW5T4p2EwToPoCSThPHHbXgmXc."
+            "kld8MXQh6YalMqKRbfs895gMjW5T4p2EwToPoCSThPHHbXgmXc."
+            "kld8MXQh6YalMqKRbfs895gMjW5T4p2EwToPoCSThPHHbXgmXc."
+            "kld8MXQh6YalMqKRbfs895gMjW5T4p2EwToPoCSThPHHbXgmXcab",
+            False,
+            False,
+        ),
         # bad (simple hostname w/ optional ports)
         ("ubuntu-pc:443080", False, False),
         ("this-pc-is-sh*t", False, False),