[DBMON-6669] Fix resolve_db_host for IPv6 loopback addresses

datadog_checks_base/changelog.d/23849.fixed

@@ -0,0 +1 @@
+Fix ``resolve_db_host`` treating loopback IP literals (e.g. ``::1``) as DNS resolution failures, which caused database checks to submit metrics with the wrong host tag and miss agent host tags.

datadog_checks_base/datadog_checks/base/utils/db/utils.py

@@ -12,7 +12,7 @@
 import time
 from concurrent.futures.thread import ThreadPoolExecutor
 from enum import Enum, auto
-from ipaddress import IPv4Address
+from ipaddress import IPv4Address, IPv6Address, ip_address
 from typing import Any, Callable, Dict, List, Optional, Tuple, Union  # noqa: F401
 
 from cachetools import TTLCache
@@ -162,12 +162,28 @@ def acquire(self, key):
         return True
 
 
+def _try_parse_db_host_ip(db_host: str) -> IPv4Address | IPv6Address | None:
+    """Try to parse db_host as an IP address."""
+    try:
+        return ip_address(db_host.strip())
+    except ValueError:
+        return None
+
+
+def _is_local_db_host(db_host: str | None) -> bool:
+    """Return True when the DB is reached via localhost, a unix socket, or a loopback IP."""
+    if not db_host or db_host == 'localhost' or db_host.startswith('/'):
+        return True
+    addr = _try_parse_db_host_ip(db_host)
+    return addr is not None and addr.is_loopback
+
+
 def resolve_db_host(db_host):
     if db_host and db_host.endswith('.local'):
         return db_host
 
     agent_hostname = datadog_agent.get_hostname()
-    if not db_host or db_host in {'localhost', '127.0.0.1'} or db_host.startswith('/'):
+    if _is_local_db_host(db_host):
         return agent_hostname
 
     try:

datadog_checks_base/tests/base/utils/db/test_util.py

@@ -32,10 +32,25 @@
 @pytest.mark.parametrize(
     "db_host, agent_hostname, want",
     [
+        # Unset host or local hostname
         (None, "agent_hostname", "agent_hostname"),
         ("localhost", "agent_hostname", "agent_hostname"),
+        # Unix socket
+        ("/var/run/mysqld.sock", "agent_hostname", "agent_hostname"),
+        # Loopback IP literals (incl. zone-scoped IPv6)
         ("127.0.0.1", "agent_hostname", "agent_hostname"),
+        ("::1", "agent_hostname", "agent_hostname"),
+        ("::1%lo0", "agent_hostname", "agent_hostname"),
+        # Non-loopback IP literals keep the configured host
+        ("169.254.169.254", "agent_hostname", "169.254.169.254"),
+        ("fe80::1", "agent_hostname", "fe80::1"),
+        ("fe80::1%eth0", "agent_hostname", "fe80::1%eth0"),
         ("192.0.2.1", "agent_hostname", "192.0.2.1"),
+        ("2001:db8::1", "agent_hostname", "2001:db8::1"),
+        # Resolved DB host shares the agent host IP
+        ("192.0.2.1", "192.0.2.1", "192.0.2.1"),
+        ("192.0.2.1", "192.0.2.254", "192.0.2.1"),
+        # Hostname resolution failures fall back to the configured db_host
         ("socket.gaierror", "agent_hostname", "socket.gaierror"),
         (
             "greater-than-or-equal-to-64-characters-causes-unicode-error-----",
@@ -44,8 +59,7 @@
         ),
         ("192.0.2.1", "socket.gaierror", "192.0.2.1"),
         ("192.0.2.1", "greater-than-or-equal-to-64-characters-causes-unicode-error-----", "192.0.2.1"),
-        ("192.0.2.1", "192.0.2.1", "192.0.2.1"),
-        ("192.0.2.1", "192.0.2.254", "192.0.2.1"),
+        # mDNS .local names are passed through unchanged
         ("postgres.svc.local", "some-pod", "postgres.svc.local"),
     ],
 )