Add dns-query-timeout option to config.yaml

Introduced a new configuration option 'dns-query-timeout' to
set the DNS query timeout (in seconds) for the charm.

Closes-Bug: #2112374
Change-Id: I552457b588183726314faaded21442f062a71ae2
Signed-off-by: Zhang Hua <joshua.zhang@canonical.com>

charmhelpers/contrib/network/ip.py

@@ -469,9 +469,21 @@ def ns_query(address):
         return None
 
     try:
-        answers = dns.resolver.query(address, rtype)
+        resolv = dns.resolver.Resolver()
+        # The dnspython library sets a default DNS query lifetime of 5.0 seconds,
+        # as defined in BaseResolver.reset() (see https://github.com/rthalley/
+        # dnspython/blob/7ed1648b/dns/resolver.py#L709, commit 7ed1648b).
+        resolv.lifetime = config('dns-query-timeout') or 5.0
+        if hasattr(resolv, 'resolve'):
+            answers = resolv.resolve(address, rtype)
+        else:
+            answers = resolv.query(address, rtype)
     except (dns.resolver.NXDOMAIN, dns.resolver.NoNameservers):
         return None
+    except dns.exception.Timeout:
+        log("DNS query timed out for address {} with rtype {} and timeout "
+            "{}".format(address, rtype, resolv.lifetime), level=WARNING)
+        return None
 
     if answers:
         return str(answers[0])

config.yaml

@@ -428,3 +428,12 @@ options:
       this option sets True as the default value, which is consistent with the
       default value 'WSGISocketRotation On' in Apache. This option should be
       used with caution. Please read the Apache doc page for more information.
+  dns-query-timeout:
+    type: float
+    default: 5.0
+    description: |
+      Sets the timeout (in seconds) for DNS queries in the charm. If not
+      configured, the default value is 5.0 seconds. Adjust this value to
+      balance query speed and reliability, especially in environments with
+      slow or unstable DNS servers. Use with caution, as very low values
+      may lead to frequent query failures.