Add DNS query function

src/ddns_service/dns.py

@@ -10,6 +10,7 @@ import dns.name
 import dns.query
 import dns.rcode
 import dns.rdatatype
+import dns.resolver
 import dns.tsigkeyring
 import dns.update
 
@@ -353,6 +354,38 @@ class DNSService:
             return hostname[:-len(zone_suffix)]
         return hostname
 
+    def query_record(self, hostname, zone, record_type):
+        """
+        Check if DNS record exists.
+
+        Args:
+            hostname: Hostname (without zone suffix).
+            zone: DNS zone name.
+            record_type: Record type string (A or AAAA).
+
+        Returns:
+            IP address string if record exists, None otherwise.
+        """
+        fqdn = f"{self._get_relative_name(hostname, zone)}.{zone}"
+        if not fqdn.endswith("."):
+            fqdn += "."
+        try:
+            resolver = dns.resolver.Resolver()
+            resolver.nameservers = [self.server]
+            resolver.port = self.port
+            resolver.lifetime = self.timeout
+            answers = resolver.resolve(fqdn, record_type)
+            return str(answers[0]) if answers else None
+        except (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer,
+                dns.resolver.NoNameservers):
+            return None
+        except Exception as e:
+            logging.warning(
+                f"DNS query failed: hostname={hostname} zone={zone} "
+                f"type={record_type}: {e}"
+            )
+            return None
+
     def update_record(self, hostname, zone, ip, ttl):
         """
         Update a DNS record for the given hostname.

src/ddns_service/models.py

@@ -2,6 +2,7 @@
 
 import logging
 import os
+import time
 
 from . import datetime_naive_utc, datetime_aware_utc, now_utc
 from .dns import encode_dnsname, EncodingError
@@ -78,11 +79,21 @@ class DateTimeFieldUTC(DateTimeField):
 
 
 class BaseModel(Model):
-    """Base model with database binding."""
+    """Base model with database binding and save retry."""
 
     class Meta:
         database = db
 
+    def save(self, *args, max_retries=3, retry_delay=0.1, **kwargs):
+        """Save with retry on DatabaseError (exponential backoff)."""
+        for attempt in range(max_retries):
+            try:
+                return super().save(*args, **kwargs)
+            except DatabaseError:
+                if attempt == max_retries - 1:
+                    raise
+                time.sleep(retry_delay * (2 ** attempt))
+
 
 class User(BaseModel):
     """User model for authentication."""
@@ -152,7 +163,10 @@ def init_database(config: dict):
         db_dir = os.path.dirname(db_path)
         if db_dir:
             os.makedirs(db_dir, exist_ok=True)
-        actual_db = SqliteDatabase(db_path)
+        actual_db = SqliteDatabase(db_path, pragmas={
+            'journal_mode': 'wal',
+            'busy_timeout': 5000,
+        })
         db.initialize(actual_db)
         logging.debug(f"Database backend: SQLite path={db_path}")