InboundXFR: optional avoiding exception when IXFR serial went backwards

dns/query.py

@@ -1495,6 +1495,7 @@ def _inbound_xfr(
     serial: int | None,
     timeout: float | None,
     expiration: float | None,
+    raise_on_serial_went_backwards: bool,
 ) -> Any:
     """Given a socket, does the zone transfer."""
     rdtype = query.question[0].rdtype
@@ -1507,7 +1508,7 @@ def _inbound_xfr(
     else:
         tcpmsg = struct.pack("!H", len(wire)) + wire
         _net_write(s, tcpmsg, expiration)
-    with dns.xfr.Inbound(txn_manager, rdtype, serial, is_udp) as inbound:
+    with dns.xfr.Inbound(txn_manager, rdtype, serial, is_udp, raise_on_serial_went_backwards) as inbound:
         done = False
         tsig_ctx = None
         r: dns.message.Message | None = None
@@ -1555,6 +1556,7 @@ def xfr(
     source_port: int = 0,
     serial: int = 0,
     use_udp: bool = False,
+    raise_on_serial_went_backwards: bool = True,
     keyalgorithm: dns.name.Name | str = dns.tsig.default_algorithm,
 ) -> Any:
     """Return a generator for the responses to a zone transfer.
@@ -1595,6 +1597,9 @@ def xfr(
     :type serial: int
     :param use_udp: If ``True``, use UDP (only meaningful for IXFR).
     :type use_udp: bool
+    :param raise_on_serial_went_backwards: If ``True`` and the IXFR response
+        has older SOA serial than queried, raise exception.
+    :type raise_on_serial_went_backwards: bool
     :param keyalgorithm: The TSIG algorithm to use.
     :type keyalgorithm: :py:class:`dns.name.Name` or str
     :returns: A generator of :py:class:`dns.message.Message` objects.
@@ -1646,7 +1651,7 @@ def __getattr__(self, _):
     sock_type = socket.SOCK_DGRAM if use_udp else socket.SOCK_STREAM
     with make_socket(af, sock_type, source) as s:
         _connect(s, destination, expiration)
-        yield from _inbound_xfr(tm, s, q, serial, timeout, expiration)
+        yield from _inbound_xfr(tm, s, q, serial, timeout, expiration, raise_on_serial_went_backwards)
 
 
 def inbound_xfr(

dns/xfr.py

@@ -61,6 +61,7 @@ def __init__(
         rdtype: dns.rdatatype.RdataType = dns.rdatatype.AXFR,
         serial: int | None = None,
         is_udp: bool = False,
+        raise_on_serial_went_backwards: bool = True,
     ):
         """Initialize an inbound zone transfer.
 
@@ -97,6 +98,7 @@ def __init__(
         self.done = False
         self.expecting_SOA = False
         self.delete_mode = False
+        self.raise_on_serial_went_backwards = raise_on_serial_went_backwards
 
     def process_message(self, message: dns.message.Message) -> bool:
         """Process one message in the transfer.
@@ -145,7 +147,9 @@ def process_message(self, message: dns.message.Message) -> bool:
                     self.done = True
                 elif dns.serial.Serial(soa.serial) < self.serial:
                     # It went backwards!
-                    raise SerialWentBackwards
+                    if self.raise_on_serial_went_backwards:
+                        raise SerialWentBackwards
+                    self.done = True
                 else:
                     if self.is_udp and len(message.answer[answer_index:]) == 0:
                         #