A DNS zone transfer can reveal every record in a zone. That may be acceptable for a secondary name server and unacceptable for an arbitrary internet client. The control is usually a small allow-transfer statement in BIND, but the effective policy depends on where it is set and which listener receives the request.
Start with the authoritative context
BIND's configuration reference describes allow-transfer as the list of hosts allowed to receive zone transfers. It can be configured at the zone, view, or global options level. A zone-level setting can override a broader setting, so the review should follow BIND's scope rules rather than searching for one line at the top of the file.
Recent BIND release notes say that an explicit allow-transfer ACL must be set to enable outgoing transfers. Do not assume that a missing ACL behaves the same way across every installed version. Read the version-specific manual for the server you actually operate.
An ACL should name the secondary servers or a narrow network that contains them. A comment saying "internal only" is not a policy. If any appears, the record needs a specific explanation and a check that the zone is meant to be public.
Test the query path
From a controlled host, request an authoritative transfer:
dig @192.0.2.53 example.com AXFR
Replace the address and zone with test values. Confirm that an intended secondary can transfer the zone and that an unauthorized client receives a refusal. Run the unauthorized test from outside the management network and from the address class most likely to be exposed.
Repeat the test for each relevant view. A server may answer differently depending on the source address, the destination address, or the zone's view. A transfer refusal from the local host does not prove that an external interface is protected.
Check secondary operations
An ACL that is too narrow can break zone synchronization. After changing the rule, inspect secondary logs for transfer failures and verify that the serial and record set are current. A successful dig from an administrative host does not prove that the real secondary has the update.
For signed zones, confirm that transfers include the intended records and that signing keys are not exposed in diagnostics. Restrict access to configuration files and key material. A zone transfer policy protects records, but it does not protect the server from every configuration or patching issue.
Keep the evidence
Save the effective configuration, the BIND version, the test source address, the command, and the response. Record the time of the test and whether a reload was required. If an ACL is managed by templates, include the rendered output so a reviewer can see what reached the server.
Review the rule when zones are added or moved. A delegation change can create a new external resolver that appears to be an internal secondary. A stale ACL may then block legitimate transfers while granting access to an old host that no longer belongs to the organization.
A complete test shows a known secondary succeeding, an unrelated network failing, and current zone content. Attach that evidence to the ACL review.