Internal zone transfer blocked by firewall

Since about one week ago, the (central) firewall started to apply internal packet inspection to the regular zone transfers of the internal view of ru.nl. The transfer was then blocked by the firewall before completion, causing our copy of the data, which is required for accessing bass.ru.nl and also internal mail delivery to @ru.nl addresses, to expire after 8 days. This period is relatively short, we have 3 weeks for our own zones, and the effect of expiry was underestimated.

After coordinated effort with our colleagues of ILS networking, we managed to trace the cause back to the firewall and once this was fixed, the zone transfers were working as they were before.