ends of the DNS servers themselves. Operational problems Serial number changes The preamble portion of zone transfer relies on the serial number, and only the serial number, to determine whether a zone's data have changed, and thus whether the actual data transfer is required. For some DNS server packages, the serial numbers of SOA resource records are maintained by administrators by hand. Every edit to the database involves making two changes, one to the record being changed and the other to the zone serial number. The process requires accuracy: the administrator may forget to change the serial number or change it incorrectly (reduce it). RFC 1912 (section 2.2 SOA records) recommends using the value YYYYMMDDnn as the number (YYYY=year, MM=month, DD=day, nn=revision number). This won't overflow until the year 4294. Some DNS server packages have overcome this problem by automatically constructing the serial number from the last modification timestamp of the database file on disk. This is the case for djbdns, for example. The operating system ensures that the last modification timestamp is updated whenever an administrator edits the database file, effectively automatically updating the serial number, and thus relieving administrators of the need to make two edits (in two different places) for every single change. Furthermore, the paradigm of database replication for which the serial number check (and indeed zone transfer itself) is designed, which involves a single central DNS server holding the primary version of the database with all other DNS servers merely holding copies, simply does not match that of many modern DNS server packages. Modern DNS server packages with sophisticated database back ends such as SQL servers and Active Directory allow administrators to make updates to the database in multiple places (such systems employ multi-master replication), with the database back end's own replication mechanism handling the replication to all other servers. This paradigm simply does not match that of a single, central, monotonically increasing number to record changes, and thus is incompatible with zone transfer to a large extent. Modern DNS server packages with sophisticated database back ends often will create a "shim" serial number, simulating the existence of a single central place where updates are made, but this is at best imperfect. Fortunately, for this and several reasons outlined later, DNS servers that use such sophisticated database back ends in general rarely use zone transfer as their database replication mechanism in the first place, and