DNS on Windows Server 2003, 3rd Edition

SOA Serial Numbers

Every zone has a serial number. Every time the data in a file is changed, the zone’s serial number must be incremented. If the serial number is not incremented, secondary name servers for the zone will not pick up the updated data. The change is simple. If the original datafile had the following SOA record:

@                       IN  SOA terminator.movie.edu.  hostmaster.movie.edu. (
                         24           ; serial number
                         900          ; refresh
                         600          ; retry
                         86400        ; expire
                         3600       ) ; default TTL

the updated datafile would have the following SOA record:

@                       IN  SOA terminator.movie.edu.  hostmaster.movie.edu. (
                         25           ; serial number
                         900          ; refresh
                         600          ; retry
                         86400        ; expire
                         3600       ) ; default TTL

(Recall from Chapter 4 that “@” expands to the current origin of the zone datafile, which is usually the name of the zone. In this example, the “@” stands for movie.edu.) This simple change is the key to distributing the data to all of your secondaries. Failing to increment the serial number is the most common mistake made when updating by hand. The first few times you make a change manually, you’ll remember to update the serial number because this process is new and you are paying close attention. After modifying zone datafiles becomes second nature (we bet you can’t wait for that), you’ll make some quick little change, forget to update the serial number . . . and none of the secondaries will pick up the new data. Eternal vigilance is the price of modifying zone datafiles by hand.

There are several good ways to manage serial numbers, which ...