DNS secondary server does not update

Sometimes in a primary / secondary DNS name server environment, the secondary server zone files do not get updated. This occurs with BIND, the Berkely Internet Name Domain, name services.

This problem can be caused by failure to change the serial number of the primary zone file. This is a typical symptom of changing the zone file but not the serial number in the zone file. Be sure to change the serial number in the reverse zone file, too.

The top of my zone file looks like this:

; Authoritative data for both.org zone
@       IN SOA  hornet.both.org.        root.hornet.both.org. (
                                        2013102101      ; serial
                                        1D              ; refresh
                                        1H              ; retry
                                        1W              ; expire
                                        3H )            ; minimum

The serial number can be anything you want it to be, so long as it is always ascending in numerical value. I do use a sequence with some meaning. My serial number is based on the date the last change was made and a 2 digit sequence number within the date and it breaks down like this:

2013 = year
10 = month
21 = day
01 = Sequence number for the date

If I were to have changed the zone file again on the date of October 21, 2013, the sequence number would be 02 and the serial number would look like this: 2013102102. If I were to change my zone file today, the serial number for the first change of today would be 2015011401.

This serial number format gives me information that can be very useful because it tells me the date of the last zone file change, and it also provides an ascending sequence. The forward and reverse zone serial numbers do not have to be the same, but they can be.