# DNS Serial Number Rollover

 Restrictions This applies only to customers with a managed DNS service from VSHN.

If you need to lower the DNS serial number, propagation to the slave name servers won’t work as expected. The problem is that the slaves only accept zone updates with a higher serial number.

A typical case where you will run into this problem is if you use the YYYYMMDDXX serial number format and switch to a Unix timestamp serial number, for example by using our SERIALAUTOUPDATE feature, which uses the Unix timestamp:

1 ; SERIALAUTOUPDATE

You can see the problem by checking the SOA records for the zone like this:

$dig +short SOA brokendomain.com @ns0.yourdomain.com$ dig +short SOA brokendomain.com @ns1.yourdomain.com

You will see that the hidden master (ns0) has the new, but lower serial number, but the slaves don’t.

To fix that, take the old serial number (the one the slaves still use, noted "O" below) and calculate (you can use most calculator apps for this):

N = (O + 2^31–1) % 2^32

Where "N" is the new serial number. Insert it without using the SERIALAUTOUPDATE feature and push the zone change.

As soon as the changes have propagated to all slaves, you can switch to the SERIALAUTOUPDATE feature, and zone updates will work fine.