"SOA Serial Number Format is Invalid" warning by mxtoolbox.com - why?

Solution 1:

There is a recommendation that the SOA serial number use a format that is four digits of year, two digits of month, two digits of day and two digits of count of changes in the same day. This format is common, but far from universal (look at .COM for a high-profile example of a zone that doesn't). The tool you got the error message from is oversensitive and should be adjusted.

Solution 2:

The SOA SERIAL field is specified to be an an unsigned integer value that has special rules for how it wraps around, and consequently also for how serial numbers are compared, etc.

RFC1035 defines this field as:

SERIAL

The unsigned 32 bit version number of the original copy of the zone. Zone transfers preserve this value. This value wraps and should be compared using sequence space arithmetic.

The arithmetic of serial numbers is explained in detail in RFC1982.

Anyway, the popular YYYYMMDDnn "format" is just a convention for picking integer values such that, when written out in decimal, convey some meaningful information to humans (may be helpful when troubleshooting). The use of such values have no special meaning in the system itself and using values that do not follow this convention is not an error.