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

Question

When testing the SOA setting for example-domain.org on http://mxtoolbox.com/, it says that

SOA Serial Number Format is Invalid

The entry is

ns-885.awsdns-46.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400

That, however, is exactly what Amazon suggest in their Route 53 documentation on http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/SOA-NSrecords.html

mxtoolbox issues a warning - why? They also consider the missing DMARC settings as an error.

Please bear with me - I am not a sysadmin. Any hint that uses a language that a developer can understand is greatly appreciated.

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.

Solution 3

mxtoolbox issues a warning when your serial does not match XXXXMMDDnn in their time zone. Assuming you are east of their time zone, your serial might be considered to be a day in the future and not meet their suggested format, thus yielding to a warning.

Try again in some hours without touching the serial and the warning will be gone.

Comments

[Andrew B]

Yeah, so long as the number is incrementing the specific syntax doesn't matter. mxtoolbox is being a nuisance here.

[Håkan Lindqvist]

It's important to note that what actually constitutes a greater/lower value is not immediately intuitive for large differences because of the special rules to allow for the controlled wrap around. See tools.ietf.org/html/rfc1982 for all the details.