Книга: Fedora™ Unleashed, 2008 edition

Maintaining Accurate Serial Numbers

Maintaining Accurate Serial Numbers

Accurate serial numbers are very important to the correct operation of slave servers. An increase in the serial number of a zone causes slaves to reload the zone and update their local caches.

A common mistake that system administrators make is forgetting to increment the serial number after a change to the zone data. If you make this mistake, secondary nameservers don't reload the zone, and continue to serve old data. If you suspect that the data on the master and slave servers is out of sync, you can use dig to view the SOA record for the zone on each server (dig @master domain SOA and dig @slave domain SOA) and compare the serial numbers in the responses.

Another common problem is setting the serial number to an incorrect value—either too small or too large. A too-small serial number causes slaves to think that they possess a more up-to-date copy of the zone data, but this is easily corrected by increasing the serial number as necessary. A too-large serial number is more problematic and requires more elaborate measures to repair.

Serial number comparisons are defined in such a way that if a serial number — when subtracted from another with no overflow correction — results in a positive number, the second number is newer than the first, and a zone transfer is required. (See RFC 1982, "Serial Number Arithmetic," for details.) You can exploit this property by temporarily setting the serial number to 2?? (4,294,967,296), waiting for all the slaves to reload the zone, and then setting it to the correct number.

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