In the last post we started talking about the SOA record in DNS and the serial number used in a zone file.  I promised that in this post we’d talk about why we use serial numbers.

Hint:  It’s to cut down on traffic.

Chaos in the Traffic

These days it seems you can stream anything, play games, play music, and there’s always enough bandwidth to go around.  What’s bandwidth?  That’s the size of your connection to the Internet and connections within the Internet.  In the old days, the amount of bandwidth was really small.  You could sit at a terminal and type things on a connection far away and hit a key, wait for it to appear, hit the next key, wait for it to appear.

Slow typing

In other words, there wasn’t much bandwidth available.

People came up with tricks to reduce bandwidth usage and one of those tricks was a serial number.

An operation name servers use to share zone files amongst themselves is called a zone transfer.  You can imagine that some of these files get really big.  If there isn’t a lot of bandwidth, then constantly passing those files back and forth is a bad thing.

Instead, you have the serial number and you ask the other side ‘excuse me, has your serial number increased from the value I have here?’ and if the answer is ‘yes’, then the zone transfer occurs.  Otherwise, the zone file stays where it is.

This means that when the administrator makes any change to the zone file, he has to update the serial number in order for the transfer to happen.

Now for the obvious question:  Can everyone do zone transfers?

Well, the answer is no.  It’s a security problem if we do let everyone transfer.

Who can do transfers?  Like we said before, there’s a master name server listed in the SOA record.  We also know that most domains have more than one name server. For example, Google has four:

• google.com name server ns3.google.com.
• google.com name server ns1.google.com.
• google.com name server ns2.google.com.
• google.com name server ns4.google.com.

Imagine having to update the zone file on each one of those.  No thank you!  That’s what the master name server is for, you update that name server and then the other three can zone transfer your zone file from it, assuming the serial number has increased.  Those other three name servers are called slaves to the master name server or secondaries to the primary name server.  We’ll call them secondary or secondaries.  It’s an older term, but I like it.