nsdc update implementation details

[Jelte Jansen]

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David Prangnell wrote:
> My NSD server is accepting notifications from two servers.  From my  
> nsd.conf:
> 	# master 1
>          allow-notify: X.X.X.X NOKEY
>          request-xfr: AXFR X.X.X.X NOKEY
>          # master 2
>          allow-notify: Y.Y.Y.Y NOKEY
>          request-xfr: AXFR Y.Y.Y.Y NOKEY
> 
> Are both servers sequentially queried each time the update is run?   
> Or is it a random one of the two servers that answers and the second  
> one ignored?
> 

Subsequent master servers are queried if the previous server either
didn't respond or had no updates. They are always queried in the order
of the configuration file.

So if master 1 answers, but with the same SOA serial as the current one,
master 2 is queried too. If master 1 has a higher serial, an XFR is
performed with master 1, and master 2 is not queried.

Jelte
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