update forwarding denied

[Len Conrad]

>2) How do I determine the IP address of the machine that is sending
>    the DDNS packet?

query logging, as always.

>   I am assuming that the IP address contained in the
>    message is the address of the master nameserver for the zone that
>    the DDNS packet is trying to update.

It sends the DNS query packet with update opcode to the ip of the slave. 
The slave's query logging will show the source of the update query.  The 
contents of the update packet would not contain the ip of the NS to be 
updated.

As you must know, you really should run updates with TSIG only.

Len