If the dns records are morrored to the slave, then there are just 2 options: Either mydns is not the dns server that is listening on the slave or mydns can not connect to the database.
Please post the output of:
netstat -tap | grep dns
from the slave server and check the mydns.conf file if the login details for the mysql user and database are correct (e.g. test login details with the mysql command).
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