add 'working DNS' to the list of mandatory requirements for the 'exit' flag
current requirements for the exit flag as per the spec:
https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2524
"Exit" -- A router is called an 'Exit' iff it allows exits to at least one /8 address space on each of ports 80 and 443. (Up until Tor version 0.3.2, the flag was assigned if relays exit to at least two of the ports 80, 443, and 6667.)
Recently the requirements for the exit flag have been changed to make 80+443 mandatory because exits only allowing 80 OR 443 would introduce to much breakage, the same is true for exits not able to resolve any DNS requests, their usefulness as an exit is limited.
https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2280
"Exit" if the router is more useful for building general-purpose exit circuits than for relay circuits.
So lets add the DNS requirement to the list of requirements for the exit flag.
The requirement should be automatically verified by dir auths by attempting DNS resolution for each exit candidate up to 5 times a day. If more than 2 resolution attempts fail the 'working DNS' requirement is not met. After 3 successful attempts no further attempts are necessary for that day.
Relays loosing the exit flag have a chance to regain it after being tested the next day again.