Should we warn when exit nodes are using opendns or google dns?

Somewhat related to discussion on legacy/trac#8093 (moved) -- people are still setting up exit nodes to use OpenDNS or Google DNS. Is that really a safe idea? That makes it distressingly easy for these DNS services (or anybody watching them) to get timing information on user DNS requests.

Furthermore, the default OpenDNS configuration blocks some stuff. If we don't warn about OpenDNS in general, maybe we should warn when configuring an OpenDNS server in a way that hasn't disabled blocking.

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