[tor-bugs] #12582 [Ooni]: Add support for detecting the DNS resolver used by a probe

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#12582: Add support for detecting the DNS resolver used by a probe
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 Reporter:  hellais      |          Owner:  hellais
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:
Component:  Ooni         |        Version:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
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 Currently when running the dns_consistency test if no resolvers are
 specified we attempt to read it from /etc/resolv.conf. Sometimes what is
 found inside of resolv.conf is the actual address of a DNS resolver, but
 it may be something in the local network.

 To understand where censorship is actually happening it would be ideal to
 know the address of the true resolver that is being used. This can be
 achieved by delegating a subdomain of ooni.nu to a nameserver that is
 controlled by us. This way every time a DNS query for
 <RANDOM_STRING>.<SUBDOMAIN>.ooni.nu is done we will see a query to our
 nameserver.

 This nameserver will simply return to every A query the IP address that
 issued the request. A test helper for this has already been implemented:
 https://github.com/TheTorProject/ooni-
 backend/blob/master/oonib/testhelpers/dns_helpers.py#L26.

 What needs to be done is added support to ooni-probe for performing this
 lookup and using the data in the report.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12582>
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