[tor-bugs] #21900 [Core Tor/Tor]: evdns fails when resolv.conf is missing, but succeeds when resolv.conf is empty

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#21900: evdns fails when resolv.conf is missing, but succeeds when resolv.conf is
empty
------------------------------+---------------------------------
     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  technical-debt, dns
Actual Points:                |  Parent ID:
       Points:  2             |   Reviewer:
      Sponsor:                |
------------------------------+---------------------------------
 When tor's ServerDNSResolvConfFile (default /etc/resolv.conf) is missing,
 evdns does not add any name servers, and therefore Exits do not allow any
 exit traffic (not even IP-based traffic):
 {{{
 [debug] configure_nameservers: stat()ing /etc/resolv.conf
 [warn] Unable to stat resolver configuration in '/etc/resolv.conf': No
 such file or directory
 [info] mark_my_descriptor_dirty: Decided to publish new relay descriptor:
 dns resolvers failed
 }}}

 This happens on macOS when the network is down. On macOS, /etc/resolv.conf
 is symlinked to /var/run/resolv.conf. When the network is down, macOS
 deletes /var/run/resolv.conf, so the stat() call on /etc/resolv.conf
 fails.

 But when tor's ServerDNSResolvConfFile is empty, evdns adds a default name
 server (127.0.0.1:53 on my macOS), and therefore Exits allow exit traffic:
 {{{
 [info] eventdns: Parsing resolv.conf file /dev/null
 [info] eventdns: Added nameserver 127.0.0.1:53 as 0x615000009e00
 [info] mark_my_descriptor_dirty: Decided to publish new relay descriptor:
 dns resolvers failed
 [info] eventdns: Parsing resolv.conf file /dev/null
 ...
 [info] mark_my_descriptor_dirty: Decided to publish new relay descriptor:
 dns resolvers back
 }}}

 We should also stop the extra descriptor upload:
 {{{
 [info] mark_my_descriptor_dirty: Decided to publish new relay descriptor:
 dns resolvers failed
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21900>
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