[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:  rl1987
     Type:  defect                               |         Status:
                                                 |  accepted
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  technical-debt, dns, crash, tor-     |  Actual Points:
  relay, macos, 032-unreached                    |
Parent ID:                                       |         Points:  2
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by rl1987):

 To reproduce, run:

 {{{
 src/app/tor -ORPort 4430 -ServerDNSResolvConfFile no_such_file -SOCKSPort
 9053 -Log debug
 }}}
 and

 {{{
 src/app/tor -ORPort 4430 -ServerDNSResolvConfFile /dev/null -SOCKSPort
 9053 -Log debug
 }}}

 The latter yields (on my macOS system):

 {{{
 Aug 13 17:07:34.000 [debug] configure_nameservers: stat()ing /dev/null
 Aug 13 17:07:34.000 [info] configure_nameservers: Parsing resolver
 configuration in '/dev/null'
 Aug 13 17:07:34.000 [info] eventdns: Parsing resolv.conf file /dev/null
 Aug 13 17:07:34.000 [info] eventdns: Added nameserver 127.0.0.1:53 as
 0x7fa1201bc490
 Aug 13 17:07:34.000 [warn] Unable to parse '/dev/null', or no nameservers
 in '/dev/null' (6)
 Aug 13 17:07:34.000 [info] mark_my_descriptor_dirty: Decided to publish
 new relay descriptor: dns resolvers failed
 Aug 13 17:07:34.000 [warn] Couldn't set up any working nameservers.
 Network not up yet?  Will try again soon.
 }}}

 Note that empty resolv.conf is still treated as error condition - by
 libevent code, not by tor. Libevent also falls back to including 127.0.0.1
 as default DNS server. See
 https://github.com/libevent/libevent/blob/master/evdns.c#L3649

 So I suppose we should:

  * When resolv.conf is empty or non-existant:
    * Don't `goto err`
    * Add 127.0.0.1 to list of DNS servers.
    * Print a scary warning.
    * Mention the above behavior in manpage.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21900#comment:9>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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