[tor-bugs] #21903 [Core Tor/Chutney]: Disable DNS in chutney by default, and add an option to enable it

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#21903: Disable DNS in chutney by default, and add an option to enable it
------------------------------+--------------------------------
 Reporter:  teor              |          Owner:  teor
     Type:  defect            |         Status:  needs_revision
 Priority:  Medium            |      Milestone:
Component:  Core Tor/Chutney  |        Version:
 Severity:  Normal            |     Resolution:
 Keywords:                    |  Actual Points:
Parent ID:  #19573            |         Points:  0.5
 Reviewer:                    |        Sponsor:
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Comment (by teor):

 Here's a design for this:
 * If the default ServerDNSResolvConfFile is missing, or is a symlink with
 a missing target (thanks, Apple!), chutney applies a workaround
 `ServerDNSResolvConfFile /dev/null`
 * An environmental variable can be used to set `ServerDNSResolvConfFile`
   * This can be used to implement --offline: CHUTNEY_DNS_CONF=/dev/null
   * This can be used to implement --dns-conf=X: CHUTNEY_DNS_CONF=X
 * An environmental variable can be used to not set
 `ServerDNSResolvConfFile`
   * This can be used to implement --dns-conf-default: CHUTNEY_DNS_CONF=""

 To fix the crash on SETCONF in #21900, the user needs to supply a DNS conf
 with at least one nameserver.

 To provide a conf that simultaneously:
 * works offline: CHUTNEY_DNS_CONF=/path/to/empty/or/working/conf
 * and doesn't crash on SETCONF:
 CHUTNEY_DNS_CONF=/path/to/conf/with/a/dns/server

 The user must supply a local DNS server that gives the right answers to
 tor (#19573).
 Or we can fix #21900, and just say /dev/null.
 (We should document this in the chutney README.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21903#comment:7>
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