[tor-bugs] #18642 [Core Tor/Tor]: Teach the OOM handler about the DNS cache

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#18642: Teach the OOM handler about the DNS cache
-------------------------------------------------+-------------------------
 Reporter:  nickm                                |          Owner:  (none)
     Type:  enhancement                          |         Status:
                                                 |  needs_review
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.5.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-dos, oom, tor-relay, dns, 035    |  Actual Points:
  -triaged-in-20180711                           |
Parent ID:                                       |         Points:  1
 Reviewer:  dgoulet                              |        Sponsor:
                                                 |  SponsorV-can
-------------------------------------------------+-------------------------

Comment (by teor):

 Replying to [comment:23 dgoulet]:
 > Replying to [comment:22 neel]:
 > > If I have to make something, I am thinking about something like:
 `OOMHandlerClearOnlyDNS (0/1)` where 0 is the default (run all OOM checks)
 and 1 is to only run OOM on DNS (if you're an exit).
 >
 > Wait no, no need for that at all. What I was saying is that we should
 only run that OOM handler in the case tor is running as an Exit and one
 way to look at that is with `ExitRelay 1` global option ;).

 That's not how ExitRelay works:

     ExitRelay 0|1|auto

     Tells Tor whether to run as an exit relay. If Tor is running as a non-
 bridge server, and ExitRelay is set to 1, then Tor allows traffic to exit
 according to the ExitPolicy option (or the default ExitPolicy if none is
 specified).

     If ExitRelay is set to 0, no traffic is allowed to exit, and the
 ExitPolicy option is ignored.

     If ExitRelay is set to "auto", then Tor behaves as if it were set to
 1, but warns the user if this would cause traffic to exit. In a future
 version, the default value will be 0. (Default: auto)

 https://www.torproject.org/docs/tor-manual.html.en

 Instead, try something like:
 {{{
 smartlist_t *exit_policy = router_get_my_routerinfo()->exit_policy;
 if (!policy_is_reject_star(exit_policy, AF_INET) ||
 !policy_is_reject_star(exit_policy, AF_INET6)) {
   /* Run the OOM handler on DNS */
 }
 }}}

 But that won't handle OOM when the operator has turned exiting off, but
 used to have it on. So instead, maybe we should:
 * always run the OOM handler on DNS
 * check the approximate size of the DNS cache before running the OOM
 handler on it, or
 * set a flag when we run the OOM handler and we're not an exit, then clear
 the flag when we become an exit.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18642#comment:27>
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