commit 98275743081728b1538f70111c891ea573127e85
parent 04d4786cc40b2d80b4cd34e7d5339b0100861c8f
Author: Sebastian Hahn <sebastian@torproject.org>
Date: Fri, 3 Nov 2017 03:06:12 +0100
Fix DNS resolution on busy exit relays
Thanks to Dhalgren who analyzed this issue two years ago and found a
solution!
Diffstat:
2 files changed, 29 insertions(+), 3 deletions(-)
diff --git a/changes/bug21394 b/changes/bug21394
@@ -0,0 +1,9 @@
+ o Major bugfixes (Exit nodes):
+ - Fix an issue causing high-bandwidth exit nodes to fail a majority
+ or all of their DNS requests, making them basically unsuitable for
+ regular usage in Tor circuits. The problem is related to
+ libevent's DNS handling, but we can work around it in Tor. Fixes
+ bugs 21394 and 18580; bugfix on 0.1.2.2-alpha which introduced
+ eventdns. Credit goes to Dhalgren for identifying and finding a
+ workaround to this bug and to gamambel, arthuredelstein and
+ arma in helping to track it down and analyze it.
diff --git a/src/or/dns.c b/src/or/dns.c
@@ -1425,14 +1425,31 @@ configure_nameservers(int force)
#define SET(k,v) evdns_base_set_option(the_evdns_base, (k), (v))
+ // If we only have one nameserver, it does not make sense to back off
+ // from it for a timeout. Unfortunately, the value for max-timeouts is
+ // currently clamped by libevent to 255, but it does not hurt to set
+ // it higher in case libevent gets a patch for this.
+ // Reducing attempts in the case of just one name server too, because
+ // it is very likely to be a local one where a network connectivity
+ // issue should not cause an attempt to fail.
if (evdns_base_count_nameservers(the_evdns_base) == 1) {
- SET("max-timeouts:", "16");
- SET("timeout:", "10");
+ SET("max-timeouts:", "1000000");
+ SET("attempts:", "1");
} else {
SET("max-timeouts:", "3");
- SET("timeout:", "5");
}
+ // Elongate the queue of maximum inflight dns requests, so if a bunch
+ // time out at the resolver (happens commonly with unbound) we won't
+ // stall every other DNS request. This potentially means some wasted
+ // CPU as there's a walk over a linear queue involved, but this is a
+ // much better tradeoff compared to just failing DNS requests because
+ // of a full queue.
+ SET("max-inflight:", "8192");
+
+ // Time out after 5 seconds if no reply.
+ SET("timeout:", "5");
+
if (options->ServerDNSRandomizeCase)
SET("randomize-case:", "1");
else
