commit 98275743081728b1538f70111c891ea573127e85 Author: Sebastian Hahn sebastian@torproject.org Date: Fri Nov 3 03:06:12 2017 +0100

Fix DNS resolution on busy exit relays

Thanks to Dhalgren who analyzed this issue two years ago and found a solution! --- changes/bug21394 | 9 +++++++++ src/or/dns.c | 23 ++++++++++++++++++++--- 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/changes/bug21394 b/changes/bug21394 new file mode 100644 index 000000000..e5452e20b --- /dev/null +++ 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 index 0ad4c0f50..c1e3c3256 100644 --- 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