relay: Reconfigure libevent options only on DNS params change (7ce17c2b) · Commits · Gus / Tor

src/feature/relay/dns.c

+59 −44

Original line number	Diff line number	Diff line
		@@ -111,6 +111,7 @@ static int answer_is_wildcarded(const char *ip);
		static int evdns_err_is_transient(int err);
		static void inform_pending_connections(cached_resolve_t *resolve);
		static void make_pending_resolve_cached(cached_resolve_t *cached);
		static void configure_libevent_options(void);

		#ifdef DEBUG_DNS_CACHE
		static void assert_cache_ok_(void);
		@@ -222,7 +223,7 @@ dns_new_consensus_params(const networkstatus_t *ns)
		/* Consensus has parameters for the Exit relay DNS side and so we only reset
		* the DNS nameservers if we are in server mode. */
		if (server_mode(get_options())) {
		dns_reset();
		configure_libevent_options();
		}
		}

		@@ -1415,6 +1416,60 @@ get_consensus_param_exit_dns_attempts(void)
		return str;
		}

		/** Configure the libevent options. This can be called after initialization.
		* This should never be called without the evdns base pointer initialized. */
		static void
		configure_libevent_options(void)
		{
		if (BUG(!the_evdns_base)) {
		return;
		}

		#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. Higher-than-
		// default maximum of 3 with multiple nameservers to avoid spuriously
		// marking one down on bursts of timeouts resulting from scans/attacks
		// against non-responding authoritative DNS servers.
		if (evdns_base_count_nameservers(the_evdns_base) == 1) {
		SET("max-timeouts:", "1000000");
		} else {
		SET("max-timeouts:", "10");
		}

		// Elongate the queue of maximum inflight dns requests, so if a bunch
		// remain pending 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");

		/* Set timeout to be 1 second. This tells libevent that it shouldn't wait
		* more than N second to drop a DNS query and consider it "timed out". It is
		* very important to differentiate here a libevent timeout and a DNS server
		* timeout. And so, by setting this to N second, libevent sends back
		* "DNS_ERR_TIMEOUT" if that N second is reached which does NOT indicate that
		* the query itself timed out in transit. */
		SET("timeout:", get_consensus_param_exit_dns_timeout());

		/* This tells libevent to attemps up to X times a DNS query if the previous
		* one failed to complete within N second. We believe that this should be
		* enough to catch temporary hiccups on the first query. But after that, it
		* should signal us that it won't be able to resolve it. */
		SET("attempts:", get_consensus_param_exit_dns_attempts());

		if (get_options()->ServerDNSRandomizeCase)
		SET("randomize-case:", "1");
		else
		SET("randomize-case:", "0");

		#undef SET
		}

		/** Configure eventdns nameservers if force is true, or if the configuration
		* has changed since the last time we called this function, or if we failed on
		* our last attempt. On Unix, this reads from /etc/resolv.conf or
		@@ -1528,50 +1583,10 @@ configure_nameservers(int force)
		}
		#endif /* defined(_WIN32) */

		#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. Higher-than-
		// default maximum of 3 with multiple nameservers to avoid spuriously
		// marking one down on bursts of timeouts resulting from scans/attacks
		// against non-responding authoritative DNS servers.
		if (evdns_base_count_nameservers(the_evdns_base) == 1) {
		SET("max-timeouts:", "1000000");
		} else {
		SET("max-timeouts:", "10");
		}

		// Elongate the queue of maximum inflight dns requests, so if a bunch
		// remain pending 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");

		/* Set timeout to be 1 second. This tells libevent that it shouldn't wait
		* more than N second to drop a DNS query and consider it "timed out". It is
		* very important to differentiate here a libevent timeout and a DNS server
		* timeout. And so, by setting this to N second, libevent sends back
		* "DNS_ERR_TIMEOUT" if that N second is reached which does NOT indicate that
		* the query itself timed out in transit. */
		SET("timeout:", get_consensus_param_exit_dns_timeout());

		/* This tells libevent to attemps up to X times a DNS query if the previous
		* one failed to complete within N second. We believe that this should be
		* enough to catch temporary hiccups on the first query. But after that, it
		* should signal us that it won't be able to resolve it. */
		SET("attempts:", get_consensus_param_exit_dns_attempts());

		if (options->ServerDNSRandomizeCase)
		SET("randomize-case:", "1");
		else
		SET("randomize-case:", "0");

		#undef SET
		/* Setup libevent options. */
		configure_libevent_options();

		/* Relaunch periodical DNS check event. */
		dns_servers_relaunch_checks();

		nameservers_configured = 1;