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relay: Reconfigure libevent options only on DNS params change

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Related #40312

Signed-off-by: David Goulet <dgoulet@torproject.org>
This commit is contained in:
David Goulet 2022-03-15 15:33:35 -04:00
parent 2cdb5ceb1d
commit 7ce17c2b00
1 changed files with 59 additions and 44 deletions
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103
src/feature/relay/dns.c
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@ -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;