Commit 347eaa32 authored by David Goulet's avatar David Goulet 🐼
Browse files

relay: Lower DNS Exit-side timeout 



Introduces two new consensus parameter:

  exit_dns_timeout: Number of seconds before libevent should consider
  the DNS request a timeout.

  exit_dns_num_attempts: Number of attempts that libeven should retry a
  previously failing query before calling it a timeout.

Closes #40312

Signed-off-by: David Goulet's avatarDavid Goulet <dgoulet@torproject.org>
parent 17a8b3c7

changes/ticket40312

0 → 100644
+5 −0
Original line number Diff line number Diff line 
  o Major bugfixes (relay, DNS):

    - Lower the DNS timeout from 3 attempts at 5 seconds each to 2 attempts at

      1 seconds each. Two new consensus parameters were added to control these

      values. See ticket for more details on why. Fixes bug 40312; bugfix on

      0.3.5.1-alpha.

src/feature/relay/dns.c

+62 −6
Original line number Diff line number Diff line
@@ -61,6 +61,7 @@ 
#include "core/or/relay.h"

#include "feature/control/control_events.h"

#include "feature/relay/dns.h"

#include "feature/nodelist/networkstatus.h"

#include "feature/relay/router.h"

#include "feature/relay/routermode.h"

#include "feature/stats/rephist.h"
@@ -1353,6 +1354,54 @@ configured_nameserver_address(const size_t idx) 
}

#endif /* defined(HAVE_EVDNS_BASE_GET_NAMESERVER_ADDR) */



/** Return a pointer to a stack allocated buffer containing the string

 * representation of the exit_dns_timeout consensus parameter. */

static const char *

get_consensus_param_exit_dns_timeout(void)

{

  static char str[4];



  /* Get the Exit DNS timeout value from the consensus or default. This is in

   * milliseconds. */

#define EXIT_DNS_TIMEOUT_DEFAULT (1000)

#define EXIT_DNS_TIMEOUT_MIN (1)

#define EXIT_DNS_TIMEOUT_MAX (120000)

  int32_t val = networkstatus_get_param(NULL, "exit_dns_timeout",

                                        EXIT_DNS_TIMEOUT_DEFAULT,

                                        EXIT_DNS_TIMEOUT_MIN,

                                        EXIT_DNS_TIMEOUT_MAX);

  /* NOTE: We convert it to seconds because libevent only supports that. In the

   * future, if we support different resolver(s), we might want to specialize

   * this call. */



  /* NOTE: We also don't allow 0 and so we must cap the division to 1 second

   * else all DNS request would fail if the consensus would ever tell us a

   * value below 1000 (1 sec). */

  val = MAX(1, val / 1000);



  tor_snprintf(str, sizeof(str), "%d", val);

  return str;

}



/** Return a pointer to a stack allocated buffer containing the string

 * representation of the exit_dns_num_attempts consensus parameter. */

static const char *

get_consensus_param_exit_dns_attempts(void)

{

  static char str[4];



  /* Get the Exit DNS number of attempt value from the consensus or default. */

#define EXIT_DNS_NUM_ATTEMPTS_DEFAULT (2)

#define EXIT_DNS_NUM_ATTEMPTS_MIN (0)

#define EXIT_DNS_NUM_ATTEMPTS_MAX (255)

  int32_t val = networkstatus_get_param(NULL, "exit_dns_num_attempts",

                                        EXIT_DNS_NUM_ATTEMPTS_DEFAULT,

                                        EXIT_DNS_NUM_ATTEMPTS_MIN,

                                        EXIT_DNS_NUM_ATTEMPTS_MAX);

  tor_snprintf(str, sizeof(str), "%d", val);

  return str;

}



/** 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
@@ -1489,12 +1538,19 @@ configure_nameservers(int force) 
  // of a full queue.

  SET("max-inflight:", "8192");



  // Two retries at 5 and 10 seconds for bind9/named which relies on

  // clients to handle retries.  Second retry for retried circuits with

  // extended 15 second timeout.  Superfluous with local-system Unbound

  // instance--has its own elaborate retry scheme.

  SET("timeout:", "5");

  SET("attempts:","3");

  /* 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");