dirauth: Make voting flag threshold tunable via torrc

Remove UPTIME_TO_GUARANTEE_STABLE, MTBF_TO_GUARANTEE_STABLE,
TIME_KNOWN_TO_GUARANTEE_FAMILIAR WFU_TO_GUARANTEE_GUARD and replace each
of them with a tunnable torrc option.

Related to #40652

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket40652

   o Minor features (dirauth):
-    - Add an AuthDirVoteGuard torrc option that can allow authorities to assign
-      the Guard flag to the given fingerprints/country code/IPs. This is a
-      needed feature mostly for defense purposes in case a DoS hits the network
-      and relay start losing the Guard flags too fast. Closes ticket 40652.
+    - Add an AuthDirVoteGuard torrc option that can allow authorities to
+      assign the Guard flag to the given fingerprints/country code/IPs. This
+      is a needed feature mostly for defense purposes in case a DoS hits the
+      network and relay start losing the Guard flags too fast.
+    - Make UPTIME_TO_GUARANTEE_STABLE, MTBF_TO_GUARANTEE_STABLE,
+      TIME_KNOWN_TO_GUARANTEE_FAMILIAR WFU_TO_GUARANTEE_GUARD tunable from
+      torrc.
+    - Add a torrc option to control the Guard flag bandwidth threshold
+      percentile. Closes ticket 40652.

doc/man/tor.1.txt

@@ -3234,6 +3234,27 @@ on the public Tor network.
     nodes to vote Guard for regardless of their uptime and bandwidth. See
     <<ExcludeNodes,ExcludeNodes>> for more information on how to specify nodes.
 
+[[AuthDirVoteGuardBwThresholdFraction]] **AuthDirVoteGuardBwThresholdFraction** __FRACTION__::
+    The Guard flag bandwidth performance threshold fraction that is the
+    fraction representing who gets the Guard flag out of all measured
+    bandwidth. (Default: 0.75)
+
+[[AuthDirVoteGuardGuaranteeTimeKnown]] **AuthDirVoteGuardGuaranteeTimeKnown** __N__ **seconds**|**minutes**|**hours**|**days**|**weeks**::
+    A relay with at least this much weighted time known can be considered
+    familiar enough to be a guard. (Default: 8 days)
+
+[[AuthDirVoteGuardGuaranteeWFU]] **AuthDirVoteGuardGuaranteeWFU** __FRACTION__::
+    A level of weighted fractional uptime (WFU) is that is sufficient to be a
+    Guard. (Default: 0.98)
+
+[[AuthDirVoteStableGuaranteeMinUptime]] **AuthDirVoteStableGuaranteeMinUptime** __N__ **seconds**|**minutes**|**hours**|**days**|**weeks**::
+    If a relay's uptime is at least this value, then it is always considered
+    stable, regardless of the rest of the network. (Default: 30 days)
+
+[[AuthDirVoteStableGuaranteeMTBF]] **AuthDirVoteStableGuaranteeMTBF** __N__ **seconds**|**minutes**|**hours**|**days**|**weeks**::
+    If a relay's mean time between failures (MTBF) is least this value, then
+    it will always be considered stable. (Default: 5 days)
+
 [[BridgePassword]] **BridgePassword** __Password__::
     If set, contains an HTTP authenticator that tells a bridge authority to
     serve all requested bridge information. Used by the (only partially

src/feature/dirauth/dirauth_config.c

@@ -434,6 +434,11 @@ dirauth_options_validate(const void *arg, char **msg)
            "Recommended*Versions.");
   }
 
+  if (options->AuthDirVoteGuardBwThresholdFraction > 1.0 ||
+      options->AuthDirVoteGuardBwThresholdFraction < 0.0) {
+    REJECT("Guard bandwdith threshold fraction is invalid.");
+  }
+
   char *t;
   /* Call these functions to produce warnings only. */
   t = format_recommended_version_list(options->RecommendedClientVersions, 1);

src/feature/dirauth/dirauth_options.inc

@@ -79,6 +79,28 @@ CONF_VAR(RecommendedServerVersions, LINELIST, 0, NULL)
 /** Relays which should be voted Guard regardless of uptime and bandwidth. */
 CONF_VAR(AuthDirVoteGuard, ROUTERSET, 0, NULL)
 
+/** If a relay's uptime is at least this value, then it is always considered
+ * stable, regardless of the rest of the network. This way we resist attacks
+ * where an attacker doubles the size of the network using allegedly
+ * high-uptime nodes, displacing all the current guards. */
+CONF_VAR(AuthDirVoteStableGuaranteeMinUptime, INTERVAL, 0, "30 days")
+
+/** If a relay's MTBF is at least this value, then it is always stable. See
+ * above. */
+CONF_VAR(AuthDirVoteStableGuaranteeMTBF, INTERVAL, 0, "5 days")
+
+/** A relay with at least this much weighted time known can be considered
+ * familiar enough to be a guard. */
+CONF_VAR(AuthDirVoteGuardGuaranteeTimeKnown, INTERVAL, 0, "8 days")
+
+/** A relay with sufficient WFU is around enough to be a guard. */
+CONF_VAR(AuthDirVoteGuardGuaranteeWFU, DOUBLE, 0, "0.98")
+
+/** The Guard flag bandwidth performance threshold fraction that is the
+ * fraction representing who gets the Guard flag out of all measured
+ * bandwidth. */
+CONF_VAR(AuthDirVoteGuardBwThresholdFraction, DOUBLE, 0, "0.75")
+
 /** If an authority has been around for less than this amount of time, it
  * does not believe its reachability information is accurate.  Only
  * altered on testing networks. */

src/feature/dirauth/voteflags.c

@@ -36,24 +36,6 @@
 
 #include "lib/container/order.h"
 
-/** If a router's uptime is at least this value, then it is always
- * considered stable, regardless of the rest of the network. This
- * way we resist attacks where an attacker doubles the size of the
- * network using allegedly high-uptime nodes, displacing all the
- * current guards. */
-#define UPTIME_TO_GUARANTEE_STABLE (3600*24*30)
-/** If a router's MTBF is at least this value, then it is always stable.
- * See above.  (Corresponds to about 7 days for current decay rates.) */
-#define MTBF_TO_GUARANTEE_STABLE (60*60*24*5)
-/** Similarly, every node with at least this much weighted time known can be
- * considered familiar enough to be a guard.  Corresponds to about 20 days for
- * current decay rates.
- */
-#define TIME_KNOWN_TO_GUARANTEE_FAMILIAR (8*24*60*60)
-/** Similarly, every node with sufficient WFU is around enough to be a guard.
- */
-#define WFU_TO_GUARANTEE_GUARD (0.98)
-
 /* Thresholds for server performance: set by
  * dirserv_compute_performance_thresholds, and used by
  * generate_v2_networkstatus */
@@ -111,13 +93,13 @@ dirserv_thinks_router_is_unreliable(time_t now,
        */
       long uptime = real_uptime(router, now);
       if ((unsigned)uptime < stable_uptime &&
-          (unsigned)uptime < UPTIME_TO_GUARANTEE_STABLE)
+          uptime < dirauth_get_options()->AuthDirVoteStableGuaranteeMinUptime)
         return 1;
     } else {
       double mtbf =
         rep_hist_get_stability(router->cache_info.identity_digest, now);
       if (mtbf < stable_mtbf &&
-          mtbf < MTBF_TO_GUARANTEE_STABLE)
+          mtbf < dirauth_get_options()->AuthDirVoteStableGuaranteeMTBF)
         return 1;
     }
   }
@@ -325,13 +307,15 @@ dirserv_compute_performance_thresholds(digestmap_t *omit_as_sybil)
     /* (Now bandwidths is sorted.) */
     if (fast_bandwidth_kb < RELAY_REQUIRED_MIN_BANDWIDTH/(2 * 1000))
       fast_bandwidth_kb = bandwidths_kb[n_active/4];
+    int nth = (int)(n_active *
+                    dirauth_options->AuthDirVoteGuardBwThresholdFraction);
     guard_bandwidth_including_exits_kb =
-      third_quartile_uint32(bandwidths_kb, n_active);
+      find_nth_uint32(bandwidths_kb, n_active, nth);
     guard_tk = find_nth_long(tks, n_active, n_active/8);
   }
 
-  if (guard_tk > TIME_KNOWN_TO_GUARANTEE_FAMILIAR)
-    guard_tk = TIME_KNOWN_TO_GUARANTEE_FAMILIAR;
+  if (guard_tk > dirauth_options->AuthDirVoteGuardGuaranteeTimeKnown)
+    guard_tk = dirauth_options->AuthDirVoteGuardGuaranteeTimeKnown;
 
   {
     /* We can vote on a parameter for the minimum and maximum. */
@@ -379,15 +363,16 @@ dirserv_compute_performance_thresholds(digestmap_t *omit_as_sybil)
   } SMARTLIST_FOREACH_END(node);
   if (n_familiar)
     guard_wfu = median_double(wfus, n_familiar);
-  if (guard_wfu > WFU_TO_GUARANTEE_GUARD)
-    guard_wfu = WFU_TO_GUARANTEE_GUARD;
+  if (guard_wfu > dirauth_options->AuthDirVoteGuardGuaranteeWFU)
+    guard_wfu = dirauth_options->AuthDirVoteGuardGuaranteeWFU;
 
   enough_mtbf_info = rep_hist_have_measured_enough_stability();
 
   if (n_active_nonexit) {
+    int nth = (int)(n_active_nonexit *
+                    dirauth_options->AuthDirVoteGuardBwThresholdFraction);
     guard_bandwidth_excluding_exits_kb =
-      find_nth_uint32(bandwidths_excluding_exits_kb,
-                      n_active_nonexit, n_active_nonexit*3/4);
+      find_nth_uint32(bandwidths_excluding_exits_kb, n_active_nonexit, nth);
   }
 
   log_info(LD_DIRSERV,