relay: Overload state on DNS timeout is now X% over Y secs

With this commit, we will only report a general overload state if we've
seen more than X% of DNS timeout errors over Y seconds. Previous
behavior was to report when a single timeout occured which is really too
small of a threshold.

The value X is a consensus parameters called
"overload_dns_timeout_scale_percent" which is a scaled percentage
(factor of 1000) so we can represent decimal points for X like 0.5% for
instance. Its default is 1000 which ends up being 1%.

The value Y is a consensus parameters called
"overload_dns_timeout_period_secs" which is the time period for which
will gather DNS errors and once over, we assess if that X% has been
reached ultimately triggering a general overload signal.

Closes #40491

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

4 files changed, 142 insertions, 10 deletions

diff --git a/src/feature/nodelist/networkstatus.c b/src/feature/nodelist/networkstatus.c
index 2ffa6da1a3..af808a6ba7 100644
--- a/src/feature/nodelist/networkstatus.c
+++ b/src/feature/nodelist/networkstatus.c
@@ -103,6 +103,7 @@
 #include "feature/dirauth/vote_microdesc_hash_st.h"
 #include "feature/nodelist/vote_routerstatus_st.h"
 #include "feature/nodelist/routerstatus_st.h"
+#include "feature/stats/rephist.h"
 
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
@@ -1663,6 +1664,7 @@ notify_before_networkstatus_changes(const networkstatus_t *old_c,
   dos_consensus_has_changed(new_c);
   relay_consensus_has_changed(new_c);
   hs_dos_consensus_has_changed(new_c);
+  rep_hist_consensus_has_changed(new_c);
 }
 
 /* Called after a new consensus has been put in the global state. It is safe
diff --git a/src/feature/relay/dns.c b/src/feature/relay/dns.c
index 22f929808e..ed8d235e92 100644
--- a/src/feature/relay/dns.c
+++ b/src/feature/relay/dns.c
@@ -1548,16 +1548,6 @@ evdns_callback(int result, char type, int count, int ttl, void *addresses,
 
   tor_addr_make_unspec(&addr);
 
-  /* Note down any DNS errors to the statistics module */
-  if (result == DNS_ERR_TIMEOUT) {
-    /* libevent timed out while resolving a name. However, because libevent
-     * handles retries and timeouts internally, this means that all attempts of
-     * libevent timed out. If we wanted to get more granular information about
-     * individual libevent attempts, we would have to implement our own DNS
-     * timeout/retry logic */
-    rep_hist_note_overload(OVERLOAD_GENERAL);
-  }
-
   /* Keep track of whether IPv6 is working */
   if (type == DNS_IPv6_AAAA) {
     if (result == DNS_ERR_TIMEOUT) {
@@ -1659,6 +1649,10 @@ evdns_callback(int result, char type, int count, int ttl, void *addresses,
     dns_found_answer(string_address, orig_query_type,
                      result, &addr, hostname, ttl);
 
+  /* The result can be changed within this function thus why we note the result
+   * at the end. */
+  rep_hist_note_dns_query(type, result);
+
   tor_free(arg_);
 }
 
diff --git a/src/feature/stats/rephist.c b/src/feature/stats/rephist.c
index e25c01331d..c3a281a8c2 100644
--- a/src/feature/stats/rephist.c
+++ b/src/feature/stats/rephist.c
@@ -84,6 +84,8 @@
 #include "feature/nodelist/networkstatus_st.h"
 #include "core/or/or_circuit_st.h"
 
+#include <event2/dns.h>
+
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>
 #endif
@@ -204,6 +206,54 @@ typedef struct {
   uint64_t overload_fd_exhausted;
 } overload_stats_t;
 
+/***** DNS statistics *****/
+
+/** Represents the statistics of DNS queries seen if it is an Exit. */
+typedef struct {
+  /** Total number of DNS request seen at an Exit. They might not all end
+   * successfully or might even be lost by tor. This counter is incremented
+   * right before the DNS request is initiated. */
+  uint64_t stats_n_request;
+
+  /** Total number of DNS timeout errors. */
+  uint64_t stats_n_error_timeout;
+
+  /** When is the next assessment time of the general overload for DNS errors.
+   * Once this time is reached, all stats are reset and this time is set to the
+   * next assessment time. */
+  time_t next_assessment_time;
+} overload_dns_stats_t;
+
+/** Keep track of the DNS requests for the general overload state. */
+static overload_dns_stats_t overload_dns_stats;
+
+/* We use a scale here so we can represent percentages with decimal points by
+ * scaling the value by this factor and so 0.5% becomes a value of 500.
+ * Default is 1% and thus min and max range is 0 to 100%. */
+#define OVERLOAD_DNS_TIMEOUT_PERCENT_SCALE 1000.0
+#define OVERLOAD_DNS_TIMEOUT_PERCENT_DEFAULT 1000
+#define OVERLOAD_DNS_TIMEOUT_PERCENT_MIN 0
+#define OVERLOAD_DNS_TIMEOUT_PERCENT_MAX 100000
+
+/** Consensus parameter: indicate what fraction of DNS timeout errors over the
+ * total number of DNS requests must be reached before we trigger a general
+ * overload signal .*/
+static double overload_dns_timeout_fraction =
+   OVERLOAD_DNS_TIMEOUT_PERCENT_DEFAULT /
+   OVERLOAD_DNS_TIMEOUT_PERCENT_SCALE / 100.0;
+
+/* Number of seconds for the assessment period. Default is 10 minutes (600) and
+ * the min max range is within a 32bit value. */
+#define OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_DEFAULT (10 * 60)
+#define OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_MIN 0
+#define OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_MAX INT32_MAX
+
+/** Consensus parameter: Period, in seconds, over which we count the number of
+ * DNS requests and timeout errors. After that period, we assess if we trigger
+ * an overload or not. */
+static int32_t overload_dns_timeout_period_secs =
+  OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_DEFAULT;
+
 /** Current state of overload stats */
 static overload_stats_t overload_stats;
 
@@ -218,6 +268,89 @@ overload_happened_recently(time_t overload_time, int n_hours)
   return false;
 }
 
+/** Assess the DNS timeout errors and if we have enough to trigger a general
+ * overload. */
+static void
+overload_general_dns_assessment(void)
+{
+  /* Initialize the time. Should be done once. */
+  if (overload_dns_stats.next_assessment_time == 0) {
+    goto reset;
+  }
+
+  /* Not the time yet. */
+  if (overload_dns_stats.next_assessment_time > approx_time()) {
+    return;
+  }
+
+  /* Lets see if we can signal a general overload. */
+  double fraction = (double) overload_dns_stats.stats_n_error_timeout /
+                    (double) overload_dns_stats.stats_n_request;
+  if (fraction >= overload_dns_timeout_fraction) {
+    log_notice(LD_HIST, "General overload -> DNS timeouts (%" PRIu64 ") "
+               "fraction %.4f%% is above threshold of %.4f%%",
+               overload_dns_stats.stats_n_error_timeout,
+               fraction * 100.0,
+               overload_dns_timeout_fraction * 100.0);
+    rep_hist_note_overload(OVERLOAD_GENERAL);
+  }
+
+ reset:
+  /* Reset counters for the next period. */
+  overload_dns_stats.stats_n_error_timeout = 0;
+  overload_dns_stats.stats_n_request = 0;
+  overload_dns_stats.next_assessment_time =
+    approx_time() + overload_dns_timeout_period_secs;
+}
+
+/** Called just before the consensus will be replaced. Update the consensus
+ * parameters in case they changed. */
+void
+rep_hist_consensus_has_changed(const networkstatus_t *ns)
+{
+  overload_dns_timeout_fraction =
+    networkstatus_get_param(ns, "overload_dns_timeout_scale_percent",
+                            OVERLOAD_DNS_TIMEOUT_PERCENT_DEFAULT,
+                            OVERLOAD_DNS_TIMEOUT_PERCENT_MIN,
+                            OVERLOAD_DNS_TIMEOUT_PERCENT_MAX) /
+    OVERLOAD_DNS_TIMEOUT_PERCENT_SCALE / 100.0;
+
+  overload_dns_timeout_period_secs =
+    networkstatus_get_param(ns, "overload_dns_timeout_period_secs",
+                            OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_DEFAULT,
+                            OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_MIN,
+                            OVERLOAD_DNS_TIMEOUT_PERIOD_SECS_MAX);
+}
+
+/** Note a DNS error for the given given libevent DNS record type and error
+ * code. Possible types are: DNS_IPv4_A, DNS_PTR, DNS_IPv6_AAAA.
+ *
+ * IMPORTANT: Libevent is _not_ returning the type in case of an error and so
+ * if error is anything but DNS_ERR_NONE, the type is not usable and set to 0.
+ *
+ * See: https://gitlab.torproject.org/tpo/core/tor/-/issues/40490 */
+void
+rep_hist_note_dns_query(int type, uint8_t error)
+{
+  (void) type;
+
+  /* Assess if we need to trigger a general overload with regards to the DNS
+   * errors or not. */
+  overload_general_dns_assessment();
+
+  /* We only care about timeouts for the moment. */
+  switch (error) {
+  case DNS_ERR_TIMEOUT:
+    overload_dns_stats.stats_n_error_timeout++;
+    break;
+  default:
+    break;
+  }
+
+  /* Increment total number of requests. */
+  overload_dns_stats.stats_n_request++;
+}
+
 /* The current version of the overload stats version */
 #define OVERLOAD_STATS_VERSION 1
 
diff --git a/src/feature/stats/rephist.h b/src/feature/stats/rephist.h
index d4a2f301cf..749b4996a8 100644
--- a/src/feature/stats/rephist.h
+++ b/src/feature/stats/rephist.h
@@ -72,11 +72,14 @@ void rep_hist_seen_new_rp_cell(bool is_v2);
 char *rep_hist_get_hs_v3_stats_string(void);
 void rep_hist_hsdir_stored_maybe_new_v3_onion(const uint8_t *blinded_key);
 
+void rep_hist_note_dns_query(int type, uint8_t error);
+
 void rep_hist_free_all(void);
 
 void rep_hist_note_negotiated_link_proto(unsigned link_proto,
                                          int started_here);
 void rep_hist_log_link_protocol_counts(void);
+void rep_hist_consensus_has_changed(const networkstatus_t *ns);
 
 extern uint64_t rephist_total_alloc;
 extern uint32_t rephist_total_num;