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>

1 files changed, 4 insertions, 10 deletions

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_);
 }