Merge #8949: Be more agressive in getting connections to peers with relevant services.

4630479 Make dnsseed's definition of acute need include relevant services. (Gregory Maxwell)
9583477 Be more aggressive in connecting to peers with relevant services. (Gregory Maxwell)

src/net.cpp

@@ -1463,12 +1463,19 @@ static std::string GetDNSHost(const CDNSSeedData& data, ServiceFlags* requiredSe
 void CConnman::ThreadDNSAddressSeed()
 {
     // goal: only query DNS seeds if address need is acute
+    // Avoiding DNS seeds when we don't need them improves user privacy by
+    //  creating fewer identifying DNS requests, reduces trust by giving seeds
+    //  less influence on the network topology, and reduces traffic to the seeds.
     if ((addrman.size() > 0) &&
         (!GetBoolArg("-forcednsseed", DEFAULT_FORCEDNSSEED))) {
         MilliSleep(11 * 1000);
 
         LOCK(cs_vNodes);
-        if (vNodes.size() >= 2) {
+        int nRelevant = 0;
+        for (auto pnode : vNodes) {
+            nRelevant += pnode->fSuccessfullyConnected && ((pnode->nServices & nRelevantServices) == nRelevantServices);
+        }
+        if (nRelevant >= 2) {
             LogPrintf("P2P peers available. Skipped DNS seeding.\n");
             return;
         }
@@ -1675,8 +1682,8 @@ void CConnman::ThreadOpenConnections()
             if (nANow - addr.nLastTry < 600 && nTries < 30)
                 continue;
 
-            // only consider nodes missing relevant services after 40 failed attempts
+            // only consider nodes missing relevant services after 40 failed attempts and only if less than half the outbound are up.
-            if ((addr.nServices & nRelevantServices) != nRelevantServices && nTries < 40)
+            if ((addr.nServices & nRelevantServices) != nRelevantServices && (nTries < 40 || nOutbound >= (nMaxOutbound >> 1)))
                 continue;
 
             // do not allow non-default ports, unless after 50 invalid addresses selected already