Make dnsseed's definition of acute need include relevant services.

We normally prefer to connect to peers offering the relevant services.

If we're not connected to enough peers with relevant services, we
 probably don't know about them and could use dnsseed's help.

Author: gmaxwell

src/net.cpp

@@ -1462,12 +1462,19 @@ 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;
         }