Do not query all DNS seed at once

Instead, when necessary, query 3. If that leads to a sufficient number
of connects, stop. If not, query 3 more, and so on.

src/net.cpp

@@ -49,6 +49,9 @@ static_assert(MINIUPNPC_API_VERSION >= 10, "miniUPnPc API version >= 10 assumed"
 // Dump addresses to peers.dat every 15 minutes (900s)
 static constexpr int DUMP_PEERS_INTERVAL = 15 * 60;
 
+/** Number of DNS seeds to query when the number of connections is low. */
+static constexpr int DNSSEEDS_TO_QUERY_AT_ONCE = 3;
+
 // We add a random period time (0 to 1 seconds) to feeler connections to prevent synchronization.
 #define FEELER_SLEEP_WINDOW 1
 
@@ -1508,35 +1511,41 @@ void StopMapPort()
 
 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) &&
-        (!gArgs.GetBoolArg("-forcednsseed", DEFAULT_FORCEDNSSEED))) {
-        if (!interruptNet.sleep_for(std::chrono::seconds(11)))
-            return;
+    FastRandomContext rng;
+    std::vector<std::string> seeds = Params().DNSSeeds();
+    Shuffle(seeds.begin(), seeds.end(), rng);
+    int seeds_right_now = 0; // Number of seeds left before testing if we have enough connections
+    int found = 0;
 
-        LOCK(cs_vNodes);
-        int nRelevant = 0;
-        for (const CNode* pnode : vNodes) {
-            nRelevant += pnode->fSuccessfullyConnected && !pnode->fFeeler && !pnode->fOneShot && !pnode->m_manual_connection && !pnode->fInbound;
-        }
-        if (nRelevant >= 2) {
-            LogPrintf("P2P peers available. Skipped DNS seeding.\n");
-            return;
-        }
+    if (gArgs.GetBoolArg("-forcednsseed", DEFAULT_FORCEDNSSEED)) {
+        // When -forcednsseed is provided, query all.
+        seeds_right_now = seeds.size();
     }
 
-    const std::vector<std::string> &vSeeds = Params().DNSSeeds();
-    int found = 0;
+    for (const std::string& seed : seeds) {
+        // goal: only query DNS seed 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 && seeds_right_now == 0) {
+            if (!interruptNet.sleep_for(std::chrono::seconds(11))) return;
 
-    LogPrintf("Loading addresses from DNS seeds (could take a while)\n");
+            LOCK(cs_vNodes);
+            int nRelevant = 0;
+            for (const CNode* pnode : vNodes) {
+                nRelevant += pnode->fSuccessfullyConnected && !pnode->fFeeler && !pnode->fOneShot && !pnode->m_manual_connection && !pnode->fInbound;
+            }
+            if (nRelevant >= 2) {
+                LogPrintf("P2P peers available. Skipped DNS seeding.\n");
+                return;
+            }
+            seeds_right_now += DNSSEEDS_TO_QUERY_AT_ONCE;
+        }
 
-    for (const std::string &seed : vSeeds) {
         if (interruptNet) {
             return;
         }
+        LogPrintf("Loading addresses from DNS seed %s\n", seed);
         if (HaveNameProxy()) {
             AddOneShot(seed);
         } else {
@@ -1549,13 +1558,11 @@ void CConnman::ThreadDNSAddressSeed()
                 continue;
             }
             unsigned int nMaxIPs = 256; // Limits number of IPs learned from a DNS seed
-            if (LookupHost(host.c_str(), vIPs, nMaxIPs, true))
-            {
-                for (const CNetAddr& ip : vIPs)
-                {
+            if (LookupHost(host.c_str(), vIPs, nMaxIPs, true)) {
+                for (const CNetAddr& ip : vIPs) {
                     int nOneDay = 24*3600;
                     CAddress addr = CAddress(CService(ip, Params().GetDefaultPort()), requiredServiceBits);
-                    addr.nTime = GetTime() - 3*nOneDay - GetRand(4*nOneDay); // use a random age between 3 and 7 days old
+                    addr.nTime = GetTime() - 3*nOneDay - rng.randrange(4*nOneDay); // use a random age between 3 and 7 days old
                     vAdd.push_back(addr);
                     found++;
                 }
@@ -1566,8 +1573,8 @@ void CConnman::ThreadDNSAddressSeed()
                 AddOneShot(seed);
             }
         }
+        --seeds_right_now;
     }
-
     LogPrintf("%d addresses found from DNS seeds\n", found);
 }