Fall back to oneshot for DNS Seeds which don't support filtering.

This allows us to not have to update the chainparams whenever a
DNS Seed changes its filtering support, as well fixes a bug
introduced in 4440710 where returned nodes will never be
attempted.

Author: TheBlueMatt

src/chainparams.cpp

@@ -125,12 +125,12 @@ class CMainParams : public CChainParams {
         assert(genesis.hashMerkleRoot == uint256S("0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"));
 
         // Note that of those with the service bits flag, most only support a subset of possible options
-        vSeeds.emplace_back("seed.bitcoin.sipa.be", true); // Pieter Wuille, only supports x1, x5, x9, and xd
-        vSeeds.emplace_back("dnsseed.bluematt.me", true); // Matt Corallo, only supports x9
-        vSeeds.emplace_back("dnsseed.bitcoin.dashjr.org", false); // Luke Dashjr
-        vSeeds.emplace_back("seed.bitcoinstats.com", true); // Christian Decker, supports x1 - xf
-        vSeeds.emplace_back("seed.bitcoin.jonasschnelli.ch", true); // Jonas Schnelli, only supports x1, x5, x9, and xd
-        vSeeds.emplace_back("seed.btc.petertodd.org", true); // Peter Todd, only supports x1, x5, x9, and xd
+        vSeeds.emplace_back("seed.bitcoin.sipa.be"); // Pieter Wuille, only supports x1, x5, x9, and xd
+        vSeeds.emplace_back("dnsseed.bluematt.me"); // Matt Corallo, only supports x9
+        vSeeds.emplace_back("dnsseed.bitcoin.dashjr.org"); // Luke Dashjr
+        vSeeds.emplace_back("seed.bitcoinstats.com"); // Christian Decker, supports x1 - xf
+        vSeeds.emplace_back("seed.bitcoin.jonasschnelli.ch"); // Jonas Schnelli, only supports x1, x5, x9, and xd
+        vSeeds.emplace_back("seed.btc.petertodd.org"); // Peter Todd, only supports x1, x5, x9, and xd
 
         base58Prefixes[PUBKEY_ADDRESS] = std::vector<unsigned char>(1,0);
         base58Prefixes[SCRIPT_ADDRESS] = std::vector<unsigned char>(1,5);
@@ -229,10 +229,10 @@ class CTestNetParams : public CChainParams {
         vFixedSeeds.clear();
         vSeeds.clear();
         // nodes with support for servicebits filtering should be at the top
-        vSeeds.emplace_back("testnet-seed.bitcoin.jonasschnelli.ch", true);
-        vSeeds.emplace_back("seed.tbtc.petertodd.org", true);
-        vSeeds.emplace_back("seed.testnet.bitcoin.sprovoost.nl", true);
-        vSeeds.emplace_back("testnet-seed.bluematt.me", true); // Just a static list of stable node(s), only supports x9
+        vSeeds.emplace_back("testnet-seed.bitcoin.jonasschnelli.ch");
+        vSeeds.emplace_back("seed.tbtc.petertodd.org");
+        vSeeds.emplace_back("seed.testnet.bitcoin.sprovoost.nl");
+        vSeeds.emplace_back("testnet-seed.bluematt.me"); // Just a static list of stable node(s), only supports x9
 
         base58Prefixes[PUBKEY_ADDRESS] = std::vector<unsigned char>(1,111);
         base58Prefixes[SCRIPT_ADDRESS] = std::vector<unsigned char>(1,196);

src/chainparams.h

@@ -14,12 +14,6 @@
 #include <memory>
 #include <vector>
 
-struct CDNSSeedData {
-    std::string host;
-    bool supportsServiceBitsFiltering;
-    CDNSSeedData(const std::string &strHost, bool supportsServiceBitsFilteringIn) : host(strHost), supportsServiceBitsFiltering(supportsServiceBitsFilteringIn) {}
-};
-
 struct SeedSpec6 {
     uint8_t addr[16];
     uint16_t port;
@@ -71,7 +65,8 @@ class CChainParams
     bool MineBlocksOnDemand() const { return fMineBlocksOnDemand; }
     /** Return the BIP70 network string (main, test or regtest) */
     std::string NetworkIDString() const { return strNetworkID; }
-    const std::vector<CDNSSeedData>& DNSSeeds() const { return vSeeds; }
+    /** Return the list of hostnames to look up for DNS seeds */
+    const std::vector<std::string>& DNSSeeds() const { return vSeeds; }
     const std::vector<unsigned char>& Base58Prefix(Base58Type type) const { return base58Prefixes[type]; }
     const std::string& Bech32HRP() const { return bech32_hrp; }
     const std::vector<SeedSpec6>& FixedSeeds() const { return vFixedSeeds; }
@@ -85,7 +80,7 @@ class CChainParams
     CMessageHeader::MessageStartChars pchMessageStart;
     int nDefaultPort;
     uint64_t nPruneAfterHeight;
-    std::vector<CDNSSeedData> vSeeds;
+    std::vector<std::string> vSeeds;
     std::vector<unsigned char> base58Prefixes[MAX_BASE58_TYPES];
     std::string bech32_hrp;
     std::string strNetworkID;

src/net.cpp

@@ -1577,19 +1577,6 @@ void MapPort(bool)
 
 
 
-static std::string GetDNSHost(const CDNSSeedData& data, ServiceFlags* requiredServiceBits)
-{
-    //use default host for non-filter-capable seeds or if we use the default service bits (NODE_NETWORK)
-    if (!data.supportsServiceBitsFiltering || *requiredServiceBits == NODE_NETWORK) {
-        *requiredServiceBits = NODE_NETWORK;
-        return data.host;
-    }
-
-    // See chainparams.cpp, most dnsseeds only support one or two possible servicebits hostnames
-    return strprintf("x%x.%s", *requiredServiceBits, data.host);
-}
-
-
 void CConnman::ThreadDNSAddressSeed()
 {
     // goal: only query DNS seeds if address need is acute
@@ -1612,22 +1599,22 @@ void CConnman::ThreadDNSAddressSeed()
         }
     }
 
-    const std::vector<CDNSSeedData> &vSeeds = Params().DNSSeeds();
+    const std::vector<std::string> &vSeeds = Params().DNSSeeds();
     int found = 0;
 
     LogPrintf("Loading addresses from DNS seeds (could take a while)\n");
 
-    for (const CDNSSeedData &seed : vSeeds) {
+    for (const std::string &seed : vSeeds) {
         if (interruptNet) {
             return;
         }
         if (HaveNameProxy()) {
-            AddOneShot(seed.host);
+            AddOneShot(seed);
         } else {
             std::vector<CNetAddr> vIPs;
             std::vector<CAddress> vAdd;
             ServiceFlags requiredServiceBits = GetDesirableServiceFlags(NODE_NONE);
-            std::string host = GetDNSHost(seed, &requiredServiceBits);
+            std::string host = strprintf("x%x.%s", requiredServiceBits, seed);
             CNetAddr resolveSource;
             if (!resolveSource.SetInternal(host)) {
                 continue;
@@ -1643,6 +1630,10 @@ void CConnman::ThreadDNSAddressSeed()
                     found++;
                 }
                 addrman.Add(vAdd, resolveSource);
+            } else {
+                // We now avoid directly using results from DNS Seeds which do not support service bit filtering,
+                // instead using them as a oneshot to get nodes with our desired service bits.
+                AddOneShot(seed);
             }
         }
     }