[test] Introduce test logic to query DNS seeds

This commit introduces a DNS seed to the regest chain params in order to add
coverage to the DNS querying logic.

The first test checks that we do not query DNS seeds if we are able to
succesfully connect to 2 outbound connections. Since we participate in ADDR
relay with those connections, including sending a GETADDR message during the
VERSION handshake, querying the DNS seeds is unnecessary.

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

4 files changed, 40 insertions, 3 deletions

diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 1b71c4db43..4f1ff1d512 100644
--- a/src/chainparams.cpp
+++ b/src/chainparams.cpp
@@ -435,7 +435,8 @@ public:
         assert(genesis.hashMerkleRoot == uint256S("0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"));
 
         vFixedSeeds.clear(); //!< Regtest mode doesn't have any fixed seeds.
-        vSeeds.clear();      //!< Regtest mode doesn't have any DNS seeds.
+        vSeeds.clear();
+        vSeeds.emplace_back("dummySeed.invalid.");
 
         fDefaultConsistencyChecks = true;
         fRequireStandard = true;
diff --git a/test/functional/feature_config_args.py b/test/functional/feature_config_args.py
index de9d0d2e80..07b74e65de 100755
--- a/test/functional/feature_config_args.py
+++ b/test/functional/feature_config_args.py
@@ -158,8 +158,9 @@ class ConfArgsTest(BitcoinTestFramework):
         self.stop_node(0)
 
         # No peers.dat exists and -dnsseed=1
-        # We expect the node will use DNS Seeds, but Regtest mode has 0 DNS seeds
-        # So after 60 seconds, the node should fallback to fixed seeds (this is a slow test)
+        # We expect the node will use DNS Seeds, but Regtest mode does not have
+        # any valid DNS seeds. So after 60 seconds, the node should fallback to
+        # fixed seeds
         assert not os.path.exists(os.path.join(default_data_dir, "peers.dat"))
         start = int(time.time())
         with self.nodes[0].assert_debug_log(expected_msgs=[
diff --git a/test/functional/p2p_dns_seeds.py b/test/functional/p2p_dns_seeds.py
new file mode 100755
index 0000000000..254f9af445
--- /dev/null
+++ b/test/functional/p2p_dns_seeds.py
@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+# Copyright (c) 2021 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test ThreadDNSAddressSeed logic for querying DNS seeds."""
+
+from test_framework.p2p import P2PInterface
+from test_framework.test_framework import BitcoinTestFramework
+
+
+class P2PDNSSeeds(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 1
+        self.extra_args = [["-dnsseed=1"]]
+
+    def run_test(self):
+        self.existing_outbound_connections_test()
+
+    def existing_outbound_connections_test(self):
+        # Make sure addrman is populated to enter the conditional where we
+        # delay and potentially skip DNS seeding.
+        self.nodes[0].addpeeraddress("192.0.0.8", 8333)
+
+        self.log.info("Check that we *do not* query DNS seeds if we have 2 outbound connections")
+
+        self.restart_node(0)
+        with self.nodes[0].assert_debug_log(expected_msgs=["P2P peers available. Skipped DNS seeding."], timeout=12):
+            for i in range(2):
+                self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i, connection_type="outbound-full-relay")
+
+
+if __name__ == '__main__':
+    P2PDNSSeeds().main()
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 00527e78f1..1a59cfc755 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -121,6 +121,7 @@ BASE_SCRIPTS = [
     'wallet_listreceivedby.py --legacy-wallet',
     'wallet_listreceivedby.py --descriptors',
     'wallet_abandonconflict.py --legacy-wallet',
+    'p2p_dns_seeds.py',
     'wallet_abandonconflict.py --descriptors',
     'feature_csv_activation.py',
     'rpc_rawtransaction.py --legacy-wallet',