test: avoid generating non-loopback traffic from p2p_seednode.py

`p2p_seednode.py` would try to connect to `0.0.0.1` and `0.0.0.2` as
seed nodes. This sends outbound TCP packets on a non-loopback interface
to the default router.

Configure an unavailable proxy for all executions of `bitcoind` during
this test. Also change `0.0.0.1` and `0.0.0.2` because connecting to
them would skip the `-proxy=` setting because for such an address:
* `CNetAddr::IsLocal()` is true, thus
* `CNetAddr::IsRoutable()` is false, thus
* `CNetAddr::GetNetwork()` is `NET_UNROUTABLE`, even though
  `CNetAddr::m_net` is `NET_IPV4`.

This speeds up the execution time of `p2p_seednode.py`
from 12.5s to 2.5s.

Author: vasild

test/functional/p2p_seednode.py

@@ -9,6 +9,7 @@
 import random
 import time
 
+from test_framework.netutil import UNREACHABLE_PROXY_ARG
 from test_framework.test_framework import BitcoinTestFramework
 
 ADD_NEXT_SEEDNODE = 10
@@ -17,32 +18,34 @@
 class P2PSeedNodes(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
+        # Specify a non-working proxy to make sure no actual connections to random IPs are attempted.
+        self.extra_args = [[UNREACHABLE_PROXY_ARG]]
         self.disable_autoconnect = False
 
     def test_no_seednode(self):
         self.log.info("Check that if no seednode is provided, the node proceeds as usual (without waiting)")
         with self.nodes[0].assert_debug_log(expected_msgs=[], unexpected_msgs=["Empty addrman, adding seednode", f"Couldn't connect to peers from addrman after {ADD_NEXT_SEEDNODE} seconds. Adding seednode"], timeout=ADD_NEXT_SEEDNODE):
-            self.restart_node(0)
+            self.restart_node(0, extra_args=self.nodes[0].extra_args)
 
     def test_seednode_empty_addrman(self):
-        seed_node = "0.0.0.1"
+        seed_node = "25.0.0.1"
         self.log.info("Check that the seednode is immediately added on bootstrap on an empty addrman")
         with self.nodes[0].assert_debug_log(expected_msgs=[f"Empty addrman, adding seednode ({seed_node}) to addrfetch"], timeout=ADD_NEXT_SEEDNODE):
-            self.restart_node(0, extra_args=[f'-seednode={seed_node}'])
+            self.restart_node(0, extra_args=self.nodes[0].extra_args + [f'-seednode={seed_node}'])
 
     def test_seednode_non_empty_addrman(self):
         self.log.info("Check that if addrman is non-empty, seednodes are queried with a delay")
-        seed_node = "0.0.0.2"
+        seed_node = "25.0.0.2"
         node = self.nodes[0]
         # Fill the addrman with unreachable nodes
         for i in range(10):
             ip = f"{random.randrange(128,169)}.{random.randrange(1,255)}.{random.randrange(1,255)}.{random.randrange(1,255)}"
             port = 8333 + i
             node.addpeeraddress(ip, port)
 
-        # Restart the node so seednode is processed again. Specify a non-working proxy to make sure no actual connections to random IPs are attempted.
+        # Restart the node so seednode is processed again.
         with node.assert_debug_log(expected_msgs=["trying v1 connection"], timeout=ADD_NEXT_SEEDNODE):
-            self.restart_node(0, extra_args=[f'-seednode={seed_node}', '-proxy=127.0.0.1:1'])
+            self.restart_node(0, extra_args=self.nodes[0].extra_args + [f'-seednode={seed_node}'])
 
         with node.assert_debug_log(expected_msgs=[f"Couldn't connect to peers from addrman after {ADD_NEXT_SEEDNODE} seconds. Adding seednode ({seed_node}) to addrfetch"], unexpected_msgs=["Empty addrman, adding seednode"], timeout=ADD_NEXT_SEEDNODE * 1.5):
             node.setmocktime(int(time.time()) + ADD_NEXT_SEEDNODE + 1)

test/functional/test_framework/netutil.py

@@ -13,6 +13,10 @@
 import array
 import os
 
+# Easily unreachable address. Attempts to connect to it will stay within the machine.
+# Used to avoid non-loopback traffic or DNS queries.
+UNREACHABLE_PROXY_ARG = '-proxy=127.0.0.1:1'
+
 # STATE_ESTABLISHED = '01'
 # STATE_SYN_SENT  = '02'
 # STATE_SYN_RECV = '03'