Added feeler connections increasing good addrs in the tried table.

Tests if addresses are online or offline by briefly connecting to them. These short lived connections are referred to as feeler connections. Feeler connections are designed to increase the number of fresh online addresses in tried by selecting and connecting to addresses in new. One feeler connection is attempted on average once every two minutes.

This change was suggested as Countermeasure 4 in
Eclipse Attacks on Bitcoin’s Peer-to-Peer Network, Ethan Heilman,
Alison Kendler, Aviv Zohar, Sharon Goldberg. ePrint Archive Report
2015/263. March 2015.

1 files changed, 51 insertions, 8 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 39c8d12e20..33dc10cbcb 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -43,6 +43,9 @@
 // Dump addresses to peers.dat and banlist.dat every 15 minutes (900s)
 #define DUMP_ADDRESSES_INTERVAL 900
 
+// We add a random period time (0 to 1 seconds) to feeler connections to prevent synchronization.
+#define FEELER_SLEEP_WINDOW 1
+
 #if !defined(HAVE_MSG_NOSIGNAL) && !defined(MSG_NOSIGNAL)
 #define MSG_NOSIGNAL 0
 #endif
@@ -61,6 +64,7 @@
 
 namespace {
     const int MAX_OUTBOUND_CONNECTIONS = 8;
+    const int MAX_FEELER_CONNECTIONS = 1;
 
     struct ListenSocket {
         SOCKET socket;
@@ -1017,7 +1021,8 @@ static void AcceptConnection(const ListenSocket& hListenSocket) {
     SOCKET hSocket = accept(hListenSocket.socket, (struct sockaddr*)&sockaddr, &len);
     CAddress addr;
     int nInbound = 0;
-    int nMaxInbound = nMaxConnections - MAX_OUTBOUND_CONNECTIONS;
+    int nMaxInbound = nMaxConnections - (MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS);
+    assert(nMaxInbound > 0);
 
     if (hSocket != INVALID_SOCKET)
         if (!addr.SetSockAddr((const struct sockaddr*)&sockaddr))
@@ -1613,6 +1618,9 @@ void ThreadOpenConnections()
 
     // Initiate network connections
     int64_t nStart = GetTime();
+
+    // Minimum time before next feeler connection (in microseconds).
+    int64_t nNextFeeler = PoissonNextSend(nStart*1000*1000, FEELER_INTERVAL);
     while (true)
     {
         ProcessOneShot();
@@ -1652,13 +1660,36 @@ void ThreadOpenConnections()
                 }
             }
         }
+        assert(nOutbound <= (MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS));
 
-        int64_t nANow = GetAdjustedTime();
+        // Feeler Connections
+        //
+        // Design goals:
+        //  * Increase the number of connectable addresses in the tried table.
+        //
+        // Method:
+        //  * Choose a random address from new and attempt to connect to it if we can connect 
+        //    successfully it is added to tried.
+        //  * Start attempting feeler connections only after node finishes making outbound 
+        //    connections.
+        //  * Only make a feeler connection once every few minutes.
+        //
+        bool fFeeler = false;
+        if (nOutbound >= MAX_OUTBOUND_CONNECTIONS) {
+            int64_t nTime = GetTimeMicros(); // The current time right now (in microseconds).
+            if (nTime > nNextFeeler) {
+                nNextFeeler = PoissonNextSend(nTime, FEELER_INTERVAL);
+                fFeeler = true;
+            } else {
+                continue;
+            }
+        }
 
+        int64_t nANow = GetAdjustedTime();
         int nTries = 0;
         while (true)
         {
-            CAddrInfo addr = addrman.Select();
+            CAddrInfo addr = addrman.Select(fFeeler);
 
             // if we selected an invalid address, restart
             if (!addr.IsValid() || setConnected.count(addr.GetGroup()) || IsLocal(addr))
@@ -1694,8 +1725,17 @@ void ThreadOpenConnections()
             break;
         }
 
-        if (addrConnect.IsValid())
-            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant);
+        if (addrConnect.IsValid()) {
+
+            if (fFeeler) {
+                // Add small amount of random noise before connection to avoid synchronization.
+                int randsleep = GetRandInt(FEELER_SLEEP_WINDOW * 1000);
+                MilliSleep(randsleep);
+                LogPrint("net", "Making feeler connection to %s\n", addrConnect.ToString());
+            }
+
+            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant, NULL, false, fFeeler);
+        }
     }
 }
 
@@ -1777,7 +1817,7 @@ void ThreadOpenAddedConnections()
 }
 
 // if successful, this moves the passed grant to the constructed node
-bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot)
+bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot, bool fFeeler)
 {
     //
     // Initiate outbound network connection
@@ -1801,6 +1841,8 @@ bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSem
     pnode->fNetworkNode = true;
     if (fOneShot)
         pnode->fOneShot = true;
+    if (fFeeler)
+        pnode->fFeeler = true;
 
     return true;
 }
@@ -2062,7 +2104,7 @@ void StartNode(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     if (semOutbound == NULL) {
         // initialize semaphore
-        int nMaxOutbound = std::min(MAX_OUTBOUND_CONNECTIONS, nMaxConnections);
+        int nMaxOutbound = std::min((MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS), nMaxConnections);
         semOutbound = new CSemaphore(nMaxOutbound);
     }
 
@@ -2107,7 +2149,7 @@ bool StopNode()
     LogPrintf("StopNode()\n");
     MapPort(false);
     if (semOutbound)
-        for (int i=0; i<MAX_OUTBOUND_CONNECTIONS; i++)
+        for (int i=0; i<(MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS); i++)
             semOutbound->post();
 
     if (fAddressesInitialized)
@@ -2448,6 +2490,7 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     fWhitelisted = false;
     fOneShot = false;
     fClient = false; // set by version message
+    fFeeler = false;
     fInbound = fInboundIn;
     fNetworkNode = false;
     fSuccessfullyConnected = false;