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, 4 insertions, 1 deletions

diff --git a/src/net.h b/src/net.h
index ea03defc40..1ba2d47da7 100644
--- a/src/net.h
+++ b/src/net.h
@@ -41,6 +41,8 @@ namespace boost {
 static const int PING_INTERVAL = 2 * 60;
 /** Time after which to disconnect, after waiting for a ping response (or inactivity). */
 static const int TIMEOUT_INTERVAL = 20 * 60;
+/** Run the feeler connection loop once every 2 minutes or 120 seconds. **/
+static const int FEELER_INTERVAL = 120;
 /** The maximum number of entries in an 'inv' protocol message */
 static const unsigned int MAX_INV_SZ = 50000;
 /** The maximum number of new addresses to accumulate before announcing. */
@@ -89,7 +91,7 @@ CNode* FindNode(const CSubNet& subNet);
 CNode* FindNode(const std::string& addrName);
 CNode* FindNode(const CService& ip);
 CNode* FindNode(const NodeId id); //TODO: Remove this
-bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false);
+bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false, bool fFeeler = false);
 void MapPort(bool fUseUPnP);
 unsigned short GetListenPort();
 bool BindListenPort(const CService &bindAddr, std::string& strError, bool fWhitelisted = false);
@@ -350,6 +352,7 @@ public:
     // the network or wire types and the cleaned string used when displayed or logged.
     std::string strSubVer, cleanSubVer;
     bool fWhitelisted; // This peer can bypass DoS banning.
+    bool fFeeler; // If true this node is being used as a short lived feeler.
     bool fOneShot;
     bool fClient;
     bool fInbound;