Always use a 50% chance to choose between tried and new entries

This change was suggested as Countermeasure 2 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.

src/addrman.cpp

@@ -332,14 +332,13 @@ void CAddrMan::Attempt_(const CService& addr, int64_t nTime)
     info.nAttempts++;
 }
 
-CAddress CAddrMan::Select_(int nUnkBias)
+CAddress CAddrMan::Select_()
 {
     if (size() == 0)
         return CAddress();
 
-    double nCorTried = sqrt(nTried) * (100.0 - nUnkBias);
-    double nCorNew = sqrt(nNew) * nUnkBias;
-    if ((nCorTried + nCorNew) * GetRandInt(1 << 30) / (1 << 30) < nCorTried) {
+    // Use a 50% chance for choosing between tried and new table entries.
+    if (nTried > 0 && (nNew == 0 || GetRandInt(2) == 0)) {
         // use a tried node
         double fChanceFactor = 1.0;
         while (1) {

src/addrman.h

@@ -231,7 +231,7 @@ protected:
 
     //! Select an address to connect to.
     //! nUnkBias determines how much to favor new addresses over tried ones (min=0, max=100)
-    CAddress Select_(int nUnkBias);
+    CAddress Select_();
 
 #ifdef DEBUG_ADDRMAN
     //! Perform consistency check. Returns an error code or zero.
@@ -533,13 +533,13 @@ public:
      * Choose an address to connect to.
      * nUnkBias determines how much "new" entries are favored over "tried" ones (0-100).
      */
-    CAddress Select(int nUnkBias = 50)
+    CAddress Select()
     {
         CAddress addrRet;
         {
             LOCK(cs);
             Check();
-            addrRet = Select_(nUnkBias);
+            addrRet = Select_();
             Check();
         }
         return addrRet;

src/net.cpp

@@ -1221,8 +1221,7 @@ void ThreadOpenConnections()
         int nTries = 0;
         while (true)
         {
-            // use an nUnkBias between 10 (no outgoing connections) and 90 (8 outgoing connections)
-            CAddress addr = addrman.Select(10 + min(nOutbound,8)*10);
+            CAddress addr = addrman.Select();
 
             // if we selected an invalid address, restart
             if (!addr.IsValid() || setConnected.count(addr.GetGroup()) || IsLocal(addr))