Don't store or send blocks forked before last checkpoint.

src/main.cpp

@@ -2185,6 +2185,11 @@ bool CBlock::AcceptBlock(CValidationState &state, CDiskBlockPos *dbp)
         if (!Checkpoints::CheckBlock(nHeight, hash))
             return state.DoS(100, error("AcceptBlock() : rejected by checkpoint lock-in at %d", nHeight));
 
+        // Don't accept any forks from the main chain prior to last checkpoint
+        CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(mapBlockIndex);
+        if (pcheckpoint && nHeight < pcheckpoint->nHeight)
+            return state.DoS(100, error("AcceptBlock() : forked chain older than last checkpoint (height %d)", nHeight));
+
         // Reject block.nVersion=1 blocks when 95% (75% on testnet) of the network has upgraded:
         if (nVersion < 2)
         {
@@ -3087,11 +3092,28 @@ void static ProcessGetData(CNode* pfrom)
 
             if (inv.type == MSG_BLOCK || inv.type == MSG_FILTERED_BLOCK)
             {
-                // Send block from disk
+                bool send = true;
                 map<uint256, CBlockIndex*>::iterator mi = mapBlockIndex.find(inv.hash);
                 pfrom->nBlocksRequested++;
                 if (mi != mapBlockIndex.end())
                 {
+                    // If the requested block is at a height below our last
+                    // checkpoint, only serve it if it's in the checkpointed chain
+                    int nHeight = ((*mi).second)->nHeight;
+                    CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(mapBlockIndex);
+                    if (pcheckpoint && nHeight < pcheckpoint->nHeight) {
+                       if (!((*mi).second)->IsInMainChain())
+                       {
+                         printf("ProcessGetData(): ignoring request for old block that isn't in the main chain\n");
+                         send = false;
+                       }
+                    }
+                } else {
+                    send = false;
+                }
+                if (send)
+                {
+                    // Send block from disk
                     CBlock block;
                     block.ReadFromDisk((*mi).second);
                     if (inv.type == MSG_BLOCK)