Add 2 outbound block-relay-only connections

Transaction relay is primarily optimized for balancing redundancy/robustness
with bandwidth minimization -- as a result transaction relay leaks information
that adversaries can use to infer the network topology.

Network topology is better kept private for (at least) two reasons:

(a) Knowledge of the network graph can make it easier to find the source IP of
a given transaction.

(b) Knowledge of the network graph could be used to split a target node or
nodes from the honest network (eg by knowing which peers to attack in order to
achieve a network split).

We can eliminate the risks of (b) by separating block relay from transaction
relay; inferring network connectivity from the relay of blocks/block headers is
much more expensive for an adversary.

After this commit, bitcoind will make 2 additional outbound connections that
are only used for block relay. (In the future, we might consider rotating our
transaction-relay peers to help limit the effects of (a).)

1 files changed, 28 insertions, 14 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 9514da8809..e589b940ad 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -352,7 +352,7 @@ static CAddress GetBindAddress(SOCKET sock)
     return addr_bind;
 }
 
-CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCountFailure, bool manual_connection)
+CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCountFailure, bool manual_connection, bool block_relay_only)
 {
     if (pszDest == nullptr) {
         if (IsLocal(addrConnect))
@@ -442,7 +442,7 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
     NodeId id = GetNewNodeId();
     uint64_t nonce = GetDeterministicRandomizer(RANDOMIZER_ID_LOCALHOSTNONCE).Write(id).Finalize();
     CAddress addr_bind = GetBindAddress(hSocket);
-    CNode* pnode = new CNode(id, nLocalServices, GetBestHeight(), hSocket, addrConnect, CalculateKeyedNetGroup(addrConnect), nonce, addr_bind, pszDest ? pszDest : "", false);
+    CNode* pnode = new CNode(id, nLocalServices, GetBestHeight(), hSocket, addrConnect, CalculateKeyedNetGroup(addrConnect), nonce, addr_bind, pszDest ? pszDest : "", false, block_relay_only);
     pnode->AddRef();
 
     return pnode;
@@ -905,7 +905,7 @@ void CConnman::AcceptConnection(const ListenSocket& hListenSocket) {
     SOCKET hSocket = accept(hListenSocket.socket, (struct sockaddr*)&sockaddr, &len);
     CAddress addr;
     int nInbound = 0;
-    int nMaxInbound = nMaxConnections - (nMaxOutbound + nMaxFeeler);
+    int nMaxInbound = nMaxConnections - m_max_outbound;
 
     if (hSocket != INVALID_SOCKET) {
         if (!addr.SetSockAddr((const struct sockaddr*)&sockaddr)) {
@@ -1666,7 +1666,7 @@ int CConnman::GetExtraOutboundCount()
             }
         }
     }
-    return std::max(nOutbound - nMaxOutbound, 0);
+    return std::max(nOutbound - m_max_outbound_full_relay - m_max_outbound_block_relay, 0);
 }
 
 void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
@@ -1726,7 +1726,8 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
         CAddress addrConnect;
 
         // Only connect out to one peer per network group (/16 for IPv4).
-        int nOutbound = 0;
+        int nOutboundFullRelay = 0;
+        int nOutboundBlockRelay = 0;
         std::set<std::vector<unsigned char> > setConnected;
         {
             LOCK(cs_vNodes);
@@ -1738,7 +1739,11 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                     // also have the added issue that they're attacker controlled and could be used
                     // to prevent us from connecting to particular hosts if we used them here.
                     setConnected.insert(pnode->addr.GetGroup());
-                    nOutbound++;
+                    if (pnode->m_tx_relay == nullptr) {
+                        nOutboundBlockRelay++;
+                    } else if (!pnode->fFeeler) {
+                        nOutboundFullRelay++;
+                    }
                 }
             }
         }
@@ -1757,7 +1762,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
         //
         bool fFeeler = false;
 
-        if (nOutbound >= nMaxOutbound && !GetTryNewOutboundPeer()) {
+        if (nOutboundFullRelay >= m_max_outbound_full_relay && nOutboundBlockRelay >= m_max_outbound_block_relay && !GetTryNewOutboundPeer()) {
             int64_t nTime = GetTimeMicros(); // The current time right now (in microseconds).
             if (nTime > nNextFeeler) {
                 nNextFeeler = PoissonNextSend(nTime, FEELER_INTERVAL);
@@ -1831,7 +1836,14 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                 LogPrint(BCLog::NET, "Making feeler connection to %s\n", addrConnect.ToString());
             }
 
-            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant, nullptr, false, fFeeler);
+            // Open this connection as block-relay-only if we're already at our
+            // full-relay capacity, but not yet at our block-relay peer limit.
+            // (It should not be possible for fFeeler to be set if we're not
+            // also at our block-relay peer limit, but check against that as
+            // well for sanity.)
+            bool block_relay_only = nOutboundBlockRelay < m_max_outbound_block_relay && !fFeeler && nOutboundFullRelay >= m_max_outbound_full_relay;
+
+            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant, nullptr, false, fFeeler, false, block_relay_only);
         }
     }
 }
@@ -1918,7 +1930,7 @@ void CConnman::ThreadOpenAddedConnections()
 }
 
 // if successful, this moves the passed grant to the constructed node
-void CConnman::OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot, bool fFeeler, bool manual_connection)
+void CConnman::OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot, bool fFeeler, bool manual_connection, bool block_relay_only)
 {
     //
     // Initiate outbound network connection
@@ -1937,7 +1949,7 @@ void CConnman::OpenNetworkConnection(const CAddress& addrConnect, bool fCountFai
     } else if (FindNode(std::string(pszDest)))
         return;
 
-    CNode* pnode = ConnectNode(addrConnect, pszDest, fCountFailure, manual_connection);
+    CNode* pnode = ConnectNode(addrConnect, pszDest, fCountFailure, manual_connection, block_relay_only);
 
     if (!pnode)
         return;
@@ -2240,7 +2252,7 @@ bool CConnman::Start(CScheduler& scheduler, const Options& connOptions)
 
     if (semOutbound == nullptr) {
         // initialize semaphore
-        semOutbound = MakeUnique<CSemaphore>(std::min((nMaxOutbound + nMaxFeeler), nMaxConnections));
+        semOutbound = MakeUnique<CSemaphore>(std::min(m_max_outbound, nMaxConnections));
     }
     if (semAddnode == nullptr) {
         // initialize semaphore
@@ -2318,7 +2330,7 @@ void CConnman::Interrupt()
     InterruptSocks5(true);
 
     if (semOutbound) {
-        for (int i=0; i<(nMaxOutbound + nMaxFeeler); i++) {
+        for (int i=0; i<m_max_outbound; i++) {
             semOutbound->post();
         }
     }
@@ -2628,7 +2640,7 @@ int CConnman::GetBestHeight() const
 
 unsigned int CConnman::GetReceiveFloodSize() const { return nReceiveFloodSize; }
 
-CNode::CNode(NodeId idIn, ServiceFlags nLocalServicesIn, int nMyStartingHeightIn, SOCKET hSocketIn, const CAddress& addrIn, uint64_t nKeyedNetGroupIn, uint64_t nLocalHostNonceIn, const CAddress& addrBindIn, const std::string& addrNameIn, bool fInboundIn)
+CNode::CNode(NodeId idIn, ServiceFlags nLocalServicesIn, int nMyStartingHeightIn, SOCKET hSocketIn, const CAddress& addrIn, uint64_t nKeyedNetGroupIn, uint64_t nLocalHostNonceIn, const CAddress& addrBindIn, const std::string& addrNameIn, bool fInboundIn, bool block_relay_only)
     : nTimeConnected(GetSystemTimeInSeconds()),
     addr(addrIn),
     addrBind(addrBindIn),
@@ -2643,7 +2655,9 @@ CNode::CNode(NodeId idIn, ServiceFlags nLocalServicesIn, int nMyStartingHeightIn
     hSocket = hSocketIn;
     addrName = addrNameIn == "" ? addr.ToStringIPPort() : addrNameIn;
     hashContinue = uint256();
-    m_tx_relay = MakeUnique<TxRelay>();
+    if (!block_relay_only) {
+        m_tx_relay = MakeUnique<TxRelay>();
+    }
 
     for (const std::string &msg : getAllNetMessageTypes())
         mapRecvBytesPerMsgCmd[msg] = 0;