Cache responses to addr requests

Prevents a spy from scraping victim's AddrMan by
reconnecting and re-requesting addrs.

src/net.cpp

@@ -2539,6 +2539,17 @@ std::vector<CAddress> CConnman::GetAddresses()
     return addresses;
 }
 
+std::vector<CAddress> CConnman::GetAddresses(Network requestor_network)
+{
+    const auto current_time = GetTime<std::chrono::microseconds>();
+    if (m_addr_response_caches.find(requestor_network) == m_addr_response_caches.end() ||
+        m_addr_response_caches[requestor_network].m_update_addr_response < current_time) {
+        m_addr_response_caches[requestor_network].m_addrs_response_cache = GetAddresses();
+        m_addr_response_caches[requestor_network].m_update_addr_response = current_time + std::chrono::hours(21) + GetRandMillis(std::chrono::hours(6));
+    }
+    return m_addr_response_caches[requestor_network].m_addrs_response_cache;
+}
+
 bool CConnman::AddNode(const std::string& strNode)
 {
     LOCK(cs_vAddedNodes);

src/net.h

@@ -27,6 +27,7 @@
 #include <atomic>
 #include <cstdint>
 #include <deque>
+#include <map>
 #include <thread>
 #include <memory>
 #include <condition_variable>
@@ -254,6 +255,13 @@ public:
     void MarkAddressGood(const CAddress& addr);
     void AddNewAddresses(const std::vector<CAddress>& vAddr, const CAddress& addrFrom, int64_t nTimePenalty = 0);
     std::vector<CAddress> GetAddresses();
+    /**
+     * Cache is used to minimize topology leaks, so it should
+     * be used for all non-trusted calls, for example, p2p.
+     * A non-malicious call (from RPC) should
+     * call the function without a parameter to avoid using the cache.
+     */
+    std::vector<CAddress> GetAddresses(Network requestor_network);
 
     // This allows temporarily exceeding m_max_outbound_full_relay, with the goal of finding
     // a peer that is better than all our current peers.
@@ -418,6 +426,29 @@ private:
     std::atomic<NodeId> nLastNodeId{0};
     unsigned int nPrevNodeCount{0};
 
+    /**
+     * Cache responses to addr requests to minimize privacy leak.
+     * Attack example: scraping addrs in real-time may allow an attacker
+     * to infer new connections of the victim by detecting new records
+     * with fresh timestamps (per self-announcement).
+     */
+    struct CachedAddrResponse {
+        std::vector<CAddress> m_addrs_response_cache;
+        std::chrono::microseconds m_update_addr_response{0};
+    };
+
+    /**
+     * Addr responses stored in different caches
+     * per network prevent cross-network node identification.
+     * If a node for example is multi-homed under Tor and IPv6,
+     * a single cache (or no cache at all) would let an attacker
+     * to easily detect that it is the same node by comparing responses.
+     * The used memory equals to 1000 CAddress records (or around 32 bytes) per
+     * distinct Network (up to 5) we have/had an inbound peer from,
+     * resulting in at most ~160 KB.
+     */
+    std::map<Network, CachedAddrResponse> m_addr_response_caches;
+
     /**
      * Services this instance offers.
      *

src/net_processing.cpp

@@ -3477,7 +3477,7 @@ void ProcessMessage(
         pfrom.fSentAddr = true;
 
         pfrom.vAddrToSend.clear();
-        std::vector<CAddress> vAddr = connman.GetAddresses();
+        std::vector<CAddress> vAddr = connman.GetAddresses(pfrom.addr.GetNetwork());
         FastRandomContext insecure_rand;
         for (const CAddress &addr : vAddr) {
             pfrom.PushAddress(addr, insecure_rand);