Block abusive IPs after repeated failed plan lookups

Port 79 mostly attracts HTTP/SIP probes, TLS handshakes, and username
guessers -- none of which resolve to a plan file. Treat any request that
fails to read a plan as an "offense" and timestamp it against the source
IP.

Add BanTracker (ban.hpp/ban.cpp): a per-IP rolling-window offender list.
When an IP has more than 3 offenses still inside a 24h window, its
connections are dropped without being read or answered; timestamps older
than the window are pruned so a blocked IP frees itself automatically.
State is in-memory (single io_context thread, no locking); the clock is
injected for testability. A periodic sweeper keeps the map bounded.

Legitimate lookups that hit a real plan never count, which also
frustrates username enumeration. Unit tests in test_ban.cpp.

README.md

@@ -59,3 +59,13 @@ and execute `docker compose up -d`
 
 # Setting your status
 within the `./users` directory, create a file named after the user you wish to have a response. That's it!
+
+# Abuse protection
+Most traffic on port 79 is not finger at all -- HTTP and SIP probes, TLS
+handshakes, and username-guessing scanners. None of these resolve to a plan
+file, so the daemon treats any request that fails to read a plan as an
+"offense" and timestamps it against the source IP. When an IP records more than
+3 failures within a rolling 24-hour window, its connections are dropped
+(without being read or answered) until those failures age back out of the
+window. Legitimate lookups that hit a real plan never count against an IP. All
+state is in-memory; thresholds live in `BanTracker::Config` (`ban.hpp`).

ban.cpp

@@ -0,0 +1,55 @@
+#include "ban.hpp"
+
+namespace {
+// Count timestamps that fall within (now - window, now]. The deque is kept in
+// ascending order, so the in-window entries are always a suffix.
+int count_in_window(const std::deque<BanTracker::clock::time_point> &ts,
+                    BanTracker::clock::time_point now,
+                    BanTracker::clock::duration window) {
+  const auto cutoff = now - window;
+  int count = 0;
+  for (auto it = ts.rbegin(); it != ts.rend() && *it > cutoff; ++it) {
+    ++count;
+  }
+  return count;
+}
+} // namespace
+
+bool BanTracker::is_blocked(const std::string &ip, clock::time_point now) const {
+  auto it = offenders_.find(ip);
+  if (it == offenders_.end()) {
+    return false;
+  }
+  return count_in_window(it->second, now, cfg_.window) > cfg_.threshold;
+}
+
+BanTracker::OffenseResult
+BanTracker::record_offense(const std::string &ip, clock::time_point now) {
+  auto &ts = offenders_[ip];
+  const auto cutoff = now - cfg_.window;
+
+  // Drop this IP's timestamps that have aged out of the window.
+  while (!ts.empty() && ts.front() <= cutoff) {
+    ts.pop_front();
+  }
+
+  ts.push_back(now);
+
+  const int count = static_cast<int>(ts.size());
+  return {count, count > cfg_.threshold};
+}
+
+void BanTracker::sweep(clock::time_point now) {
+  const auto cutoff = now - cfg_.window;
+  for (auto it = offenders_.begin(); it != offenders_.end();) {
+    auto &ts = it->second;
+    while (!ts.empty() && ts.front() <= cutoff) {
+      ts.pop_front();
+    }
+    if (ts.empty()) {
+      it = offenders_.erase(it);
+    } else {
+      ++it;
+    }
+  }
+}

ban.hpp

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <chrono>
+#include <cstddef>
+#include <deque>
+#include <string>
+#include <unordered_map>
+
+// BanTracker records the timestamps of "offenses" -- requests that are
+// obviously not finger queries -- per client IP, over a rolling time window.
+// When an IP has more than `threshold` offenses still inside the window, it is
+// blocked and its connections are dropped. Offense timestamps older than the
+// window are pruned, so a blocked IP automatically frees itself once its old
+// offenses age out.
+//
+// All state is in-memory: the daemon runs a single io_context thread, so every
+// call happens on the same thread and no locking is required. Time is passed
+// in as a steady_clock time_point rather than read internally, so the logic is
+// deterministic and unit-testable.
+class BanTracker {
+public:
+  using clock = std::chrono::steady_clock;
+
+  struct Config {
+    int threshold = 3;                              // block when offenses exceed this
+    clock::duration window = std::chrono::hours(24); // rolling window length
+  };
+
+  struct OffenseResult {
+    int count;    // offenses within the window, including this one
+    bool blocked; // true if the IP is now blocked (count > threshold)
+  };
+
+  BanTracker() = default;
+  explicit BanTracker(Config cfg) : cfg_(cfg) {}
+
+  // True if ip currently has more than `threshold` offenses inside the rolling
+  // window. Does not mutate state.
+  bool is_blocked(const std::string &ip, clock::time_point now) const;
+
+  // Record one offense from ip at `now`. Prunes that IP's expired timestamps,
+  // appends this one, and reports the in-window count and whether it is now
+  // blocked.
+  OffenseResult record_offense(const std::string &ip, clock::time_point now);
+
+  // Drop timestamps older than the window across all IPs, removing any IP left
+  // with no offenses. Safe to call periodically to keep the map bounded.
+  void sweep(clock::time_point now);
+
+  // Number of tracked IPs (for introspection and tests).
+  std::size_t tracked() const { return offenders_.size(); }
+
+  const Config &config() const { return cfg_; }
+
+private:
+  Config cfg_{};
+  // Per-IP offense timestamps, kept in ascending order (steady_clock is
+  // monotonic, so appends are always newest-last).
+  std::unordered_map<std::string, std::deque<clock::time_point>> offenders_;
+};

main.cpp

@@ -6,9 +6,12 @@
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/ip/tcp.hpp>
 #include <boost/asio/signal_set.hpp>
+#include <boost/asio/steady_timer.hpp>
 #include <boost/asio/write.hpp>
+#include <chrono>
 #include <cstdio>
 
+#include "ban.hpp"
 #include "handler.hpp"
 
 using boost::asio::awaitable;
@@ -22,8 +25,18 @@ awaitable<std::string> dofinger(const std::string &username) {
   co_return process(username);
 }
 
-awaitable<void> echo(tcp::socket socket, std::string client_addr) {
+awaitable<void> echo(tcp::socket socket, std::string client_addr,
+                     BanTracker &bans) {
   try {
+    auto now = std::chrono::steady_clock::now();
+
+    // An IP that has racked up too many failed lookups (scanners, username
+    // guessers, non-finger junk) is dropped without being read or answered.
+    if (bans.is_blocked(client_addr, now)) {
+      std::printf("finger drop from %s: blocked\n", client_addr.c_str());
+      co_return;
+    }
+
     char data[1024];
     auto bytes_read =
         co_await socket.async_read_some(boost::asio::buffer(data), deferred);
@@ -36,8 +49,19 @@ awaitable<void> echo(tcp::socket socket, std::string client_addr) {
     std::printf("finger request from %s for user '%s'\n",
                 client_addr.c_str(), username.c_str());
     auto response = co_await dofinger(username);
-    if (response.compare(std::string(username)) == 0) {
+
-      // No plan found
+    // A "failure" is simply any request that does not resolve to a readable
+    // plan file: an unknown user, rejected input, or non-finger junk. Each
+    // failure is timestamped against the client IP; once an IP exceeds the
+    // threshold within the rolling window, the is_blocked() check above starts
+    // dropping its connections. This also frustrates username guessing.
+    bool plan_served =
+        response != username && response.rfind("InvalidInput:", 0) != 0;
+    if (!plan_served) {
+      auto res = bans.record_offense(client_addr, now);
+      std::printf("finger miss from %s for '%s' (%d failures in window)%s\n",
+                  client_addr.c_str(), username.c_str(), res.count,
+                  res.blocked ? " -- now blocked" : "");
       co_await async_write(
           socket, boost::asio::buffer(std::string("No plan found\r\n")),
           deferred);
@@ -50,7 +74,7 @@ awaitable<void> echo(tcp::socket socket, std::string client_addr) {
   }
 }
 
-awaitable<void> listener() {
+awaitable<void> listener(BanTracker &bans) {
   auto executor = co_await this_coro::executor;
   tcp::acceptor acceptor(executor, {tcp::v4(), 79});
   for (;;) {
@@ -59,21 +83,34 @@ awaitable<void> listener() {
     auto endpoint = socket.remote_endpoint(ec);
     std::string client_addr =
         ec ? std::string("unknown") : endpoint.address().to_string();
-    co_spawn(executor, echo(std::move(socket), std::move(client_addr)),
+    co_spawn(executor, echo(std::move(socket), std::move(client_addr), bans),
              detached);
   }
 }
 
+// Periodically prune offense records that have aged out of the window so the
+// tracker's memory stays bounded even for IPs that never reconnect.
+awaitable<void> sweeper(BanTracker &bans) {
+  boost::asio::steady_timer timer(co_await this_coro::executor);
+  for (;;) {
+    timer.expires_after(std::chrono::minutes(10));
+    co_await timer.async_wait(deferred);
+    bans.sweep(std::chrono::steady_clock::now());
+  }
+}
+
 int main() {
   // Line-buffer stdout so docker logs / tail -f see entries in real time.
   std::setvbuf(stdout, nullptr, _IOLBF, 0);
   try {
     boost::asio::io_context io_context(1);
+    BanTracker bans;
 
     boost::asio::signal_set signals(io_context, SIGINT, SIGTERM);
     signals.async_wait([&](auto, auto) { io_context.stop(); });
 
-    co_spawn(io_context, listener(), detached);
+    co_spawn(io_context, listener(bans), detached);
+    co_spawn(io_context, sweeper(bans), detached);
 
     io_context.run();
   } catch (std::exception &e) {

meson.build

@@ -14,7 +14,7 @@ gtest_dep = dependency('gtest', main : true, required : true)
 gmock_dep = dependency('gmock', main : true, required : true)
 
 executable('finger',
-  'main.cpp','handler.cpp',
+  'main.cpp','handler.cpp','ban.cpp',
   dependencies : [boost_dep, threads_dep],
   install : true)
 
@@ -33,7 +33,13 @@ test_real_fs_exe = executable('test_handler_real_filesystem',
   'test_handler_real_filesystem.cpp', 'handler.cpp',
   dependencies : [boost_dep, threads_dep, gtest_dep, gmock_dep])
 
+# Ban tracker test executable
+test_ban_exe = executable('test_ban',
+  'test_ban.cpp', 'ban.cpp',
+  dependencies : [boost_dep, threads_dep, gtest_dep, gmock_dep])
+
 # Register the tests
 test('handler_tests', test_exe)
 test('handler_mock_tests', test_mock_exe)
 test('handler_real_filesystem_tests', test_real_fs_exe)
+test('ban_tests', test_ban_exe)

test_ban.cpp

@@ -0,0 +1,89 @@
+#include "ban.hpp"
+#include <gtest/gtest.h>
+
+using namespace std::chrono_literals;
+using clock_t_ = BanTracker::clock;
+
+// Work well away from the steady_clock epoch so that subtracting the window
+// never underflows and default-constructed time_points are unambiguous.
+static const clock_t_::time_point kBase = clock_t_::time_point{} + 1000h;
+
+TEST(BanTracker, UnknownIpIsNotBlocked) {
+  BanTracker bt;
+  EXPECT_FALSE(bt.is_blocked("1.2.3.4", kBase));
+}
+
+TEST(BanTracker, BlocksOnlyAfterMoreThanThreshold) {
+  BanTracker bt; // default threshold = 3, so block on the 4th failure
+  EXPECT_FALSE(bt.record_offense("1.2.3.4", kBase).blocked); // 1
+  EXPECT_FALSE(bt.record_offense("1.2.3.4", kBase).blocked); // 2
+  EXPECT_FALSE(bt.record_offense("1.2.3.4", kBase).blocked); // 3
+  EXPECT_FALSE(bt.is_blocked("1.2.3.4", kBase));
+  auto r = bt.record_offense("1.2.3.4", kBase); // 4
+  EXPECT_TRUE(r.blocked);
+  EXPECT_EQ(r.count, 4);
+  EXPECT_TRUE(bt.is_blocked("1.2.3.4", kBase));
+}
+
+TEST(BanTracker, TracksEachIpIndependently) {
+  BanTracker bt;
+  for (int i = 0; i < 4; ++i) {
+    bt.record_offense("1.1.1.1", kBase);
+  }
+  EXPECT_TRUE(bt.is_blocked("1.1.1.1", kBase));
+  EXPECT_FALSE(bt.is_blocked("2.2.2.2", kBase));
+}
+
+TEST(BanTracker, OffensesAgeOutOfRollingWindow) {
+  BanTracker bt;
+  // Four failures spread over a couple of hours -> blocked.
+  for (int i = 0; i < 4; ++i) {
+    bt.record_offense("1.2.3.4", kBase + i * 1h);
+  }
+  EXPECT_TRUE(bt.is_blocked("1.2.3.4", kBase + 3h));
+
+  // 24h after the first failure, that one drops out of the window: only 3
+  // remain, so the IP is no longer blocked.
+  EXPECT_FALSE(bt.is_blocked("1.2.3.4", kBase + 24h + 1min));
+}
+
+TEST(BanTracker, WindowBoundaryIsExclusiveAtCutoff) {
+  BanTracker bt;
+  // Exactly window-old timestamps are pruned (cutoff is inclusive of <=).
+  bt.record_offense("1.2.3.4", kBase);
+  auto r = bt.record_offense("1.2.3.4", kBase + 24h);
+  EXPECT_EQ(r.count, 1); // the kBase entry was pruned before appending
+}
+
+TEST(BanTracker, SweepRemovesFullyExpiredIp) {
+  BanTracker bt;
+  for (int i = 0; i < 4; ++i) {
+    bt.record_offense("1.2.3.4", kBase);
+  }
+  EXPECT_EQ(bt.tracked(), 1u);
+  bt.sweep(kBase + 24h + 1min); // all offenses aged out
+  EXPECT_EQ(bt.tracked(), 0u);
+}
+
+TEST(BanTracker, SweepKeepsStillActiveIp) {
+  BanTracker bt;
+  for (int i = 0; i < 4; ++i) {
+    bt.record_offense("1.2.3.4", kBase);
+  }
+  bt.sweep(kBase + 1h); // still inside the window
+  EXPECT_EQ(bt.tracked(), 1u);
+  EXPECT_TRUE(bt.is_blocked("1.2.3.4", kBase + 1h));
+}
+
+TEST(BanTracker, RespectsCustomConfig) {
+  BanTracker bt(BanTracker::Config{/*threshold=*/1, /*window=*/1h});
+  EXPECT_FALSE(bt.record_offense("9.9.9.9", kBase).blocked); // 1, not > 1
+  EXPECT_TRUE(bt.record_offense("9.9.9.9", kBase).blocked);  // 2 > 1
+  EXPECT_TRUE(bt.is_blocked("9.9.9.9", kBase));
+  EXPECT_FALSE(bt.is_blocked("9.9.9.9", kBase + 1h + 1min)); // window elapsed
+}
+
+int main(int argc, char **argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}