Agent: Add TCPPortSelector

monkey/infection_monkey/network/info.py

@@ -4,10 +4,12 @@ import struct
 from dataclasses import dataclass
 from ipaddress import IPv4Interface
 from random import shuffle  # noqa: DUO102
-from typing import List
+from threading import Lock
+from typing import Dict, List, Set
 
 import netifaces
 import psutil
+from egg_timer import EggTimer
 
 from infection_monkey.utils.environment import is_windows_os
 
@@ -120,20 +122,93 @@ else:
         return routes
 
 
-def get_free_tcp_port(min_range=1024, max_range=65535):
+class TCPPortSelector:
+    """
+    Select an available TCP port that a new server can listen on
 
-    in_use = {conn.laddr[1] for conn in psutil.net_connections()}
+    Examines the system to find which ports are not in use and makes an intelligent decision
+    regarding what port can be used to host a server. In multithreaded applications, a race occurs
+    between the time when the OS reports that a port is free and when the port is actually used. In
+    other words, two threads which request a free port simultaneously may be handed the same port,
+    as the OS will report that the port is not in use. To combat this, the TCPPortSelector will
+    reserve a port for a period of time to give the requester ample time to start their server. Once
+    the requester's server is listening on the port, the OS will report the port as "LISTEN".
+    """
 
-    for port in COMMON_PORTS:
-        if port not in in_use:
-            return port
+    def __init__(self):
+        self._leases: Dict[int, EggTimer] = {}
+        self._lock = Lock()
 
-    min_range = max(1, min_range)
-    max_range = min(65535, max_range)
-    ports = list(range(min_range, max_range))
-    shuffle(ports)
-    for port in ports:
-        if port not in in_use:
-            return port
+    def get_free_tcp_port(
+        self, min_range: int = 1024, max_range: int = 65535, lease_time_sec: float = 30
+    ):
+        """
+        Get a free TCP port that a new server can listen on
 
-    return None
+        This function will attempt to provide a well-known port that the caller can listen on. If no
+        well-known ports are available, a random port will be selected.
+
+        :param min_range: The smallest port number a random port can be chosen from, defaults to
+                          1024
+        :param max_range: The largest port number a random port can be chosen from, defaults to
+                          65535
+        :lease_time_sec: The amount of time a port should be reserved for if the OS does not report
+                         it as in use, defaults to 30 seconds
+        :return: A TCP port number
+        """
+        with self._lock:
+            ports_in_use = {conn.laddr[1] for conn in psutil.net_connections()}
+
+            common_port = self._get_free_common_port(ports_in_use, lease_time_sec)
+            if common_port is not None:
+                return common_port
+
+            return self._get_free_random_port(ports_in_use, min_range, max_range, lease_time_sec)
+
+    def _get_free_common_port(self, ports_in_use: Set[int], lease_time_sec):
+        for port in COMMON_PORTS:
+            if self._port_is_available(port, ports_in_use):
+                self._reserve_port(port, lease_time_sec)
+                return port
+
+        return None
+
+    def _get_free_random_port(
+        self, ports_in_use: Set[int], min_range: int, max_range: int, lease_time_sec: float
+    ):
+        min_range = max(1, min_range)
+        max_range = min(65535, max_range)
+        ports = list(range(min_range, max_range))
+        shuffle(ports)
+        for port in ports:
+            if self._port_is_available(port, ports_in_use):
+                self._reserve_port(port, lease_time_sec)
+                return port
+
+        return None
+
+    def _port_is_available(self, port: int, ports_in_use: Set[int]):
+        if port in ports_in_use:
+            return False
+
+        if port not in self._leases:
+            return True
+
+        if self._leases[port].is_expired():
+            return True
+
+        return False
+
+    def _reserve_port(self, port: int, lease_time_sec: float):
+        timer = EggTimer()
+        timer.set(lease_time_sec)
+        self._leases[port] = timer
+
+
+# TODO: This function is here because existing components rely on it. Refactor these components to
+#       accept a TCPPortSelector instance and use that instead.
+def get_free_tcp_port(min_range=1024, max_range=65535, lease_time_sec=30):
+    return get_free_tcp_port.port_selector.get_free_tcp_port(min_range, max_range, lease_time_sec)
+
+
+get_free_tcp_port.port_selector = TCPPortSelector()  # type: ignore[attr-defined]

monkey/tests/unit_tests/infection_monkey/network/test_info.py

@@ -3,7 +3,7 @@ from typing import Tuple
 
 import pytest
 
-from infection_monkey.network.info import get_free_tcp_port
+from infection_monkey.network.info import TCPPortSelector
 from infection_monkey.network.ports import COMMON_PORTS
 
 
@@ -13,28 +13,48 @@ class Connection:
 
 
 @pytest.mark.parametrize("port", COMMON_PORTS)
-def test_get_free_tcp_port__checks_common_ports(port: int, monkeypatch):
+def test_tcp_port_selector__checks_common_ports(port: int, monkeypatch):
+    tcp_port_selector = TCPPortSelector()
     unavailable_ports = [Connection(("", p)) for p in COMMON_PORTS if p is not port]
 
     monkeypatch.setattr(
         "infection_monkey.network.info.psutil.net_connections", lambda: unavailable_ports
     )
-    assert get_free_tcp_port() is port
+    assert tcp_port_selector.get_free_tcp_port() is port
 
 
-def test_get_free_tcp_port__checks_other_ports_if_common_ports_unavailable(monkeypatch):
+def test_tcp_port_selector__checks_other_ports_if_common_ports_unavailable(monkeypatch):
+    tcp_port_selector = TCPPortSelector()
     unavailable_ports = [Connection(("", p)) for p in COMMON_PORTS]
     monkeypatch.setattr(
         "infection_monkey.network.info.psutil.net_connections", lambda: unavailable_ports
     )
 
-    assert get_free_tcp_port() is not None
+    assert tcp_port_selector.get_free_tcp_port() is not None
 
 
-def test_get_free_tcp_port__none_if_no_available_ports(monkeypatch):
+def test_tcp_port_selector__none_if_no_available_ports(monkeypatch):
+    tcp_port_selector = TCPPortSelector()
     unavailable_ports = [Connection(("", p)) for p in range(65535)]
     monkeypatch.setattr(
         "infection_monkey.network.info.psutil.net_connections", lambda: unavailable_ports
     )
 
-    assert get_free_tcp_port() is None
+    assert tcp_port_selector.get_free_tcp_port() is None
+
+
+@pytest.mark.parametrize("common_port", COMMON_PORTS)
+def test_tcp_port_selector__checks_common_ports_leases(common_port: int, monkeypatch):
+    tcp_port_selector = TCPPortSelector()
+    unavailable_ports = [Connection(("", p)) for p in COMMON_PORTS if p is not common_port]
+    monkeypatch.setattr(
+        "infection_monkey.network.info.psutil.net_connections", lambda: unavailable_ports
+    )
+
+    free_port_1 = tcp_port_selector.get_free_tcp_port()
+    free_port_2 = tcp_port_selector.get_free_tcp_port()
+
+    assert free_port_1 == common_port
+    assert free_port_2 != common_port
+    assert free_port_2 is not None
+    assert free_port_2 not in COMMON_PORTS