don't connect to privileged ports by default

ChangeLog

@@ -66,6 +66,7 @@
 	  incoming connection
 	* added more detailed instrumentation of the disk I/O thread
 
+	* avoid connecting to peers listening on ports < 1024
 	* optimized piece picking to not cause busy loops in some end-game modes
 	* fixed python bindings for tcp::endpoint
 	* fixed edge case of pad file support

include/libtorrent/session_settings.hpp

@@ -254,6 +254,7 @@ namespace libtorrent
 			, announce_double_nat(false)
 			, torrent_connect_boost(10)
 			, seeding_outgoing_connections(true)
+			, no_connect_privileged_ports(true)
 		{}
 
 		// libtorrent version. Used for forward binary compatibility
@@ -1014,6 +1015,11 @@ namespace libtorrent
 		// making outgoing connections is costly and known to not
 		// add any benefits
 		bool seeding_outgoing_connections;
+
+		// when this is true, libtorrent will not attempt to make outgoing
+		// connections to peers whose port is < 1024. This is a safety
+		// precaution to avoid being part of a DDoS attack
+		bool no_connect_privileged_ports;
 	};
 
 #ifndef TORRENT_DISABLE_DHT

src/policy.cpp

@@ -557,6 +557,10 @@ namespace libtorrent
 		aux::session_impl const& ses = m_torrent->session();
 		if (ses.m_port_filter.access(p.port) & port_filter::blocked)
 			return false;
+
+		if (ses.m_settings.no_connect_privileged_ports && p.port < 1024)
+			return false;
+
 		return true;
 	}
 
@@ -1180,6 +1184,13 @@ namespace libtorrent
 			return 0;
 		}
 
+		if (ses.m_settings.no_connect_privileged_ports && remote.port() < 1024)
+		{
+			if (ses.m_alerts.should_post<peer_blocked_alert>())
+				ses.m_alerts.post_alert(peer_blocked_alert(m_torrent->get_handle(), remote.address()));
+			return 0;
+		}
+
 		// if the IP is blocked, don't add it
 		if (ses.m_ip_filter.access(remote.address()) & ip_filter::blocked)
 		{