nic_router: use increasing src port for new nat

The NAT feature of the NIC router used to prefer re-using source ports that
have been freed recently. From an external server's perspective, if a client
dies and restarts, chances are high that the new connect arrives with the same
source-IP/source-port as the old connection. The server has to forcefully reset
the connection. If that happens a lot, the server may even start to ignore
further connections from this IP/port combination for a while as a mitigation.

This patch adds a continuous counter feature that makes sure that every new
port allocation will increment and result in a port that hasn't been used for a
long time.

The NAT feature of the nic_router is now more in line with RFC 6056 chapter 4.

Ref #4086

repos/os/src/server/nic_router/domain.cc

@@ -313,7 +313,7 @@ void Domain::init(Domain_tree &domains)
 		try {
 			Nat_rule &rule = *new (_alloc)
 				Nat_rule(domains, _tcp_port_alloc, _udp_port_alloc,
-				         _icmp_port_alloc, node);
+				         _icmp_port_alloc, node, _config.verbose());
 			_nat_rules.insert(&rule);
 			if (_config.verbose()) {
 				log("[", *this, "] NAT rule: ", rule); }

repos/os/src/server/nic_router/nat_rule.cc

@@ -47,12 +47,13 @@ Nat_rule::Nat_rule(Domain_tree    &domains,
                    Port_allocator &tcp_port_alloc,
                    Port_allocator &udp_port_alloc,
                    Port_allocator &icmp_port_alloc,
-                   Xml_node const  node)
+                   Xml_node const  node,
+                   bool     const  verbose)
 :
 	_domain(_find_domain(domains, node)),
-	_tcp_port_alloc (tcp_port_alloc,  node.attribute_value("tcp-ports", 0UL)),
-	_udp_port_alloc (udp_port_alloc,  node.attribute_value("udp-ports", 0UL)),
-	_icmp_port_alloc(icmp_port_alloc, node.attribute_value("icmp-ids", 0UL))
+	_tcp_port_alloc (tcp_port_alloc,  node.attribute_value("tcp-ports", 0UL), verbose),
+	_udp_port_alloc (udp_port_alloc,  node.attribute_value("udp-ports", 0UL), verbose),
+	_icmp_port_alloc(icmp_port_alloc, node.attribute_value("icmp-ids", 0UL), verbose)
 { }
 
 

repos/os/src/server/nic_router/nat_rule.h

@@ -54,7 +54,8 @@ class Net::Nat_rule : public Genode::Avl_node<Nat_rule>
 		         Port_allocator         &tcp_port_alloc,
 		         Port_allocator         &udp_port_alloc,
 		         Port_allocator         &icmp_port_alloc,
-		         Genode::Xml_node const  node);
+		         Genode::Xml_node const  node,
+		         bool             const  verbose);
 
 		Nat_rule &find_by_domain(Domain &domain);
 

repos/os/src/server/nic_router/port_allocator.cc

@@ -11,6 +11,9 @@
  * under the terms of the GNU Affero General Public License version 3.
  */
 
+/* Genode includes */
+#include <base/log.h>
+
 /* local includes */
 #include <port_allocator.h>
 
@@ -26,6 +29,24 @@ bool Net::dynamic_port(Port const port) {
  ** Port_allocator **
  ********************/
 
+Port Net::Port_allocator::alloc()
+{
+	for (unsigned nr_of_trials { 0 };
+	     nr_of_trials < COUNT;
+	     nr_of_trials++) {
+
+		uint16_t const port_offset = _next_port_offset;
+		_next_port_offset = (_next_port_offset + 1) % COUNT;
+		try {
+			_alloc.alloc_addr(port_offset);
+			return Port { (uint16_t)(port_offset + FIRST) };
+		}
+		catch (Bit_allocator<COUNT>::Range_conflict) { }
+	}
+	throw Out_of_indices();
+}
+
+
 void Net::Port_allocator::alloc(Port const port)
 {
 	try { _alloc.alloc_addr(port.value - FIRST); }
@@ -34,6 +55,12 @@ void Net::Port_allocator::alloc(Port const port)
 }
 
 
+void Port_allocator::free(Port const port)
+{
+	_alloc.free(port.value - FIRST);
+}
+
+
 /**************************
  ** Port_allocator_guard **
  **************************/
@@ -67,7 +94,15 @@ void Port_allocator_guard::free(Port const port)
 
 
 Port_allocator_guard::Port_allocator_guard(Port_allocator &port_alloc,
-                                           unsigned const  max)
+                                           unsigned const  max,
+                                           bool     const  verbose)
 :
-	_port_alloc(port_alloc), _max(max)
-{ }
+	_port_alloc(port_alloc),
+	_max(min(max, static_cast<uint16_t>(Port_allocator::COUNT)))
+{
+	if (verbose && max > (Port_allocator::COUNT)) {
+
+		warning("number of ports was truncated to capacity of allocator");
+	}
+}
+

repos/os/src/server/nic_router/port_allocator.h

@@ -39,16 +39,18 @@ class Net::Port_allocator
 	private:
 
 		Genode::Bit_allocator<COUNT> _alloc            { };
+		Genode::uint16_t             _next_port_offset { 0 };
 
 	public:
 
 		struct Allocation_conflict : Genode::Exception { };
+		struct Out_of_indices      : Genode::Exception { };
 
-		Port alloc() { return Port(_alloc.alloc() + FIRST); }
+		Port alloc();
 
 		void alloc(Port const port);
 
-		void free(Port const port) { _alloc.free(port.value - FIRST); }
+		void free(Port const port);
 };
 
 
@@ -70,7 +72,9 @@ class Net::Port_allocator_guard
 
 		void free(Port const port);
 
-		Port_allocator_guard(Port_allocator & port_alloc, unsigned const max);
+		Port_allocator_guard(Port_allocator &port_alloc,
+		                     unsigned const  max,
+		                     bool     const  verbose);
 
 		unsigned max() const { return _max; }
 };