Add system stack

system_nat.go

@@ -0,0 +1,68 @@
+package tun
+
+import (
+	"net/netip"
+	"sync"
+)
+
+type TCPNat struct {
+	portIndex  uint16
+	portAccess sync.RWMutex
+	addrAccess sync.RWMutex
+	addrMap    map[netip.AddrPort]uint16
+	portMap    map[uint16]*TCPSession
+}
+
+type TCPSession struct {
+	Source      netip.AddrPort
+	Destination netip.AddrPort
+}
+
+func NewNat() *TCPNat {
+	return &TCPNat{
+		portIndex: 10000,
+		addrMap:   make(map[netip.AddrPort]uint16),
+		portMap:   make(map[uint16]*TCPSession),
+	}
+}
+
+func (n *TCPNat) LookupBack(port uint16) *TCPSession {
+	n.portAccess.RLock()
+	defer n.portAccess.RUnlock()
+	return n.portMap[port]
+}
+
+func (n *TCPNat) Lookup(source netip.AddrPort, destination netip.AddrPort) uint16 {
+	n.addrAccess.RLock()
+	port, loaded := n.addrMap[source]
+	n.addrAccess.RUnlock()
+	if loaded {
+		return port
+	}
+	n.addrAccess.Lock()
+	nextPort := n.portIndex
+	if nextPort == 0 {
+		nextPort = 10000
+		n.portIndex = 10001
+	} else {
+		n.portIndex++
+	}
+	n.addrMap[source] = nextPort
+	n.addrAccess.Unlock()
+	n.portAccess.Lock()
+	n.portMap[nextPort] = &TCPSession{
+		Source:      source,
+		Destination: destination,
+	}
+	n.portAccess.Unlock()
+	return nextPort
+}
+
+func (n *TCPNat) Revoke(natPort uint16, session *TCPSession) {
+	n.addrAccess.Lock()
+	delete(n.addrMap, session.Source)
+	n.addrAccess.Unlock()
+	n.portAccess.Lock()
+	delete(n.portMap, natPort)
+	n.portAccess.Unlock()
+}