add start of Hard NAT impl

untested so far

Change-Id: I682b604d0e90debf9eae3f1814663f336d03f57c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

natlab/natlabd/nat.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"math/rand/v2"
 	"net/netip"
 	"time"
 )
@@ -43,6 +44,8 @@ type oneToOneNAT struct {
 	wanIP netip.Addr
 }
 
+var _ NATTable = (*oneToOneNAT)(nil)
+
 func (n *oneToOneNAT) PickOutgoingSrc(src, dst netip.AddrPort, at time.Time) (wanSrc netip.AddrPort) {
 	return netip.AddrPortFrom(n.wanIP, src.Port())
 }
@@ -50,3 +53,75 @@ func (n *oneToOneNAT) PickOutgoingSrc(src, dst netip.AddrPort, at time.Time) (wa
 func (n *oneToOneNAT) PickIncomingDst(src, dst netip.AddrPort, at time.Time) (lanDst netip.AddrPort) {
 	return netip.AddrPortFrom(n.lanIP, dst.Port())
 }
+
+type hardKeyOut struct {
+	lanIP netip.Addr
+	dst   netip.AddrPort
+}
+
+type hardKeyIn struct {
+	wanPort uint16
+	src     netip.AddrPort
+}
+
+type portMappingAndTime struct {
+	port uint16
+	at   time.Time
+}
+
+type lanAddrAndTime struct {
+	lanAddr netip.AddrPort
+	at      time.Time
+}
+
+// hardNAT is an "Endpoint Dependent" NAT, like FreeBSD/pfSense/OPNsense.
+// This is shown as "MappingVariesByDestIP: true" by netcheck, and what
+// Tailscale calls "Hard NAT".
+type hardNAT struct {
+	wanIP netip.Addr
+
+	out map[hardKeyOut]portMappingAndTime
+	in  map[hardKeyIn]lanAddrAndTime
+}
+
+var _ NATTable = (*hardNAT)(nil)
+
+func (n *hardNAT) PickOutgoingSrc(src, dst netip.AddrPort, at time.Time) (wanSrc netip.AddrPort) {
+	ko := hardKeyOut{src.Addr(), dst}
+	if pm, ok := n.out[ko]; ok {
+		// Existing flow.
+		// TODO: bump timestamp
+		return netip.AddrPortFrom(n.wanIP, pm.port)
+	}
+
+	// No existing mapping exists. Create one.
+
+	// TODO: clean up old expired mappings
+
+	// Instead of proper data structures that would be efficient, we instead
+	// just loop a bunch and look for a free port. This project is only used
+	// by tests and doesn't care about performance, this is good enough.
+	port := src.Port() // start with the port the client used? (TODO: does FreeBSD do this? make knob?)
+	for {
+		ki := hardKeyIn{wanPort: port, src: dst}
+		if _, ok := n.in[ki]; ok {
+			// Collision. Try again.
+			port = rand.N(uint16(32<<10)) + 32<<10 // pick some "ephemeral" port
+			continue
+		}
+		n.in[ki] = lanAddrAndTime{lanAddr: src, at: at}
+		n.out[ko] = portMappingAndTime{port: port, at: at}
+	}
+}
+
+func (n *hardNAT) PickIncomingDst(src, dst netip.AddrPort, at time.Time) (lanDst netip.AddrPort) {
+	if dst.Addr() != n.wanIP {
+		return netip.AddrPort{} // drop; not for us. shouldn't happen if natlabd routing isn't broken.
+	}
+	ki := hardKeyIn{wanPort: dst.Port(), src: src}
+	if pm, ok := n.in[ki]; ok {
+		// Existing flow.
+		return pm.lanAddr
+	}
+	return netip.AddrPort{} // drop; no mapping
+}