conn,device: enable cryptorouting via PeerAwareEndpoint

Introduce an optional extension point for Endpoint that enables a path
for WireGuard to inform an integration about the peer public key that is
associated with an Endpoint.

The API is expected to return either the same or a new Endpoint in
response to this function. A future version of this patch could
potentially remove the returned Endpoint, but would require larger
integrator changes downstream.

This adds a small per-packet cost that could later be removed with a
larger refactor of the wireguard-go interface and Tailscale magicsock
code, as well as introducing a generic bound for Endpoint in a device &
bind instance.

Updates tailscale/corp#20732

conn/conn.go

@@ -84,6 +84,20 @@ type Endpoint interface {
 	SrcIP() netip.Addr
 }
 
+// PeerAwareEndpoint is an optional Endpoint specialization for
+// integrations that want to know about the outcome of cryptorouting
+// identification.
+//
+// If they receive a packet from a source they had not pre-identified,
+// to learn the identification WireGuard can derive from the session
+// or handshake.
+//
+// If GetPeerEndpoint returns nil, WireGuard will be unable to respond
+// to the peer until a new endpoint is written by a later packet.
+type PeerAwareEndpoint interface {
+	GetPeerEndpoint(peerPublicKey [32]byte) Endpoint
+}
+
 var (
 	ErrBindAlreadyOpen   = errors.New("bind is already open")
 	ErrWrongEndpointType = errors.New("endpoint type does not correspond with bind type")

device/noise-protocol.go

@@ -124,7 +124,7 @@ type Handshake struct {
 	localEphemeral            NoisePrivateKey          // ephemeral secret key
 	localIndex                uint32                   // used to clear hash-table
 	remoteIndex               uint32                   // index for sending
-	remoteStatic              NoisePublicKey           // long term key
+	remoteStatic              NoisePublicKey           // long term key, never changes, can be accessed without mutex
 	remoteEphemeral           NoisePublicKey           // ephemeral public key
 	precomputedStaticStatic   [NoisePublicKeySize]byte // precomputed shared secret
 	lastTimestamp             tai64n.Timestamp

device/peer.go

@@ -283,6 +283,9 @@ func (peer *Peer) SetEndpointFromPacket(endpoint conn.Endpoint) {
 		return
 	}
 	peer.endpoint.clearSrcOnTx = false
+	if ep, ok := endpoint.(conn.PeerAwareEndpoint); ok {
+		endpoint = ep.GetPeerEndpoint(peer.handshake.remoteStatic)
+	}
 	peer.endpoint.val = endpoint
 }