tsnet,wgengine: fix src to primary Tailscale IP for TCP dials

Ensure that the src address for a connection is one of the primary
addresses assigned by Tailscale. Not, for example, a virtual IP address.

Updates #14667

Signed-off-by: Fran Bull <fran@tailscale.com>

cmd/k8s-operator/depaware.txt

@@ -904,6 +904,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         tailscale.com/tstime/rate                                    from tailscale.com/derp+
         tailscale.com/tsweb/varz                                     from tailscale.com/util/usermetric
         tailscale.com/types/appctype                                 from tailscale.com/ipn/ipnlocal
+        tailscale.com/types/bools                                    from tailscale.com/tsnet
         tailscale.com/types/dnstype                                  from tailscale.com/ipn/ipnlocal+
         tailscale.com/types/empty                                    from tailscale.com/ipn+
         tailscale.com/types/ipproto                                  from tailscale.com/net/flowtrack+

tsnet/tsnet.go

@@ -49,6 +49,7 @@ import (
 	"tailscale.com/net/socks5"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tsd"
+	"tailscale.com/types/bools"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
 	"tailscale.com/types/nettype"
@@ -601,7 +602,9 @@ func (s *Server) start() (reterr error) {
 		// Note: don't just return ns.DialContextTCP or we'll return
 		// *gonet.TCPConn(nil) instead of a nil interface which trips up
 		// callers.
-		tcpConn, err := ns.DialContextTCP(ctx, dst)
+		v4, v6 := s.TailscaleIPs()
+		src := bools.IfElse(dst.Addr().Is6(), v6, v4)
+		tcpConn, err := ns.DialContextTCPWithBind(ctx, src, dst)
 		if err != nil {
 			return nil, err
 		}
@@ -611,7 +614,9 @@ func (s *Server) start() (reterr error) {
 		// Note: don't just return ns.DialContextUDP or we'll return
 		// *gonet.UDPConn(nil) instead of a nil interface which trips up
 		// callers.
-		udpConn, err := ns.DialContextUDP(ctx, dst)
+		v4, v6 := s.TailscaleIPs()
+		src := bools.IfElse(dst.Addr().Is6(), v6, v4)
+		udpConn, err := ns.DialContextUDPWithBind(ctx, src, dst)
 		if err != nil {
 			return nil, err
 		}

wgengine/netstack/netstack.go

@@ -843,6 +843,27 @@ func (ns *Impl) DialContextTCP(ctx context.Context, ipp netip.AddrPort) (*gonet.
 	return gonet.DialContextTCP(ctx, ns.ipstack, remoteAddress, ipType)
 }
 
+// DialContextTCPWithBind creates a new gonet.TCPConn connected to the specified
+// remoteAddress with its local address bound to localAddr on an available port.
+func (ns *Impl) DialContextTCPWithBind(ctx context.Context, localAddr netip.Addr, remoteAddr netip.AddrPort) (*gonet.TCPConn, error) {
+	remoteAddress := tcpip.FullAddress{
+		NIC:  nicID,
+		Addr: tcpip.AddrFromSlice(remoteAddr.Addr().AsSlice()),
+		Port: remoteAddr.Port(),
+	}
+	localAddress := tcpip.FullAddress{
+		NIC:  nicID,
+		Addr: tcpip.AddrFromSlice(localAddr.AsSlice()),
+	}
+	var ipType tcpip.NetworkProtocolNumber
+	if remoteAddr.Addr().Is4() {
+		ipType = ipv4.ProtocolNumber
+	} else {
+		ipType = ipv6.ProtocolNumber
+	}
+	return gonet.DialTCPWithBind(ctx, ns.ipstack, localAddress, remoteAddress, ipType)
+}
+
 func (ns *Impl) DialContextUDP(ctx context.Context, ipp netip.AddrPort) (*gonet.UDPConn, error) {
 	remoteAddress := &tcpip.FullAddress{
 		NIC:  nicID,
@@ -859,6 +880,28 @@ func (ns *Impl) DialContextUDP(ctx context.Context, ipp netip.AddrPort) (*gonet.
 	return gonet.DialUDP(ns.ipstack, nil, remoteAddress, ipType)
 }
 
+// DialContextUDPWithBind creates a new gonet.UDPConn. Connected to remoteAddr.
+// With its local address bound to localAddr on an available port.
+func (ns *Impl) DialContextUDPWithBind(ctx context.Context, localAddr netip.Addr, remoteAddr netip.AddrPort) (*gonet.UDPConn, error) {
+	remoteAddress := &tcpip.FullAddress{
+		NIC:  nicID,
+		Addr: tcpip.AddrFromSlice(remoteAddr.Addr().AsSlice()),
+		Port: remoteAddr.Port(),
+	}
+	localAddress := &tcpip.FullAddress{
+		NIC:  nicID,
+		Addr: tcpip.AddrFromSlice(localAddr.AsSlice()),
+	}
+	var ipType tcpip.NetworkProtocolNumber
+	if remoteAddr.Addr().Is4() {
+		ipType = ipv4.ProtocolNumber
+	} else {
+		ipType = ipv6.ProtocolNumber
+	}
+
+	return gonet.DialUDP(ns.ipstack, localAddress, remoteAddress, ipType)
+}
+
 // getInjectInboundBuffsSizes returns packet memory and a sizes slice for usage
 // when calling tstun.Wrapper.InjectInboundPacketBuffer(). These are sized with
 // consideration for MTU and GSO support on ns.linkEP. They should be recycled