wgengine/netstack: start SOCKS5 server in netstack mode

Updates #707
Updates #504

Signed-off-by: Naman Sood <mail@nsood.in>

cmd/tailscaled/depaware.txt

@@ -96,6 +96,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
      💣 tailscale.com/net/netstat                                    from tailscale.com/ipn/ipnserver
         tailscale.com/net/packet                                     from tailscale.com/wgengine+
         tailscale.com/net/portmapper                                 from tailscale.com/net/netcheck+
+        tailscale.com/net/socks5                                     from tailscale.com/wgengine/netstack
         tailscale.com/net/stun                                       from tailscale.com/net/netcheck+
         tailscale.com/net/tlsdial                                    from tailscale.com/control/controlclient+
         tailscale.com/net/tsaddr                                     from tailscale.com/ipn/ipnlocal+

wgengine/netstack/netstack.go

@@ -14,6 +14,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"net"
 	"strings"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
@@ -30,6 +31,7 @@ import (
 	"gvisor.dev/gvisor/pkg/waiter"
 	"inet.af/netaddr"
 	"tailscale.com/net/packet"
+	"tailscale.com/net/socks5"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine"
@@ -113,6 +115,7 @@ func (ns *Impl) Start() error {
 	ns.ipstack.SetTransportProtocolHandler(udp.ProtocolNumber, udpFwd.HandlePacket)
 	go ns.injectOutbound()
 	ns.tundev.PostFilterIn = ns.injectInbound
+	go ns.socks5Server()
 
 	return nil
 }
@@ -282,6 +285,21 @@ func (ns *Impl) forwardTCP(client *gonet.TCPConn, wq *waiter.Queue, address stri
 	ns.logf("[v2] netstack: forwarder connection to %s closed", address)
 }
 
+func (ns *Impl) socks5Server() {
+	ln, err := net.Listen("tcp", "localhost:1080")
+	if err != nil {
+		ns.logf("could not start SOCKS5 listener: %v", err)
+		return
+	}
+	srv := &socks5.Server{
+		Logf: ns.logf,
+		Dialer: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			return ns.dialContextTCP(ctx, addr)
+		},
+	}
+	ns.logf("SOCKS5 server exited: %v", srv.Serve(ln))
+}
+
 func (ns *Impl) acceptUDP(r *udp.ForwarderRequest) {
 	ns.logf("[v2] UDP ForwarderRequest: %v", r)
 	var wq waiter.Queue