net/interfaces: add State.PAC field, populate it on Windows

Not used for anything yet (except logging), but populate the current
proxy autoconfig PAC URL in Interfaces.State.

A future change will do things based on it.

net/interfaces/interfaces.go

@@ -177,6 +177,9 @@ type State struct {
 
 	// HTTPProxy is the HTTP proxy to use.
 	HTTPProxy string
+
+	// PAC is the URL to the Proxy Autoconfig URL, if applicable.
+	PAC string
 }
 
 func (s *State) Equal(s2 *State) bool {
@@ -197,6 +200,9 @@ func (s *State) RemoveTailscaleInterfaces() {
 	}
 }
 
+// getPAC, if non-nil, returns the current PAC file URL.
+var getPAC func() string
+
 // GetState returns the state of all the current machine's network interfaces.
 //
 // It does not set the returned State.IsExpensive. The caller can populate that.
@@ -222,6 +228,9 @@ func GetState() (*State, error) {
 	if u, err := tshttpproxy.ProxyFromEnvironment(req); err == nil && u != nil {
 		s.HTTPProxy = u.String()
 	}
+	if getPAC != nil {
+		s.PAC = getPAC()
+	}
 
 	return s, nil
 }

net/interfaces/interfaces_windows.go

@@ -6,11 +6,14 @@ package interfaces
 
 import (
 	"fmt"
+	"log"
 	"os/exec"
 	"syscall"
+	"unsafe"
 
 	"github.com/tailscale/winipcfg-go"
 	"go4.org/mem"
+	"golang.org/x/sys/windows"
 	"inet.af/netaddr"
 	"tailscale.com/tsconst"
 	"tailscale.com/util/lineread"
@@ -18,6 +21,7 @@ import (
 
 func init() {
 	likelyHomeRouterIP = likelyHomeRouterIPWindows
+	getPAC = getPACWindows
 }
 
 /*
@@ -148,3 +152,32 @@ func DefaultRouteInterface() (string, error) {
 	}
 	return fmt.Sprintf("%s (%s)", iface.FriendlyName, iface.Description), nil
 }
+
+var (
+	winHTTP                  = windows.NewLazySystemDLL("winhttp.dll")
+	detectAutoProxyConfigURL = winHTTP.NewProc("WinHttpDetectAutoProxyConfigUrl")
+
+	kernel32   = windows.NewLazySystemDLL("kernel32.dll")
+	globalFree = kernel32.NewProc("GlobalFree")
+)
+
+const (
+	winHTTP_AUTO_DETECT_TYPE_DHCP  = 0x00000001
+	winHTTP_AUTO_DETECT_TYPE_DNS_A = 0x00000002
+)
+
+func getPACWindows() string {
+	var res *uint16
+	r, _, err := detectAutoProxyConfigURL.Call(
+		winHTTP_AUTO_DETECT_TYPE_DHCP|winHTTP_AUTO_DETECT_TYPE_DNS_A,
+		uintptr(unsafe.Pointer(&res)),
+	)
+	var got string
+	if res != nil {
+		got = windows.UTF16PtrToString(res)
+		globalFree.Call(uintptr(unsafe.Pointer(res)))
+	} else {
+		log.Printf("getPACWindows: r=%v, err=%#v", r, err)
+	}
+	return got
+}