cmd/tailscale: fix default for tailscale set --accept-routes

The default values for `tailscale up` and `tailscale set` are supposed
to agree for all common flags. But they don’t for `--accept-routes`
on Windows and from the Mac OS App Store, because `tailscale up`
computes this value based on the operating system:

    user@host:~$ tailscale up --help 2>&1 | grep -A1 accept-routes
      --accept-dns, --accept-dns=false
            accept DNS configuration from the admin panel (default true)
    user@host:~$ tailscale set --help 2>&1 | grep -A1 accept-routes
      --accept-dns, --accept-dns=false
            accept DNS configuration from the admin panel

Luckily, `tailscale set` uses `ipn.MaskedPrefs`, so the default values
don’t logically matter. But someone will get the wrong idea if they
trust the `tailscale set --help` documentation.

In addition, `ipn.Prefs.RouteAll` defaults to true so it disagrees
with both of the flags above.

This patch makes `--accept-routes` use the same logic for in both
commands by hoisting the logic that was buried in `cmd/tailscale/cli`
to `ipn.Prefs.DefaultRouteAll`. Then, all three of defaults can agree.

Fixes: #15319

Signed-off-by: Simon Law <sfllaw@sfllaw.ca>

ipn/prefs.go

@@ -29,6 +29,7 @@ import (
 	"tailscale.com/types/views"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/syspolicy"
+	"tailscale.com/version"
 )
 
 // DefaultControlURL is the URL base of the control plane
@@ -664,7 +665,7 @@ func NewPrefs() *Prefs {
 	// Provide default values for options which might be missing
 	// from the json data for any reason. The json can still
 	// override them to false.
-	return &Prefs{
+	p := &Prefs{
 		// ControlURL is explicitly not set to signal that
 		// it's not yet configured, which relaxes the CLI "up"
 		// safety net features. It will get set to DefaultControlURL
@@ -672,7 +673,6 @@ func NewPrefs() *Prefs {
 		// later anyway.
 		ControlURL: "",
 
-		RouteAll:            true,
 		CorpDNS:             true,
 		WantRunning:         false,
 		NetfilterMode:       preftype.NetfilterOn,
@@ -682,6 +682,8 @@ func NewPrefs() *Prefs {
 			Apply: opt.Bool("unset"),
 		},
 	}
+	p.RouteAll = p.DefaultRouteAll(runtime.GOOS)
+	return p
 }
 
 // ControlURLOrDefault returns the coordination server's URL base.
@@ -711,6 +713,19 @@ func (p *Prefs) ControlURLOrDefault() string {
 	return DefaultControlURL
 }
 
+// DefaultRouteAll returns the default value of [Prefs.RouteAll] as a function
+// of the platform it's running on.
+func (p *Prefs) DefaultRouteAll(goos string) bool {
+	switch goos {
+	case "windows":
+		return true
+	case "darwin":
+		return version.IsSandboxedMacOS()
+	default:
+		return false
+	}
+}
+
 // AdminPageURL returns the admin web site URL for the current ControlURL.
 func (p PrefsView) AdminPageURL() string { return p.ж.AdminPageURL() }