ipn: add AcceptRoutesFilter preference for filtering peer routes

The preference is stored in string form as the internal representation
netipx.IPSetBuilder does not serialize. The string form is reasonably
compact and readable.

Signed-off-by: James Tucker <james@tailscale.com>

ipn/ipn_clone.go

@@ -35,6 +35,7 @@ func (src *Prefs) Clone() *Prefs {
 var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	ControlURL             string
 	RouteAll               bool
+	AcceptRoutesFilter 		 string
 	AllowSingleHosts       bool
 	ExitNodeID             tailcfg.StableNodeID
 	ExitNodeIP             netip.Addr

ipn/prefs.go

@@ -70,6 +70,13 @@ type Prefs struct {
 	// controlled by ExitNodeID/IP below.
 	RouteAll bool
 
+	// AcceptRoutesFilter specifies an ordered list of IP ranges that are to be
+	// included or excluded from peer routes. The value is comma-seprated IP CIDRs
+	// with an optional leading `-` prefix indicating an exclusion, e.g.
+	// "0.0.0.0/0,-192.168.20.0/24" meaning "all routes except those intersecting
+	// 192.168.20.0/24".
+	AcceptRoutesFilter string
+
 	// AllowSingleHosts specifies whether to install routes for each
 	// node IP on the tailscale network, in addition to a route for
 	// the whole network.
@@ -206,6 +213,7 @@ type MaskedPrefs struct {
 
 	ControlURLSet             bool `json:",omitempty"`
 	RouteAllSet               bool `json:",omitempty"`
+	AcceptRoutesFilterSet     bool `json:",omitempty"`
 	AllowSingleHostsSet       bool `json:",omitempty"`
 	ExitNodeIDSet             bool `json:",omitempty"`
 	ExitNodeIPSet             bool `json:",omitempty"`
@@ -293,6 +301,9 @@ func (p *Prefs) pretty(goos string) string {
 	var sb strings.Builder
 	sb.WriteString("Prefs{")
 	fmt.Fprintf(&sb, "ra=%v ", p.RouteAll)
+	if p.RouteAll || p.AcceptRoutesFilter != "" {
+		fmt.Fprintf(&sb, "acceptfilter=%q ", p.AcceptRoutesFilter)
+	}
 	if !p.AllowSingleHosts {
 		sb.WriteString("mesh=false ")
 	}
@@ -366,6 +377,7 @@ func (p *Prefs) Equals(p2 *Prefs) bool {
 	return p != nil && p2 != nil &&
 		p.ControlURL == p2.ControlURL &&
 		p.RouteAll == p2.RouteAll &&
+		p.AcceptRoutesFilter == p2.AcceptRoutesFilter &&
 		p.AllowSingleHosts == p2.AllowSingleHosts &&
 		p.ExitNodeID == p2.ExitNodeID &&
 		p.ExitNodeIP == p2.ExitNodeIP &&
@@ -424,6 +436,7 @@ func NewPrefs() *Prefs {
 		ControlURL: "",
 
 		RouteAll:           true,
+		AcceptRoutesFilter: "0.0.0.0/0,::/0",
 		AllowSingleHosts:   true,
 		CorpDNS:            true,
 		WantRunning:        false,

ipn/prefs_test.go

@@ -38,6 +38,7 @@ func TestPrefsEqual(t *testing.T) {
 	prefsHandles := []string{
 		"ControlURL",
 		"RouteAll",
+		"AcceptRoutesFilter",
 		"AllowSingleHosts",
 		"ExitNodeID",
 		"ExitNodeIP",