route: fix RTM_GET netmask parsing on Darwin

route/address.go

@@ -396,13 +396,19 @@ func marshalAddrs(b []byte, as []Addr) (uint, error) {
 func parseAddrs(attrs uint, fn func(int, []byte) (int, Addr, error), b []byte) ([]Addr, error) {
 	var as [syscall.RTAX_MAX]Addr
 	af := int(syscall.AF_UNSPEC)
+	isInet := func(fam int) bool {
+		return fam == syscall.AF_INET || fam == syscall.AF_INET6
+	}
+	isMask := func(addrType uint) bool {
+		return addrType == syscall.RTAX_NETMASK || addrType == syscall.RTAX_GENMASK
+	}
 	for i := uint(0); i < syscall.RTAX_MAX && len(b) >= roundup(0); i++ {
 		if attrs&(1<<i) == 0 {
 			continue
 		}
 		if i <= syscall.RTAX_BRD {
-			switch b[1] {
-			case syscall.AF_LINK:
+			switch {
+			case b[1] == syscall.AF_LINK:
 				a, err := parseLinkAddr(b)
 				if err != nil {
 					return nil, err
@@ -413,8 +419,10 @@ func parseAddrs(attrs uint, fn func(int, []byte) (int, Addr, error), b []byte) (
 					return nil, errMessageTooShort
 				}
 				b = b[l:]
-			case syscall.AF_INET, syscall.AF_INET6:
-				af = int(b[1])
+			case isInet(int(b[1])) || (isMask(i) && isInet(af)):
+				if isInet(int(b[1])) {
+					af = int(b[1])
+				}
 				a, err := parseInetAddr(af, b)
 				if err != nil {
 					return nil, err

route/address_darwin_test.go

@@ -29,12 +29,12 @@ var parseAddrsOnDarwinLittleEndianTests = []parseAddrsOnDarwinTest{
 			0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
 			0x0, 0x0, 0x0, 0x0,
 
-			0x7, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+			0x7, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0,
 		},
 		[]Addr{
 			&Inet4Addr{IP: [4]byte{192, 168, 86, 0}},
 			&LinkAddr{Index: 4},
-			&Inet4Addr{IP: [4]byte{255, 255, 255, 255}},
+			&Inet4Addr{IP: [4]byte{255, 255, 255, 0}},
 			nil,
 			nil,
 			nil,

route/example_darwin_test.go

@@ -0,0 +1,70 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package route_test
+
+import (
+	"fmt"
+	"net/netip"
+	"os"
+	"syscall"
+
+	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
+)
+
+// This example demonstrates how to parse a response to RTM_GET request.
+func ExampleParseRIB() {
+	fd, err := unix.Socket(unix.AF_ROUTE, unix.SOCK_RAW, unix.AF_UNSPEC)
+	if err != nil {
+		return
+	}
+	defer unix.Close(fd)
+
+	// Create a RouteMessage with RTM_GET type
+	rtm := &route.RouteMessage{
+		Version: syscall.RTM_VERSION,
+		Type:    unix.RTM_GET,
+		ID:      uintptr(os.Getpid()),
+		Seq:     0,
+		Addrs: []route.Addr{
+			&route.Inet4Addr{IP: [4]byte{127, 0, 0, 0}},
+		},
+	}
+
+	// Marshal the message into bytes
+	msgBytes, err := rtm.Marshal()
+	if err != nil {
+		return
+	}
+
+	// Send the message over the routing socket
+	_, err = unix.Write(fd, msgBytes)
+	if err != nil {
+		return
+	}
+
+	// Read the response from the routing socket
+	var buf [2 << 10]byte
+	n, err := unix.Read(fd, buf[:])
+	if err != nil {
+		return
+	}
+
+	// Parse the response messages
+	msgs, err := route.ParseRIB(route.RIBTypeRoute, buf[:n])
+	if err != nil {
+		return
+	}
+	routeMsg, ok := msgs[0].(*route.RouteMessage)
+	if !ok {
+		return
+	}
+	netmask, ok := routeMsg.Addrs[2].(*route.Inet4Addr)
+	if !ok {
+		return
+	}
+	fmt.Println(netip.AddrFrom4(netmask.IP))
+	// Output: 255.0.0.0
+}