route: fix RTM_GET netmask parsing on Darwin

On Darwin, the AF_FAMILY byte of a sockaddr for a netmask or genmask
can be ignored if unreasonable. In such cases, it is the family of the
DST address that should instead be used.

Depends on golang#231.
Fixes golang/go#71578.

Author: hurricanehrndz

route/address.go

@@ -401,8 +401,8 @@ func parseAddrs(attrs uint, fn func(int, []byte) (int, Addr, error), b []byte) (
 			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 +413,10 @@ 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 ((b[1] == syscall.AF_INET || b[1] == syscall.AF_INET6) || ((i == syscall.RTAX_NETMASK || i == syscall.RTAX_GENMASK) && (af == syscall.AF_INET || af == syscall.AF_INET6))):
+				if (i != syscall.RTAX_NETMASK && i != syscall.RTAX_GENMASK) {
+					af = int(b[1])
+				}
 				a, err := parseInetAddr(af, b)
 				if err != nil {
 					return nil, err

route/example_darwin_test.go

@@ -0,0 +1,66 @@
+package route_test
+
+import (
+	"fmt"
+	"net/netip"
+	"os"
+	"syscall"
+
+	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
+)
+
+func ExampleParseRIB_RTMGet() {
+	// This example demonstrates how to parse a response to RTM_GET request.
+	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
+}