net/netutil: add function to check rp_filter value (#5703)

Updates #4432


Change-Id: Ifc332a5747fc1feffdbb87437308cf8ecb21b0b0

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

net/netutil/netutil_test.go

@@ -6,6 +6,7 @@ package netutil
 import (
 	"io"
 	"net"
+	"net/netip"
 	"runtime"
 	"testing"
 )
@@ -65,3 +66,14 @@ func TestIPForwardingEnabledLinux(t *testing.T) {
 		t.Errorf("got true; want false")
 	}
 }
+
+func TestCheckReversePathFiltering(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skipf("skipping on %s", runtime.GOOS)
+	}
+	warn, err := CheckReversePathFiltering([]netip.Prefix{
+		netip.MustParsePrefix("192.168.1.1/24"),
+	}, nil)
+	t.Logf("err: %v", err)
+	t.Logf("warnings: %v", warn)
+}