Feature: set open FD offset for iOS (#418)

Co-authored-by: Jason Lyu <xjasonlyu@gmail.com>

core/device/fdbased/open_unix.go

@@ -12,7 +12,6 @@ import (
 
 func open(fd int, mtu uint32, offset int) (device.Device, error) {
 	f := &FD{fd: fd, mtu: mtu}
-
 	ep, err := iobased.New(os.NewFile(uintptr(fd), f.Name()), mtu, offset)
 	if err != nil {
 		return nil, fmt.Errorf("create endpoint: %w", err)

engine/parse.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/netip"
 	"net/url"
+	"runtime"
 	"strings"
 
 	"github.com/gorilla/schema"
@@ -67,7 +68,13 @@ func parseDevice(s string, mtu uint32) (device.Device, error) {
 }
 
 func parseFD(u *url.URL, mtu uint32) (device.Device, error) {
-	return fdbased.Open(u.Host, mtu, 0)
+	offset := 0
+	// fd offset in ios
+	// https://stackoverflow.com/questions/69260852/ios-network-extension-packet-parsing/69487795#69487795
+	if runtime.GOOS == "ios" {
+		offset = 4
+	}
+	return fdbased.Open(u.Host, mtu, offset)
 }
 
 func parseProxy(s string) (proxy.Proxy, error) {