improve condition

core/device/fdbased/open_unix.go

@@ -5,19 +5,13 @@ package fdbased
 import (
 	"fmt"
 	"os"
-	"runtime"
 
 	"github.com/xjasonlyu/tun2socks/v2/core/device"
 	"github.com/xjasonlyu/tun2socks/v2/core/device/iobased"
 )
 
 func open(fd int, mtu uint32, offset int) (device.Device, error) {
 	f := &FD{fd: fd, mtu: mtu}
-	// fd offset in ios
-	// https://stackoverflow.com/questions/69260852/ios-network-extension-packet-parsing/69487795#69487795
-	if offset == 0 && runtime.GOOS == "ios" {
-		offset = 4
-	}
 	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"
@@ -67,6 +68,12 @@ func parseDevice(s string, mtu uint32) (device.Device, error) {
 }
 
 func parseFD(u *url.URL, mtu uint32) (device.Device, error) {
+
+	// fd offset in ios
+	// https://stackoverflow.com/questions/69260852/ios-network-extension-packet-parsing/69487795#69487795
+	if runtime.GOOS == "ios" {
+		return fdbased.Open(u.Host, mtu, 4)
+	}
 	return fdbased.Open(u.Host, mtu, 0)
 }