XTLS/Xray-core · error

invalid xray.tun.fd: file descriptor must be >= 3

Error message

invalid xray.tun.fd: file descriptor must be >= 3

What it means

Error "invalid xray.tun.fd: file descriptor must be >= 3" thrown in XTLS/Xray-core.

Source

Thrown at proxy/tun/tun_linux.go:84

		options: options,
		ownsTun: true,
	}

	return linuxTun, nil
}

func openFromEnv(expectedName string) (int, netlink.Link, bool, error) {
	fdStr := platform.NewEnvFlag(platform.TunFdKey).GetValue(func() string { return "" })
	if fdStr == "" {
		return -1, nil, false, nil
	}

	fd, err := strconv.Atoi(fdStr)
	if err != nil {
		return -1, nil, true, errors.New("invalid ", platform.TunFdKey).Base(err)
	}
	if fd < 3 {
		return -1, nil, true, errors.New("invalid ", platform.TunFdKey, ": file descriptor must be >= 3")
	}

	ifr, err := unix.NewIfreq("")
	if err != nil {
		return -1, nil, true, err
	}
	if err = unix.IoctlIfreq(fd, unix.TUNGETIFF, ifr); err != nil {
		return -1, nil, true, err
	}

	flags := ifr.Uint16()
	if flags&unix.IFF_TUN == 0 {
		return -1, nil, true, errors.New("invalid ", platform.TunFdKey, ": file descriptor is not a TUN device")
	}
	if flags&unix.IFF_NO_PI == 0 {
		return -1, nil, true, errors.New("invalid ", platform.TunFdKey, ": TUN device must use IFF_NO_PI")
	}

View on GitHub (pinned to 7d214f8b09)

When it happens

Trigger: Thrown at proxy/tun/tun_linux.go:84 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/30482c5584f2994b. Report an issue: GitHub.