The D6686 may be vulnerable to becoming inoperative if a telnet session is left open by any means not intended by its owner. Enabling a password for Telnet sessions will prevent an internet attacker from attempting to utilize the second available tunnel.
How to add a password to protect the telnet session:
How to remove the password from the telnet session:
How to return the D6686 to factory settings.
Changes will take effect on next reboot.
After the reboot cycle finishes, the unit will now again be at the factory default settings.