Options. For example, periodically difficulty trying to obtain information about the domain using wbinfo, in case of failure to change the config and do a squid-k reconfigure. Or you can try to replace the authenticator with its script, which will proxy the standard authenticator if the domain is available, and emulate his positive response in case of a fall the domain. In any case, I do not advise to fence these crutches, lift up on the dev backup domain controller. Otherwise one day You will have to go through all the machines and perewesti them to the new domain.