Inexplicably, users could not authenticate to their OD accounts

Everything has been running fairly well in the small environment for months. After applying some recent updates to a rather stock 10.4.11 server supporting six users, acting as a Open Directory Master, and running basic web, mail, and file services, the server just started refusing all connections and bouncing email.

I was a bit puzzled and spent the last four hours trying to figure it out. I found nothing interesting in any of the logs, tried rebooting and, googling for similar symptoms. I began to think that maybe two desktops that I was working on (one upgrading to Leopard and another computer being replaced by a new model (with Tiger). All the work was done on the machines over the last couple of days and everything was working fine.

After getting to the point that I was going to try rebuilding the whole environment, I noodled around (I like noodling), the server and looked at the Directory Access utility on the server (and OD Master). Authentication was set to search /NetInfo/DefaultLocalNode and not /LDAPv3/127.0.0.1. As soon as I made that change everything starting working instantly.