php monogo driver 1.3.4
After demoting old replicaset primary to secondary, and promoting old replicaset second into primary, we began seeing "No candidate servers found" MongoException at initial attempt to connect to (new) replicaset primary (via this hint in the /etc/mongo.conf: replSet = rs1/pri.eastghost.com)
Fix seems to be
1. NEVER list "localhost" in the bind= line of /etc/mongo.conf
2. ALWAYS list every replica set member in every member's /etc/hosts file -- there seems to be something wrong with DNS lookup timing.