Trivial - P5 Master/Slave replication is deprecated yet in /etc/mongod.conf we have this:
# Replication Options # in replicated mongo databases, specify here whether this is a slave or master #slave=true #source=master.example.com # Slave only: specify a single database to replicate #only=master.example.com # or #master=true #source=slave.example.com
It would be better to replace these with replSet= for example.