I'm setting up a MongoDB replica set to authenticate via LDAP, here: http://docs.mongodb.org/manual/tutorial/configure-ldap-sasl-openldap/

I ran into a very difficult to debug problem we may want to mention (In the "Test the saslauthd configuration" section):

Even if "testsaslauthd -u testuser -p testpassword -f /var/run/saslauthd/mux" is successful, the "/var/run/saslauthd" directory must have permissions set to 755 for MongoDB to successfully authenticate.