Mongos cannot do slaveOk queries when primary is down

https://jira.mongodb.org/browse/SERVER-7246

In v2.4.9 only (this is set by default in v2.6.0 and later), it is necessary to use the following two startup parameters for mongos:

--setParameter ignoreInitialVersionFailure=true
--setParameter authOnPrimaryOnly=false

These parameters can also be set on a MongoS after launch with the following commands

db.adminCommand({setParameter:1,ignoreInitialVersionFailure:true})
db.adminCommand({setParameter:1,authOnPrimaryOnly:false})