checkServerIdentity What does it look at?

When using ssl I get `failed to connect to server [127.0.0.1:27017] on first connect` unless I remove checkServerIdentity:true from the serveroptions on node.js mongodb 2.2.4

What file is it checking?

require(__dirname+'/../node_modules/mongodb').MongoClient.connect(
'mongodb://user:pass@127.0.0.1:27017/db?ssl=true&authMechanism=SCRAM-SHA-1&authSource=db'
, {server:{
sslValidate:true
//, checkServerIdentity:true
, sslCA:[require('fs').readFileSync('/etc/letsencrypt/identrust-root-download-x3.pem')]
, sslKey:require('fs').readFileSync('/etc/letsencrypt/live/example.com/privkey.pem')
, sslCert:require('fs').readFileSync('/etc/letsencrypt/live/example.com/mongod-sslCert.pem')
}}
, function(e,db)

);