reverseDNS error logs

[koertkuipers]

i am seeing a lot of error messages like this:
13/11/04 16:01:04 ERROR hbase.HBaseInputFormatGranular: Cannot resolve the host name for /192.168.1.56 because of javax.naming.NameNotFoundException: DNS name not found [response code 3]; remaining name '56.1.168.192.in-addr.arpa'

Our cluster uses hosts files only. We have no DNS. The job completes successfully. Are these real errors? If not, the logging level should probably be changed.

I tried to understand what is going on, but got confused by this call:
hostName = Strings.domainNamePointerToHostName(DNS.reverseDns(
ipAddress, this.nameServer));

this.nameServer is a private variable initialized to nulll. I cannot find where in the code it gets set.

Best, Koert