NFS must be set up for anonymous access and it must accept NFS version 3.
Your exports file should look something like
/backup 192.168.0.0/255.255.255.0(rw,anonuid=96,anongid=96,secure,root_squash,no_wdelay,async)
The anonuid and anongid should match up with the anonymous nfs user (nfsnobody ???) that nfs runs under.