Yes, you can point NxFilter by its IPv6 address. However, we disabled its socket server on IPv6 address at default. It's because if we accept DNS requests on both IPv4 and IPv6, you may need to login twice when you use authentication. To prevent such confusion, we recommend you to set nothing for the DNS server of your IPv6 network. Your users will use the DNS sever set for your IPv4 network.

If you still want to set NxFilter as the DNS server for your IPv6 network, one possible approach is to use an IPv4 over IPv6 address. Suppose that your NxFilter is runninig on 192.168.0.100. Then you can use an IPv6 address like below to point the server,

Code:
::ffff:192.168.0.100
If you have to use a real IPv6 address for NxFilter, set the value of 'java.net.preferIPv4Stack' to 'false' in /nxfilter/bin/startup.sh.

Code:
java -Djava.net.preferIPv4Stack=false -Xmx768m -cp $NX_HOME/nxd.jar:$NX_HOME//lib/*: nxd.Main