ping inet6 yes source 2001:db8:1:1::2 host 2001:4860:4860::8888

A IPv6 Bogon address that can be used for sinkholing is

2600:5200::1/128

Clientless VPN works fine from outside the network when the target webserver is dual stacked AND has both public AAAA record and A record.

However, if you are inside the network, Clientless VPN will break if the target webserver is dual stacked AND has both an AAAA record and an A record..If you are inside the network, Clientless VPN will only work if there is no AAAA record for the target web server. This is based on what the endpoint can see. It doesn't matter about the DNS Proxy used by the Portal.