Citrix Gateway VPN users are not able to resolve IPv4 DNS when their ISP has IPv6 enabled.  

In order for ADC to resolve IPV6 addresses, DNS parameters would need to be configured as AThenAAAAQuery. This sends the query for an A record, and then sends a query for an AAAA record if the query for the A record results in a NODATA response from the name server. For more information refer to Citrix Developer Docs - dns-parameter .

As a workaround, you also disable IPv6 on the Windows machine for users reporting the issue.

Problem Cause

DNS parameters were set to OnlyAquery. In order for ADC to resolve IPV6 addresses the DNS parameters would need to be configured as AThenAAAAQuery.