Domain Name System (DNS) is a fundamental component for today’s Internet communications, enabling the domain-to-IP translations for billions of users and numerous applications. Yet, the operational failures of DNS are not rare and sometimes lead to severe consequences like Internet outages. To gain a better understanding of DNS operational failures, previous works examined large-scale DNS logs (DNS queries and responses between Internet users and DNS servers), but the DNS logs cannot offer a comprehensive view of the failures (e.g., errors at domain registrars) and explain the failures at a finer grain. In this paper, we try to assess DNS operational failures from another data source, the supporting forums built by DNS service providers. Specifically, we mined 4 DNS forums and crawled more than 10000 posts and 50000 replies. With a new analysis framework developed by us, we are able to tag the forum posts by different categories (e.g., general concerns, issue locations, and record types), and gain new insights regarding how and why users encounter DNS failures. In the end, we offer suggestions to DNS service providers and users to mitigate DNS operational issues.