The Domain Name System (DNS) is one of the most crucial parts of the Internet. Although the original standard defined the usage of DNS over UDP (DoUDP) as well as DNS over TCP (DoTCP), UDP has become the predominant protocol used in the DNS. With the introduction of new Resource Records (RRs), the sizes of DNS responses have increased considerably. Since this can lead to truncation or IP fragmentation, the fallback to DoTCP as required by the standard ensures successful DNS responses by overcoming the size limitations of DoUDP. However, the effects of the usage of DoTCP by stub resolvers are not extensively studied to this date. We close this gap by presenting a view at DoTCP from the Edge, issuing 12.1M DNS requests from 2,500 probes toward Public as well as Probe DNS recursive resolvers. In our measurement study, we observe that DoTCP is generally slower than DoUDP, where the relative increase in Response Time is less than 37% for most resolvers. While optimizations to DoTCP can be leveraged to further reduce the response times, we show that support on Public resolvers is still missing, hence leaving room for optimizations in the future. Moreover, we also find that Public resolvers generally have comparable reliability for DoTCP and DoUDP. However, Probe resolvers show a significantly different behavior: DoTCP queries targeting Probe resolvers fail in 3 out of 4 cases, and, therefore, do not comply with the standard. This problem will only aggravate in the future: As DNS response sizes will continue to grow, the need for DoTCP will solidify.

域名系统 (DNS) 是 Internet 中最重要的部分之一。尽管最初的标准定义了 DNS over UDP (DoUDP) 和 DNS over TCP (DoTCP) 的使用，但 UDP 已成为 DNS 中使用的主要协议。随着新资源记录 (RR) 的引入，DNS 响应的大小显着增加。由于这可能导致截断或IP 碎片，因此按照标准要求回退到 DoTCP 通过克服 DoUDP 的大小限制来确保成功的 DNS 响应。然而，到目前为止，还没有广泛研究存根解析器使用 DoTCP 的影响。我们通过在 DoTCP 从边缘展示一个视图来缩小这一差距，从 2,500 个探针向公共发出 1210 万个 DNS 请求以及探测DNS 递归解析器。在我们的测量研究中，我们观察到 DoTCP 通常比 DoUDP 慢，其中大多数解析器的响应时间相对增加不到 37%。虽然可以利用对 DoTCP 的优化来进一步减少响应时间，但我们表明仍然缺少对公共解析器的支持，因此为未来的优化留出了空间。此外，我们还发现公共解析器通常具有与 DoTCP 和 DoUDP 相当的可靠性。但是，Probe解析器表现出明显不同的行为：DoTCP 查询以Probe为目标解析器在 4 个案例中有 3 个失败，因此不符合标准。这个问题在未来只会更加严重：随着 DNS 响应大小将继续增长，对 DoTCP 的需求将更加稳固。