DNS has always been available over TCP. The DNS packet header has a Truncated (TC) bit that tells you when you should query over TCP, as UDP responses are usual

DNS has always been available over TCP. The DNS packet header has a Truncated (TC) bit that tells you when you should query over TCP, as UDP responses are usually truncated at ~500 bytes. DNS over TCP can often provide better throughput and lower latency, especially on networks with a lot of packet loss (which can be your network if you're, say, batch resolving IP address PTR records from a log where your DNS UDP requests overflow the kernel and NIC TX buffers).