4.3 TCP support
MyDNS will process all TCP requests it receives if the configuration option
is true. This is not usually necessary or recommended.
TCP support will make the server run a little slower, and a
denial-of-service attack is easier if TCP is allowed.
Some very large sites may require TCP support, however. If a response set
would exceed the UDP message size limit (512 bytes), MyDNS will set the
TC (truncated) flag on its answer. Some clients will then fall back to TCP,
which can handle such large answers. If TCP support is enabled, those
clients can get their responses. Also, TCP support is required to perform
DNS-based zone transfers.