The edns-tcp-keepalive EDNS0 Option
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DNS messages between clients and servers may be received over either
UDP or TCP. UDP transport involves keeping less state on a busy
server, but can cause truncation and retries over TCP. Additionally,
UDP can be exploited for reflection attacks. Using TCP would reduce
retransmits and amplification. However, clients commonly use TCP only
for retries and servers typically use idle timeouts on the order of
seconds. This document defines an EDNS0 option ("edns-tcp-keepalive")
that allows DNS servers to signal a variable idle timeout. This
signalling encourages the use of long-lived TCP connections by
allowing the state associated with TCP transport to be managed
effectively with minimal impact on the DNS transaction time.