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TCP Fast Open? Not so fast (2021)
As TCP becomes more important to DNS, overhead is becoming an issue.
If a client never communicated before with the target server it will add a TCP option requesting a TFO cookie to the initial SYN packet. Figure 3 —SYN with request and (rejected) TFO cookie causes resend of request.After some head scratching it turns out that I was using the wrong servers to test. This is a known issue since 2017 and is caused by the anycast cluster not sharing the secret used to generate cookies between nodes, but this can be done by setting a sysctl or a specific option on the listening socket.
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