Interesting problem, and this may very well be a kernel issue. I am wondering if your issue is not in the SYN Q, but in the Accept queue overflowing and because of the ordering it is dropping a packet that is causing the SYN Q to stall. A bit of googling with that thought in mind brought me to this article which I think may help you debug your issue a bit further. https://www.alibabacloud.com/blog/599203