Hi, This is my first post, hope I'm not using the mailing list wrongly.
As proposed in an old thread (https://www.spinics.net/lists/netdev/msg329144.html), when we send SYN cookies and then exit this mode, tcp_synq_no_recent_overflow() returns false if cookies are received until 2 minutes later (TCP_SYNCOOKIE_VALID). Despite the rest of the SYN cookie code allows ACKs be aged up to 2 minutes, we should not accept cookies for that long if the SYN cookie episode is over. As we dont keep state, an ACK with a previous cookie will be assumed as a valid third packet of a TCP connection. In this scenario, the validity range allows a client not to wait for a SYN-ACK most of the time. A client could intentionally send the required number of packets to fill the queue (eg, with a spoofed IP address). Then, it could open a connection, collect the cookie, and reuse it to speed up the opening of successive connections for 2 minutes. This cheat -specially attractive in low-end devices, where the SYN queue is rather small- may behave similarly to TCP Fast Open, but without the awareness of the server. Decreasing TCP_SYNCOOKIE_VALID would prevent the replay of cookies.
