On Wed, 2017-06-07 at 00:12 +0200, Juan José Echevarria wrote: > 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.
No idea why someone would use this unreliable convoluted way, instead of Fast Open ;) > > Decreasing TCP_SYNCOOKIE_VALID would prevent the replay of cookies. It will also prevent connections from innocent users with RTT of say 30 seconds (seen in real world conditions)
