On Sat, Jan 29, 2022 at 09:25:05PM +0100, Jozsef Kadlecsik wrote: > Hi Florian, > > On Sat, 29 Jan 2022, Florian Westphal wrote: > > > TCP conntrack assumes that a syn-ack retransmit is identical to the > > previous syn-ack. This isn't correct and causes stuck 3whs in some more > > esoteric scenarios. tcpdump to illustrate the problem: > > > > client > server: Flags [S] seq 1365731894, win 29200, [mss 1460,sackOK,TS val 2083035583 ecr 0,wscale 7] > > server > client: Flags [S.] seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215367629 ecr 2082921663] > > > > Note the invalid/outdated synack ack number. > > Conntrack marks this syn-ack as out-of-window/invalid, but it did > > initialize the reply direction parameters based on this packets content. > > > > client > server: Flags [S] seq 1365731894, win 29200, [mss 1460,sackOK,TS val 2083036623 ecr 0,wscale 7] > > > > ... retransmit... > > > > server > client: Flags [S.], seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215368644 ecr 2082921663] > > > > and another bogus synack. This repeats, then client re-uses for a new > > attempt: > > > > client > server: Flags [S], seq 2375731741, win 29200, [mss 1460,sackOK,TS val 2083100223 ecr 0,wscale 7] > > server > client: Flags [S.], seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215430754 ecr 2082921663] > > > > ... but still gets a invalid syn-ack. > > > > This repeats until: > > > > server > client: Flags [S.], seq 145824453, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215437785 ecr 2082921663] > > server > client: Flags [R.], seq 145824454, ack 643160523, win 65535, [mss 8952,wscale 5,TS val 3215443451 ecr 2082921663] > > client > server: Flags [S], seq 2375731741, win 29200, [mss 1460,sackOK,TS val 2083115583 ecr 0,wscale 7] > > server > client: Flags [S.], seq 162602410, ack 2375731742, win 65535, [mss 8952,wscale 5,TS val 3215445754 ecr 2083115583] > > > > This syn-ack has the correct ack number, but conntrack flags it as > > invalid: The internal state was created from the first syn-ack seen > > so the sequence number of the syn-ack is treated as being outside of > > the announced window. > > I can only assume that the client is/are behind like a carrier-grade NAT > and the bogus SYN-ACK sent by the server is replying a connection attempt > from another client. Yes, the best thing to do is to reinit the state. > > > Don't assume that retransmitted syn-ack is identical to previous one. > > Treat it like the first syn-ack and reinit state. > > > > Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx> > > Signed-off-by: Florian Westphal <fw@xxxxxxxxx> > > Acked-by: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx> Applied, thanks
