On Tue, Mar 19, 2024 at 10:52:44AM +0800, Jason Xing wrote: > Hello Simon, > > On Tue, Mar 19, 2024 at 4:16 AM Simon Horman <horms@xxxxxxxxxx> wrote: > > > > On Mon, Mar 11, 2024 at 03:05:50PM +0800, Jason Xing wrote: > > > From: Jason Xing <kernelxing@xxxxxxxxxxx> > > > > > > Supposing we set DNAT policy converting a_port to b_port on the > > > server at the beginning, the socket is set up by using 4-tuple: > > > > > > client_ip:client_port <--> server_ip:b_port > > > > > > Then, some strange skbs from client or gateway, say, out-of-window > > > skbs are eventually sent to the server_ip:a_port (not b_port) > > > in TCP layer due to netfilter clearing skb->_nfct value in > > > nf_conntrack_in() function. Why? Because the tcp_in_window() > > > considers the incoming skb as an invalid skb by returning > > > NFCT_TCP_INVALID. > > > > > > At last, the TCP layer process the out-of-window > > > skb (client_ip,client_port,server_ip,a_port) and try to look up > > > such an socket in tcp_v4_rcv(), as we can see, it will fail for sure > > > because the port is a_port not our expected b_port and then send > > > back an RST to the client. > > > > > > The detailed call graphs go like this: > > > 1) > > > nf_conntrack_in() > > > -> nf_conntrack_handle_packet() > > > -> nf_conntrack_tcp_packet() > > > -> tcp_in_window() // tests if the skb is out-of-window > > > -> return -NF_ACCEPT; > > > -> skb->_nfct = 0; // if the above line returns a negative value > > > 2) > > > tcp_v4_rcv() > > > -> __inet_lookup_skb() // fails, then jump to no_tcp_socket > > > -> tcp_v4_send_reset() > > > > > > The moment the client receives the RST, it will drop. So the RST > > > skb doesn't hurt the client (maybe hurt some gateway which cancels > > > the session when filtering the RST without validating > > > the sequence because of performance reason). Well, it doesn't > > > matter. However, we can see many strange RST in flight. > > > > > > The key reason why I wrote this patch is that I don't think > > > the behaviour is expected because the RFC 793 defines this > > > case: > > > > > > "If the connection is in a synchronized state (ESTABLISHED, > > > FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), > > > any unacceptable segment (out of window sequence number or > > > unacceptible acknowledgment number) must elicit only an empty > > > > Not for those following along, it appears that RFC 793 does misspell > > unacceptable as above. Perhaps spelling was different in 1981 :) > > Thanks for the check. Yes, it did misspell that word. Should I correct > that word in my quotation? No, I think you should keep the quote the same as the original text. > > > acknowledgment segment containing the current send-sequence number > > > and an acknowledgment..." > > > > > > I think, even we have set DNAT policy, it would be better if the > > > whole process/behaviour adheres to the original TCP behaviour as > > > default. > > > > > > Suggested-by: Florian Westphal <fw@xxxxxxxxx> > > > Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx> > > > > ... >
