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 :) > 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> ...
