On Thu, Sep 18, 2025 at 6:25 PM Michael Tuexen <michael.tue...@lurchi.franken.de> wrote: > > 2. Sending RST to segment with old sequence SYN-RECEIVED instead of > > acknowledgement > > According to RFC793 page 69: If an incoming segment is not acceptable, an > > acknowledgement should be sent in reply. (here `should` is not capitalized). > > This should be applied to all states including and after SYN-RECEIVED. But > > it's > > not the case with FreeBSD TCP socket. I found this with manually > > constructed TCP > > segment: > > A > B: Flags [S], seq 1, win 8192, length 0 > > B > A: Flags [S.], seq 4054810353, ack 2, win 65535, length 0 > > A > B: Flags [.], ack 1, win 8192, length 0 > > B > A: Flags [R], seq 4054810354, win 0, length 0 > I am not sure which scenario are you considering. Could you provide SEG.SEQ > for the this TCP segment? > > Expected behavior is to send an empty ack: > > A > B: Flags [S], seq 1, win 8192, length 0 > > B > A: Flags [S.], seq 3620804602, ack 2, win 65495, length 0 > > A > B: Flags [.], ack 1, win 8192, length 0 > > B > A: Flags [.], ack 1, win 65495, length 0 > > Which is the case with Linux.
I'd be happy to explain the scenario in more detail. Consider the following TCP handshake sequence: 1. Socket A sends a SYN segment: <CTL=SYN><SEQ=x> to Socket B, which is in the TCP_LISTEN state. 2. Socket B transitions to TCP_SYN_RECV and responds with <CTL=SYN,ACK><SEQ=y><ACK=x+1>. 3. Instead of sending the expected <CTL=ACK><SEQ=x+1><ACK=y+1> to complete the three-way handshake, Socket A incorrectly sends <CTL=ACK><SEQ=x><ACK=y+1>. According to the RFC, the appropriate response to such a malformed ACK should be an empty ACK segment: <CTL=ACK><SEQ=y+1><ACK=x+1>. After that, Socket B should either wait for a valid ACK or retransmit the SYN-ACK if necessary. However, in FreeBSD’s current implementation, a RST segment is sent instead: <CTL=RST><SEQ=y+1>, which aborts the connection prematurely. This behavior appears to deviate from the RFC guidance and may lead to unnecessary connection resets in edge cases. Best regards Tilnel
