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Re: ACK is being sent with a sequence of 0 every time.
From: David Fifield <david () bamsoftware com>
Date: Wed, 13 Jun 2012 12:51:06 -0700

On Wed, Jun 13, 2012 at 12:47:32PM -0700, David Fifield wrote:
This goes along with another long comment in tcp_probe_match:

    /* We are looking to recover the tryno and pingseq of the probe, which are
       encoded in the ACK field for probes with the ACK flag set and in the SEQ
       field for all other probes. According to RFC 793, section 3.9, under
       "SEGMENT ARRIVES", it's supposed to work like this: If our probe had ACK
       set, our ACK number is reflected in the response's SEQ field. If our
       probe had SYN or FIN set (and not ACK), then our SEQ is one less than the
       returned ACK value because SYN and FIN consume a sequence number (section
       3.3). Otherwise, our SEQ is the returned ACK.

       However, nmap-os-db shows that these assumptions can't be relied on, so
       we try all three possibilities for each probe. */

If I were you, I would experiment with the matching condition in

  if (o.magic_port_set) {
    goodseq = seq32_decode(USI, ntohl(tcp->th_ack) - 1, &tryno, &pingseq)
              || seq32_decode(USI, ntohl(tcp->th_ack), &tryno, &pingseq)
              || seq32_decode(USI, ntohl(tcp->th_seq), &tryno, &pingseq);
  } else {
    /* Get the values from the destination port (our source port). */
    sport_decode(USI, o.magic_port, ntohs(tcp->th_dport), &tryno, &pingseq);
    goodseq = true;

Step through that carefully in a debugger, because this is where your
ACK reply is getting matched up with a SYN probe and vice versa. In the
case when o.magic_port_set is false, it appears that we don't do any
enforcement of matching SEQ and ACK values; what happens if you try
enforcing that?

As a related, simple test: See if you can reproduce the spurious closed
port issue when you use the -g option (which sets o.magic_port_set to

David Fifield
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