TCP mode: mysterious connect which did not happen

[pspacek]

Version: 638e7e7

For mysterious reasons, dnsperf sometimes thinks it is connected to TCP when it really is not - because nothing is listening on the target port.

Reproducer:

while true; do time dnsperf -s 127.0.0.2 -d /tmp/qlist -l 0.0001 -m tcp; done

• /tmp/qlist content: net. SOA
• nothing is listening on 127.0.0.2:53
• repeat until you get non-zero connection statistics, e.g.

DNS Performance Testing Tool
Version 2.10.0

[Status] Command line: dnsperf -s 127.0.0.2 -d /tmp/qlist -l 0.0001 -m tcp
[Status] Sending queries (to 127.0.0.2:53)
[Status] Started at: Mon Jan 16 18:05:21 2023
[Status] Stopping after 0.000100 seconds
[Timeout] Query timed out: msg id 0
[Status] Testing complete (time limit)

Statistics:

  Queries sent:         1
  Queries completed:    0 (0.00%)
  Queries lost:         1 (100.00%)

  Response codes:       
  Average packet size:  request 21, response 0
  Run time (s):         0.001089
  Queries per second:   0.000000

  Average Latency (s):  0.000000 (min 0.000000, max 0.000000)

Connection Statistics:

  Reconnections:        26

  Average Latency (s):  0.000041 (min 0.000025, max 0.000294)
  Latency StdDev (s):   0.000054


real	0m5.008s
user	0m1.765s
sys	0m3.235s

This should be "impossible" because nothing is listening on 127.0.0.2 port 53. To verify that I've captured PCAP with all attempts leading to this output:
dnsperf.pcap.zip

[jelu]

Duplicate #208

[pspacek]

This is not a duplicate, it's still reproducible on 2e648bb (which includes fix for #208).