Description of problem:
dnsmasq does not forward all of the received queries although it does in previous versions.

When dnsmasq received the same queries in a short period of time, some of the queries were not forwarded to outside nameservers.
In the reproducer below, if we execute the dig command five times then 30 queries are sent to dnsmasq as per /etc/resolv.conf.
According to dnsmasq.log and the result of strace, dnsmasq received 30 queries as expected.
But dnsmasq missed to forward 4 queries as per tcpdump and strace.

Version-Release number of selected component (if applicable):

• dnsmasq-2.79-24.el8_7.1.x86_64 (The latest of RHEL8)
• dnsmasq-2.79-24.el8.x86_64
• dnsmasq-2.79-24.el8.aarch64

How reproducible:
Setup dnsmasq with below configuration and execute multiple dig commands.

Steps to Reproduce:
1. Start dnsmasq with below settings

  # cat /etc/dnsmasq.conf
  ~~~
  domain-needed
  bogus-priv
  interface=lo
  bind-interfaces
  listen-address=127.0.0.1
  cache-size=1000
  resolv-file=/etc/resolv.dnsmasq
  no-poll
  ## Can append below two parameters to log host queries
  log-queries
  log-facility=/var/log/dnsmasq.log
  ~~~

  # cat /etc/resolv.conf
  ~~~
  # Generated by NetworkManager
  nameserver 127.0.0.1
  search ap-northeast-1.compute.internal
  options timeout:2 attempts:5
  ~~~

Note that the nameserver 192.168.122.39 is non-existent DNS server.
Hence dig commands in the next step are expected to be timed out.

  # cat /etc/resolv.dnsmasq
  ~~~
  nameserver 192.168.122.39
  ~~~

  # systemctl start dnsmasq

2. Execute the dig command 5 times continuously in a short period of time.

  ~~~
  [root@rhel7u9-3 ~]# dig google.com &
  [2] 22689
  [root@rhel7u9-3 ~]# dig google.com &
  [3] 22693
  [root@rhel7u9-3 ~]# dig google.com &
  [4] 22697
  [root@rhel7u9-3 ~]# dig google.com &
  [5] 22701
  [root@rhel7u9-3 ~]# dig google.com &
  [6] 22705
  ~~~

Actual results:

1. 1. Note
      We set tcpdump and attach strace to dnsmasq before step2 for investigation purpose.
        # tcpdump -nn port 53
        # strace -ttTvfyyx -s 2048 -o /tmp/dnsmasq.strace -p <PID> &
      It is not required to reproduce the issue.
      ##

dnsmasq received 30 queries but forwarded only 26 queries.

As per tcpdump:
~~~
[root@rhel8u4 ~]# tcpdump -nn port 53
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp1s0, link-type EN10MB (Ethernet), capture size 262144 bytes
02:05:34.053295 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:36.055429 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:36.693097 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:37.251990 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:37.809456 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:38.052601 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:38.355890 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:38.693503 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:39.252428 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:39.809996 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:40.053837 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:40.357148 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:40.693653 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:41.251730 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:41.810211 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:42.054093 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:42.356754 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:42.693896 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:43.251842 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:43.810632 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:44.054270 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:44.357849 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:44.693101 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:45.253189 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:45.810437 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
02:05:46.358110 IP 192.168.122.244.31032 > 192.168.122.39.53: 5020+ [1au] A? google.com. (51)
^C
26 packets captured
26 packets received by filter
0 packets dropped by kernel
~~~

As per strace:
~~~
[root@rhel8u4 ~]# grep recvmsg /tmp/dnsmasq.strace | wc -l
30
[root@rhel8u4 ~]#
[root@rhel8u4 ~]# grep sendto /tmp/dnsmasq.strace | wc -l
26
[root@rhel8u4 ~]#
~~~

Expected results:
dnsmasq should forward all of received queries(30 queries should be forward in above case).

Additional info:
We checked several versions of dnsmasq in RHEL8 and RHEL7 with the exact same configuration and testing procedure and confirmed them below.
Details can be seen in the support case 03454752.

RHEL8
dnsmasq-2.79-13.el8_3.1 : Reproduced
dnsmasq-2.79-13.el8     : Not reproduced (which means dnsmasq forwarded all of 30 queries)
dnsmasq-2.79-4.el8      : Not reproduced (the oldest version of RHEL8)

RHEL7
dnsmasq-2.76-17.el7_9.3 : Not reproduced (the latest of RHEL7)
dnsmasq-2.76-16.el7_9.1 : Reproduced

Best Regards,
Ryo Nakamura.