[Bug 1765477] Re: 5s delay in AAAA dns resolving; artful and earlier is quick

[Benjamin DeCamp]

Any five second delay in name resolution is most likely caused by this
open bug in systemd-resolved:

https://github.com/systemd/systemd/issues/11849

The root cause is a delay when resolved has to downgrade the feature set
of its requests to upstream DNS servers. It correctly discovers a
compatible feature set, but doesn't implement the discovered feature set
until the next new query attempt. The retry logic is supposed to handle
this, but the bug is that it goes crazy and retries with a known not
working feature set until the max retry counter is hit. Then 5 seconds
later, a new attempt is made, and works perfectly, the compatible
feature set is used on the first attempt of the new query.



** Bug watch added: github.com/systemd/systemd/issues #11849
   https://github.com/systemd/systemd/issues/11849

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https://bugs.launchpad.net/bugs/1765477

Title:
  5s delay in AAAA dns resolving; artful and earlier is quick

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  Basic steps:
  lxc launch ubuntu-daily:bionic bionic-daily
  lxc exec bionic-daily bash
  sleep 10s (because of #1765173)
  time host -d bionic-daily.lxd # or whatever domain your containers use by default

  In bionic, there is a 5s delay when requesting the AAAA record:

  
  root at wondrous-grackle:~# time host -d wondrous-grackle.lxd
  Trying "wondrous-grackle.lxd"
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50934
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

  ;; QUESTION SECTION:
  ;wondrous-grackle.lxd.		IN	A

  ;; ANSWER SECTION:
  wondrous-grackle.lxd.	0	IN	A	10.0.100.125

  Received 54 bytes from 127.0.0.53#53 in 0 ms
  Trying "wondrous-grackle.lxd"

  (5s delay here)

  
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17153
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

  ;; QUESTION SECTION:
  ;wondrous-grackle.lxd.		IN	AAAA

  Received 38 bytes from 127.0.0.53#53 in 1 ms
  Trying "wondrous-grackle.lxd"
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12145
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

  ;; QUESTION SECTION:
  ;wondrous-grackle.lxd.		IN	MX

  Received 38 bytes from 127.0.0.53#53 in 0 ms

  real    0m5.023s

  
  In artful and xenial, there is no such delay and the (correct) empty reply is almost instantaneous.

  See https://pastebin.ubuntu.com/p/KP5DBcKyJr/

  
  In both cases, /etc/resolv.conf points at 127.0.0.53

  
  systemd version:
   *** 237-3ubuntu8 500
          500 http://br.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

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