[Bug 1818527] [NEW] Stub resolver cache is corrupted

[Abam]

Public bug reported:

It seems that when systemd-resolve cache an non-existent CNAME record
for a domain, any attempt to resolve A record for the same domain fail.

systemd version the issue has been seen with
Installed: 237-3ubuntu10.13
Used distribution

Distributor ID: Ubuntu
Description: Ubuntu 18.04.2 LTS
Release: 18.04
Codename: bionic

Expected behaviour you didn't see

Return A record for a domain when it exists.

Unexpected behaviour you saw

Resolution failed.

Steps to reproduce the problem

Whait for 1 minutes (github.com TTL for A record)

Try to resolv github.com CNAME record dig CNAME github.com

This will return an empty result.

Then try to resolve github.com A record dig A github.com.

This will now return empty result unless you restart systemd-resolved or
wait for cache expiration.

At the same time using another DNS will resolve correctly dig A
github.com @8.8.8.8.

Exemple :

Wait for 1 minutes to let cache expire, then run

dig CNAME github.com
dig A github.com
# no result
dig A github.com @8.8.8.8
# ;; ANSWER SECTION:
# github.com.		59	IN	A	192.30.253.113
# github.com.		59	IN	A	192.30.253.112

PS: Don't forget to restart systemd-resolve, before trying to post an
answer.

This bug was first reported in github
https://github.com/systemd/systemd/issues/11789 but systemd version in
ubuntu is too  old.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1818527

Title:
  Stub resolver cache is corrupted

Status in systemd package in Ubuntu:
  New

Bug description:
  It seems that when systemd-resolve cache an non-existent CNAME record
  for a domain, any attempt to resolve A record for the same domain
  fail.

  systemd version the issue has been seen with
  Installed: 237-3ubuntu10.13
  Used distribution

  Distributor ID: Ubuntu
  Description: Ubuntu 18.04.2 LTS
  Release: 18.04
  Codename: bionic

  Expected behaviour you didn't see

  Return A record for a domain when it exists.

  Unexpected behaviour you saw

  Resolution failed.

  Steps to reproduce the problem

  Whait for 1 minutes (github.com TTL for A record)

  Try to resolv github.com CNAME record dig CNAME github.com

  This will return an empty result.

  Then try to resolve github.com A record dig A github.com.

  This will now return empty result unless you restart systemd-resolved
  or wait for cache expiration.

  At the same time using another DNS will resolve correctly dig A
  github.com @8.8.8.8.

  Exemple :

  Wait for 1 minutes to let cache expire, then run

  dig CNAME github.com
  dig A github.com
  # no result
  dig A github.com @8.8.8.8
  # ;; ANSWER SECTION:
  # github.com.		59	IN	A	192.30.253.113
  # github.com.		59	IN	A	192.30.253.112

  PS: Don't forget to restart systemd-resolve, before trying to post an
  answer.

  This bug was first reported in github
  https://github.com/systemd/systemd/issues/11789 but systemd version in
  ubuntu is too  old.

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