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You have been subscribed to a public bug by Heitor Alves de Siqueira (halves):

[Impact]
systemd-resolved fails to resolve A records

[Description]
When systemd-resolve caches a non-existent CNAME record for a specific domain, further attempts at resolving A records for that same domain  fail. This has been fixed upstream in v240.

Upstream commit: https://github.com/systemd/systemd/commit/3740146a4cbd

$ git describe --contains 3740146a4cbd
v240~839

$ rmadison systemd --arch amd64
 systemd | 229-4ubuntu4     | xenial          | source, ...
 systemd | 229-4ubuntu21.21 | xenial-security | source, ...
 systemd | 229-4ubuntu21.21 | xenial-updates  | source, ...
 systemd | 237-3ubuntu10    | bionic          | source, ...
 systemd | 237-3ubuntu10.19 | bionic-security | source, ...
 systemd | 237-3ubuntu10.21 | bionic-updates  | source, ...
 systemd | 237-3ubuntu10.22 | bionic-proposed | source, ...
 systemd | 239-7ubuntu10    | cosmic          | source, ...
 systemd | 239-7ubuntu10.12 | cosmic-security | source, ...
 systemd | 239-7ubuntu10.13 | cosmic-updates  | source, ...
 systemd | 239-7ubuntu10.14 | cosmic-proposed | source, ...
 systemd | 240-6ubuntu5     | disco           | source, ...
 systemd | 240-6ubuntu5.1   | disco-proposed  | source, ...
 systemd | 240-6ubuntu9     | eoan            | source, ...

Despite the package versions above, only Bionic is affected. Cosmic
already includes a backported fix, and Xenial doesn't seem affected due
to resolvconf handling DNS resolution.

[Test Case]
Flush resolved's caches and try resolving a non-existent CNAME record. Further resolution attempts for the corresponding A record will fail:

$ systemd-resolve --flush-caches
$ dig github.com CNAME
$ dig github.com A

[Regression Potential]
The regression potential for this fix should be very low, as it's a direct cherry-pick from upstream systemd. It has seen extensive testing  in both upstream and other Ubuntu releases, and was verified for Bionic through autopkgtests.

================================

[Original 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.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: systemd (Ubuntu Xenial)
     Importance: Undecided
         Status: Invalid

** Affects: systemd (Ubuntu Bionic)
     Importance: Medium
     Assignee: Heitor Alves de Siqueira (halves)
         Status: In Progress


** Tags: sts sts-sponsor
-- 
Stub resolver cache is corrupted
https://bugs.launchpad.net/bugs/1818527
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