[BUG] coredns-custom ConfigMap somehow causes "no such host" error

[mikutas]

What did you do

• How was the cluster created?

  • k3d cluster create -p "54321:80@loadbalancer

• What did you do afterwards?

  • create dnsutils Pod
    • https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/
  • kubectl exec -i -t dnsutils -- nslookup github.com

What did you expect to happen

github.com successfully resolved

Screenshots or terminal output

Server:		10.43.0.10
Address:	10.43.0.10#53

** server can't find github.com: NXDOMAIN

command terminated with exit code 1

After deleting coredns-custom cm:

Server:		10.43.0.10
Address:	10.43.0.10#53

Non-authoritative answer:
Name:	github.com
Address: 20.27.177.113

Which OS & Architecture

• output of k3d runtime-info

arch: aarch64
cgroupdriver: cgroupfs
cgroupversion: "2"
endpoint: /var/run/docker.sock
filesystem: extfs
infoname: docker-desktop
name: docker
os: Docker Desktop
ostype: linux
version: 26.0.0

Which version of k3d

• output of k3d version

k3d version v5.7.0
k3s version v1.29.6-k3s1 (default)

Which version of docker

• output of docker version and docker info

Client:
 Cloud integration: v1.0.35+desktop.13
 Version:           26.0.0
 API version:       1.45
 Go version:        go1.21.8
 Git commit:        2ae903e
 Built:             Wed Mar 20 15:14:46 2024
 OS/Arch:           darwin/arm64
 Context:           desktop-linux

Server: Docker Desktop 4.29.0 (145265)
 Engine:
  Version:          26.0.0
  API version:      1.45 (minimum version 1.24)
  Go version:       go1.21.8
  Git commit:       8b79278
  Built:            Wed Mar 20 15:18:02 2024
  OS/Arch:          linux/arm64
  Experimental:     false
 containerd:
  Version:          1.6.28
  GitCommit:        ae07eda36dd25f8a1b98dfbf587313b99c0190bb
 runc:
  Version:          1.1.12
  GitCommit:        v1.1.12-0-g51d5e94
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

[umutsahin]

Have the same issue

[iwilltry42]

cc @cjc7373
Weird that this didn't occur during my testing.
However, I can confirm that this is a big, so I reverted those changes and released V5.7.1.

Thanks for reporting!

[cjc7373]

It turns out that coredns' file plugin doesn't support fallthrough (coredns/coredns#4773), so that the idea of using the file plugin was totally wrong.

Since the related commits were reverted, I think we should reopen #1221.

As pointed out in #1453 (comment), we can use a different server block. But this way since we need a suffix, we can't deal with names of containers (like k3d-test-serverlb).

[iwilltry42]

Regression resolved - we're following up on the overall issue in the issue #1221