I created Kubernetes cluster using kubeadm.

It consists of 1 control plane and 3 worker nodes.

Kubernetes version: 1.28.2

I installed Flannel CNI using Helm: flannel-v0.25.5

Now when I attempted to install GitLab agent I noticed errors:

{"level":"error","time":"2024-07-22T13:01:38.020Z","msg":"Failed to register agent pod. Please make sure the agent version matches the server version","mod_name":"agent_registrar","error":"rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing: failed to WebSocket dial: failed to send handshake request: Get \"https://gitlab.net/-/kubernetes-agent/\": dial tcp: lookup gitlab.net: i/o timeout""}

I started debugging the cluster. First I checked the CoreDNS logs:

CoreDNS-1.10.1
linux/amd64, go1.20, 055b2c3
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:45566->103.132.192.238:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:45428->103.132.192.239:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:44693->103.132.192.239:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:47252->103.132.192.239:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:59979->103.132.192.238:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:45005->103.132.192.238:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:48939->103.132.192.239:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:51245->103.132.192.238:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:37896->103.132.192.239:53: i/o timeout
[ERROR] plugin/errors: 2 1599281134285358708.3212447837645310758. HINFO: read udp 10.244.0.3:50890->103.132.192.239:53: i/o timeout

I started a pod with nginx/busybox and checked if I can reach external addresses.

/ # nslookup google.com
;; connection timed out; no servers could be reached

However from host control plane or Flannel pod nslookup works without any issues.