Kubernetes DNS lookg not working from worker node - connection timed out; no servers could be reached - kubernetes

Question

I have build new Kubernetes cluster v1.20.1 single master and single node with Calico CNI.
I deployed the busybox pod in default namespace.
# kubectl get pods busybox -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
busybox 1/1 Running 0 12m 10.203.0.129 node02 <none> <none>
nslookup not working
kubectl exec -ti busybox -- nslookup kubernetes.default
Server: 10.96.0.10
Address 1: 10.96.0.10
nslookup: can't resolve 'kubernetes.default'
cluster is running RHEL 8 with latest update
followed this steps: https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/
nslookup command not able to reach nameserver
# kubectl exec -i -t dnsutils -- nslookup kubernetes.default
;; connection timed out; no servers could be reached
command terminated with exit code 1
resolve.conf file
# kubectl exec -ti dnsutils -- cat /etc/resolv.conf
search default.svc.cluster.local svc.cluster.local cluster.local
nameserver 10.96.0.10
options ndots:5
DNS pods running
# kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
NAME READY STATUS RESTARTS AGE
coredns-74ff55c5b-472vx 1/1 Running 1 85m
coredns-74ff55c5b-c75bq 1/1 Running 1 85m
DNS pod logs
kubectl logs --namespace=kube-system -l k8s-app=kube-dns
.:53
[INFO] plugin/reload: Running configuration MD5 = db32ca3650231d74073ff4cf814959a7
CoreDNS-1.7.0
linux/amd64, go1.14.4, f59c03d
.:53
[INFO] plugin/reload: Running configuration MD5 = db32ca3650231d74073ff4cf814959a7
CoreDNS-1.7.0
linux/amd64, go1.14.4, f59c03d
Service is defined
# kubectl get svc --namespace=kube-system
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 86m
**I can see the endpoints of DNS pod**
# kubectl get endpoints kube-dns --namespace=kube-system
NAME ENDPOINTS AGE
kube-dns 10.203.0.5:53,10.203.0.6:53,10.203.0.5:53 + 3 more... 86m
enabled the logging, but didn't see traffic coming to DNS pod
# kubectl logs --namespace=kube-system -l k8s-app=kube-dns
.:53
[INFO] plugin/reload: Running configuration MD5 = db32ca3650231d74073ff4cf814959a7
CoreDNS-1.7.0
linux/amd64, go1.14.4, f59c03d
.:53
[INFO] plugin/reload: Running configuration MD5 = db32ca3650231d74073ff4cf814959a7
CoreDNS-1.7.0
linux/amd64, go1.14.4, f59c03d
I can ping DNS POD
# kubectl exec -i -t dnsutils -- ping 10.203.0.5
PING 10.203.0.5 (10.203.0.5): 56 data bytes
64 bytes from 10.203.0.5: seq=0 ttl=62 time=6.024 ms
64 bytes from 10.203.0.5: seq=1 ttl=62 time=6.052 ms
64 bytes from 10.203.0.5: seq=2 ttl=62 time=6.175 ms
64 bytes from 10.203.0.5: seq=3 ttl=62 time=6.000 ms
^C
--- 10.203.0.5 ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 6.000/6.062/6.175 ms
nmap show port filtered
# ke netshoot-6f677d4fdf-5t5cb -- nmap 10.203.0.5
Starting Nmap 7.80 ( https://nmap.org ) at 2021-01-15 22:29 UTC
Nmap scan report for 10.203.0.5
Host is up (0.0060s latency).
Not shown: 997 closed ports
PORT STATE SERVICE
53/tcp filtered domain
8080/tcp filtered http-proxy
8181/tcp filtered intermapper
Nmap done: 1 IP address (1 host up) scanned in 14.33 seconds
If I schedule the POD on master node, nslookup works nmap show port open?
# ke netshoot -- bash
bash-5.0# nslookup kubernetes.default
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: kubernetes.default.svc.cluster.local
Address: 10.96.0.1
nmap -p 53 10.96.0.10
Starting Nmap 7.80 ( https://nmap.org ) at 2021-01-15 22:46 UTC
Nmap scan report for kube-dns.kube-system.svc.cluster.local (10.96.0.10)
Host is up (0.000098s latency).
PORT STATE SERVICE
53/tcp open domain
Nmap done: 1 IP address (1 host up) scanned in 0.14 seconds
Why nslookup from POD running on worker node is not working? how to troubleshoot this issue?
I re-build the server two times, still same issue.
Thanks
SR
Update adding kubeadm config file
# cat kubeadm-config.yaml
---
apiVersion: kubeadm.k8s.io/v1beta2
bootstrapTokens:
- groups:
- system:bootstrappers:kubeadm:default-node-token
token: abcdef.0123456789abcdef
ttl: 24h0m0s
usages:
- signing
- authentication
kind: InitConfiguration
nodeRegistration:
criSocket: unix:///run/containerd/containerd.sock
taints:
- effect: NoSchedule
key: node-role.kubernetes.io/master
kubeletExtraArgs:
cgroup-driver: "systemd"
---
apiVersion: kubeadm.k8s.io/v1beta2
kind: ClusterConfiguration
kubernetesVersion: stable
controlPlaneEndpoint: "master01:6443"
networking:
dnsDomain: cluster.local
podSubnet: 10.0.0.0/14
serviceSubnet: 10.96.0.0/12
---
apiVersion: kubeproxy.config.k8s.io/v1alpha1
kind: KubeProxyConfiguration
mode: "ipvs
"

Answer 1

First of all, according to the docs - please note that Calico and kubeadm support Centos/RHEL 7+.
In both Calico and kubeadm documentation we can see that they only support RHEL7+.
By default RHEL8 uses nftables instead of iptables ( we can still use iptables but "iptables" on RHEL8 is actually using the kernel's nft framework in the background - look at "Running Iptables on RHEL 8").
9.2.1. nftables replaces iptables as the default network packet filtering framework
I believe that nftables may cause this network issues because as we can find on nftables adoption page:
Kubernetes does not support nftables yet.
Note: For now I highly recommend you to use RHEL7 instead of RHEL8.
With that in mind, I'll present some information that may help you with RHEL8.
I have reproduced your issue and found a solution that works for me.
First I opened ports required by Calico - these ports can be found
here under "Network requirements".
As workaround:
Next I reverted to the old iptables backend on all cluster
nodes, you can easily do so by setting FirewallBackend in
/etc/firewalld/firewalld.conf to iptables as described
here.
Finally I restarted firewalld to make the new rules active.
I've tried nslookup from Pod running on worker node (kworker) and it seems to work correctly.
root#kmaster:~# kubectl get pod,svc -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
pod/web 1/1 Running 0 112s 10.99.32.1 kworker <none> <none>
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
service/kubernetes ClusterIP 10.99.0.1 <none> 443/TCP 5m51s <none>
root#kmaster:~# kubectl exec -it web -- bash
root#web:/# nslookup kubernetes.default
Server: 10.99.0.10
Address: 10.99.0.10#53
Name: kubernetes.default.svc.cluster.local
Address: 10.99.0.1
root#web:/#

Answer 2

In my situation, we're using the K3S cluster. And the new agent couldn't make the default(ClusterFirst) DNS query. After lots of research, I found I need to change the kube-proxy cluster-cidr args to make the DNS work successfully.
Hope this info is useful for others.

Answer 3

I ran into the same issue setting up a vanilla kubeadm 1.25 cluster on RHEL8 and #matt_j's answer lead me to another solution that avoids nftables by using ipvs mode in kube-proxy.
Just modify the kube-proxy ConfigMap in kube-system namespace so the config.conf file has this value;
...
data:
config.conf:
...
mode: "ipvs"
...
And ensure kube-proxy or your nodes are restarted.