Lab 9.3 issue with coredns

rzadzins

Hello,

I am stuck on the lab 9.3. to the point that I've recreated the cluster from scratch (following the k8s docs on teardown) and I cannot get DNS to work on the nettool pod. I checked kubectl describe coredns-pod and it shows it's listening on the cluster IP port 53. The same IP is found in the nettool-pod's /etc/resolv.conf and yet I get the following:

ubuntu@master:~$ kubectl create -f sols/s_09/nettool.yaml
pod/nettool created
ubuntu@master:~$ kubectl get pods -A
NAMESPACE NAME READY STATUS RESTARTS AGE
default nettool 1/1 Running 0 5s
kube-system calico-kube-controllers-5c6f6b67db-b67kz 1/1 Running 0 4m58s
kube-system calico-node-hqbcg 1/1 Running 1 3m54s
kube-system calico-node-tjwzd 1/1 Running 0 4m58s
kube-system coredns-f9fd979d6-k9kc4 1/1 Running 0 8m16s
kube-system coredns-f9fd979d6-z5dtb 1/1 Running 0 8m16s
kube-system etcd-ip-172-31-12-197 1/1 Running 0 8m33s
kube-system kube-apiserver-ip-172-31-12-197 1/1 Running 0 8m33s
kube-system kube-controller-manager-ip-172-31-12-197 1/1 Running 0 8m33s
kube-system kube-proxy-dnqq4 1/1 Running 0 8m17s
kube-system kube-proxy-hc8lh 1/1 Running 1 3m54s
kube-system kube-scheduler-ip-172-31-12-197 1/1 Running 0 8m32s
ubuntu@master:~$ kubectl exec -it nettool -- /bin/bash
root@nettool:/# apt-get update
Err:1 http://security.ubuntu.com/ubuntu focal-security InRelease
Temporary failure resolving 'security.ubuntu.com'
Err:2 http://archive.ubuntu.com/ubuntu focal InRelease
Temporary failure resolving 'archive.ubuntu.com'
Err:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Temporary failure resolving 'archive.ubuntu.com'
Err:4 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Temporary failure resolving 'archive.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal/InRelease Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal-updates/InRelease Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal-backports/InRelease Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/focal-security/InRelease Temporary failure resolving 'security.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.
root@nettool:/# cat /etc/resolv.conf
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local eu-central-1.compute.internal
options ndots:5
root@nettool:/#

I would appreciate any leads so I could learn to troubleshoot such issues please.

chrispokorni

Hi @rzadzins,

Similar errors have been reported on Kubernetes clusters running on AWS. It typically has to do with firewalls, at VPC and/or EC2 level.

Are you able to reach ubuntu.com from your node (not from a container)? Can you compare the resolv.conf of your node vs the nettool/ubuntu container?

Can you provide details about the coredns configMap object?
kubectl -n kube-system get cm coredns -o yaml

Can you provide the logs of acoredns Pod?
kubectl -n kube-system logs coredns-...

Regards,
-Chris

rzadzins

Hi @chrispokorni

Yes, ubuntu.com resolves from the node. The /etc/resolv.conf config on the node has been populated by systemd:

nameserver 127.0.0.53
options edns0 trust-ad
search eu-central-1.compute.internal

On the pod however, it points to the cluster IP:
root@nettool:/# cat /etc/resolv.conf
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local eu-central-1.compute.internal
options ndots:5

The coredns configmap is pristine:
ubuntu@master:~$ kubectl -n kube-system get cm coredns -o yaml
apiVersion: v1
data:
Corefile: |
.:53 {
errors
health {
lameduck 5s
}
ready
kubernetes cluster.local in-addr.arpa ip6.arpa {
pods insecure
fallthrough in-addr.arpa ip6.arpa
ttl 30
}
prometheus :9153
forward . /etc/resolv.conf {
max_concurrent 1000
}
cache 30
loop
reload
loadbalance
}
kind: ConfigMap
metadata:
creationTimestamp: "2020-11-17T21:50:49Z"
managedFields:
- apiVersion: v1
fieldsType: FieldsV1
fieldsV1:
f:data:
.: {}
f:Corefile: {}
manager: kubeadm
operation: Update
time: "2020-11-17T21:50:49Z"
name: coredns
namespace: kube-system
resourceVersion: "193"
selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
uid: e78e3cc4-c931-4bc7-ac1a-18af785909fb

The logs of the coredns pods show timeouts:
ubuntu@master:~$ kubectl -n kube-system logs coredns-f9fd979d6-g5bsd
.:53
[INFO] plugin/reload: Running configuration MD5 = db32ca3650231d74073ff4cf814959a7
CoreDNS-1.7.0
linux/amd64, go1.14.4, f59c03d
[ERROR] plugin/errors: 2 3509682171098248668.7412719447730720321. HINFO: read udp 172.31.4.1:54497->172.31.0.2:53: i/o timeout
[ERROR] plugin/errors: 2 3509682171098248668.7412719447730720321. HINFO: read udp 172.31.4.1:59711->172.31.0.2:53: i/o timeout
[ERROR] plugin/errors: 2 3509682171098248668.7412719447730720321. HINFO: read udp 172.31.4.1:55194->172.31.0.2:53: i/o timeout
[ERROR] plugin/errors: 2 3509682171098248668.7412719447730720321. HINFO: read udp 172.31.4.1:41570->172.31.0.2:53: i/o timeout
[ERROR] plugin/errors: 2 3509682171098248668.7412719447730720321. HINFO: read udp 172.31.4.1:51632->172.31.0.2:53: i/o timeout
[ERROR] plugin/errors: 2 archive.ubuntu.com.eu-central-1.compute.internal. A: read udp 172.31.4.1:35942->172.31.0.2:53: i/o timeout
(that last one is probably me trying to run apt-get update on the pod)

I've run iptables cleanups before creating the cluster and made sure ufw is disabled:
root@master:~# ufw status
Status: inactive

Next I checked the VPC, its subnets and ACLs but they show all traffic is allowed to all destinations. The subnet in this VPC is 172.31.0.0/20, so the above communcation that times out belongs to it.

chrispokorni

Are the firewall rules built on top of existing default rules in a default VPC? Or did you build a custom/new VPC with an all-open firewall rule?

I have experienced such conflicts in the past when I used default VPCs with default rules as a foundation to my cluster specific rules. My rules for this class allow all traffic - all protocols, from all sources, to all ports - no restrictions of any kind.

Regards,
-Chris

rzadzins

@chrispokorni I've set up a new AWS account for the sake of the labs, in it a new VPC with all traffic allowed, and then the 2 new EC2 instances to use this VPC and subnets.

I checked one other thing - the DNS settings in the yaml file of the pod result in having the google DNS in /etc/resolv.conf in the pod which then allows domain resolution. I'm not sure where else traffic could be filtered out...

chrispokorni

One would expect container DNS to be configured on EC2 instances similarly to other environments. It must be something specific to how AWS handles network and DNS configuration.

Regards,
-Chris

rzadzins

I solved the problem - it was my mistake.

After some more research, I noticed I am getting timeouts also for other pod activity, not only DNS. After a lot of trial and error (and cluster tear downs) I noticed I had misread the instructions - there must be zero overlap in the network configuration between the local interfaces and the cluster (I somehow got confused by the lab asking to check the IP on the interface of the instance). Now DNS is smooth. Thanks for the help and patience!

PS. one thing that bothered me - why the CoreDNS service is called kube-dns? From what I understood kube-dns got replaced by CoreDNS.

chrispokorni

Hi @rzadzins,

It is common to have one application (foo) exposed via a service of a different name (bar). By exposing the newly introduced CoreDNS application via the kube-dns service helps with backward compatibility.

Regards,
-Chris

rzadzins

Good point, thank you Chris!

akarthik

rzadzins, I seem to having the same issue for the name resolution. I see that the worker node pod is not able to reach to the control node.
23:45:00.823 INFO [SelfRegisteringRemote$1.run] - Couldn't register this node: The hub is down or not responding: selenium-hub: Temporary failure in name resolution
23:45:05.825 INFO [SelfRegisteringRemote$1.run] - Couldn't register this node: The hub is down or not responding: selenium-hub

Trying to register to a selenium hub. On removing the service name and changing that to the IP, getting a connection time out

23:48:41.336 INFO [GridLauncherV3.lambda$buildLaunchers$7] - Selenium Grid node is up and ready to register to the hub
23:48:41.398 INFO [SelfRegisteringRemote$1.run] - Starting auto registration thread. Will try to register every 5000 ms.
23:50:41.628 WARN [SelfRegisteringRemote.registerToHub] - Error getting the parameters from the hub. The node may end up with wrong timeouts.connect timed out
23:50:41.629 INFO [SelfRegisteringRemote.registerToHub] - Registering the node to the hub: http://10.0.0.6:4444/grid/register
23:52:41.730 INFO [SelfRegisteringRemote$1.run] - Couldn't register this node: Error sending the registration request: connect timed out
23:54:46.748 INFO [SelfRegisteringRemote$1.run] - Couldn't register this node: The hub is down or not responding: connect timed out

Would really appreciate any pointers on this.

chrispokorni

Hi @akarthik,

Is your Kubernetes cluster bootstrapped with the kubeadm tool, following the lab guide?

Regards,
-Chris

rebxpat

I was able to work around this issue by following these steps.
ssh into the ubutnu container as described in the course.
create a copy of the /etc/resolv.conf file
cp /etc/resolv.conf /etc/resolv.conf.orig
replace the resolv.conf by executing
echo "nameserver 8.8.8.8" | tee /etc/resolv.conf > /dev/null
run the apt-get commands as described in the course
replace the resolv.conf with the original
cp /etc/resolv.conf.orig /etc/resolv.conf
continue with the course materials for Lab 9.3.

dandrade.dev

Hi @rzadzins

I'm facing the same issue right now.
What did you change in network installation?

---

" I noticed I had misread the instructions - there must be zero overlap in the network configuration between the local interfaces and the cluster (I somehow got confused by the lab asking to check the IP on the interface of the instance)"

Could you share the mistake?

chrispokorni

Hi @dandrade.dev,

In summary, at bootstrapping time the cluster admin should pay close attention to all networks involved, and insure no overlap between them. With that said, the Kubernetes cluster will use by default 10.96.0.0/12 network for Services, the Calico network plugin will use by default (but can be modified at bootstrapping time) 192.168.0.0/16 for application Pod network, therefore the VM/node network should not overlap with either of the networks above, it should be a distinct network - for example 10.200.0.0/16.

Regards,
-Chris

dandrade.dev

Hi @chrispokorni ! Thanks for your response.

Ok, I got it.

Let me show that I made:

-> Remove all nodes from the cluster (from the cp):
kubectl cordon node-1
kubectl drain node-1 --force --ignore-daemonsets
kubectl delete node node-1
... more clear net config and restarted services
kubeadm reset
rm -rf /etc/cni/net.d && iptables -F
systemctl restart kubelet && systemctl restart containerd

-> Restart cp (as root):
1 - kubeadm reset
2 - rm -rf /etc/cni/net.d && iptables -F
3 - systemctl restart kubelet && systemctl restart containerd

-> Changed the Calico and kubeadm-config manifests with the new CIDR block
calico.yaml - [...] - name: CALICO_IPV4POOL_CIDR value: "10.200.0.0/16 [...]"
kubeadm-config.yaml - [...] - podSubnet: 10.200.0.0/16 [...]"

-> Init the cp (as root):
kubeadm init --config=kubeadm-config.yaml --upload-cert
... and reconfigure kubectl access (as admin user):
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config

-> Calico Applied
kubectl apply -f calico.yaml

-> Rejoin the nodes to cluster:
sudo kubeadm join cp_server:6443 --token XXX --discovery-token-ca-cert-hash YYY
sudo kubeadm init

Until here, it seems ok. The nodes joined to the cluster

When I return to the exercise (LAB 9.3) and launch the Ubuntu POD, I'm still ok. But, when I try perform, as the exercise said, $ apt-get update , I still stuck.

Please, could you help me with this?

---

FYI:

Calico ConfigMap:

_apiVersion: v1
data:
calico_backend: bird
cni_network_config: |-
{
"name": "k8s-pod-network",
"cniVersion": "0.3.1",
"plugins": [
{
"type": "calico",
"log_level": "info",
"log_file_path": "/var/log/calico/cni/cni.log",
"datastore_type": "kubernetes",
"nodename": "KUBERNETES_NODE_NAME",
"mtu": CNI_MTU,
"ipam": {
"type": "calico-ipam"
},
"policy": {
"type": "k8s"
},
"kubernetes": {
"kubeconfig": "KUBECONFIG_FILEPATH"
}
},
{
"type": "portmap",
"snat": true,
"capabilities": {"portMappings": true}
},
{
"type": "bandwidth",
"capabilities": {"bandwidth": true}
}
]
}
typha_service_name: none
veth_mtu: "0"
kind: ConfigMap`

My VPC CIDR: 172.31.0.0/16 (172.31.0.0 - 172.31.255.255)
Default k8S network CIDR: 10.96.0.0/12 (10.96.0.0 - 10.111.255.255)
Calico and Cluster CIDR: 10.200.0.0/16 (10.200.0.0 - 10.200.255.255)

My SecurityGroups on AWS EC2 have outbound for the world allowed (0.0.0.0/0) for all ports/protocols

apt-get update commands work well inside the node, not into the pod

chrispokorni

Hi @dandrade.dev,

On EC2 instances there was no need to go through all that trouble and modify the pod network, since all IP networks were already distinct, with no overlaps.

For the purposes of this course, Kubernetes 1.26.1 or better yet 1.27.1 (instead of 1.26.0) is recommended.

What is the output of kubectl get pods -A -o wide

On the "ubuntu" container, can you check the /etc/resolv.conf file? Can you try adding nameserver 8.8.8.8 as suggested in an earlier comment of this thread?

Regards,
-Chris

dandrade.dev

Hi @chrispokorni

Sure!

The PODS

The /etc/resolv.conf

If I modify the /etc/resolv.conf file to nameserver 8.8.8.8, the apt-get update and install commands work well. But it not seems the right way to solve that. Beyond this, further, in the exercise, we need to perform some curl commands to service-lab.accounting.svc.cluster.local... and this not works as expected with this nameserver on resolv.conf.

Cheers!

chrispokorni

Hi @dandrade.dev,

How are your VPC and SG configured? A possible firewall misconfiguration could impact pod-to-pod and pod-to-service communication across nodes in your cluster.

When setting up your VPC, SG, and EC2 instances, did you happen to watch and follow the video guide from the introductory chapter?

Regards,
-Chris

dandrade.dev

Hey @chrispokorni

Yep!
I followed the steps presented here: https://cm.lf.training/LFS258/LabSetup-AWS.mp4

Btw, the VPC used was created by default from AWS.

And, although not the best approach, I allow all traffic as the instruct video said.

Cheers and sorry for the situation!

chrispokorni

Hi @dandrade.dev,

It seems you are blocking all inbound protocols with the exception of TCP. Please open inbound traffic of All protocols.

Regards,
-Chris

dandrade.dev

Hi @chrispokorni

That's it!!
Thanks a lot!

Just a side note here: It is not needed open all inbound traffic, just the UDP on port 53 and just for the VPC CIDR.

Cheers!!!