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:

[email protected]:~$ kubectl create -f sols/s_09/nettool.yaml
pod/nettool created
[email protected]:~$ 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
[email protected]:~$ kubectl exec -it nettool -- /bin/bash
[email protected]:/# 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.
[email protected]:/# 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
[email protected]:/#

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:
[email protected]:/# 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:
[email protected]:~$ 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:
[email protected]:~$ 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:
[email protected]:~# 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.