<p>I am trying to troubleshoot a DNS issue in our K8 cluster v1.19. There are 3 nodes (1 controller, 2 workers) all running vanilla Ubuntu 20.04 using Calico network with <a href="https://metallb.universe.tf/" rel="nofollow noreferrer">Metallb</a> for inbound load balancing. This is all hosted on premise and has full access to the internet. There is also a proxy server (Traefik) in front of it that is handling the SSL to the K8 cluster and other services in the infrastructure.</p>
<p>This issue happened when I upgraded the helm chart for the pod that was/is connecting to the redis pod, but otherwise had been happy to run for the past 36 days.</p>
<p>In the log of one of the pods it is showing an error that it cannot determine where the redis pod(s) is/are:</p>
<pre><code>2020-11-09 00:00:00 [1] [verbose]:      [Cache] Attempting connection to redis.
2020-11-09 00:00:00 [1] [verbose]:      [Cache] Successfully connected to redis.
2020-11-09 00:00:00 [1] [verbose]:      [PubSub] Attempting connection to redis.
2020-11-09 00:00:00 [1] [verbose]:      [PubSub] Successfully connected to redis.
2020-11-09 00:00:00 [1] [warn]:         Secret key is weak. Please consider lengthening it for better security.
2020-11-09 00:00:00 [1] [verbose]:      [Database] Connecting to database...
2020-11-09 00:00:00 [1] [info]:         [Database] Successfully connected .
2020-11-09 00:00:00 [1] [verbose]:      [Database] Ran 0 migration(s).
2020-11-09 00:00:00 [1] [verbose]:      Sending request for public key.
Error: getaddrinfo EAI_AGAIN oct-2020-redis-master
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:67:26) {
  errno: -3001,
  code: 'EAI_AGAIN',
  syscall: 'getaddrinfo',
  hostname: 'oct-2020-redis-master'
}
[ioredis] Unhandled error event: Error: getaddrinfo EAI_AGAIN oct-2020-redis-master
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:67:26)
Error: connect ETIMEDOUT
    at Socket.&lt;anonymous&gt; (/app/node_modules/ioredis/built/redis/index.js:307:37)
    at Object.onceWrapper (events.js:421:28)
    at Socket.emit (events.js:315:20)
    at Socket.EventEmitter.emit (domain.js:486:12)
    at Socket._onTimeout (net.js:483:8)
    at listOnTimeout (internal/timers.js:554:17)
    at processTimers (internal/timers.js:497:7) {
  errorno: 'ETIMEDOUT',
  code: 'ETIMEDOUT',
  syscall: 'connect'
}
</code></pre>
<p>I have gone through the steps outlined in <a href="https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/" rel="nofollow noreferrer">https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/</a></p>
<pre class="lang-sh prettyprint-override"><code>ubuntu@k8-01:~$ kubectl exec -i -t dnsutils -- nslookup kubernetes.default
;; connection timed out; no servers could be reached

command terminated with exit code 1
ubuntu@k8-01:~$ kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
NAME                      READY   STATUS    RESTARTS   AGE
coredns-f9fd979d6-lfm5t   1/1     Running   17         37d
coredns-f9fd979d6-sw2qp   1/1     Running   18         37d
ubuntu@k8-01:~$ kubectl logs --namespace=kube-system -l k8s-app=kube-dns
CoreDNS-1.7.0
linux/amd64, go1.14.4, f59c03d
[INFO] Reloading
[INFO] plugin/health: Going into lameduck mode for 5s
[INFO] plugin/reload: Running configuration MD5 = 3d3f6363f05ccd60e0f885f0eca6c5ff
[INFO] Reloading complete
[INFO] 10.244.210.238:34288 - 28733 &quot;A IN oct-2020-redis-master.default.svc.cluster.local. udp 75 false 512&quot; NOERROR qr,aa,rd 148 0.001300712s
[INFO] 10.244.210.238:44532 - 12032 &quot;A IN oct-2020-redis-master.default.svc.cluster.local. udp 75 false 512&quot; NOERROR qr,aa,rd 148 0.001279312s
[INFO] 10.244.210.235:44595 - 65094 &quot;A IN oct-2020-redis-master.default.svc.cluster.local. udp 75 false 512&quot; NOERROR qr,aa,rd 148 0.000163001s
[INFO] 10.244.210.235:55945 - 20758 &quot;A IN oct-2020-redis-master.default.svc.cluster.local. udp 75 false 512&quot; NOERROR qr,aa,rd 148 0.000141202s
ubuntu@k8-01:~$ kubectl get services --all-namespaces
NAMESPACE     NAME                                               TYPE           CLUSTER-IP       EXTERNAL-IP     PORT(S)                      AGE
default       oct-2020-api                                       ClusterIP      10.107.89.213    &lt;none&gt;          80/TCP                       37d
default       oct-2020-nginx-ingress-controller                  LoadBalancer   10.110.235.175   192.168.2.150   80:30194/TCP,443:31514/TCP   37d
default       oct-2020-nginx-ingress-default-backend             ClusterIP      10.98.147.246    &lt;none&gt;          80/TCP                       37d
default       oct-2020-redis-headless                            ClusterIP      None             &lt;none&gt;          6379/TCP                     37d
default       oct-2020-redis-master                              ClusterIP      10.109.58.236    &lt;none&gt;          6379/TCP                     37d
default       oct-2020-webclient                                 ClusterIP      10.111.204.251   &lt;none&gt;          80/TCP                       37d
default       kubernetes                                         ClusterIP      10.96.0.1        &lt;none&gt;          443/TCP                      37d
kube-system   coredns                                            NodePort       10.101.104.114   &lt;none&gt;          53:31245/UDP                 15h
kube-system   kube-dns                                           ClusterIP      10.96.0.10       &lt;none&gt;          53/UDP,53/TCP,9153/TCP       37d
</code></pre>
<p>When I enter the pod:</p>
<pre><code>/app # grep &quot;nameserver&quot; /etc/resolv.conf
nameserver 10.96.0.10
/app # nslookup
BusyBox v1.31.1 () multi-call binary.

Usage: nslookup [-type=QUERY_TYPE] [-debug] HOST [DNS_SERVER]

Query DNS about HOST

QUERY_TYPE: soa,ns,a,aaaa,cname,mx,txt,ptr,any
/app # ping 10.96.0.10
PING 10.96.0.10 (10.96.0.10): 56 data bytes
^C
--- 10.96.0.10 ping statistics ---
3 packets transmitted, 0 packets received, 100% packet loss
/app # nslookup oct-20-redis-master
;; connection timed out; no servers could be reached
</code></pre>
<p>Any ideas on troubleshooting would be greatly appreciated.</p>

SkywalkerIsNull
<p>To answer my own question, I deleted the DNS pods and then it worked again. The command was the following:</p>
<pre class="lang-sh prettyprint-override"><code>kubectl delete pod coredns-f9fd979d6-sw2qp --namespace=kube-system
</code></pre>
<p>This doesn't get to the underlying problem of why this is happening, or why K8 isn't detecting that something is wrong with those pods and recreating them. I am going to keep digging into this and put some more instrumenting on the DNS pods to see what it actually is that is causing this problem.</p>
<p>If anyone has any ideas on instrumenting to hook up or look at specifically, that would be appreciated.</p>
