Monday, 21 September 2020

Debugging Kubernetes DNS

This is totally cribbed from here -> Debugging DNS Resolution


 Create dnsutils.yaml file

cat << EOF > ~/dnsutils.yaml
apiVersion: v1
kind: Pod
metadata:
  name: dnsutils
  namespace: default
spec:
  containers:
  - name: dnsutils
    image: gcr.io/kubernetes-e2e-test-images/dnsutils:1.3
    command:
      - sleep
      - "3600"
    imagePullPolicy: IfNotPresent
  restartPolicy: Always
EOF

Validate YAML


cat ~/dnsutils.yaml

apiVersion: v1
kind: Pod
metadata:
  name: dnsutils
  namespace: default
spec:
  containers:
  - name: dnsutils
    image: gcr.io/kubernetes-e2e-test-images/dnsutils:1.3
    command:
      - sleep
      - "3600"
    imagePullPolicy: IfNotPresent
  restartPolicy: Always


Apply to cluster


kubectl apply -f ~/dnsutils.yaml


pod/dnsutils created


Check running pods


kubectl get pods


NAME                                  READY   STATUS      RESTARTS   AGE

dnsutils                              1/1     Running     0          47s


Use dnsutils to lookup default K8s servicename


kubectl exec -ti dnsutils -- 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


Use dnsutils to inspect /etc/resolv.conf


kubectl exec -ti dnsutils -- cat /etc/resolv.conf


nameserver 10.96.0.10

search default.svc.cluster.local svc.cluster.local cluster.local

options ndots:5


Use dnsutils to get help for the ping command


kubectl exec -ti dnsutils -- ping --help


BusyBox v1.26.2 (2018-05-30 13:53:45 GMT) multi-call binary.


Usage: ping [OPTIONS] HOST


Send ICMP ECHO_REQUEST packets to network hosts


-4,-6 Force IP or IPv6 name resolution

-c CNT Send only CNT pings

-s SIZE Send SIZE data bytes in packets (default:56)

-t TTL Set TTL

-I IFACE/IP Use interface or IP address as source

-W SEC Seconds to wait for the first response (default:10)

(after all -c CNT packets are sent)

-w SEC Seconds until ping exits (default:infinite)

(can exit earlier with -c CNT)

-q Quiet, only display output at start

and when finished

-p Pattern to use for payload


Use dnsutils to ping google.com


kubectl exec -ti dnsutils -- ping www.google.com


PING www.google.com (172.217.2.100): 56 data bytes

64 bytes from 172.217.2.100: seq=0 ttl=114 time=1.216 ms

64 bytes from 172.217.2.100: seq=1 ttl=114 time=1.328 ms

64 bytes from 172.217.2.100: seq=2 ttl=114 time=1.344 ms

64 bytes from 172.217.2.100: seq=3 ttl=114 time=1.278 ms

64 bytes from 172.217.2.100: seq=4 ttl=114 time=1.483 ms

64 bytes from 172.217.2.100: seq=5 ttl=114 time=1.393 ms

64 bytes from 172.217.2.100: seq=6 ttl=114 time=1.227 ms

64 bytes from 172.217.2.100: seq=7 ttl=114 time=1.343 ms

^C

--- www.google.com ping statistics ---

8 packets transmitted, 8 packets received, 0% packet loss

round-trip min/avg/max = 1.216/1.326/1.483 ms


Use dnsutils to send only 5 pings to google.com


kubectl exec -ti dnsutils -- ping -c 5 www.google.com


PING www.google.com (172.217.2.100): 56 data bytes

64 bytes from 172.217.2.100: seq=0 ttl=114 time=1.382 ms

64 bytes from 172.217.2.100: seq=1 ttl=114 time=1.480 ms

64 bytes from 172.217.2.100: seq=2 ttl=114 time=1.414 ms

64 bytes from 172.217.2.100: seq=3 ttl=114 time=1.326 ms

64 bytes from 172.217.2.100: seq=4 ttl=114 time=1.276 ms


--- www.google.com ping statistics ---

5 packets transmitted, 5 packets received, 0% packet loss

round-trip min/avg/max = 1.276/1.375/1.480 ms


Use dnsutils to ping google.com with a 5-second Time To Live (TTL)


kubectl exec -ti dnsutils -- ping -t 5 www.google.com


PING www.google.com (172.217.2.100): 56 data bytes

^C

--- www.google.com ping statistics ---

276 packets transmitted, 0 packets received, 100% packet loss


Use dnsutils to ping google.com with a 5 second wait


kubectl exec -ti dnsutils -- ping -w 5 www.google.com


PING www.google.com (172.217.2.100): 56 data bytes

64 bytes from 172.217.2.100: seq=0 ttl=114 time=1.153 ms

64 bytes from 172.217.2.100: seq=1 ttl=114 time=1.660 ms

64 bytes from 172.217.2.100: seq=2 ttl=114 time=1.312 ms

64 bytes from 172.217.2.100: seq=3 ttl=114 time=1.144 ms

64 bytes from 172.217.2.100: seq=4 ttl=114 time=1.289 ms


--- www.google.com ping statistics ---

5 packets transmitted, 5 packets received, 0% packet loss

round-trip min/avg/max = 1.144/1.311/1.660 ms


Delete pod

delete pod dnsutils

pod "dnsutils" deleted


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