Troubleshoot kube-dns in GKE

Autopilot Standard

This page shows you how to resolve issues with kube-dns in Google Kubernetes Engine (GKE).

Identify the source of DNS issues in kube-dns

Errors like dial tcp: i/o timeout, no such host, or Could not resolve host often signal problems with the ability of kube-dns to resolve queries.

If you've seen one of those errors, but don't know the cause, use the following sections to help you find it. The following sections are arranged to start with the steps that are most likely to help you, so try each section in order.

Check if kube-dns Pods are running

Kube-dns Pods are critical for name resolution within the cluster. If they're not running you're likely to experience issues with DNS resolution.

To verify that the kube-dns Pods are running without any recent restarts, view the status of these Pods:

kubectl get pods -l k8s-app=kube-dns -n kube-system

The output is similar to the following:

NAME                   READY          STATUS          RESTARTS       AGE
kube-dns-POD_ID_1      5/5            Running         0              16d
kube-dns-POD_ID_2      0/5            Terminating     0              16d

In this output, POD_ID_1 and POD_ID_2 represent unique identifiers that are automatically appended to the kube-dns Pods.

If your output shows that any of your kube-dns Pods don't have a status of Running, work though the following steps:

Use the Admin Activity audit logs to investigate if there's been any recent changes such as cluster or node pool version upgrades, or changes to the kube-dns ConfigMap. To learn more about audit logs, see GKE audit logging information. If you find changes, revert them and view the status of the Pods again.
If you don't find any relevant recent changes, investigate if you're experiencing an OOM error on the node that the kube-dns Pod runs on. If you see an error similar to the following in your Cloud Logging log messages, then these Pods are experiencing an OOM error:
```
Warning: OOMKilling Memory cgroup out of memory
```
To resolve this error, see Error Message: "Warning: OOMKilling Memory cgroup out of memory".
If you don't find any OOM error messages, restart the kube-dns Deployment:
```
kubectl rollout restart deployment/kube-dns --namespace=kube-system
```
After you've restarted the Deployment, check if your kube-dns Pods are running.

If these steps don't work, or all of your kube-dns Pods have a status of Running, but you're still having DNS issues, check that the /etc/resolv.conf file is configured correctly.

Check that `/etc/resolv.conf` is configured correctly

Review the /etc/resolv.conf file of the Pods experiencing DNS issues and make sure that the entries it contains are correct:

View the /etc/resolv.conf file of the Pod:
```
kubectl exec -it POD_NAME -- cat /etc/resolv.conf
```
Replace POD_NAME with the name of the Pod that's experiencing DNS issues. If there are multiple Pods that are experiencing issues, repeat the steps in this section for each Pod.

If the Pod binary doesn't support the kubectl exec command, this command might fail. If this happens, create a simple Pod to use as a test environment. This procedure lets you run a test Pod in the same namespace as your problematic Pod.
Verify that the name server IP address in /etc/resolv.conf file is correct:
- Pods that are using a host network should use the values in the node's /etc/resolv.conf file. The name server IP address should be 169.254.169.254.
- For Pods that aren't using a host network, the kube-dns Service IP address should be the same as the name server IP address. To compare the IP addresses, complete the following steps:
  1. Get the IP address of the kube-dns Service:
```
kubectl get svc kube-dns -n kube-system
```
    The output is similar to the following:
```
NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)         AGE
kube-dns   ClusterIP   192.0.2.10   <none>        53/UDP,53/TCP   64d
```
  2. Make note of the value in the Cluster IP column. In this example, it's 192.0.2.10.
  3. Compare the kube-dns Service IP address with the IP address from the /etc/resolv.conf file:
```
# cat /etc/resolv.conf

search default.svc.cluster.local svc.cluster.local cluster.local c.PROJECT_NAME google.internal
nameserver 192.0.2.10
options ndots:5
```
    In this example, the two values match, so an incorrect name server IP address isn't the cause of your problem.
    
    However, if the IP addresses don't match, that means that a dnsConfig field is configured in the manifest of the application Pod.
    
    If the value in the dnsConfig.nameservers field is correct, investigate your DNS server and make sure it's functioning correctly.
    
    If you don't want to use the custom name server, remove the field and perform a rolling restart of the Pod:
```
kubectl rollout restart deployment POD_NAME
```
    Replace POD_NAME with the name of your Pod.
Verify the search and ndots entries in /etc/resolv.conf. Make sure there are no spelling errors, stale configurations and the failing request points to existing service in the correct namespace.

Perform a DNS lookup

After you've confirmed that /etc/resolv.conf is configured correctly and the DNS record is correct, use the dig command-line tool to perform DNS lookups from the Pod that's reporting DNS errors:

Directly query a Pod by opening a shell inside of it:
```
kubectl exec -it POD_NAME -n NAMESPACE_NAME -- SHELL_NAME
```
Replace the following:
- POD_NAME: the name of Pod that's reporting DNS errors.
- NAMESPACE_NAME: the namespace that the Pod belongs to.
- SHELL_NAME: The name of the shell that you want to open. For example, sh or /bin/bash.
This command might fail if your Pod doesn't permit the kubectl exec command or if the Pod doesn't have the dig binary. If this happens, create a test Pod with an image that has dig installed:
```
kubectl run "test-$RANDOM" ti --restart=Never --image=thockin/dnsutils - bash
```
Check if the Pod can correctly resolve the internal DNS Service of the cluster:
```
dig kubernetes
```
Because the /etc/resolv.conf file is pointing to the kube-dns Service IP address, when you run this command the DNS server is the kube-dns Service.

You should see a successful DNS response with the IP address of the Kubernetes API Service (often something like 10.96.0.1). If you see SERVFAIL or no response, this usually indicates that the kube-dns Pod is unable to resolve the internal service names.
Check if the kube-dns Service can resolve an external domain name:
```
dig example.com
```

If you're experiencing difficulties with a particular kube-dns Pod responding to DNS queries, check if that Pod can resolve an external domain name:

 dig example.com @KUBE_DNS_POD_IP

Replace KUBE_DNS_POD_IP with the IP address of the kube-dns Pod. If you don't know the value of this IP address, run the following command:

 kubectl get pods -n kube-system -l k8s-app=kube-dns -o wide

The IP address is in the IP column.

If the resolution of the command is successful, then you see status: NOERROR and details of the A record as shown in the following example:

 ; <<>> DiG 9.16.27 <<>> example.com
 ;; global options: +cmd
 ;; Got answer:
 ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31256
 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

 ;; OPT PSEUDOSECTION:
 ; EDNS: version: 0, flags:; udp: 512
 ;; QUESTION SECTION:
 ;example.com.                   IN      A

 ;; ANSWER SECTION:
 example.com.            30      IN      A       93.184.215.14

 ;; Query time: 6 msec
 ;; SERVER: 10.76.0.10#53(10.76.0.10)
 ;; WHEN: Tue Oct 15 16:45:26 UTC 2024
 ;; MSG SIZE  rcvd: 56

Exit the shell:
```
exit
```

If any of these commands fail, perform a rolling restart of the kube-dns Deployment:

kubectl rollout restart deployment/kube-dns --namespace=kube-system

After you've completed the restart, re-try the dig commands and see if they now succeed. If they still fail, proceed to take a packet capture.

Take a packet capture

Take a packet capture to verify if the DNS queries are being received and answered appropriately by the kube-dns Pods:

Using SSH, connect to the node running the kube-dns Pod. For example:
1. In the Google Cloud console, go to the VM Instances page.
  
  Go to VM Instances
2. Locate the node that you want to connect to. If you don't know the name of the node on your kube-dns Pod, run the following command:
```
kubectl get pods -n kube-system -l k8s-app=kube-dns -o wide
```
  The name of the node is listed in the Node column.
3. In the Connect column, click SSH.
In the terminal, start toolbox; a pre-installed debugging tool:
```
toolbox
```

At the root prompt, install the tcpdump package:

apt update -y && apt install -y tcpdump

Using tcpdump, take a packet capture of your DNS traffic:
```
tcpdump -i eth0 port 53" -w FILE_LOCATION
```
Replace FILE_LOCATION with a path to where you want to save the capture.
Review the packet capture. Check if there are packets with destination IP addresses that match the kube-dns Service IP address. This ensures that the DNS requests are reaching the right destination for resolution. Failure to see DNS traffic landing on the correct Pods, might indicate presence of a network policy that's blocking the requests.

Check for a network policy

Restrictive network policies can sometimes disrupt DNS traffic. To verify if a network policy exists in the kube-system namespace, run the following command:

kubectl get networkpolicy -n kube-system

If you find a network policy, review it and ensure the policy allows necessary DNS communication. For example, if you have a network policy which blocks all egress traffic, the policy would also block DNS requests.

If the output is No resources found in kube-system namespace then you don't have any network policies and you can rule this out as the cause of your issue. Investigating logs can help you find more points of failure.

Enable temporary DNS query logging

To help you identify issues such incorrect DNS responses, temporarily enable debug logging of DNS queries.

This is a resource intensive procedure, so we recommend that you disable this logging after you've collected a suitable sample of logs.

Investigate the kube-dns Pod

Review how kube-dns Pods receive and resolve DNS queries with Cloud Logging.

To view log entries related to the kube-dns Pod, complete the following steps:

In the Google Cloud console, go to the Logs Explorer page.

Go to Logs Explorer
In the query pane, enter the following filter to view events related to the kube-dns container:
```
resource.type="k8s_container"
resource.labels.namespace_name="kube-system"
resource.labels.Pod_name:"kube-dns"
resource.labels.cluster_name="CLUSTER_NAME"
resource.labels.location="CLUSTER_LOCATION"
```
Replace the following:
- CLUSTER_NAME: the name of the cluster that the kube-dns Pod belongs to.
- CLUSTER_LOCATION: the location of your cluster.
Click Run query.

Review the output. The following example output shows one possible error that you might see:

{
   "timestamp": "2024-10-10T15:32:16.789Z",
   "severity": "ERROR",
   "resource": {
      "type": "k8s_container",
      "labels": {
      "namespace_name": "kube-system",
      "Pod_name": "kube-dns",
      "cluster_name": "CLUSTER_NAME",
      "location": "CLUSTER_LOCATION"
      }
   },
   "message": "Failed to resolve 'example.com': Timeout."
},

In this example, kube-dns couldn't resolve example.com in a reasonable time. This type of error can be caused by multiple issues. For example, the upstream server might be incorrectly configured in the kube-dns ConfigMap, or there might be high network traffic.

If you don't have Cloud Logging enabled, view the Kubernetes logs instead:

Pod=$(kubectl get Pods -n kube-system -l k8s-app=kube-dns -o name | head -n1)
kubectl logs -n kube-system $Pod -c dnsmasq
kubectl logs -n kube-system $Pod -c kubedns
kubectl logs -n kube-system $Pod -c sidecar

Investigate recent changes in the kube-dns ConfigMap

If you suddenly encounter DNS resolution failures in your cluster, one cause is an incorrect configuration change made to the kube-dns ConfigMap. In particular, configuration changes to the stub domains and upstream servers definitions can cause issues.

To check for updates to the stub domain settings, complete the following steps:

In the Google Cloud console, go to the Logs Explorer page.

Go to Logs Explorer

In the query pane, enter the following query:

resource.labels.cluster_name="clouddns"
resource.type="k8s_container"
resource.labels.namespace_name="kube-system"
labels.k8s-pod/k8s-app="kube-dns" jsonPayload.message=~"Updated stubDomains to"

Click Run query.
Review the output. If there have been any updates, the output is similar to the following:
```
Updated stubDomains to map[example.com: [8.8.8.8 8.8.4.4 1.1.3.3 1.0.8.111]]
```
If you see an update, expand the result to learn more about the changes. Verify that any stub domains and their corresponding upstream DNS servers are correctly defined. Incorrect entries here can lead to resolution failures for those domains.

To check for changes to the upstream server, complete the following steps:

In the Google Cloud console, go to the Logs Explorer page.

Go to Logs Explorer

In the query pane, enter the following query:

resource.labels.cluster_name="clouddns"
resource.type="k8s_container" resource.labels.namespace_name="kube-system"
labels.k8s-pod/k8s-app="kube-dns" jsonPayload.message=~"Updated upstreamNameservers to"

Click Run query.
Review the output. If there have been any changes, the output is similar to the following:
```
Updated upstreamNameservers to [8.8.8.8]
```
Expand the result to learn more about the changes. Verify that the list of upstream DNS servers is accurate and that these servers are reachable from your cluster. If these servers are unavailable or misconfigured, general DNS resolution might fail.

If you've checked for changes to the stub domains and upstream servers, but didn't find any results, check for all changes with the following filter:

resource.type="k8s_cluster"
protoPayload.resourceName:"namespaces/kube-system/configmaps/kube-dns"
protoPayload.methodName=~"io.k8s.core.v1.configmaps."

Review any listed changes to see if they've caused the error.

Contact Cloud Customer Care

If you've worked through the preceding sections, but still can't diagnose the cause of your issue, contact Cloud Customer Care.

Resolve common issues

If you've experienced a specific error or issue, use the advice in the following sections.

Issue: Intermittent DNS timeouts

If you notice intermittent DNS resolution timeouts that occur when there's an increase in DNS traffic or when business hours start, try the following solutions to optimize your DNS performance:

Check the number of kube-dns Pods running on the cluster and compare it to the total number of GKE nodes. If there aren't enough resources, consider scaling up the kube-dns Pods.
To improve average DNS lookup time, enable NodeLocal DNS Cache.
DNS resolution to external names can overload the kube-dns Pod. To reduce the number of queries, adjust the ndots setting in the /etc/resolv.conf file. ndots represents the number of dots that must appear in a domain name to resolve a query before the initial absolute query.

The following example is the /etc/resolv.conf file of an application Pod:
```
search default.svc.cluster.local svc.cluster.local cluster.local c.PROJECT_ID.internal google.internal
nameserver 10.52.16.10
options ndots:5
```
In this example, kube-dns looks for five dots in the domain that's queried. If the Pod makes a DNS resolution call for example.com, then your logs look similar to the following example:
```
"A IN example.com.default.svc.cluster.local." NXDOMAIN
"A IN example.com.svc.cluster.local." NXDOMAIN
"A IN example.com.cluster.local." NXDOMAIN
"A IN example.com.google.internal." NXDOMAIN
"A IN example.com.c.PROJECT_ID.internal." NXDOMAIN
"A IN example.com." NOERROR
```
To resolve this issue, either change the value of ndots to 1 to look only for a single dot or append a dot (.) at the end of the domain that you query or use. For example:
```
dig example.com.
```

Issue: DNS queries fail intermittently from some nodes

If you notice DNS queries failing intermittently from some nodes, you might see the following symptoms:

When you run dig commands to the kube-dns Service IP address or Pod IP address, the DNS queries fail intermittently with timeouts.
Running dig commands from a Pod on the same node as the kube-dns Pod fails.

To resolve this issue, complete the following steps:

Perform a Connectivity Test. Set the problematic Pod or node as the source, and destination as the IP address of the kube-dns Pod. This lets you check if you have the required firewall rules in place to allow this traffic.
If the test is not successful, and traffic is being blocked by a firewall rule, use Cloud Logging to list any manual changes made to the firewall rules. Look for changes that block a specific kind of traffic:
1. In the Google Cloud console, go to the Logs Explorer page.
  
  Go to Logs Explorer
2. In the query pane, enter the following query:
```
logName="projects/project-name/logs/cloudaudit.googleapis.com/activity"
resource.type="gce_firewall_rule"
```
3. Click Run query. Use the output of the query to determine if any changes have been made. If you notice any errors, correct them and reapply the firewall rule.
  
  Make sure you don't make changes to any automated firewall rules.
If there haven't been any changes to the firewall rules, check the node pool version and make sure it's compatible with the control plane and other working node pools. If any of the cluster's node pools are more than two minor versions older than the control plane, this might be causing issues. For more information about this incompatibility, see Node version not compatible with control plane version.
To determine if the requests are being sent to the correct kube-dns service IP, capture network traffic on the problematic node and filter for port 53 (DNS traffic). Capture traffic on the kube-dns Pods themselves to see if the requests are reaching the intended Pods and if they're being successfully resolved.

What's next

For general information about diagnosing Kubernetes DNS issues, see Debugging DNS Resolution.

If you need additional assistance, reach out to Cloud Customer Care.