website/content/en/docs/tasks/administer-cluster/dns-debugging-resolution.md

---
reviewers:
- bowei
- zihongz
title:  Debugging DNS Resolution
content_template: templates/task
---

{{% capture overview %}}
This page provides hints on diagnosing DNS problems.
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{{% capture prerequisites %}}
* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
* Kubernetes version 1.6 and above.
* The cluster must be configured to use the `coredns` (or `kube-dns`) addons.
{{% /capture %}}

{{% capture steps %}}

### Create a simple Pod to use as a test environment

Create a file named busybox.yaml with the following contents:

{{< codenew file="admin/dns/busybox.yaml" >}}

Then create a pod using this file and verify its status:

```shell
kubectl apply -f https://k8s.io/examples/admin/dns/busybox.yaml
pod/busybox created

kubectl get pods busybox
NAME      READY     STATUS    RESTARTS   AGE
busybox   1/1       Running   0          <some-time>
```

Once that pod is running, you can exec `nslookup` in that environment.
If you see something like the following, DNS is working correctly.

```shell
kubectl exec -ti busybox -- nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10

Name:      kubernetes.default
Address 1: 10.0.0.1
```

If the `nslookup` command fails, check the following:

### Check the local DNS configuration first

Take a look inside the resolv.conf file.
(See [Inheriting DNS from the node](/docs/tasks/administer-cluster/dns-custom-nameservers/#inheriting-dns-from-the-node) and
[Known issues](#known-issues) below for more information)

```shell
kubectl exec busybox cat /etc/resolv.conf
```

Verify that the search path and name server are set up like the following
(note that search path may vary for different cloud providers):

```
search default.svc.cluster.local svc.cluster.local cluster.local google.internal c.gce_project_id.internal
nameserver 10.0.0.10
options ndots:5
```

Errors such as the following indicate a problem with the coredns/kube-dns add-on or
associated Services:

```
kubectl exec -ti busybox -- nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10

nslookup: can't resolve 'kubernetes.default'
```

or

```
kubectl exec -ti busybox -- nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

nslookup: can't resolve 'kubernetes.default'
```

### Check if the DNS pod is running

Use the `kubectl get pods` command to verify that the DNS pod is running.

For CoreDNS:
```shell
kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
NAME                       READY     STATUS    RESTARTS   AGE
...
coredns-7b96bf9f76-5hsxb   1/1       Running   0           1h
coredns-7b96bf9f76-mvmmt   1/1       Running   0           1h
...
```

Or for kube-dns:
```shell
kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
NAME                    READY     STATUS    RESTARTS   AGE
...
kube-dns-v19-ezo1y      3/3       Running   0           1h
...
```

If you see that no pod is running or that the pod has failed/completed, the DNS
add-on may not be deployed by default in your current environment and you will
have to deploy it manually.

### Check for Errors in the DNS pod

Use `kubectl logs` command to see logs for the DNS containers.

For CoreDNS:
```shell
for p in $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name); do kubectl logs --namespace=kube-system $p; done
```

Here is an example of a healthy CoreDNS log:

```
.:53
2018/08/15 14:37:17 [INFO] CoreDNS-1.2.2
2018/08/15 14:37:17 [INFO] linux/amd64, go1.10.3, 2e322f6
CoreDNS-1.2.2
linux/amd64, go1.10.3, 2e322f6
2018/08/15 14:37:17 [INFO] plugin/reload: Running configuration MD5 = 24e6c59e83ce706f07bcc82c31b1ea1c
```


For kube-dns, there are 3 sets of logs:
```shell
kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name | head -1) -c kubedns

kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name | head -1) -c dnsmasq

kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name | head -1) -c sidecar
```

See if there are any suspicious error messages in the logs. In kube-dns, a '`W`', '`E`' or '`F`' at the beginning
of a line represents a Warning, Error or Failure. Please search for entries that have these
as the logging level and use
[kubernetes issues](https://github.com/kubernetes/kubernetes/issues)
to report unexpected errors.

### Is DNS service up?

Verify that the DNS service is up by using the `kubectl get service` command.

```shell
kubectl get svc --namespace=kube-system
NAME         TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)             AGE
...
kube-dns     ClusterIP   10.0.0.10      <none>        53/UDP,53/TCP        1h
...
```


Note that the service name will be "kube-dns" for both CoreDNS and kube-dns deployments.
If you have created the service or in the case it should be created by default
but it does not appear, see 
[debugging services](/docs/tasks/debug-application-cluster/debug-service/) for
more information.

### Are DNS endpoints exposed?

You can verify that DNS endpoints are exposed by using the `kubectl get endpoints`
command.

```shell
kubectl get ep kube-dns --namespace=kube-system
NAME       ENDPOINTS                       AGE
kube-dns   10.180.3.17:53,10.180.3.17:53    1h
```

If you do not see the endpoints, see endpoints section in the
[debugging services](/docs/tasks/debug-application-cluster/debug-service/) documentation.

For additional Kubernetes DNS examples, see the
[cluster-dns examples](https://github.com/kubernetes/examples/tree/master/staging/cluster-dns)
in the Kubernetes GitHub repository.


### Are DNS queries being received/processed?

You can verify if queries are being received by CoreDNS by adding the `log` plugin to the CoreDNS configuration (aka Corefile).
The CoreDNS Corefile is held in a ConfigMap named `coredns`. To edit it, use the command ...

```
kubectl -n kube-system edit configmap coredns
```

Then add `log` in the Corefile section per the example below.

```
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        log
        errors
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          upstream
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        proxy . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }

```

After saving the changes, it may take up to minute or two for Kubernetes to propagate these changes to the CoreDNS pods.

Next, make some queries and view the logs per the sections above in this document. If CoreDNS pods are receiving the queries, you should see them in the logs.

Here is an example of a query in the log.

```
.:53
2018/08/15 14:37:15 [INFO] CoreDNS-1.2.0
2018/08/15 14:37:15 [INFO] linux/amd64, go1.10.3, 2e322f6
CoreDNS-1.2.0
linux/amd64, go1.10.3, 2e322f6
2018/09/07 15:29:04 [INFO] plugin/reload: Running configuration MD5 = 162475cdf272d8aa601e6fe67a6ad42f
2018/09/07 15:29:04 [INFO] Reloading complete
172.17.0.18:41675 - [07/Sep/2018:15:29:11 +0000] 59925 "A IN kubernetes.default.svc.cluster.local. udp 54 false 512" NOERROR qr,aa,rd,ra 106 0.000066649s

```

## Known issues

Some Linux distributions (e.g. Ubuntu), use a local DNS resolver by default (systemd-resolved).
Systemd-resolved moves and replaces `/etc/resolv.conf` with a stub file that can cause a fatal forwarding
loop when resolving names in upstream servers. This can be fixed manually by using kubelet's `--resolv-conf` flag
to point to the correct `resolv.conf` (With `systemd-resolved`, this is `/run/systemd/resolve/resolv.conf`).
kubeadm (>= 1.11) automatically detects `systemd-resolved`, and adjusts the kubelet flags accordingly.

Kubernetes installs do not configure the nodes' `resolv.conf` files to use the
cluster DNS by default, because that process is inherently distribution-specific.
This should probably be implemented eventually.

Linux's libc is impossibly stuck ([see this bug from
2005](https://bugzilla.redhat.com/show_bug.cgi?id=168253)) with limits of just
3 DNS `nameserver` records and 6 DNS `search` records. Kubernetes needs to
consume 1 `nameserver` record and 3 `search` records. This means that if a
local installation already uses 3 `nameserver`s or uses more than 3 `search`es,
some of those settings will be lost. As a partial workaround, the node can run
`dnsmasq` which will provide more `nameserver` entries, but not more `search`
entries. You can also use kubelet's `--resolv-conf` flag.

If you are using Alpine version 3.3 or earlier as your base image, DNS may not
work properly owing to a known issue with Alpine.
Check [here](https://github.com/kubernetes/kubernetes/issues/30215)
for more information.

## Kubernetes Federation (Multiple Zone support)

Release 1.3 introduced Cluster Federation support for multi-site Kubernetes
installations. This required some minor (backward-compatible) changes to the
way the Kubernetes cluster DNS server processes DNS queries, to facilitate
the lookup of federated services (which span multiple Kubernetes clusters).
See the [Cluster Federation Administrators' Guide](/docs/concepts/cluster-administration/federation/)
for more details on Cluster Federation and multi-site support.

## References

- [DNS for Services and Pods](/docs/concepts/services-networking/dns-pod-service/)
- [Docs for the kube-dns DNS cluster addon](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/kube-dns/README.md)

## What's next
- [Autoscaling the DNS Service in a Cluster](/docs/tasks/administer-cluster/dns-horizontal-autoscaling/).

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