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Add CoreDNS details to DNS Debug docs (#10201)
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* add coredns details

* address nits, add query logging section
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@chrisohaver @k8s-ci-robot
chrisohaver authored and k8s-ci-robot committed Sep 10, 2018
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116 changes: 105 additions & 11 deletions content/en/docs/tasks/administer-cluster/dns-debugging-resolution.md
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ This page provides hints on diagnosing DNS problems.
{{% capture prerequisites %}}
* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
* Kubernetes version 1.6 and above.
* The cluster must be configured to use the `kube-dns` addon.
* The cluster must be configured to use the `coredns` (or `kube-dns`) addons.
{{% /capture %}}

{{% capture steps %}}
Expand Down Expand Up @@ -68,7 +68,7 @@ nameserver 10.0.0.10
options ndots:5
```

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

```
Expand All @@ -93,6 +93,17 @@ nslookup: can't resolve 'kubernetes.default'

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
Expand All @@ -107,8 +118,26 @@ have to deploy it manually.

### Check for Errors in the DNS pod

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

Expand All @@ -117,8 +146,8 @@ kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system
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 is any suspicious log. Letter '`W`', '`E`', '`F`' at the beginning
represent Warning, Error and Failure. Please search for entries that have these
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.
Expand All @@ -135,6 +164,8 @@ 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
Expand All @@ -158,20 +189,83 @@ 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

Kubernetes installs do not configure the nodes' resolv.conf files to use the
cluster DNS by default, because that process is inherently distro-specific.
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
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
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.
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.
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