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Reformat the node local DNS cache page
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tengqm committed Mar 11, 2022
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---

<!-- overview -->
{{< feature-state for_k8s_version="v1.18" state="stable" >}}
This page provides an overview of NodeLocal DNSCache feature in Kubernetes.

{{< feature-state for_k8s_version="v1.18" state="stable" >}}

This page provides an overview of NodeLocal DNSCache feature in Kubernetes.

## {{% heading "prerequisites" %}}


{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}

{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}

<!-- steps -->

## Introduction

NodeLocal DNSCache improves Cluster DNS performance by running a dns caching agent on cluster nodes as a DaemonSet. In today's architecture, Pods in ClusterFirst DNS mode reach out to a kube-dns serviceIP for DNS queries. This is translated to a kube-dns/CoreDNS endpoint via iptables rules added by kube-proxy. With this new architecture, Pods will reach out to the dns caching agent running on the same node, thereby avoiding iptables DNAT rules and connection tracking. The local caching agent will query kube-dns service for cache misses of cluster hostnames(cluster.local suffix by default).

NodeLocal DNSCache improves Cluster DNS performance by running a DNS caching agent
on cluster nodes as a DaemonSet. In today's architecture, Pods in 'ClusterFirst' DNS mode
reach out to a kube-dns `serviceIP` for DNS queries. This is translated to a
kube-dns/CoreDNS endpoint via iptables rules added by kube-proxy.
With this new architecture, Pods will reach out to the DNS caching agent
running on the same node, thereby avoiding iptables DNAT rules and connection tracking.
The local caching agent will query kube-dns service for cache misses of cluster
hostnames ("`cluster.local`" suffix by default).

## Motivation

* With the current DNS architecture, it is possible that Pods with the highest DNS QPS have to reach out to a different node, if there is no local kube-dns/CoreDNS instance.
Having a local cache will help improve the latency in such scenarios.
* With the current DNS architecture, it is possible that Pods with the highest DNS QPS
have to reach out to a different node, if there is no local kube-dns/CoreDNS instance.
Having a local cache will help improve the latency in such scenarios.

* Skipping iptables DNAT and connection tracking will help reduce [conntrack races](https://github.com/kubernetes/kubernetes/issues/56903) and avoid UDP DNS entries filling up conntrack table.
* Skipping iptables DNAT and connection tracking will help reduce
[conntrack races](https://github.com/kubernetes/kubernetes/issues/56903)
and avoid UDP DNS entries filling up conntrack table.

* Connections from local caching agent to kube-dns service can be upgraded to TCP. TCP conntrack entries will be removed on connection close in contrast with UDP entries that have to timeout ([default](https://www.kernel.org/doc/Documentation/networking/nf_conntrack-sysctl.txt) `nf_conntrack_udp_timeout` is 30 seconds)
* Connections from local caching agent to kube-dns service can be upgraded to TCP.
TCP conntrack entries will be removed on connection close in contrast with
UDP entries that have to timeout
([default](https://www.kernel.org/doc/Documentation/networking/nf_conntrack-sysctl.txt)
`nf_conntrack_udp_timeout` is 30 seconds)

* Upgrading DNS queries from UDP to TCP would reduce tail latency attributed to dropped UDP packets and DNS timeouts usually up to 30s (3 retries + 10s timeout). Since the nodelocal cache listens for UDP DNS queries, applications don't need to be changed.
* Upgrading DNS queries from UDP to TCP would reduce tail latency attributed to
dropped UDP packets and DNS timeouts usually up to 30s (3 retries + 10s timeout).
Since the nodelocal cache listens for UDP DNS queries, applications don't need to be changed.

* Metrics & visibility into dns requests at a node level.
* Metrics & visibility into DNS requests at a node level.

* Negative caching can be re-enabled, thereby reducing number of queries to kube-dns service.

Expand All @@ -49,48 +62,75 @@ This is the path followed by DNS Queries after NodeLocal DNSCache is enabled:
{{< figure src="/images/docs/nodelocaldns.svg" alt="NodeLocal DNSCache flow" title="Nodelocal DNSCache flow" caption="This image shows how NodeLocal DNSCache handles DNS queries." class="diagram-medium" >}}

## Configuration
{{< note >}} The local listen IP address for NodeLocal DNSCache can be any address that can be guaranteed to not collide with any existing IP in your cluster. It's recommended to use an address with a local scope, per example, from the link-local range 169.254.0.0/16 for IPv4 or from the Unique Local Address range in IPv6 fd00::/8.

{{< note >}}
The local listen IP address for NodeLocal DNSCache can be any address that
can be guaranteed to not collide with any existing IP in your cluster.
It's recommended to use an address with a local scope, per example,
from the 'link-local' range '169.254.0.0/16' for IPv4 or from the
'Unique Local Address' range in IPv6 'fd00::/8'.
{{< /note >}}

This feature can be enabled using the following steps:

* Prepare a manifest similar to the sample [`nodelocaldns.yaml`](https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/dns/nodelocaldns/nodelocaldns.yaml) and save it as `nodelocaldns.yaml.`
* If using IPv6, the CoreDNS configuration file need to enclose all the IPv6 addresses into square brackets if used in IP:Port format.
If you are using the sample manifest from the previous point, this will require to modify [the configuration line L70](https://github.com/kubernetes/kubernetes/blob/b2ecd1b3a3192fbbe2b9e348e095326f51dc43dd/cluster/addons/dns/nodelocaldns/nodelocaldns.yaml#L70) like this `health [__PILLAR__LOCAL__DNS__]:8080`
* Substitute the variables in the manifest with the right values:
* Prepare a manifest similar to the sample
[`nodelocaldns.yaml`](https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/dns/nodelocaldns/nodelocaldns.yaml)
and save it as `nodelocaldns.yaml.`

* kubedns=`kubectl get svc kube-dns -n kube-system -o jsonpath={.spec.clusterIP}`
* If using IPv6, the CoreDNS configuration file need to enclose all the IPv6 addresses
into square brackets if used in 'IP:Port' format.
If you are using the sample manifest from the previous point, this will require to modify
[the configuration line L70](https://github.com/kubernetes/kubernetes/blob/b2ecd1b3a3192fbbe2b9e348e095326f51dc43dd/cluster/addons/dns/nodelocaldns/nodelocaldns.yaml#L70)
like this: "`health [__PILLAR__LOCAL__DNS__]:8080`"

* domain=`<cluster-domain>`
* Substitute the variables in the manifest with the right values:

* localdns=`<node-local-address>`
```shell
kubedns=`kubectl get svc kube-dns -n kube-system -o jsonpath={.spec.clusterIP}`
domain=<cluster-domain>
localdns=<node-local-address>
```

`<cluster-domain>` is "cluster.local" by default. `<node-local-address>` is the local listen IP address chosen for NodeLocal DNSCache.
`<cluster-domain>` is "`cluster.local`" by default. `<node-local-address>` is the
local listen IP address chosen for NodeLocal DNSCache.

* If kube-proxy is running in IPTABLES mode:
* If kube-proxy is running in IPTABLES mode:

``` bash
sed -i "s/__PILLAR__LOCAL__DNS__/$localdns/g; s/__PILLAR__DNS__DOMAIN__/$domain/g; s/__PILLAR__DNS__SERVER__/$kubedns/g" nodelocaldns.yaml
```
``` bash
sed -i "s/__PILLAR__LOCAL__DNS__/$localdns/g; s/__PILLAR__DNS__DOMAIN__/$domain/g; s/__PILLAR__DNS__SERVER__/$kubedns/g" nodelocaldns.yaml
```

`__PILLAR__CLUSTER__DNS__` and `__PILLAR__UPSTREAM__SERVERS__` will be populated by the node-local-dns pods.
In this mode, node-local-dns pods listen on both the kube-dns service IP as well as `<node-local-address>`, so pods can lookup DNS records using either IP address.
`__PILLAR__CLUSTER__DNS__` and `__PILLAR__UPSTREAM__SERVERS__` will be populated by
the `node-local-dns` pods.
In this mode, the `node-local-dns` pods listen on both the kube-dns service IP
as well as `<node-local-address>`, so pods can lookup DNS records using either IP address.

* If kube-proxy is running in IPVS mode:

``` bash
sed -i "s/__PILLAR__LOCAL__DNS__/$localdns/g; s/__PILLAR__DNS__DOMAIN__/$domain/g; s/,__PILLAR__DNS__SERVER__//g; s/__PILLAR__CLUSTER__DNS__/$kubedns/g" nodelocaldns.yaml
sed -i "s/__PILLAR__LOCAL__DNS__/$localdns/g; s/__PILLAR__DNS__DOMAIN__/$domain/g; s/,__PILLAR__DNS__SERVER__//g; s/__PILLAR__CLUSTER__DNS__/$kubedns/g" nodelocaldns.yaml
```
In this mode, node-local-dns pods listen only on `<node-local-address>`. The node-local-dns interface cannot bind the kube-dns cluster IP since the interface used for IPVS loadbalancing already uses this address.
`__PILLAR__UPSTREAM__SERVERS__` will be populated by the node-local-dns pods.

In this mode, the `node-local-dns` pods listen only on `<node-local-address>`.
The `node-local-dns` interface cannot bind the kube-dns cluster IP since the
interface used for IPVS loadbalancing already uses this address.
`__PILLAR__UPSTREAM__SERVERS__` will be populated by the node-local-dns pods.

* Run `kubectl create -f nodelocaldns.yaml`
* If using kube-proxy in IPVS mode, `--cluster-dns` flag to kubelet needs to be modified to use `<node-local-address>` that NodeLocal DNSCache is listening on.
Otherwise, there is no need to modify the value of the `--cluster-dns` flag, since NodeLocal DNSCache listens on both the kube-dns service IP as well as `<node-local-address>`.

Once enabled, node-local-dns Pods will run in the kube-system namespace on each of the cluster nodes. This Pod runs [CoreDNS](https://github.com/coredns/coredns) in cache mode, so all CoreDNS metrics exposed by the different plugins will be available on a per-node basis.
* If using kube-proxy in IPVS mode, `--cluster-dns` flag to kubelet needs to be modified
to use `<node-local-address>` that NodeLocal DNSCache is listening on.
Otherwise, there is no need to modify the value of the `--cluster-dns` flag,
since NodeLocal DNSCache listens on both the kube-dns service IP as well as
`<node-local-address>`.

Once enabled, the `node-local-dns` Pods will run in the `kube-system` namespace
on each of the cluster nodes. This Pod runs [CoreDNS](https://github.com/coredns/coredns)
in cache mode, so all CoreDNS metrics exposed by the different plugins will
be available on a per-node basis.

You can disable this feature by removing the DaemonSet, using `kubectl delete -f <manifest>` . You should also revert any changes you made to the kubelet configuration.
You can disable this feature by removing the DaemonSet, using `kubectl delete -f <manifest>`.
You should also revert any changes you made to the kubelet configuration.

## StubDomains and Upstream server Configuration

Expand All @@ -103,7 +143,9 @@ In those cases, the `kube-dns` ConfigMap can be updated.

## Setting memory limits

node-local-dns pods use memory for storing cache entries and processing queries. Since they do not watch Kubernetes objects, the cluster size or the number of Services/Endpoints do not directly affect memory usage. Memory usage is influenced by the DNS query pattern.
The `node-local-dns` Pods use memory for storing cache entries and processing queries.
Since they do not watch Kubernetes objects, the cluster size or the number of Services/Endpoints
do not directly affect memory usage. Memory usage is influenced by the DNS query pattern.
From [CoreDNS docs](https://github.com/coredns/deployment/blob/master/kubernetes/Scaling_CoreDNS.md),
> The default cache size is 10000 entries, which uses about 30 MB when completely filled.

Expand All @@ -114,17 +156,18 @@ The number of concurrent queries is linked to the memory demand, because each ex
goroutine used for handling a query requires an amount of memory. You can set an upper limit
using the `max_concurrent` option in the forward plugin.

If a node-local-dns pod attempts to use more memory than is available (because of total system
If a `node-local-dns` Pod attempts to use more memory than is available (because of total system
resources, or because of a configured
[resource limit](/docs/concepts/configuration/manage-resources-containers/)), the operating system
may shut down that pod's container.
If this happens, the container that is terminated (“OOMKilled”) does not clean up the custom
packet filtering rules that it previously added during startup.
The node-local-dns container should get restarted (since managed as part of a DaemonSet), but this
The `node-local-dns` container should get restarted (since managed as part of a DaemonSet), but this
will lead to a brief DNS downtime each time that the container fails: the packet filtering rules direct
DNS queries to a local Pod that is unhealthy.
You can determine a suitable memory limit by running node-local-dns pods without a limit and
measuring the peak usage. You can also set up and use a
[VerticalPodAutoscaler](https://github.com/kubernetes/autoscaler/tree/master/vertical-pod-autoscaler)
in _recommender mode_, and then check its recommendations.

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