Using NodeLocal DNSCache to Improve DNS Performance
Challenges
When the number of DNS requests in a cluster increases, the load of CoreDNS increases and the following issues may occur:
- Increased delay: CoreDNS needs to process more requests, which may slow down the DNS query and affect service performance.
- Increased resource usage: To ensure DNS performance, CoreDNS requires higher specifications.
Solution
To minimize the impact of DNS delay, deploy NodeLocal DNSCache in the cluster to improve the networking stability and performance. NodeLocal DNSCache runs a DNS cache proxy on cluster nodes. All pods with DNS configurations use the DNS cache proxy running on nodes instead of the CoreDNS service for domain name resolution. This reduces CoreDNS's load and improves the cluster DNS performance.
After NodeLocal DNSCache is enabled, a DNS query goes through the path as shown below.
![](https://support.huaweicloud.com/intl/en-us/my-kualalumpur-1-usermanual-cce/en-us_image_0000001647417924.png)
Constraints
node-local-dns-injection is a system label used by NodeLocal DNSCache. Use this label only in the scenarios described in Preventing Automatic DNSConfig Injection.
Installing the Add-on
CCE provides add-on node-local-dns for you to install NodeLocal DNSCache.
![](https://support.huaweicloud.com/intl/en-us/my-kualalumpur-1-usermanual-cce/public_sys-resources/note_3.0-en-us.png)
- The node-local-dns add-on supports only clusters of v1.19 and later.
- NodeLocal DNSCache serves as a transparent caching proxy for CoreDNS and does not provide plug-ins such as hosts or rewrite. If you want to enable these plug-ins, modify the CoreDNS configurations.
- Pods cannot be automatically injected into system namespaces such as kube-system.
- (Optional) Modify the CoreDNS configuration so that the CoreDNS preferentially uses UDP to communicate with the upstream DNS server.
The NodeLocal DNSCache uses TCP to communicate with the CoreDNS. The CoreDNS communicates with the upstream DNS server based on the protocol used by the request source. However, the cloud server does not support TCP. To use NodeLocal DNSCache, modify the CoreDNS configuration so that UDP is preferentially used to communicate with the upstream DNS server, preventing resolution exceptions.
Perform the following operations. In the forward add-on, specify prefer_udp as the protocol used by requests. After the modification, CoreDNS preferentially uses UDP to communicate with the upstream system.
- Log in to the CCE console and click the cluster name to access the cluster console.
- In the navigation pane, choose Add-ons. On the displayed page, click Edit under CoreDNS.
- Edit the advanced configuration under Parameters and the following content to the plugins field:
{ "configBlock": "prefer_udp", "name": "forward", "parameters": ". /etc/resolv.conf" }
- Log in to the CCE console and access the cluster console. Choose Add-ons in the navigation pane, locate node-local-dns on the right, and click Install.
- On the Install Add-on page, select the add-on specifications and set related parameters.
- enable_dnsconfig_admission: After this function is enabled, a DNSConfig dynamic injection controller will be created. The controller intercepts pod creation requests in the namespace labeled with node-localdns-injection=enabled based on Admission Webhook, and automatically configures Pod dnsConfig that uses the DNS cache. If this function is disabled or the pod belongs to a non-target namespace, you must manually configure DNSConfig for the pod.
- Click Install.
Using NodeLocal DNSCache
By default, application requests are sent through the CoreDNS proxy. To use node-local-dns as the DNS cache proxy, use any of the following methods:
- Auto injection: Automatically configure the dnsConfig field of the pod when creating the pod. (Pods cannot be automatically injected into system namespaces such as kube-system.)
- Manual configuration: Manually configure the dnsConfig field of the pod.
Auto injection
The following conditions must be met:
- Automatic DNSConfig injection has been enabled during the add-on installation.
- The node-local-dns-injection=enabled label has been added to the namespace. For example, run the following command to add the label to the default namespace:
kubectl label namespace default node-local-dns-injection=enabled
- The new pod does not run in system namespaces such as kube-system and kube-public namespace.
- The node-local-dns-injection=disabled label for disabling DNS injection is not added to the new pod.
- The new pod uses the host network and DNSPolicy is ClusterFirstWithHostNet. Alternatively, the pod does not use the host network and DNSPolicy is ClusterFirst.
After auto injection is enabled, the following dnsConfig settings are automatically added to the created pod. In addition to the NodeLocal DNSCache address 169.254.20.10, the CoreDNS address 10.247.3.10 is added to nameservers, ensuring high availability of the service DNS server.
... dnsConfig: nameservers: - 169.254.20.10 - 10.247.3.10 searches: - default.svc.cluster.local - svc.cluster.local - cluster.local options: - name: timeout value: '' - name: ndots value: '5' - name: single-request-reopen ...
Manual configuration
Manually add the dnsConfig settings to the pod.
Create a pod and add the NodeLocal DNSCache IP address 169.254.20.10 to the DNSConfig nameservers configuration.
apiVersion: v1 kind: Pod metadata: name: nginx spec: containers: - image: nginx:alpine name: container-0 dnsConfig: nameservers: - 169.254.20.10 - 10.247.3.10 searches: - default.svc.cluster.local - svc.cluster.local - cluster.local options: - name: ndots value: '2' imagePullSecrets: - name: default-secret
Preventing Automatic DNSConfig Injection
apiVersion: apps/v1 kind: Deployment metadata: name: test namespace: default spec: replicas: 2 selector: matchLabels: app: test template: metadata: labels: app: test node-local-dns-injection: disabled # Prevent automatic DNSConfig injection. spec: containers: - name: container-1 image: nginx:latest imagePullPolicy: IfNotPresent imagePullSecrets: - name: default-secret
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