Chapter 6. DNS Operator in OpenShift Container Platform

1. Use the oc get command to view the deployment status:

   $ oc get -n openshift-dns-operator deployment/dns-operator

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   Example output

   NAME           READY     UP-TO-DATE   AVAILABLE   AGE
   dns-operator   1/1       1            1           23h

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2. Use the oc get command to view the state of the DNS Operator:

   $ oc get clusteroperator/dns

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   Example output

   NAME      VERSION     AVAILABLE   PROGRESSING   DEGRADED   SINCE
   dns       4.1.0-0.11  True        False         False      92m

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   AVAILABLE, PROGRESSING and DEGRADED provide information about the status of the operator. AVAILABLE is True when at least 1 pod from the CoreDNS daemon set reports an Available status condition.

Procedure

1. Use the oc describe command to view the default dns:

   $ oc describe dns.operator/default

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   Example output

   Name:         default
   Namespace:
   Labels:       <none>
   Annotations:  <none>
   API Version:  operator.openshift.io/v1
   Kind:         DNS
   ...
   Status:
     Cluster Domain:  cluster.local 

   1

   
     Cluster IP:      172.30.0.10 

   2

   
   ...

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   1

   The Cluster Domain field is the base DNS domain used to construct fully qualified pod and service domain names.

   2

   The Cluster IP is the address pods query for name resolution. The IP is defined as the 10th address in the service CIDR range.

2. To find the service CIDR of your cluster, use the oc get command:

   $ oc get networks.config/cluster -o jsonpath='{$.status.serviceNetwork}'

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Procedure

1. Modify the DNS Operator object named default:

   $ oc edit dns.operator/default

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   After you issue the previous command, the Operator creates and updates the config map named dns-default with additional server configuration blocks based on Server. If none of the servers have a zone that matches the query, then name resolution falls back to the upstream DNS servers.

   Configuring DNS forwarding

   apiVersion: operator.openshift.io/v1
   kind: DNS
   metadata:
     name: default
   spec:
     servers:
     - name: example-server 

   1

   
       zones: 

   2

   
       - example.com
       forwardPlugin:
         policy: Random 

   3

   
         upstreams: 

   4

   
         - 1.1.1.1
         - 2.2.2.2:5353
     upstreamResolvers: 

   5

   
       policy: Random 

   6

   
       upstreams: 

   7

   
       - type: SystemResolvConf 

   8

   
       - type: Network
         address: 1.2.3.4 

   9

   
         port: 53 

   10

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   1

   Must comply with the rfc6335 service name syntax.

   2

   Must conform to the definition of a subdomain in the rfc1123 service name syntax. The cluster domain, cluster.local, is an invalid subdomain for the zones field.

   3

   Defines the policy to select upstream resolvers. Default value is Random. You can also use the values RoundRobin, and Sequential.

   4

   A maximum of 15 upstreams is allowed per forwardPlugin.

   5

   Optional. You can use it to override the default policy and forward DNS resolution to the specified DNS resolvers (upstream resolvers) for the default domain. If you do not provide any upstream resolvers, the DNS name queries go to the servers in /etc/resolv.conf.

   6

   Determines the order in which upstream servers are selected for querying. You can specify one of these values: Random, RoundRobin, or Sequential. The default value is Sequential.

   7

   Optional. You can use it to provide upstream resolvers.

   8

   You can specify two types of upstreams - SystemResolvConf and Network. SystemResolvConf configures the upstream to use /etc/resolv.conf and Network defines a Networkresolver. You can specify one or both.

   9

   If the specified type is Network, you must provide an IP address. The address field must be a valid IPv4 or IPv6 address.

   10

   If the specified type is Network, you can optionally provide a port. The port field must have a value between 1 and 65535. If you do not specify a port for the upstream, by default port 853 is tried.

2. Optional: When working in a highly regulated environment, you might need the ability to secure DNS traffic when forwarding requests to upstream resolvers so that you can ensure additional DNS traffic and data privacy. Cluster administrators can configure transport layer security (TLS) for forwarded DNS queries.

   Configuring DNS forwarding with TLS

   apiVersion: operator.openshift.io/v1
   kind: DNS
   metadata:
     name: default
   spec:
     servers:
     - name: example-server 

   1

   
       zones: 

   2

   
       - example.com
       forwardPlugin:
         transportConfig:
           transport: TLS 

   3

   
           tls:
             caBundle:
               name: mycacert
             serverName: dnstls.example.com  

   4

   
         policy: Random 

   5

   
         upstreams: 

   6

   
         - 1.1.1.1
         - 2.2.2.2:5353
     upstreamResolvers: 

   7

   
       transportConfig:
         transport: TLS
         tls:
           caBundle:
             name: mycacert
           serverName: dnstls.example.com
       upstreams:
       - type: Network 

   8

   
         address: 1.2.3.4 

   9

   
         port: 53 

   10

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   1

   Must comply with the rfc6335 service name syntax.

   2

   Must conform to the definition of a subdomain in the rfc1123 service name syntax. The cluster domain, cluster.local, is an invalid subdomain for the zones field. The cluster domain, cluster.local, is an invalid subdomain for zones.

   3

   When configuring TLS for forwarded DNS queries, set the transport field to have the value TLS. By default, CoreDNS caches forwarded connections for 10 seconds. CoreDNS will hold a TCP connection open for those 10 seconds if no request is issued. With large clusters, ensure that your DNS server is aware that it might get many new connections to hold open because you can initiate a connection per node. Set up your DNS hierarchy accordingly to avoid performance issues.

   4

   When configuring TLS for forwarded DNS queries, this is a mandatory server name used as part of the server name indication (SNI) to validate the upstream TLS server certificate.

   5

   Defines the policy to select upstream resolvers. Default value is Random. You can also use the values RoundRobin, and Sequential.

   6

   Required. You can use it to provide upstream resolvers. A maximum of 15 upstreams entries are allowed per forwardPlugin entry.

   7

   Optional. You can use it to override the default policy and forward DNS resolution to the specified DNS resolvers (upstream resolvers) for the default domain. If you do not provide any upstream resolvers, the DNS name queries go to the servers in /etc/resolv.conf.

   8

   Network type indicates that this upstream resolver should handle forwarded requests separately from the upstream resolvers listed in /etc/resolv.conf. Only the Network type is allowed when using TLS and you must provide an IP address.

   9

   The address field must be a valid IPv4 or IPv6 address.

   10

   You can optionally provide a port. The port must have a value between 1 and 65535. If you do not specify a port for the upstream, by default port 853 is tried.

   Note

   If servers is undefined or invalid, the config map only contains the default server.

Verification

1. View the config map:

   $ oc get configmap/dns-default -n openshift-dns -o yaml

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   Sample DNS ConfigMap based on previous sample DNS

   apiVersion: v1
   data:
     Corefile: |
       example.com:5353 {
           forward . 1.1.1.1 2.2.2.2:5353
       }
       bar.com:5353 example.com:5353 {
           forward . 3.3.3.3 4.4.4.4:5454 

   1

   
       }
       .:5353 {
           errors
           health
           kubernetes cluster.local in-addr.arpa ip6.arpa {
               pods insecure
               upstream
               fallthrough in-addr.arpa ip6.arpa
           }
           prometheus :9153
           forward . /etc/resolv.conf 1.2.3.4:53 {
               policy Random
           }
           cache 30
           reload
       }
   kind: ConfigMap
   metadata:
     labels:
       dns.operator.openshift.io/owning-dns: default
     name: dns-default
     namespace: openshift-dns

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   1

   Changes to the forwardPlugin triggers a rolling update of the CoreDNS daemon set.

Procedure

1. Edit the DNS Operator object named default by running the following command:

   $ oc edit dns.operator.openshift.io/default

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2. Modify the time-to-live (TTL) caching values:

   Configuring DNS caching

   apiVersion: operator.openshift.io/v1
   kind: DNS
   metadata:
     name: default
   spec:
     cache:
       positiveTTL: 1h 

   1

   
       negativeTTL: 0.5h10m 

   2

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   1

   The string value 1h is converted to its respective number of seconds by CoreDNS. If this field is omitted, the value is assumed to be 0s and the cluster uses the internal default value of 900s as a fallback.

   2

   The string value can be a combination of units such as 0.5h10m and is converted to its respective number of seconds by CoreDNS. If this field is omitted, the value is assumed to be 0s and the cluster uses the internal default value of 30s as a fallback.

   Warning

   Setting TTL fields to low values could lead to an increased load on the cluster, any upstream resolvers, or both.