Configuring interface-level network sysctls

In Linux, sysctl allows an administrator to modify kernel parameters at runtime. You can modify interface-level network sysctls using the tuning Container Network Interface (CNI) meta plugin. The tuning CNI meta plugin operates in a chain with a main CNI plugin as illustrated.

The main CNI plugin assigns the interface and passes this to the tuning CNI meta plugin at runtime. You can change some sysctls and several interface attributes (promiscuous mode, all-multicast mode, MTU, and MAC address) in the network namespace by using the tuning CNI meta plugin. In the tuning CNI meta plugin configuration, the interface name is represented by the IFNAME token, and is replaced with the actual name of the interface at runtime.

Configuring the tuning CNI

The following procedure configures the tuning CNI to change the interface-level network net.ipv4.conf.IFNAME.accept_redirects sysctl. This example enables accepting and sending ICMP-redirected packets.

Procedure

1. Create a network attachment definition, such as tuning-example.yaml, with the following content:

   apiVersion: "k8s.cni.cncf.io/v1"
   kind: NetworkAttachmentDefinition
   metadata:
     name: <name> (1)
     namespace: default (2)
   spec:
     config: '{
       "cniVersion": "0.4.0", (3)
       "name": "<name>", (4)
       "plugins": [{
          "type": "<main_CNI_plugin>" (5)
         },
         {
          "type": "tuning", (6)
          "sysctl": {
               "net.ipv4.conf.IFNAME.accept_redirects": "1" (7)
           }
         }
        ]
   }

   1	Specifies the name for the additional network attachment to create. The name must be unique within the specified namespace.
   2	Specifies the namespace that the object is associated with.
   3	Specifies the CNI specification version.
   4	Specifies the name for the configuration. It is recommended to match the configuration name to the name value of the network attachment definition.
   5	Specifies the name of the main CNI plugin to configure.
   6	Specifies the name of the CNI meta plugin.
   7	Specifies the sysctl to set.

   An example yaml file is shown here:

   apiVersion: "k8s.cni.cncf.io/v1"
   kind: NetworkAttachmentDefinition
   metadata:
     name: tuningnad
     namespace: default
   spec:
     config: '{
       "cniVersion": "0.4.0",
       "name": "tuningnad",
       "plugins": [{
         "type": "bridge"
         },
         {
         "type": "tuning",
         "sysctl": {
            "net.ipv4.conf.IFNAME.accept_redirects": "1"
           }
       }
     ]
   }'

2. Apply the yaml by running the following command:

   $ oc apply -f tuning-example.yaml

   Example output

   networkattachmentdefinition.k8.cni.cncf.io/tuningnad created

3. Create a pod such as examplepod.yaml with the network attachment definition similar to the following:

   apiVersion: v1
   kind: Pod
   metadata:
     name: tunepod
     namespace: default
     annotations:
       k8s.v1.cni.cncf.io/networks: tuningnad (1)
   spec:
     containers:
     - name: podexample
       image: centos
       command: ["/bin/bash", "-c", "sleep INF"]
       securityContext:
         runAsUser: 2000 (2)
         runAsGroup: 3000 (3)
         allowPrivilegeEscalation: false (4)
         capabilities: (5)
           drop: ["ALL"]
     securityContext:
       runAsNonRoot: true (6)
       seccompProfile: (7)
         type: RuntimeDefault

   1	Specify the name of the configured NetworkAttachmentDefinition.
   2	runAsUser controls which user ID the container is run with.
   3	runAsGroup controls which primary group ID the containers is run with.
   4	allowPrivilegeEscalation determines if a pod can request to allow privilege escalation. If unspecified, it defaults to true. This boolean directly controls whether the no_new_privs flag gets set on the container process.
   5	capabilities permit privileged actions without giving full root access. This policy ensures all capabilities are dropped from the pod.
   6	runAsNonRoot: true requires that the container will run with a user with any UID other than 0.
   7	RuntimeDefault enables the default seccomp profile for a pod or container workload.

4. Apply the yaml by running the following command:

   $ oc apply -f examplepod.yaml

5. Verify that the pod is created by running the following command:

   $ oc get pod

   Example output

   NAME      READY   STATUS    RESTARTS   AGE
   tunepod   1/1     Running   0          47s

6. Log in to the pod by running the following command:

   $ oc rsh tunepod

7. Verify the values of the configured sysctl flags. For example, find the value net.ipv4.conf.net1.accept_redirects by running the following command:

   sh-4.4# sysctl net.ipv4.conf.net1.accept_redirects

   Expected output

   net.ipv4.conf.net1.accept_redirects = 1

Additional resources

• Using sysctls in containers