Test kubernetes network policies
Kubernetes Network Policy Tester
Network security is often overlooked in Kubernetes. Without network policies kubernetes deployments are basically identical to a traditional infrastructure without firewalls. Network policies in Kubernetes can provide the required network segmentation, see the introduction post.
However, network policies must be verified since it is easy to make a mistake in configuration. This is where the policy tester comes in. The tool is written in python and uses existing pods running in a kubernetes cluster to verify network commmunication. It does this by attaching an ephemeral container to an existing pod so it can verify communications from this pod to other pods and the internet. Ephemeral contains are a beta feature since kubernetes 1.23 which allows to add new containers to existing pods. The main purpose of this is for debugging (testing), so these containers are sometimes also called debug containers.
This approach allows to test network communication within the cluster and also from within the cluster to the internet. The advantage of using ephemeral containers is that it uses existing pods, so the testing environment is identical to the actual environmnet, since no separate test pods are created. Therefore, the tests are representative of the actual situation. The only thing that cannot be tested are rules from traffic coming in from outside the cluster to the cluster.
pip install policytester
# prepare pods by adding debug containers to them policytester prepare tests.yaml # execute tests for pods policytester execute tests.yaml # delete pods that got debug containers attached in any previous prepare steps. policytester cleanup tests.yaml
The separation of prepare, execute, and cleanup allows for tests toe be rerun without having to wait for instrumentation of pods.
The current version of the policy tester uses the default kubectl context to connect to. Future versions of the tool can provide ways to explicitly define a connection to a cluster.
Verify that a httpd pod can reach the nexus pod
Verify that a pod can reach the java maven repository but (most likely) not any other hosts
pods: - name: httpd-wamblee-org namespace: exposure podname: httpd-wamblee-org addresses: - name: internet hosts: - github.com - google.com - name: maven-repo hosts: - repo1.maven.org connections: - name: internet addresses: - internet ports: - port: 80 - port: 443 - name: maven-repo addresses: - maven-repo ports: - port: 80 - port: 443 rules: - name: internet-access from: - httpd-wamblee-org allowed: - maven-repo denied: - internet
A more complete example is here.
Structure of the input file
The example shows the structure of an input file. First, we need to define the pods that we will use. This is done by the following attributes:
- name: a symbolic name of the pod by which it can be referred to in the connections section,
or as pod in the
frompart of a rule.
- namespace: the namespace of the pod
- podname: the string that the actual pod name must start with. For instance for deployments, pod names are composed of the deployment name followed by a unique id.
At this moment no other ways of identifying a pod are possible. Future versions of the tool could support podSelectors in the same syntax as kubernetes does in for instance deployment yaml files. Based on a pod identification, a single pod that matches the specification is chosen. This is to avoid a combinatoric explosion of tests.
Next, addresses must be defined. Each address has the following fields:
- name: a symbolic name of the address by which it can be referred to in the connections section.
- hosts: A list of host names or IP adddresses
Addresses are simply fixed IP addresses or hostnames.
They are an alternative to a pod address which is simply the cluster IP of a pod.
The difference between a pod and address is that a pod can not only be used as the
target (server side) of a network check but also as the client part from which a connection
to a server is established. The names of addresses and pods may not conflict.
In the example above, the
address is used which refers to an address, but this could also have been the
a pod defined in the pods section.
The next section defines the connections that can be tested as a combination of pod/address in the addresses field, and ports:
- name: a symbolic name of the connection by which it can be referenced in the rules section of by other connection objects.
- addresses: A list of names of pods or addresses
- ports: A list of ports that must be tested. Each port is defined by its
portwhich defines the numeric port and by an optional
type(UDP or TCP) which by default is TCP.
Finally, the rules section describes from which source pods, the
connections are allowed or denied. In the example above, we specify that the
httpd-wamblee-org pod can connect to the maven repo at ports 80 and 443, but that
it cannot connect to github.com and google.com:
- name: the name of the rule
- from: pod references. Because of pod groups (see later) this can resolve to more than one pod
- allowed: A list of connections that are allowed.
- denied: A list of connections that are denied.
Since it is annoying to repeat the same rules, it is possible to define
groups of pods in the pods section. These pod groups may be referenced just like
any other pod or address using its
pods: - name: httpd-wamblee-org namespace: exposure podname: httpd-wamblee-org - name: httpd-brakkee-org namespace: exposure podname: httpd-brakkee-org # pod group - name: all-exposure-pods pods: - httpd-wamblee-org # refers to earlier pod name - httpd-brakkee-org # same...
Similar to pod groups address groups can be defined.
addresses: - name: internet hosts: - github.com - google.com - name: dns hosts: - 192.168.178.1 - 188.8.131.52 # address group - name: alladdresses addresses: - internet - dns
The default protocl is TCP, but UDP may also be specified using the
TCP may also be explicitly specified in this way but it is default. Note however, that
UDP tests are unreliable. This is because of the nature of UDP. To work around this, a
future version of this tool could use protocol specific tests for UDP based protocols.
connections - name: dns addresses: - dns ports: - port: 53 type: TCP - port: 53 type: UDP
The test will output a
junit.xml file which is suitable for continuous integration.
The test will also show on screen output.
Under the hood
In the prepare step, the policy tester examines all rules and identifies the pod
specifications that are referenced in the
from parts of the rules. From each
pod specification, the pods that match the specification are looked up. If one of
the pods that is found already contains a debug contqiner than that pod is used.
Otherwise, the debug container is added to one of these pods, so-called instrumenting
the pod for test. The policy tester only uses
this single pod for testing even when more than one eligble pod was found.
This is to avoid a combinatoric explosion in testing, and
also assumes that the network policies are such that it equally effects all pods in
for the same pod specification.
Each pod is given a specific label to identify it as a pod to which a debug container is added. The label is set before the debug container is added to make sure we can never have pods with debug containers but without a label. This label is used in the cleanup phase to delete pods.
In the execute phase all rules are processed. This means that all source pods
mentioned in the
from part of the rule are looked up and for each target connection
denied sections a command is run in the ephemeral container
that verifies network access. This can be a
In the cleanup phase, the policy tester simply deletes the pods with debug containers using the label that was added.
- incoming network connections cannot be tested by this tool
- UDP tests are, for obvious reasons, not really possible in a generic way. Even though policytester supports it, these tests are unreliabnle
- the cleanup phase simply deletes instrumented pods. Your setup must be able to handle this. Use this tool on a staging production-like environmnet or use with greatest care on a production system. In particular, downtime can occur if you are testing network access from deployments and replicasets with replica count 1.
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