Package: policytester
Kubernetes Network Policy TesterNetwork 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, seethe 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.Installationpip install policytesterUsage# 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.yamlThe 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.ExampleVerify that a pod can reach the java maven repository but (most likely) not any other hostspods:
  - 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:
      - internetA more complete example ishere.Structure of the input fileThe 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 thefrompart of a rule.namespace: the namespace of the podpodname: 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 adddressesAddresses 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, theinternetaddress is used which refers to an address, but this could also have been thenameof
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 addressesports: A list of ports that must be tested. Each port is defined by itsportwhich
defines the numeric port and by an optionaltype(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 thehttpd-wamblee-orgpod 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 rulefrom: pod references. Because of pod groups (see later)
this can resolve to more than one podallowed: A list of connections that are allowed.denied: A list of connections that are denied.Pod groupsSince 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 itsname.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...Address groupsSimilar to pod groups address groups can be defined.addresses:
  - name: internet
    hosts:
      - github.com
      - google.com 
  - name: dns
    hosts:
      - 192.168.178.1
      - 8.8.8.8
  # address group
  - name: alladdresses
    addresses:
      - internet
      - dnsProtocolsThe default protocl is TCP, but UDP may also be specified using thetypefield.
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: UDPTest outputThe test will output ajunit.xmlfile which is suitable for continuous integration.
The test will also show on screen output.Under the hoodPrepareIn the prepare step, the policy tester examines all rules and identifies the pod
specifications that are referenced in thefromparts 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.ExecuteIn the execute phase all rules are processed. This means that all source pods
mentioned in thefrompart of the rule are looked up and for each target connection
in theallowedanddeniedsections a command is run in the ephemeral container
that verifies network access. This can be anetcatornmapcommand.CleanupIn the cleanup phase, the policy tester simply deletes the pods with debug containers
using the label that was added.Caveatsincoming network connections cannot be tested by this toolUDP tests are, for obvious reasons, not really possible in a generic way. Even though
policytester supports it, these tests are unreliabnlethe 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.