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Tracing Openflow with ovnkube-trace

OVN and OVS traffic flows can be simulated in a single utility called ovnkube-trace. The ovnkube-trace utility runs ovn-trace, ovs-appctl ofproto/trace and ovn-detrace and correlates that information in a single output.

You can execute the ovnkube-trace binary from a dedicated container. For releases after OpenShift Container Platform 4.7, you can also copy the binary to a local host and execute it from that host.

The binaries in the Quay images do not currently work for Dual IP stack or IPv6 only environments. For those environments, you must build from source.

Installing the ovnkube-trace on local host

The ovnkube-trace tool traces packet simulations for arbitrary UDP or TCP traffic between points in an OVN-Kubernetes driven OpenShift Container Platform cluster. Copy the ovnkube-trace binary to your local host making it available to run against the cluster.

Prerequisites

  • You installed the OpenShift CLI (oc).

  • You are logged in to the cluster with a user with cluster-admin privileges.

Procedure

  1. Create a pod variable by using the following command:

    $  POD=$(oc get pods -n openshift-ovn-kubernetes -l app=ovnkube-master -o name | head -1 | awk -F '/' '{print $NF}')

  2. Run the following command on your local host to copy the binary from the ovnkube-master pods:

    $  oc cp -n openshift-ovn-kubernetes $POD:/usr/bin/ovnkube-trace ovnkube-trace

  3. Make ovnkube-trace executable by running the following command:

    $  chmod +x ovnkube-trace

  4. Display the options available with ovnkube-trace by running the following command:

    $  ./ovnkube-trace -help
    Expected output
    I0111 15:05:27.973305  204872 ovs.go:90] Maximum command line arguments set to: 191102
Usage of ./ovnkube-trace:
  -dst string
    	dest: destination pod name
  -dst-ip string
    	destination IP address (meant for tests to external targets)
  -dst-namespace string
    	k8s namespace of dest pod (default "default")
  -dst-port string
    	dst-port: destination port (default "80")
  -kubeconfig string
    	absolute path to the kubeconfig file
  -loglevel string
    	loglevel: klog level (default "0")
  -ovn-config-namespace string
    	namespace used by ovn-config itself
  -service string
    	service: destination service name
  -skip-detrace
    	skip ovn-detrace command
  -src string
    	src: source pod name
  -src-namespace string
    	k8s namespace of source pod (default "default")
  -tcp
    	use tcp transport protocol
  -udp
    	use udp transport protocol

    The command-line arguments supported are familiar Kubernetes constructs, such as namespaces, pods, services so you do not need to find the MAC address, the IP address of the destination nodes, or the ICMP type.

    The log levels are:

    • 0 (minimal output)

    • 2 (more verbose output showing results of trace commands)

    • 5 (debug output)

Running ovnkube-trace

Run ovn-trace to simulate packet forwarding within an OVN logical network.

Prerequisites

  • You installed the OpenShift CLI (oc).

  • You are logged in to the cluster with a user with cluster-admin privileges.

  • You have installed ovnkube-trace on local host

Example: Testing that DNS resolution works from a deployed pod

This example illustrates how to test the DNS resolution from a deployed pod to the core DNS pod that runs in the cluster.

Procedure

  1. Start a web service in the default namespace by entering the following command:

    $ oc run web --namespace=default --image=nginx --labels="app=web" --expose --port=80

  2. List the pods running in the openshift-dns namespace:

    oc get pods -n openshift-dns
    Example output
    NAME                  READY   STATUS    RESTARTS   AGE
dns-default-467qw     2/2     Running   0          49m
dns-default-6prvx     2/2     Running   0          53m
dns-default-fkqr8     2/2     Running   0          53m
dns-default-qv2rg     2/2     Running   0          49m
dns-default-s29vr     2/2     Running   0          49m
dns-default-vdsbn     2/2     Running   0          53m
node-resolver-6thtt   1/1     Running   0          53m
node-resolver-7ksdn   1/1     Running   0          49m
node-resolver-8sthh   1/1     Running   0          53m
node-resolver-c5ksw   1/1     Running   0          50m
node-resolver-gbvdp   1/1     Running   0          53m
node-resolver-sxhkd   1/1     Running   0          50m

  3. Run the following ovn-kube-trace command to verify DNS resolution is working:

    $ ./ovnkube-trace \
  -src-namespace default \ (1)
  -src web \ (2)
  -dst-namespace openshift-dns \ (3)
  -dst dns-default-467qw \ (4)
  -udp -dst-port 53 \ (5)
  -loglevel 0 (6)
    1 Namespace of the source pod
    2 Source pod name
    3 Namespace of destination pod
    4 Destination pod name
    5 Use the udp transport protocol. Port 53 is the port the DNS service uses.
    6 Set the log level to 1 (0 is minimal and 5 is debug)
    Expected output
    I0116 10:19:35.601303   17900 ovs.go:90] Maximum command line arguments set to: 191102
ovn-trace source pod to destination pod indicates success from web to dns-default-467qw
ovn-trace destination pod to source pod indicates success from dns-default-467qw to web
ovs-appctl ofproto/trace source pod to destination pod indicates success from web to dns-default-467qw
ovs-appctl ofproto/trace destination pod to source pod indicates success from dns-default-467qw to web
ovn-detrace source pod to destination pod indicates success from web to dns-default-467qw
ovn-detrace destination pod to source pod indicates success from dns-default-467qw to web

    The ouput indicates success from the deployed pod to the DNS port and also indicates that it is successful going back in the other direction. So you know bi-directional traffic is supported on UDP port 53 if my web pod wants to do dns resolution from core DNS.

If for example that did not work and you wanted to get the ovn-trace, the ovs-appctl ofproto/trace and ovn-detrace, and more debug type information increase the log level to 2 and run the command again as follows:

$ ./ovnkube-trace \
  -src-namespace default \
  -src web \
  -dst-namespace openshift-dns \
  -dst dns-default-467qw \
  -udp -dst-port 53 \
  -loglevel 2

The output from this increased log level is too much to list here. In a failure situation the output of this command shows which flow is dropping that traffic. For example an egress or ingress network policy may be configured on the cluster that does not allow that traffic.

Example: Verifying by using debug output a configured default deny

This example illustrates how to identify by using the debug output that an ingress default deny policy blocks traffic.

Procedure

  1. Create the following YAML that defines a deny-by-default policy to deny ingress from all pods in all namespaces. Save the YAML in the deny-by-default.yaml file:

    kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: deny-by-default
  namespace: default
spec:
  podSelector: {}
  ingress: []

  2. Apply the policy by entering the following command:

    $ oc apply -f deny-by-default.yaml
    Example output
    networkpolicy.networking.k8s.io/deny-by-default created

  3. Start a web service in the default namespace by entering the following command:

    $ oc run web --namespace=default --image=nginx --labels="app=web" --expose --port=80

  4. Run the following command to create the prod namespace:

    $ oc create namespace prod

  5. Run the following command to label the prod namespace:

    $ oc label namespace/prod purpose=production

  6. Run the following command to deploy an alpine image in the prod namespace and start a shell:

    $ oc run test-6459 --namespace=prod --rm -i -t --image=alpine -- sh

  7. Open another terminal session.

  8. In this new terminal session run ovn-trace to verify the failure in communication between the source pod test-6459 running in namespace prod and destination pod running in the default namespace:

    $ ./ovnkube-trace \
 -src-namespace prod \
 -src test-6459 \
 -dst-namespace default \
 -dst web \
 -tcp -dst-port 80 \
 -loglevel 0
    Expected output
    I0116 14:20:47.380775   50822 ovs.go:90] Maximum command line arguments set to: 191102
ovn-trace source pod to destination pod indicates failure from test-6459 to web

  9. Increase the log level to 2 to expose the reason for the failure by running the following command:

    $ ./ovnkube-trace \
 -src-namespace prod \
 -src test-6459 \
 -dst-namespace default \
 -dst web \
 -tcp -dst-port 80 \
 -loglevel 2
    Expected output
    ct_lb_mark /* default (use --ct to customize) */
------------------------------------------------
 3. ls_out_acl_hint (northd.c:6092): !ct.new && ct.est && !ct.rpl && ct_mark.blocked == 0, priority 4, uuid 32d45ad4
    reg0[8] = 1;
    reg0[10] = 1;
    next;
 4. ls_out_acl (northd.c:6435): reg0[10] == 1 && (outport == @a16982411286042166782_ingressDefaultDeny), priority 2000, uuid f730a887 (1)
    ct_commit { ct_mark.blocked = 1; };
    1 Ingress traffic is blocked due to the default deny policy being in place

  10. Create a policy that allows traffic from all pods in a particular namespaces with a label purpose=production. Save the YAML in the web-allow-prod.yaml file:

    kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: web-allow-prod
  namespace: default
spec:
  podSelector:
    matchLabels:
      app: web
  policyTypes:
  - Ingress
  ingress:
  - from:
    - namespaceSelector:
        matchLabels:
          purpose: production

  11. Apply the policy by entering the following command:

    $ oc apply -f web-allow-prod.yaml

  12. Run ovnkube-trace to verify that traffic is now allowed by entering the following command:

    $ ./ovnkube-trace \
 -src-namespace prod \
 -src test-6459 \
 -dst-namespace default \
 -dst web \
 -tcp -dst-port 80 \
 -loglevel 0
    Expected output
    I0116 14:25:44.055207   51695 ovs.go:90] Maximum command line arguments set to: 191102
ovn-trace source pod to destination pod indicates success from test-6459 to web
ovn-trace destination pod to source pod indicates success from web to test-6459
ovs-appctl ofproto/trace source pod to destination pod indicates success from test-6459 to web
ovs-appctl ofproto/trace destination pod to source pod indicates success from web to test-6459
ovn-detrace source pod to destination pod indicates success from test-6459 to web
ovn-detrace destination pod to source pod indicates success from web to test-6459

  13. In the open shell run the following command:

     wget -qO- --timeout=2 http://web.default
    Expected output
    <!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
html { color-scheme: light dark; }
body { width: 35em; margin: 0 auto;
font-family: Tahoma, Verdana, Arial, sans-serif; }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>

<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>

<p><em>Thank you for using nginx.</em></p>
</body>
</html>

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