Tracing Openflow with ovnkube-trace

To trace Open vSwitch and OVN traffic flows in OpenShift Container Platform, you can use the ovnkube-trace utility, which runs ovn-trace, ovs-appctl ofproto/trace, and ovn-detrace in a single correlated 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.

Installing the ovnkube-trace on local host

To run ovnkube-trace from your local host against a cluster, you can copy the binary from an ovnkube-control-plane pod and use familiar Kubernetes namespace and pod arguments.

The ovnkube-trace tool traces packet simulations for arbitrary UDP or TCP traffic between points in an OVN-Kubernetes driven OpenShift Container Platform cluster.

Prerequisites

You have installed the OpenShift CLI (oc).
You are logged in to the cluster with cluster-admin privileges.

Procedure

Create a pod variable by using the following command:

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

Run the following command on your local host to copy the binary from the ovnkube-control-plane pods:
```
$  oc cp -n openshift-ovn-kubernetes $POD:/usr/bin/ovnkube-trace -c ovnkube-cluster-manager ovnkube-trace
```
note
If you are using Red Hat Enterprise Linux (RHEL) 8 to run the ovnkube-trace tool, you must copy the file /usr/lib/rhel8/ovnkube-trace to your local host.
Make ovnkube-trace executable by running the following command:
```
$  chmod +x ovnkube-trace
```

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

$  ./ovnkube-trace -help

Example output
Usage of ./ovnkube-trace:
  -addr-family string
    	Address family (ip4 or ip6) to be used for tracing (default "ip4")
  -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

To simulate packet forwarding in an OVN logical network, you can run ovnkube-trace with source and destination pods, ports, and log levels.

The examples in the following procedure show DNS resolution and default-deny network policy troubleshooting.

Prerequisites

You have installed the OpenShift CLI (oc).
You are logged in to the cluster with a user with cluster-admin privileges.
You have installed the ovnkube-trace binary on your local host.

Procedure

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

$ oc run web --namespace=default --image=quay.io/openshifttest/nginx --labels="app=web" --expose --port=80

List the pods running in the openshift-dns namespace:

$ oc get pods -n openshift-dns

The output is similar to the following:

NAME                  READY   STATUS    RESTARTS   AGE
dns-default-8s42x     2/2     Running   0          5h8m
dns-default-mdw6r     2/2     Running   0          4h58m
dns-default-p8t5h     2/2     Running   0          4h58m
dns-default-rl6nk     2/2     Running   0          5h8m
dns-default-xbgqx     2/2     Running   0          5h8m
dns-default-zv8f6     2/2     Running   0          4h58m
node-resolver-62jjb   1/1     Running   0          5h8m
node-resolver-8z4cj   1/1     Running   0          4h59m
node-resolver-bq244   1/1     Running   0          5h8m
node-resolver-hc58n   1/1     Running   0          4h59m
node-resolver-lm6z4   1/1     Running   0          5h8m
node-resolver-zfx5k   1/1     Running   0          5h

Run the following ovnkube-trace command to verify DNS resolution is working:

$ ./ovnkube-trace \
  -src-namespace default \
  -src web \
  -dst-namespace openshift-dns \
  -dst dns-default-p8t5h \
  -udp -dst-port 53 \
  -loglevel 0

where:

-src-namespace: Specifies namespace of the source pod.
-src: Specifies source pod name.
-dst-namespace: Specifies namespace of destination pod.
-dst: Specifies destination pod name.
-udp: Specifies use of the udp transport protocol. Port 53 is the port the DNS service uses.
-loglevel: Specifies the log level to 0 (0 is minimal and 5 is debug). If the src&dst pod lands on the same node, the output is similar to the following:

ovn-trace source pod to destination pod indicates success from web to dns-default-p8t5h
ovn-trace destination pod to source pod indicates success from dns-default-p8t5h to web
ovs-appctl ofproto/trace source pod to destination pod indicates success from web to dns-default-p8t5h
ovs-appctl ofproto/trace destination pod to source pod indicates success from dns-default-p8t5h to web
ovn-detrace source pod to destination pod indicates success from web to dns-default-p8t5h
ovn-detrace destination pod to source pod indicates success from dns-default-p8t5h to web

If the src&dst pod lands on a different node, the output is similar to the following:

ovn-trace source pod to destination pod indicates success from web to dns-default-8s42x
ovn-trace (remote) source pod to destination pod indicates success from web to dns-default-8s42x
ovn-trace destination pod to source pod indicates success from dns-default-8s42x to web
ovn-trace (remote) destination pod to source pod indicates success from dns-default-8s42x to web
ovs-appctl ofproto/trace source pod to destination pod indicates success from web to dns-default-8s42x
ovs-appctl ofproto/trace destination pod to source pod indicates success from dns-default-8s42x to web
ovn-detrace source pod to destination pod indicates success from web to dns-default-8s42x
ovn-detrace destination pod to source pod indicates success from dns-default-8s42x to web

The output 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 of proto/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.

Testing a default deny policy with ovnkube-trace

To verify that an ingress default deny network policy blocks traffic in OpenShift Container Platform, you can run ovnkube-trace with a higher log level and read the ACL debug output. You can add an allow policy for labeled namespaces and confirm that traffic succeeds.

Prerequisites

You have installed the OpenShift CLI (oc).
You are logged in to the cluster with a user with cluster-admin privileges.
You have installed the ovnkube-trace binary on your local host.

Procedure

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: []
```

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

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

$ oc run web --namespace=default --image=quay.io/openshifttest/nginx --labels="app=web" --expose --port=80

Run the following command to create the prod namespace:
```
$ oc create namespace prod
```
Run the following command to label the prod namespace:
```
$ oc label namespace/prod purpose=production
```
To deploy an alpine image in the prod namespace and start a shell, run the following command:
```
$ oc run test-6459 --namespace=prod --rm -i -t --image=alpine -- sh
```
Open another terminal session.

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

Example output
ovn-trace source pod to destination pod indicates failure from test-6459 to web

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

Example output
...
------------------------------------------------
 3. ls_out_acl_hint (northd.c:7454): !ct.new && ct.est && !ct.rpl && ct_mark.blocked == 0, priority 4, uuid 12efc456
    reg0[8] = 1;
    reg0[10] = 1;
    next;
 5. ls_out_acl_action (northd.c:7835): reg8[30..31] == 0, priority 500, uuid 69372c5d
    reg8[30..31] = 1;
    next(4);
 5. ls_out_acl_action (northd.c:7835): reg8[30..31] == 1, priority 500, uuid 2fa0af89
    reg8[30..31] = 2;
    next(4);
 4. ls_out_acl_eval (northd.c:7691): reg8[30..31] == 2 && reg0[10] == 1 && (outport == @a16982411286042166782_ingressDefaultDeny), priority 2000, uuid 447d0dab
    reg8[17] = 1;
    ct_commit { ct_mark.blocked = 1; };
    next;
...

where:

ct_commit { ct_mark.blocked = 1; };: Specifies that ingress traffic is blocked due to the default deny policy being in place.

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

Apply the policy by entering the following command:
```
$ oc apply -f web-allow-prod.yaml
```

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

Example output
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

Run the following command in the shell that was opened in step six to connect nginx to the web-server:

$ wget -qO- --timeout=2 http://web.default

Example output
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
  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>

Additional resources

Installing the ovnkube-trace on local host​

Running ovnkube-trace​

Testing a default deny policy with ovnkube-trace​

Installing the ovnkube-trace on local host

Running ovnkube-trace

Testing a default deny policy with ovnkube-trace