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Using trace bind
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Trace the kernel functions performing socket binding.

Screencast of the trace bind gadget

The trace bind gadget is used to stream socket binding syscalls.

On Kubernetes

First, we need to create one pod for us to play with:

$ kubectl run test-pod --image busybox:latest sleep inf

You can now use the gadget, but output will be empty:

$ kubectl gadget trace bind
K8S.NODE         K8S.NAMESPACE    K8S.POD          K8S.CONTAINER    PID    COMM             PROTO  ADDR             PORT   OPTS   IF

Indeed, it is waiting for socket binding to occur. So, in another terminal, exec the container and use nc:

$ kubectl exec -ti test-pod -- nc -l -p 4242 -w 1
nc: timeout
command terminated with exit code 1

Go back to the first terminal and see:

K8S.NODE         K8S.NAMESPACE    K8S.POD          K8S.CONTAINER    PID    COMM             PROTO  ADDR             PORT   OPTS   IF
minikube         default          test-pod         test-pod         58208  nc               IP     ::               4242   .R...  0

This line corresponds to the socket binding operation initiated by nc.

Clean everything

Congratulations! You reached the end of this guide! You can now delete the pod you created:

$ kubectl delete pod test-pod
pod "test-pod" deleted

With ig

Start the gadget first

$ sudo ig trace bind -c test-trace-bind
K8S.CONTAINER    PID     COMM             PROTO  ADDR             PORT    OPTS    IF

In another terminal, run a container that performs a bind operation

$ docker run -it --rm --name test-trace-bind busybox /bin/sh -c "nc -l -p 4242"

The gadget will print the event on the first terminal:

$ sudo ig trace bind -c test-trace-bind
K8S.CONTAINER    PID     COMM             PROTO  ADDR             PORT    OPTS    IF
test-trace-bind  380299  nc               TCP    ::               4242    .R...   0

Restricting output to certain PID, ports or succeeded and failed port bindings

With the following options, you can restrict the output:

  • --pid only prints events where socket binding is done by the given PID.
  • -P/--ports only prints events where these ports are used for socket bindings.
  • -i/--ignore-errors only prints events where the bind succeeded.

So, this command will print all (i.e. succeeded and failed) attempts to bind a socket on port 4242 or 4343 by PID 42:

$ kubectl gadget trace bind -i=false --pid 42 -P=4242,4343

$ sudo ig trace bind -i=false --pid 42 -P=4242,4343