Simon Krenger

In the past few months, on all my machines I have replaced Docker with Podman and mostly the transition has been quite smooth. There are still some rough edges here and there, but the overall experience of using Podman has been great!

However, when trying to start a very simple container, one often runs into the following issue:

$ podman run -p80:80 nginx:latest 
Error: error from slirp4netns while setting up port redirection: map[desc:bad request: add_hostfwd: slirp_add_hostfwd failed]

The error message looks very cryptic, but the issue is quite simple: As a regular user, one is typically not allowed to bind ports < 1024. So by trying to bind port 80, you will get the error above.

The fix is trivial, just use a port greater than 1024:

$ podman run -p8080:80 -d nginx:latest 
22d2be2966e9cb77246a8b698f9024de89f4e6d1a0edfe44209bbe4fd27aa8b5
$ curl localhost:8080
[..]
Welcome to nginx!
[..]

If you really need to use a port number lower than 1024, there are multiple ways to configure that:

• Set net.ipv4.ip_unprivileged_port_start=80or similar in your sysctl
• Add the CAP_NET_BIND_SERVICE capability to your process or user

At their core, containers are just Linux processes that are namespaced. This means in practice, many containers still run as processes on the same host machine. While namespacing processes using cgroups creates very good boundaries between processes, the isolation is still not perfect.

So when working with a lot of different namespaces in Kubernetes and you only know the “oc project” command from OpenShift, you start to miss an easy way to change namespaces in Kubernetes.

The official documentation to switch namespaces proposes something like this:

$ kubectl config set-context $(kubectl config current-context) --namespace=<insert-namespace-name-here>

Not something that I want to type regularly. First I tried to create a BASH alias or something, which did not work. So I looked around for BASH functions. I found that Jon Whitcraft proposed a nice BASH function in a GitHub issue. I lightly modified this and placed this in my own .bashrc file:

function kubectlns() {
  ctx=`kubectl config current-context`
  ns=$1

  # verify that the namespace exists
  ns=`kubectl get namespace $1 --no-headers --output=go-template={{.metadata.name}} 2>/dev/null`
  if [ -z "${ns}" ]; then
    echo "Namespace (${1}) not found, using default"
    ns="default"
  fi

  kubectl config set-context ${ctx} --namespace="${ns}"
}

So to change your namespace, use something like this:

$ kubectlns simon
Context "kubernetes-admin@kubernetes" modified.

Nice and short.

So when using NodeSelectors in OpenShift, you’ll also have to set labels on your nodes. You can find more information on labeling nodes in the OpenShift documentation. Here is how you can add or remove a label from a node or pod:

To add a label to a node or pod:

# oc label node node001.krenger.ch mylabel=myvalue
# oc label pod mypod-34-g0f7k mylabel=myvalue

To remove a label (in the example “mylabel”) from a node or pod:

# oc label node node001.krenger.ch mylabel-
# oc label pod mypod-34-g0f7k mylabel-

You can also use oc label -h to see more options for the oc label command.

So in any larger container orchestrator installation, be it Kubernetes or OpenShift, you will encounter pods that crash regularly and enter the “CrashLoopBackOff” status.

$ oc get pod --all-namespaces 
NAMESPACE         NAME                       READY     STATUS             RESTARTS   AGE
[..]
my-project-1      helloworld-11-9w3ud        1/1       Running            0          7h
my-project-2      myapp-simon-43-7macd       0/1       CrashLoopBackOff   3774       9h

Note the container that has status “CrashLoopBackOff” and 3774 restarts.

I recently started working with OpenShift and needed to get a list of all pods on the cluster. I quickly glanced at the documentation but could not find what I wanted. My colleagues quickly pointed me in the right direction:

oc get pod --all-namespaces -o wide

Here is the command with some example output of what to expect:

# oc get pod --all-namespaces -o wide
NAMESPACE                                 NAME                                                       READY     STATUS               RESTARTS   AGE       IP               NODE
my-project                                my-pod-43-d9mo6                                            1/1       Running              0          1d        192.168.0.183    node3.krenger.local
yet-another-project                       another-pod-43-7g3r0                                       1/1       Running              0          2d        192.168.0.184    node4.krenger.local
[..]

If you just want to know which pods are on a certain node, use oc adm manage-node:

oc adm manage-node node3.krenger.local --list-pods