Kubernetes Explained Simply: #2 Kubectl Hack (List Running Images)

Running Kubernetes in production means taking inventory. A LOT. Are any of our pods running that version of Ubuntu base image affected by the new CVE?

Do we even use Alpine Linux anywhere? What versions of MySQL are we currently running (and where)? The standard output of

kubectl get pods

doesn't help to answer any of these questions.

That's okay, though, because we have the

custom-columns

output format!

$ kubectl get pods -A -o \
  custom-columns=NAMESPACE:metadata.namespace,NAME:metadata.name

NAMESPACE     NAME
fail          fail-856f678c66-dn282
interactive   interactive-797dbc7d9-ch9bd
kube-system   calico-kube-controllers-dc6cb64cb-pfhqr
kube-system   calico-node-nk854
kube-system   coredns-5644d7b6d9-9776g
kube-system   coredns-5644d7b6d9-zccn5
kube-system   csi-linode-controller-0
kube-system   csi-linode-node-82qgt
kube-system   kube-proxy-xxpvf

Whew! That's a fingerful to type. If you'd like, you can put the details of the format in a file, and reference that file via the

custom-columns-file

output format:

$ cat pods.fmt
NAMESPACE            NAME
metadata.namespace   metadata.name

$ kubectl  get pods -A -o custom-columns-file=pods.fmt
NAMESPACE     NAME
fail          fail-856f678c66-dn282
interactive   interactive-797dbc7d9-ch9bd
kube-system   calico-kube-controllers-dc6cb64cb-pfhqr
kube-system   calico-node-nk854
kube-system   coredns-5644d7b6d9-9776g
kube-system   coredns-5644d7b6d9-zccn5
kube-system   csi-linode-controller-0
kube-system   csi-linode-node-82qgt
kube-system   kube-proxy-xxpvf

Doing so allows you to swap out selectors, like namespace and

-l

label filters, while re-using the same format (and not having to retype it!)

We can do so much more than just list off names and namespaces. We can go so far as to list out the images in use by the first container of each pod:

$ cat images.fmt
NAMESPACE            NAME           IMAGE
metadata.namespace   metadata.name  spec.containers[0].image

$ kubectl get pods -A -o custom-columns-file=images.fmt
NAMESPACE     NAME                                      IMAGE
fail          fail-856f678c66-dn282                     huntprod/run
interactive   interactive-797dbc7d9-ch9bd               huntprod/run
kube-system   calico-kube-controllers-dc6cb64cb-pfhqr   calico/kube-controllers:v3.9.2
kube-system   calico-node-nk854                         calico/node:v3.9.2
kube-system   coredns-5644d7b6d9-9776g                  k8s.gcr.io/coredns:1.6.2
kube-system   coredns-5644d7b6d9-zccn5                  k8s.gcr.io/coredns:1.6.2
kube-system   csi-linode-controller-0                   quay.io/k8scsi/csi-provisioner:v1.0.0
kube-system   csi-linode-node-82qgt                     quay.io/k8scsi/driver-registrar:v1.0-canary
kube-system   kube-proxy-xxpvf                          k8s.gcr.io/kube-proxy:v1.16.3

Now we're talkin'!

Keep in mind that the tag reported is whatever was in the pod spec – if it is missing, you can assume

:latest

, but that doesn't tell you much about the actual version in use. If you want anything more specific out of Kubernetes, you'll need to use the

status

object, instead of

spec

$ cat versions.fmt
NAMESPACE            NAME           IMAGE
metadata.namespace   metadata.name  status.containerStatuses[0].imageID

$ kubectl get pods -A -o custom-columns-file=versions.fmt
NAMESPACE     NAME                                      IMAGE
fail          fail-856f678c66-dn282                     docker-pullable://huntprod/run@sha256:1d8debb90a76fcc434cd5452e61eb9f55fb71d82b8fbbe2fd54ad423e17a996d
interactive   interactive-797dbc7d9-ch9bd               docker-pullable://huntprod/run@sha256:1d8debb90a76fcc434cd5452e61eb9f55fb71d82b8fbbe2fd54ad423e17a996d
kube-system   calico-kube-controllers-dc6cb64cb-pfhqr   docker-pullable://calico/kube-controllers@sha256:5d525a6c6cec7f1e9a2b35723ffc63223f9fd067619cc1db209339792927dd02
kube-system   calico-node-nk854                         docker-pullable://calico/node@sha256:ffbe7b00344065007154b81f50d0f3960ce35fc790cdba0c2e2f0ae60e08cae2
kube-system   coredns-5644d7b6d9-9776g                  docker-pullable://k8s.gcr.io/coredns@sha256:12eb885b8685b1b13a04ecf5c23bc809c2e57917252fd7b0be9e9c00644e8ee5
kube-system   coredns-5644d7b6d9-zccn5                  docker-pullable://k8s.gcr.io/coredns@sha256:12eb885b8685b1b13a04ecf5c23bc809c2e57917252fd7b0be9e9c00644e8ee5
kube-system   csi-linode-controller-0                   docker-pullable://quay.io/k8scsi/csi-attacher@sha256:e57bb6abf0d78e638f70d38bdb07ee30ffe42d423a14fb2f910c11afab3a5e01
kube-system   csi-linode-node-82qgt                     docker-pullable://linode/linode-blockstorage-csi-driver@sha256:6a466fea4f597f274d646839ff1f40363e6c5bac5871fb2f4e23ee0eaadb56ee
kube-system   kube-proxy-xxpvf                          docker-pullable://k8s.gcr.io/kube-proxy@sha256:6c09387bbee4e58eb923695da4fdfa3c37adec632862e79f419f0b5b16865f34

Kubernetes tracks the raw SHA256 checksum of each image it executes, in

status.containerStatuses[n].imageID

, and the resolved image name and its tag in

.image

. We use those two, above, to get a more complete picture of our running image inventory.

So far we've been using the

[0]

-th index, which limits us to the first container in any multi-container pods. We could instead use

[*]

, and

kubectl

will separate multiple values with commas. This gets unwieldy fast, especially with the

imageID

fields, but it is there when you need it.

Pro Tip: Since we know that the only instance of a comma in our output will be to separate multiple image IDs, we can use
column
(a POSIX utility) to reformat the table to ease readability:

$ kubectl get pods -A -o \
    'custom-columns=POD:metadata.name,IMAGE:spec.containers[*].image' | \
    column -t -s,
POD                                       IMAGE
fail-856f678c66-dn282                     huntprod/run
interactive-797dbc7d9-ch9bd               huntprod/run
calico-kube-controllers-dc6cb64cb-pfhqr   calico/kube-controllers:v3.9.2
calico-node-nk854                         calico/node:v3.9.2
coredns-5644d7b6d9-9776g                  k8s.gcr.io/coredns:1.6.2
coredns-5644d7b6d9-zccn5                  k8s.gcr.io/coredns:1.6.2
csi-linode-controller-0                   quay.io/k8scsi/csi-provisioner:v1.0.0        quay.io/k8scsi/csi-attacher:v1.0.0            linode/linode-blockstorage-csi-driver:v0.1.3
csi-linode-node-82qgt                     quay.io/k8scsi/driver-registrar:v1.0-canary  linode/linode-blockstorage-csi-driver:v0.1.3
kube-proxy-xxpvf                          k8s.gcr.io/kube-proxy:v1.16.3

It's also important to note that our above formats completely ignore any and all init containers, in case those are important to you.

If you like that, check out this video adaptation, wherein I go a bit more in-depth, and touch on some stuff you can do with files holding all your sweet, sweet custom output formats.

Previously published at https://starkandwayne.com/blog/silly-kubectl-trick-2-listing-images/