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A _posts/2021-01-14-kaniko-mtab.md

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+---
+title: "Creating an OS image with Kaniko"
+categories:
+  - Container
+tags:
+  - kaniko
+  - linux
+  - en
+---
+
+It is common to create an OS image with a Containerfile like following:
+
+```Dockerfile
+FROM scratch
+ADD rootfs.tar /
+```
+
+However, with Kaniko, `/etc/mtab` would be missing from the image.
+
+In Kaniko, some predefined paths, including `/etc/mtab`, are in a [ignorelist][1]. If a path is in that list, it will be [ignored when taking a snapshot][2]. Currently only `/var/run` is able to be [configured][3] as excluded from ignoring.
+
+On most systems, `/etc/mtab` is a symlink to `../proc/self/mounts`.
+
+A workaround is using a minimal image with such symlink as base instead of building from scratch. We can build that image with a Containerfile like following:
+
+```Dockerfile
+FROM scratch
+ADD busybox /
+RUN ["/busybox", "ash", "-c", "/busybox ln -s ../proc/self/mounts /etc/mtab && /busybox rm /busybox"]
+```
+
+As there are no instruction to create a symlink, we use busybox for `ln`. Add busybox in the image, use busybox to create the symlink and delete itself. There is no shell at `/bin/sh`, so the `RUN` need to be in "exec form".
+
+[1]: https://github.com/GoogleContainerTools/kaniko/blob/ece215c18113020f9151fb25e69fc4ecc157c395/pkg/util/fs_util.go#L58
+[2]: https://github.com/GoogleContainerTools/kaniko/blob/ece215c18113020f9151fb25e69fc4ecc157c395/docs/designdoc.md#snapshotting-snapshotting
+[3]: https://github.com/GoogleContainerTools/kaniko/blob/ece215c18113020f9151fb25e69fc4ecc157c395/cmd/executor/cmd/root.go#L177