4

In order to debug why this issue occurs the following command is run on a container:

docker run -it <dockerImageName> dmesg

results in:

dmesg: read kernel buffer failed: Permission denied

Attempts

  1. Running docker run -it <dockerImageName> sudo dmesg returns the same issue
  • You aren't allowed to do that in a container. What's the purpose of this? – Michael Hampton Feb 4 '16 at 20:03
2

as Michael Hampton mentioned, containers are meant to run only single atomic service. As a matter of fact, one should understand that containers are not virtual machines but a single process by itself on your localhost.

Nevertheless, I got to know that, its a bit more harder to get SystemD working inside a container from here

I was able to get SystemD working inside an image built FROM centos:centos7 with:

docker run --privileged -ti -e "container=docker" -v /sys/fs/cgroup:/sys/fs/cgroup trinitronx/ansible-base:stable-centos7 /usr/sbin/init

For some undocumented reason the variable container=docker is apparently required. /sys/fs/cgroup is also required, as SystemD needs cgroups to work properly according to RedHat Bug 1033604.

After doing this, try to login to the container using docker exec -it <container> /bin/bash and then you could execute your systemctl commands.

  • Would this be safe enough to run in production? I will try to run the binary instead of using systemd. – 030 Feb 4 '16 at 23:29
  • No, I suppose as its using "privileged" and also mouting /sys directories into the container. I thought this was just for debugging temporarily. Services should not be started with systemctl after containers are up. It should be part of CMD. e.g. This is how httpd is started. However, have you every thought, how do you automate the starting of those processes after the container has started? – Maniankara Feb 5 '16 at 9:36
  • So the solution is to use --privileged :) . Helped me viewing dmesg and mounting using CIFS! Thanks. – yair Feb 12 '19 at 6:42

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