Issues running cron in Docker on different hosts

Question

Im trying to get a docker container running to mange my cronjobs

im running a very simple cron as a test in a docker container using centOS 6.5 base

* * * * * /bin/echo "it works!" >> test.log

it works fine if the container is running on CoreOS host, however if I run the container on a ubuntu 13.10 host the cron is not executed. (I though the host did not effect what was running in the container)

both hosts are running docker 0.8

am I missing something obvious, or it this a bug?

thanks

Answer 1

short answer

add this line to your dockerfile

RUN sed -i '/session    required   pam_loginuid.so/c\#session    required   pam_loginuid.so' /etc/pam.d/crond

the long answer

from what I understand issue is related to differences in the kernal between CoreOS & Unbutu. this in-turn causes a pam security issue.

to figure it our first needed to turn on logging for cron (since we are in docker normal startup is not executed). run

service rsyslog start
service crond restart

the cron log had this error (located here /var/log/cron)

FAILED to open PAM security session (Cannot make/remove an entry for the specified session)

so then I took a look at the security log, and it had this error (located here /var/log/secure)

pam_loginuid(crond:session): set_loginuid failed

some more googling and found I needed to modify my pam cond config (found here /etc/pam.d/crond) edit this file and comment out the following line

#session    required   pam_loginuid.so

restart crond and all should be good

Answer 2

the base Docker containers don't start services like cron - they only start what you specify in the ENTRYPOINT/CMD

some 'fatter' containers use things like supervisord to start services - but where possible, its more maintainable to separate services into different containers and share data using with volume containers, or --link

Answer 3

@dwitz's answer is correct but I had to alter the sed command a bit to make it work for me on Ubuntu 16.04 within a docker container.

cat /etc/pam.d/cron |sed -e "s/required     pam_loginuid.so/optional     pam_loginuid.so/g" > /tmp/cron && mv /tmp/cron /etc/pam.d/cron 

Answer 4

In my case, I debug the cron:

$ apt-get install rsyslog
$ rsyslogd
$ service cron restart
$ tail -f /var/log/syslog

And found NUMBER OF HARD LINKS > 1 error in logs.

The solution was to put this in entrypoint.sh

touch /etc/crontab /etc/cron.d/*

and boom!