[atomic-devel] The atomic command and setting hostname for containers

[Jan Pazdziora <jpazdziora redhat com>]

Hello,

with the atomic command, we can run

	atomic install ...

to configure a service in/via a container and

	atomic run ...

to start it.

If the software in the container would like to have a particual fixed
hostname set, specified by the user/admin, the LABEL INSTALL and/or RUN
have to specify

	docker run --cap-add=SYS_ADMIN

for hostname command in the container to be able to change the
hostname, which is an overkill especially give the fact that docker
itself gives us a way to specify the hostname.

It'd be nice if the atomic command could support -h option, and make
that value available for use in the INSTALL / RUN / UNINSTALL LABELs.
I envision something like

	docker install -h ipa.example.com freeipa-server --realm EXAMPLE.COM ...

and

	docker run freeipa-server

and be able to use

	LABEL INSTALL 'docker run -h ${OPT_HOSTNAME} ... ${IMAGE} ...'

and

	LABEL RUN 'docker run -h ${OPT_HOSTNAME} ... ${IMAGE} ...'

and have that OPT_HOSTNAME expand to

	-h ipa.example.com

Of course, if we wanted for admin to only specify the hostname once
during atomic install, the value would have to survive somewhere for
subsequent atomic run events. I'm not sure how to do that though,
to fit the rather state-less nature of atomic (utility).

One possibility is to specify for example

	the value of -h option of atomic install is stored in
	/etc/${NAME}.hostname file and that file is consulted for
	all atomic operations (that touch containers) and its content
	made available in OPT_HOSTNAME

But this really gets us into the business of defining "storage" for
container configuration. Another possibility would be for the INSTALL
container to handle it. For example, in the install.sh, the current
value of $(hostname) could be stored to whatever /host/... location
the container would like, and then we could come up with new LABEL
RUN_HOSTNAME_LOCATION that atomic would evaluate and use for
OPT_HOSTNAME.

So the Dockerfile bits would be

	LABEL INSTALL 'docker run -h ${OPT_HOSTNAME} -h /:/host ${IMAGE} ...'
				# Just use the -h option of atomic here
	LABEL RUN_HOSTNAME_LOCATION '/etc/${NAME}/hostname'
	LABEL RUN 'docker run -h ${OPT_HOSTNAME} ... ${IMAGE} ...'
				# Get content of /etc/${NAME}/hostname
				# and if not empty, replace OPT_HOSTNAME
				# with it

and install.sh would do

	hostname > /host/etc/${NAME}/hostname

Is the idea of having hostname handling support in atomic command
acceptable?

Is introducing new LABEL like RUN_HOSTNAME_LOCATION which would be
consulted for location of the hostname value acceptable?

I've checked that

	docker run -h '' ...

sets the hostname to the container id so if the ${OPT_HOSTNAME}
replacement correctly puts in empty string parameter, things should
work.

I can prepare patch but I wanted to get some preliminary feedback
about the idea and direction.

Thank you,

-- 
Jan Pazdziora
Sr. Principal Software Engineer, Identity Management Engineering, Red Hat