Garden MTU is ignored when starting docker daemon about docker-image-resource

[concourse-bot]

Hi there!

We use Pivotal Tracker to provide visibility into what our team is working on. A story for this issue has been automatically created.

The current status is as follows:

• #130024023 Garden MTU is ignored when starting docker daemon

This comment, as well as the labels on the issue, will be automatically updated as the status in Tracker changes.

[databus23]

Since docker 1.11 (moby/moby@fd9d7c0) docker is not adopting the MTU from the default interface anymore.

For the docker resource this could be easily fixed by adding something like this to start_docker

local mtu=$(cat /sys/class/net/$(ip route get 8.8.8.8|awk '{ print $5 }')/mtu)
local server_args="--mtu ${mtu}"

With this change the guardian setting --garden-mtu= would trickle down to the networking setup by the (inner) docker daemon which I think makes sense.

[mdomke]

I have the same problem with running docker inside a garden container on OpenStack with concourse. The MTU setting is not being promoted properly and especially SSL based connections (e.g.: downloading a tar-archive over HTTPS) fail. I manually verified that the proposed changes from @databus23 would solve the problem. Is anyone planing on creating a pull-request therefore or should I take my chances?

[mdomke]

Another option would obviously be to make the server_args a configuration value of the resource. @vito what do you think about this?

[mdomke]

I created a drop in replacement image with the suggestions from @databus23, which people can use until this issue is fixed here. It's mdomke/docker-image-resource

[vito]

@mdomke I'm not a fan of open-ended params like server_args in general as it tightly couples us to the docker CLI. check for example was rewritten to not use the daemon at all, and if we had done configs like that everyone would have to change their pipelines.

A PR would be great!