Docker and Security

At $WORK, we have discussed at length security issues as they relate to Docker and the Linux lxc technologies. Our general takeaway is that you cannot ever really trust an image that you didn’t build yourself. As Daniel Walsh of Red Hat explains in his article Docker Security with SELinux, it appears that those concerns are valid.


The problem here revolves around namespaces, and the fact that not everything in Linux is namespaced. Consider /sys, /proc, /dev/sd*, /dev/mem, etc. So what we have here is that root inside the containers effectively has root access to any of these file systems or devices. If you can somehow communicate with them, then consequently, you can own the host with little effort.

Then, What Do?

You should only run Docker images that fit one of the following criteria:

  • You have built the image yourself, from scratch
  • You have received the image from a trusted source
  • You have built the image from a third-party’s Dockerfile, which you have fully read, and understood

Also be careful with your “trusted” source. The base images are probably OK. Images released by, say, Red Hat or Canonical are probably OK. Images in Docker Hub, Docker’s official image registry, might not be. That’s not a hit on the Docker guys – it’s because there are over 15,000 images they’d have to have verified manually to be sure.

Don’t Forget Traditional Security Practices

Finally, you need to be concerned with the security of your Docker containers as well – just as concerned as if the stuff in the container were running on a regular server. For example, if someone were to compromise the webserver you have running in a container, and get escalated root privileges, then you effectively have a compromised host.

Be careful out there. Maintain the security of your containers, and only run images you can fully trust.

Puppet: "Error: Could not request certificate: stack level too deep"

This is going to be a stub, because I have no idea what the cause is., The “Error: Could not request certificate: stack level too deep” message when running puppet has been such a pain in the rear end that I need to document it. I’m a firm believer that just the act of documenting a fix ensures that the problem will never arise again. Here’s hoping.


$ puppet agent -tv
Info: Not using expired certificate for ca from cache; expired at Tue May 20 00:16:15 UTC 2014 Error: Could not request certificate: stack level too deep Exiting; failed to retrieve certificate and waitforcert is disabled

Sounds like an expired CA cert, but replacing it didn’t fix it. All the posts online with this error talk about a three-year-old activerecord bug, so that’s not valid either.  Having no Google-fu solutions, I did the following:

Update 2018-11-13:

A comment by otheus below drew my attention back to this post now years later.  His suggestion is likely a better one:

DONT remove /var/lib/puppet/ssl on the client. no! simply do:

rm -f /var/lib/puppet/ssl/certs/ca.pem

Renew the server-side cert and then re-run puppet agent on the local host.

*** Original Content Follows ***

Resolution (perhaps – again, I am unsure. Cargo Cult fix incoming):

On the puppet node:

$ rm -rf /var/lib/puppet/ssl

…and because we’re doing something weird at $WORK:

$ rm -rf /etc/puppet/ssl
$ puppet agent -t # Regenerates the SSL certificates for the agent

On the puppet master:

$ puppet cert sign # Signs the new node certificate

…and that fixed the node, somehow. It’s got to be SSL related, but who knows how it got into that state, or why updating the CA cert didn’t fix it.

Public Service Announcement: Server Name Indication (SNI)

Server Name Indication, or SNI, is an extension to the TLS protocol. It’s function, in plain English, is to allow a browser to tell a web serverwhich website it’s coming to see before starting the SSL connection. The browser then knows which SSL credentials to send back to the browser and an SSL connection can be established.

SNI is supported by all modern browsers. In fact, it’s even supported by a ton of positively ancient browsers. It’s not, however, supported by any version of Internet Explorer on Windows XP.


If you see an error like the one below, and you are using Internet Explorer on Windows XP, please, take it seriously. Don’t click through. But DO get a REAL browser, like Firefox or Chrome and use that instead, for everything you do .
Internet Explorer 8 SSL Certificate Warning


You should ideally upgrade to a newer version of Windows if you’re still on XP. You’re not getting any patches. You will get hacked. It’s not a question of if. It’s not even a question of when. You probably already have been. But I know that’s not feasible for everyone.

A couple of stats for you. Only 5.29% of the entire internet is still using IE8. IE7 holds their impressive 0.17%, and IE6 is actually 0.3%. All together, those three make up 5.76% of the internet.

The take home from all of this?

The rest of the world has moved on, and you should too.

Use Firefox or Chrome. Just do it.


This was a technology-related rant post I made on Google+. I’m copying it to this blog, mostly so I have an easy-to-read record. This one was originally published on 25 June 2014: Public Service Announcement: Server Name Indication (SNI)

How ’bout CoreOS as your Cloud base?

I’ve heard the name CoreOS around a little bit over the last two months or so, but it hadn’t really jumped out at me until last week when Mark McCahill mentioned it in a meeting. He’d read some pretty cool things about it: minimal OS, designed for running Docker containers, easy distributed configuration, default clustering and service discovery. In particular the use of etcd to manage data between clustered servers caught my eye – we’ve been struggling at $WORK with how to securely get particular types of data into Docker containers in a way that will scale out well if we ever need to start bringing up containers on hosts that don’t physically reside in our datacenters.

CoreOSI haven’t even gotten into the meat of CoreOS yet, but just now, I accomplished a task that was surely lifted out of science fiction. With the assistance of Cobbler as a PXE server and a Docker container (what else!) as a quick host for a cloud-config file, I was able to install CoreOS in seconds and SSH into it with an SSH public-key that I provided it in the cloud-config file. I was legitimately shocked by how quick and easy it was.

Docker containers start instantly – it’s one of their best features. It allows us to ship them around with impunity; perform seamless maintenance. CoreOS hosts live in the same timescale, meaning we an PXE boot and configure new hosts, specifically designed for hosting Docker containers, in seconds, and from anywhere. CoreOS offers support for installing onto bare metal, and that would surely give you the best performance, but take a moment to comprehend the flexibility given to you by using virtual machines instead.

Make an API call to your VMWare or Xen/KVM cluster in your local or remote datacenters to create and start a virtual machine. Or do the same thing with an Amazon host. Or Google. The VM PXE boots into CoreOS, within seconds and joins its cluster, and begins getting data from the rest of the cluster. Within minutes, Docker images are downloaded and built, and containers are spinning up into production. It doesn’t get any more flexible than that. At this point scaling and disaster recovery are hindered only by your ability to produce applications that can handle it. It doesn’t matter if a particular Docker container; something happens to it, you just bring up another somewhere else. Along those lines, it doesn’t matter if an entire host is up or down. That’s what it means to be in the same timescale. Containers and their hosts can be brought up and down with impunity, with no impact to the service.

Another benefit to abstracting the CoreOS away from the bare metal is the freeing of ties to a particular technology. If you can design your systems to use the APIs of your local VM solution and the remote APIs from various cloud vendors, then you can move your services wherever you need them. As long as you can control the routing of traffic in some way (load balances, DNS, Hipache, some of the cool new things being done by Cisco), and the DHCP PXE options for your host servers, then your services are effectively ephemeral and not tied to a particular location or vendor.

For now this is all still very beta, both Docker and CoreOS, but the promise being shown is real. Everyone, from the largest internet giants to the smallest one-room startups, will benefit from the coming revolution in computing.

Docker "Best Practices" (that don’t exist yet)

I’ve noticed the ways in which I set up new Docker images have shifted the more I work with the technology. For example, when I first started with Docker, I put almost all my configurations into the Dockerfile. This is easy – and the way Docker suggests it on their site – and the biggest benefit is how each command ends up being it’s own layer, and they can be cached for quick building if you make a mistake. However, it gets kind of tedious trying to manage tons of bash commands or copy a bunch of files using the RUN and ADD commands. Also, each line counts against the layer limit (though hopefully that will be fixed in a newer version of Docker). The kicker though is that complex bash commands are just hard to pull off inside the Docker file, and lack the real flexibility that running a script offers. Summary: complex bash in a Dockerfile is complex.

So next I moved to having my Dockerfile copy in a large bash script, and run just run that script. This method has the advantage of easy configuration (Hey – 2 whole lines!) and a lot of flexibility. Unfortunately, there’s really only a single checkpoint layer to use with the Docker cache – any single change to the script and the entire image has to be rebuilt from scratch. This makes development time considerably longer, and quick development is one of the big selling points with Docker. Don’t get me wrong – spinning up a container is much quicker than configuring a whole server, but with this method of building, I end up spending a lot of time staring at the screen as my images build. The big, single script also contained a lot of code that ended up being very similar between different images. I’d copy whole swaths of useful base configs out into another giant script to run in another container.

So that brought me to the current way of doing things: copying in the entire image directory (the one that contains the Dockerfile) to /build on the container, and then having the Dockerfile run a few scripts to build the image (3, in fact) Each of THOSE scripts, in turn, run other scripts specifically designed to setup one aspect of the image (one for email, if needed, or syslog, etc). This has the advantage of letting me drop in only those scripts I need for that particular image, and makes the scripts themselves a little friendlier to look at. Unfortunately, it doesn’t solve the long build times, though. The gotcha is that I’m copying everything into /build, so I don’t have to enumerate every file I’m using withing the Dockerfile. But that means that each script I change rewinds Docker back to the ADD where it’s copied in, and every script has to run from scratch without cache.

This is now slowly leading me to have a separate script for every piece except that which distinguishes the container’s main function. This makes for very portable, easy to organize configurations, but requires that each image has dozens of scripts with it. Managing all these scripts is usually accomplished (in other technology worlds) with a central version control repository like git. That way you can clone the scripts you need, and they’re all maintained in one location for easy updating. This method has an inherent drawback too, though. Each Dockerfile and it’s resulting image are dependent on an entirely separate repo for them to work, or even build.

I thought about making a base image, similar to what the Phusion guys are doing with phusion/baseimage-docker. This would allow me to put all the bits that I copy into each image every time (the email, syslog, etc) into a single image, and then add on only the relevant bits for the each container’s main function. This makes managing each image easier, but also makes them less portable, since you have to have both the Dockerfile for the image you want, and the base image for it to pull from when it builds.

It’s not yet clear what the best way to accomplish this will be. I imagine there will be best practices for any given situation.

Fully generic, basic demo image shared with the world? All in the Dockerfile.
Complicated, multi-service images shared with the world? Many little scripts, or a single large one.
Many images with custom configs more easily managed for $WORK? Base Image.

I am looking forward to seeing how other people use Docker, and what evolves as the technology, and the community using it, matures.

Email and Docker-based Drupal Containers


Got email support working in my DockerDemos Drupal Docker image. SSMTP is the way to go with these containers. There’s no running daemon to have to manage, or to take up resources.

The image is setup to either do nothing with mail, use a default SSL setup if you pass your own SMTP server as an environmental variable (perfect for $WORK!) or let you use your own custom ssmpt.conf file.

Yeah, EMAIL! Lost password messages galore!

I’m beginning to copy over my technology-related posts from Google+ to this blog, mostly so I have an easy-to-read record of them. This one was originally published on 19 May 2014: Email and Docker Drupal Containers

"Cloud-style" Docker Demo Container

Completed a first pass at a minimal “Cloud-style”#Docker container. It’s sort of like an EC2 instance. You generate an ssh pem file, and pass the public key in as an environmental variable at docker run:

sudo docker run -i -t -d -P \
-e PUBKEY="$(cat ~/.ssh/" cloudbase

You end up with a CentOS container, and a user “clouduser” that has sudo w/no password rights.

I think this would be a good way to get some folks interested in Docker – perhaps offering something like this as a playground/sandbox to build interest.

Visit a website, get a Docker CentOS container!


I’m beginning to copy over my technology-related posts from Google+ to this blog, mostly so I have an easy-to-read record of them. This one was originally published on 19 May 2014: Cloud-style Docker Container