Tuesday, September 13, 2016

Exploring Rkt on Ubuntu

I have been using docker in my home server since 0.4 in 2013. For me the most attractive property of docker is that it provides a way to decrease the amount of stuff one has to install on a server. I have only one server, but it has many different tasks of which some need to be rock solid (my family email) and other are experimental. Containers provide a nice way to clean up experiments. Unfortunately, docker has never been stable. I have had many fights with docker during upgrades and I have never fully understood how docker interacts with the iptables setup from my firewall (Shorewall).

Today, I was trying to run docker-syncthing. Unfortunately, the container went into 'restarting' without any indication what was wrong. Inspired by "Can docker be ousted" and boring and stable containers I decide to give up and try something new: CoreOS' rkt.


The installation instructions are a bit hidden. This page refers to the script install-rkt.sh. Unfortunately, no link was given. A search on github finally gave the answer. However, in the end, I liked these instructions better: ask ubuntu: Is it possible to install rkt in Ubuntu?.

Because I wanted to run Syncthing and I only had a Docker recipe, I also needed the tool docker2aci. The instructions there are clear except that you need to install golang first (docs are fixed now):

$ sudo apt-get install golang $ git clone git://github.com/appc/docker2aci $ cd docker2aci $ ./build.sh

The docker2aci binary is located in the bin folder. Put it somewhere so you can find it back.

Building and converting a docker image

After building the docker image:

# docker build -t syncthing .

I had the following:

# docker images REPOSITORY TAG IMAGE ID CREATED SIZE syncthing latest 8ea0931f1196 29 hours ago 197.1 MB

Next step is to fetch the image into the rkt image repository. The rkt fetch command can fetch an image directly from a docker repository. However, I found no way to fetch directly from the local docker image repository. This is the work around:

# docker save -o syncthing-docker-image.tar syncthing # docker2aci syncthing-docker-image.tar # rkt --insecure-options=image fetch syncthing-latest.aci # rkt image list ID NAME SIZE IMPORT TIME LAST USED sha512-8161ad07a42e syncthing:latest 168MiB 7 hours ago 7 hours ago # rkt image cat-manifest syncthing:latest | less

Running the image

Now comes the time to run the image:

# rkt --insecure-options=image run --net=host \ --dns=$(awk '/nameserver/ {print $2}' < /etc/resolv.conf) \ --volume=volume-srv-config,kind=host,source=/media/nas/syncthing/config,readOnly=false \ --volume=volume-srv-data,kind=host,source=/media/nas/syncthing/data,readOnly=false \ syncthing

Creating that statement took actually longer then I expected. Here are the highlights:

  • Option --dns=... copies the nameserver from you local /etc/resolv.conf to the same file inside the container. This makes DNS work inside the container also. Depending on your DNS setup, you may need to pass more --dns* options.
  • The --net=host gives you the easiest access to the network. If you want a bit more security, you will have to dive deeper.
  • I also tried to add the options --user=nobody --group=nogroup. However, that resulted in my container not starting up at all with weird error messages.

You now have something like this:

# rkt list UUID APP IMAGE NAME STATE CREATED STARTED NETWORKS 17baa16c syncthing syncthing:latest running 1 minute ago 1 minute ago

Inspecting the container

Inspecting a running container is easy. With rkt enter you can directly open a shell in the container:

# rkt enter 17baa16c enter: no command specified, assuming "/bin/bash" root@rkt-5e9ad759-82b4-4b27-b03a-b6b5074b2ac2:/#


Every time you start a new container, the old one stays around. With rkt gc you cleanup containers that stopped some time ago (more then half an hour?). This command should be run from cron:

# echo -e '#!/bin/sh\nexec rkt gc' > /etc/cron.daily/rkt-gc # chmod +x /etc/cron.daily/rkt-gc


As a Shorewall user you need to add a rule to open the ports for the syncthing application in /etc/shorewall/rules and that's it. This might get a lot more hairy when you are not using host networking.

Automating startup

Modern Ubuntu's use systemd to start applications. Rkt's systemd manual seems well written but can be used as an introduction as best. Their suggestion to use systemd-run failed me:

# systemd-run --slice=machine /usr/bin/rkt --insecure-options=image run --net=host --dns= --volume=volume-srv-config,kind=host,source=/media/nas/syncthing/config,readOnly=false --volume=volume-srv-data,kind=host,source=/media/nas/syncthing/data,readOnly=false syncthing Failed to start transient service unit: Cannot set property ExecStart, or unknown property.

Also, it fails to mention where to put the systemd unit file. After lots of reading (in particular the systemd section of this manual) I created /etc/systemd/system/rkt-syncthing.service with the following content:

[Unit] Description=Rkt syncthing Requires=remote-fs.target After=remote-fs.target [Service] Slice=machine.slice ExecStart=/usr/bin/rkt --insecure-options=image run --net=host --dns= --volume=volume-srv-config,kind=host,source=/media/nas/syncthing/config,readOnly=false --volume=volume-srv-data,kind=host,source=/media/nas/syncthing/data,readOnly=false syncthing KillMode=mixed Restart=always [Install] WantedBy=multi-user.target

And finally:

# systemctl daemon-reload # systemctl enable syncthing.service # systemctl start syncthing.service

Ending thoughts

Although I got far in a few hours there are still a few open problems.

  1. None of the above software was available as a Ubuntu package. This results in me spending more time to keep everything up to date.
  2. Rkt's documentation is okay but not amazing. For example, hyperlinks are missing in crucial places (see above), and in the manual page for rkt-export does not tell you how to indicate which container you want to export.
  3. Rkt containers run from an image and store changes as a layer on top. As soon the container exists, you can not start it again with those changes. This means that everything that needs to be persisted must be external to the container. For other changes you can run rkt export to create a new image, or you rebuild your image from scratch.
  4. Having to use Docker to build an image for rkt is a bit weird. Next step is to create the image directly with acbuild.


2017-02-05 Fixed commands for enabling rkt garbage collection via cron.

Thursday, March 3, 2016

Don’t call your state ‘state’

In the OO world you are frowned upon if you call something object. Its time to extend this principle: don’t call your state state. This post is about why this is a bad idea and what you can do about it.

Recently we sat down to discuss the new data model for our messaging system at eBay’s Classifieds Group. One of the things we inherited from the past is the entity called conversation with a field called state. Possible values were Ok, On hold and Blocked.

So what was the problem?
A field called ‘state’ almost always has a very intuitive meaning. Unfortunately, the word is so vague that the meaning can easily warp, depending on the problem at hand. I noticed this in a couple of projects: the state field started to collect more and more possible values. With more values came increasingly difficult state transitions. This lead to code that was way more messy then necessary.

For example, in our conversation entity we could introduce the state Closed to indicate that a participant wants to stop the conversation. Then we continue by adding the state Archived to indicate that the conversation should be hidden until a new messages arrive.

What can we do?
The key observation is that each state value represents multiple behaviors. Think about it, what behavior is needed in each state? How do these behaviors change for each state? These questions will lead you to multiple fields that can represent the entire state of your entities.

Within a couple of minutes we found three behaviors we wanted to have for our conversations: a conversation is either visible or not (field visibility with values Displayed and Hidden), it will accept new messages or not (field acceptNew with values Accept and Reject) and we want to notify the recipient of a new message (or not) (field notifyOnNew with values Notify and Mute).Not only did our code become easier to extend and reason about, as a bonus we found a feature that would have been really hard with the old model: muting a conversation.

Don’t call your state ‘state’, instead, think about the behavior each state represents and model that instead.

Sunday, February 14, 2016

Generate a certificate signing request (CSR) as a 1 liner

As I run my own (secure) web and mail server I frequently have to get certificates. You could run with a selfsigned certificate, but that is not ideal; desktop browsers are very noisy about them, and mobile browsers are outright hostile. Installing certificates on a mobile phone are no fun (though, hat of for CAdroid).

Now that Letsencrypt is live, I wanted to try it out. However, Letsencrypt does not make it easy for you to keep using the same private key. This is necessary as the Android's http client pins your certificates (the one I needed anyway). Luckily Letsencrypt allows you to use a CSR that is generated outside of their tools.

Letsencrypt certificates are only valid for a short time (90 days) so automation is key. Unfortunately, openssl does not make it easy to fully automate creating CSRs, especially when you need 'Alternative Names', a requirement from Letsencrypt.

Luckily I found a solution from Andrew Leahy. Here it is:

openssl req -new -nodes -sha256 -key private-key.pem \ -subj "/C=US/ST=CA/O=Acme, Inc./OU=Acme Example/CN=example.com" \ -reqexts SAN \ -config <(cat /etc/ssl/openssl.cnf <(printf "[SAN]\nsubjectAltName=DNS:example.com,DNS:www.example.com")) \ -out domain.csr

This will generate a CSR in file domain.csr with set country (C), state (ST), organization (O), organization unit (OU) and most importantly the common name (CN) and in addition two alternative names example.com and www.example.com. Thanks Andrew!