Brussels / 31 January & 1 February 2015


Interview with Holger Levsen
Stretching out for trustworthy reproducible builds. The status of reproducing byte-for-byte identical binary packages from a given source

Holger Levsen will give a talk about Stretching out for trustworthy reproducible builds. The status of reproducing byte-for-byte identical binary packages from a given source at FOSDEM 2015.
Q: Could you briefly introduce yourself?

My name is Holger Levsen and I’m part of the teams maintaining,, and now

Obviously I do other stuff as well, both in and out Debian, but the above is probably my current focus.

Q: What will your talk be about, exactly? Why is this topic important?

Reproducible builds, the subject of the talk, is about re-creating identical binaries from a given source in a defined environment. With reproducible builds one can be sure that these sources were used to build that binary.

I actually consider this to be an essential part of the “old” promise of free software, ie. that we know and control the software we are running. It is somewhat peculiar that it was only recently that this has started to be considered achievable.

Q: What do you hope to accomplish by giving this talk? What do you expect?

I have been quite surprised by the amount of progress we made for such a young project. I want to share this excitement and also tell the world that reproducible builds are doable, even for a huge software collection like Debian. I think it would be great if developers of many other free software projects — especially other distros — would become enthusiastic about this idea as well so that reproducible builds become the norm one day. I certainly think there are many aspects and lessons we have learned which can be applied elsewhere.

Within the Debian community this project has attracted a lot of unforeseen support already even though it’s currently just an experiment to prove that it’s even possible. We hope the talk will help to shape the goals for Debian “stretch” — aka Debian 9—the release after the upcoming Debian 8 “jessie” release.

So, this is not at all only about Debian — it’s about reliability in all software development and deployment!

Q: What’s the history of the Debian Reproducible Builds project? Why was it started and how did it evolve?

The idea of reproducible builds is far from new. It was first mentioned on the Debian development mailing list in 2000 and then again in 2007 and more recently it was picked up again and successfully realised for the Bitcoin client and the Tor Browser. These successes inspired the current work within Debian which only started just over a year ago. The first presentation happened at the previous FOSDEM.

This is a very short version of the history which omits many important points, especially how this would not be possible if Debian could not be rebuilt so easily. We are standing on the shoulders of giants!

Q: What infrastructure is the Debian Reproducible Builds project using? has been used since 2012 for continuous QA testing of several parts of Debian. In autumn 2014 I extended it to attempt a reproducible build for every package in the development branch of Debian and to output some very simple web pages as a result. Today, we have a rather more sophisticated website - for example, we track progress for large package sets, include notes about investigated packages and many other aspects. We have also introduced variations of time, file ordering, user, group, hostname, locale, etc. into the rebuild process.

In other words, we are not currently using any official Debian infrastructure for our experiment. Hopefully after the release of Jessie our changes will be included in the normal toolchain so that then all regular Debian builds will support this.

Once the infrastructure supports it we would like build reproducibility to become mandatory for Debian packages. Personally, I have high hopes for 2015 already, but others in the team would like me to be more modest ;)

Q: What are the biggest challenges in making all Debian packages reproducible?

Debian is huge. It currently contains more than 21,000 source packages and more than a thousand people are actively maintaining them. Testing, understanding, and patching so many different pieces of software is a big challenge. Working with so many different developers, different people, different worlds is another..

Q: Looking at the common issues for reproducibility, it’s clear that timestamps, e.g. generated in documentation files, are the biggest source of issues. What’s the general approach to fix this?

Don’t use build timestamps. Either drop timestamps completely or somehow use some last modification date of the source which will stay the same on rebuilds.

Q: When will it be possible for users to be sure that the complete Debian GNU/Linux installation medium they’re using is reproducible from the source?

My crystal ball is a bit blurry at the moment. The first step is to ensure every binary package we ship can be reproducibly built and we are tracking specific sets of packages in order to better target our efforts here. Then, installation media needs to be reproducible, which might be another challenge altogether. Finally, to make “live” systems reproducible we will encounter yet another challenge: the installation itself should be reproducible.

I certainly hope that most packages that will make up Debian 9 can be rebuilt reproducibly by individuals. Of course, I’d like that percentage to be 100% but whether that remains a dream depends on the efforts of many people.

Q: How can interested people contribute to the Debian Reproducible Builds project?

First, I’d suggest they read through our wiki which will provide a good overview about the current status of the project. Maintainers of Debian packages can easily check whether their packages can be built reproducibly today by going to$pkg. And for anybody interested in giving a hand, we have a nice “how to help” section as well!

Q: Have you enjoyed previous FOSDEM editions?

Yes, a lot! The reception of the Lunar’s initial talk about reproducible builds in 2014 has helped the project tremendously to gain momentum which we have managed to keep going until today. Let’s hope it’s going to be even better this year!

Creative Commons License
Creative Commons License

This interview is licensed under a Creative Commons Attribution 2.0 Belgium License.