Outage postmortem: Replacing Rust Buildbot’s outdated cert

At the end of the day on July 14th, 2015, the certificate that Rust’s buildbot slaves were using to communicate with the buildmaster expired. This broke things. The problem started at midnight on July 15th, and was only fully resolved at the end of July 16th. Much of the reason for this outage’s duration was that I was learning about Buildbot as I went along.

Here’s how the outage got resolved, just in case anyone (especially future-me) finds themself Googling a similar problem.


Dave Huseby pointed out the problem on IRC when the slaves that he runs were unable to connect to the buildmaster:

16:48:23 <&brson> edunham: dhuseby said this earlier <huseby> it seems like the verify=3 in the stunnel config is the problem
16:48:39 <&brson> if he changed 'verify' to some other value in the stunnel config it worked

A quick check fo the stunnel docs shows that verify=3 is the strictest setting, and will fail if the locally installed cert isn’t right. This supports the hypothesis that our cert might be expired. On the buildmaster, I found the cert and examined its metadata:

$ find . -type f -name "*.pem"
$ openssl x509 -noout -issuer -subject -dates -in certname.pem

On the old cert, the results contained:

$ openssl x509 -noout -issuer -subject -dates -in rust-bot-cert.pem
issuer= /
    O=Rust Project/
subject= /
    O=Rust Project/
notBefore=Jul 14 02:28:50 2012 GMT
notAfter=Jul 14 02:28:50 2015 GMT

This tells me that the cert was created in 2012 and had its expiry set for the seemingly distant future of 2015.

Make a New Cert

To determine whether the old key had a passphrase on it, go openssl rsa -check -in keyname.pem. It writes the private key to your terminal if there’s no password, or prompt for a passphrase if the key has one.

The stunnel docs give most of the relevant incantation. Since no file in our buildbot directory is named precisely stunnel.conf, make cert doesn’t quite work right. But it works fine to manually run a variant of the command given in the docs:

$ openssl req -new -x509 -days 3650 -nodes -out cert.pem -keyout key.pem

That prompted me for a variety of information, which I entered where applicable and left blank where it wasn’t. The metadata is primarily for the benefit of others verifying that a cert belongs to the correct person, which isn’t a relevant concern in our use case.

I then backed up the old key and cert (although they’re no longer usable, they contain a bunch of metadata that I didn’t know whether I’d need later) and moved the new key and cert to match the old ones’ original file names.

Finally, I updated the repository with the new certificate.

Make Buildbot spin up AMIs with the new cert

This was the tricky bit. Since the slave image does not pull updates to its copy of the Rust Buildbot github repo when it boots, the file had to be statically edited and then the AMIs re-saved. But Buildbot makes its instance requests based on AMI ID, and the IDs are unique to a particular image. So the workflow goes:

  • Figure out which AMI Buildbot will spin up for a given job
  • Spin up an instance of the AMI
  • Remote into it and manually update the cert
  • Save the instance into a new AMI, noting its ID
  • Update Buildbot’s configuration with the new ID

Figure out which AMI Buildbot will use

The AMI IDs used for spot requests are stored in /home/rustbuild/rust-buildbot/master/slave-list.txt on the buildmaster. From that file I determined that we only had 4 unique AMIs in use:

ami-b74fa1f3 -- windows
ami-7fd23e3b -- generic linux
ami-381e197d -- android
ami-dbac5f9f -- centos5 (builds snapshots that work with ancient Linux)

The slave list also told me that all requests would be made for instance type c3.2xlarge.

Spin up an instance of the AMI

Since there were only 4, I did this manually. If it was a recurring task or there had been more AMIs, I would have automated this part of the process.

In the AWS Console, go to EC2, then click the AMIs link under “Images” at the left.

Search for the AMI ID in the search box. Only one AMI is found, because IDs are unique. Then click the big blue Launch button up at the left.


The only “gotcha” in the ensuing 7-step process is making sure to put the instance into the correct security group. I spun my temporary instances up into the same group as a host I know I can get to from the Bastion server with my credentials, to reduce the number of steps I’d have to troubleshoot if they were difficult to access.

It also helps to tag the spot request with the name of the AMI it was created from, when processing several at once.

Remote into the instance and update the cert

For Linux-flavored instances, this just meant ssh rustbuild@ (using the instance’s public IP, visible in the main instances list) from the Bastion. I then found the old cert on the host, and verified that it matched the old cert on the buildmaster. Checking that the certs matched was as simple as running md5sum cert.pem on both and visually comparing the results, and reassured me that I was overwriting the correct file.

Getting into the Windows hosts requires using RDP after setting up an SSH tunnel to the bastion. Since I was on airport wifi at the time, I had a teammate stick the cert onto the Windows instances instead.

The cert that stunnel actually uses on a Windows host with our configurations actually lives at C:\Program Files (x86)\stunnel\cert.pem, not in the actual repo like on all the sensible operating systems. Although there exists a C:\bot\cert.pem, replacing it does not cause stunnel to connect successfully.

Save the instance into a new AMI

Check the box by the instance’s name in the EC2 instances list, then follow the menus around for Actions -> Image -> Create Image. Note the new AMI’s ID, and replace all instances of the old AMI’s ID with the new one in the buildmaster’s slave-list.txt.

Kick Buildbot a bit

After the AMIs and FreeBSD, Bitrig, and Mac builders all had the new cert, I restarted Buildbot on the buildmaster and reran its script for creating the stunnels. Althoug it didn’t gracefully pick up where it had left off on partially built pull requests, closing then re-opening the PRs caused it to notice them and resume building successfully.

Hopefully TaskCluster gets OSX support soon, so we can start switching off of Buildbot.

Prevent it from happening again

After I first published this post, Gerv pointed out that the correct final step would be “Add an alarm to the shared IT calendar for a month before the new cert expires”. In my case, the analog to that alarm is “Make sure we move away from Buildbot in less than a decade”. However, if you’re reading this post to solve a similar problem in an infrastructure that will still exist at the date of the cert’s expiry, you should automate a reminder so that you or your successor doesn’t get the same unpleasant surprise.