Archive for August, 2015

Cloudflare & Python’s urllib

TL;DR: Trying to diagnose why my copr builds were abruptly failing, I found an interesting thing: Cloudflare’s Browser Integrity Check apparently doesn’t like Python’s urllib sending requests.

The symptoms in Copr were weird: builds would try importing, and then fail with no log output. To me, trying to diagnose the problem made no sense – I could download the file just fine through Chrome, Firefox and wget, so I thought the issue was with copr. I was all set to file a bug with them, when I decided to look at what other people were building and see if URL importing worked for them.

Surprise surprise, it did. This clearly meant it was isolated to me and my server, so I held off on the bug filing until I could find out more. On my stuff things accessing Jenkins worked fine, so I tried other tools. Hurl.it said there were no issues accessing Jenkins, while RequestBin said… not much at all, I couldn’t get it to work with copr’s URL detection – copr wants the path to end with src.rpm, requestbin uses the path to determine which bin to route stuff to.

Next I looked at Jenkins behind an Nginx proxy. I found an extra Nginx option needed to be set to allow Jenkins CSRF to work properly, which was a plus. (I also found something on serving static files through nginx instead of jenkins, which is now part of my todo list.) I ran through steps on DO’s setting up Jenkins behind nginx tutorial just in case I had missed something, everything looked fine.

Without logs, I couldn’t really trace down what was happening, so I looked at using Github to host the SRPMs, since Github clearly worked. First thing I realized was that keeping 3 branches in an attempt to reduce the number of repos I spawned was a bad idea, and I should fix that. (and maybe follow the git flow strategy…) GitHub has a releases API, so I’ll be spending some time in the near future with it (also, streaming uploads for the 100MB+ pagespeed releases…) Or try the github-release script that someone wrote. Either way, I’d had enough for the night, and stopped.

Today I picked it back up, and took another look. Still thinking it was my server, I tried to isolate which component was failing by serving the src.rpm file on a different domain. Instead of downloading the file and sticking it in a folder, I symlinked a folder to the Jenkins’ workspace directory, and submitted a build. Surprisingly, it worked, so I took a look at my nginx logfile, which revealed that the user agent of what downloaded it was Python-urllib/1.17. That looked like something from the Python stdlib that I could try to replicate the issue with. Started up ipython, googled for python urrlib downfile, and found the urlretrieve function. Tried it against the alternate src.rpm URL, and it worked. Tried it against Jenkins directly, and it failed.

Now I could replicate the symptoms! Next step was to google “Cloudflare blocking urllib”, which suggested that the (lack of) headers was tripping Cloudflare’s Browser Integrity Check. Toggled it off in Cloudflare, and sure enough I could now get the src.rpm file through urllib.

Deciding I now had enough info to file a bug, I went to the Copr codebase to find the relevant line. Downloaded the latest release, unzipped it to grep the code, found the file and line, went to grab the line from the Git repo – and couldn’t find the line. Turns out between last week and this week, the bug was fixed, just not pushed to Copr.

So for now I’ll be regenerating all the missed versions of nginx with the integrity check disabled. Fingers crossed there’s nothing else wrong with Copr in the meantime…

 

 

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