Skip to main content

Using varnish in front of a webhosting canada or other shared hosting site? Pay attention to your probe!

I've had a varnish installation protecting http://www.fairvote.ca -- a wordpress site hosted at webhosting canada (whc.ca) -- for about a year. Last Friday in the evening, it started spitting out 503 errors, for no obvious reason. I spent a few hours yesterday in vain trying to get the hosts to tell me what, if anything might have changed at their end.

What I eventually figured out is that they changed how their servers respond to http requests without a domain - previously it was a 200, and it became something else (a redirect, I believe). That's not unreasonable (though relatively pointless to change at this stage, I would imagine), except that, by default, varnish probes its backends with requests that don't have a domain. What that meant is that varnish started thinking that the backend was broken, so stopped talking to it (even though it was perfectly capable of doing so).

The fix was just to add a customized probe that included a valid domain to the request.

Popular posts from this blog

Confused by online payment processing? You're not alone.

In the old days during "polite" conversation, it was considered rude to talk about sex, politics, religion and money. You might think we're done with taboos, we're not (and I'll leave Steven Pinker to make the general argument about that, as he does so well in The Better Angels of Our Nature).

The taboo I'm wrestling with is about money - not how much you make, but about online payment processing, how it works, and what it costs. In this case, I think the taboo exists mainly because of the stakes at hand (i.e. lots of money) and the fact that most of those who are involved don't get much out of explaining how it really works - i.e. the more nuanced communications are overwhelmed by sales-driven messaging, and the nuanced stuff is either proprietary secrets or likely to get slapped down by the sales department.

In other words, if you want to really understand about online payment processing because you want to decide between one system and another, you'…

Me and varnish win against a DDOS attack.

This past month one of my servers experienced her first DDOS - a distributed denial of service attack. A denial of service attack (or DOS) just means an attempt to shut down an internet-based service by overwhelming it with requests. A simple DOS attack is usually relatively easy to deal with using the standard linux firewall called iptables.  The way iptables works is by filtering the traffic based on the incoming request source (i.e., the IP of the attacking machine). The attacking machine's IP can be added into your custom ip tables 'blacklist' to block all traffic from it, and it's quite scalable so the only thing that can be overwhelmed is your actual internet connection, which is hard to do.

The reason a distributed DOS is harder is because the attack is distributed from multiple machines. I first noticed an increase in my traffic about a day after it had started - it wasn't slowing down my machine, but it did show up as a spike in traffic. I quickly saw that…

IATS and CiviCRM

Update, Nov 2009: I've just discovered and fixed a bug I introduced in the 2.2 branch for the IATS plugin. The bug was introduced when i updated the API files from IATS and failed to notice that the legacy method for C$ one-time donations was no longer supported.
If you're using a version greater than or equal to 2.2.7, and are using IATS for C$, non-recurring donations, then you're affected.
To fix it edit the file : CRM/Core/Payment/IATS.php, and remove the line that looks like this:

$canDollar = ($params['currencyID'] == 'CAD'); //define currency type The full fix removes a conditional branch based on that value a little further on, but by removing this line, it'll never actually use that branch. Drop me a line if you have any questions.
Update, May 2009: This post is still getting quite a bit of traffic, which is great. Here are a few important things to note:
The IATS plugin code is in CiviCRM, you don't need to add any code.You do still …