[thelist] Tip: Error 412 for sites on TextDrive

Ian Anderson ian at zstudio.co.uk
Thu Feb 16 12:20:59 CST 2006

Hi All,

this was causing a couple of people trouble when trying to add code to 
Snippetz.net recently, and it may be of use to others, so...

<tip type="Precondition failed errors on TextDrive" author="Ian Anderson">
Users may sometimes receive terse error messages when interacting with 
sites hosted on TextDrive (and, perhaps, other ISPs that use mod_security)

e.g. "The precondition on the request for the URL /foo.php evaluated to 
false" or HTTP error 412.

This can happen in at least two situations, it seems.

1. From [1]:
"TextDrive uses mod_security to block comment and referrer spam, but 
sometimes it gets triggered by mistake. This can happen if something on 
your site (like a URL) matches a common pattern (like ”-poker”, 
”-pictures”, or ”-sex”)."

In addition to the cases quoted here, it seems other suspicious strings 
may trigger mod_security to prevent the HTTP request from being 
permitted. In one case, it was the presence of "../" being present in a 
snippet that someone was trying to add through a form submission.

We've followed the advice at [1] to prevent this happening again, 
because in our situation users must be able to use such strings in their 
snippets. We spent some time pulling our code apart to absolutely no 
purpose, since as it turned out the form submission didn't even get as 
far as our PHP code before the HTTP request was rejected.

2. Kerim Friedman writes [2] of being prevented from seeing certain 
sites hosted on TextDrive at all, and of the reason being found to be 
that his (dynamically assigned) IP address had been added to a blacklist 
that TextDrive check when HTTP requests enter their network. On 
appealing to TextDrive, they removed the block. The symptom seems to 
have been identical to the previous issue, though.

[1] http://help.textdrive.com/index.php?pg=kb.page&id=85
[2] http://keywords.oxus.net/archives/2005/05/10/precondition-failed/

Hope this helps others perplexed by this error message.

zStudio - Web development and accessibility

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