slowloris, accf_http and POST requests

Ruben de Groot mail25 at
Mon Jun 22 17:15:25 UTC 2009

On Mon, Jun 22, 2009 at 08:45:23AM -0700, Norbert Papke typed:
> On June 22, 2009, Ruben de Groot wrote:
> > Can enybody explain why the http accept filter only works on GET/HEAD
> > requests?
> >
> > The reason I ask is I was checking up on the slowloris DOS tool
> > ( and, like others before me,
> > found that the -httpready switch (which uses POST instead of GET) renders
> > the accf_http module useless as a protection against this kind of attack.
> With the POST request, the client sends additional data after the header.  
> This additonal data is the form data (the x-www-form-urlencoded encoded 
> name-value pairs).  The filter will allow the request to proceed to the 
> application after the header as been received but before the form data has 
> been received.
> A "slowloris" attack could exploit this fact by sending a complete header but 
> then slowing doling out the form data.

Apparently, the current incarnation of the slowloris script doesn't do that, 
so adding POST to the methods handled by the http accept filter would 
protect me from script kiddies who want to attack my servers by this method.

My main concern here is if applying the trivial patch I posted would break
anything in the http protocol layer. And if not, why isn't the POST method
included in the http accept filter in the first place?

> To protect against this scenario, the filter would need to be modified to 
> collect the form data as well.  Of course, it doesn't stop there.  The filter 
> would also have to deal with multi-part forms.

Yes. It's an ongoing struggle. And the filter would probably soon become
too complex to maintain in a kernel module :(

> Disclaimer: This is based on cursory reading of the code.

Thank you for you input.


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