[Haskell] Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package

Magnus Therning magnus at therning.org
Fri Aug 20 17:58:43 EDT 2010

On 20/08/10 22:32, John Millikin wrote:
> On Fri, Aug 20, 2010 at 12:52, Magnus Therning <magnus at therning.org> wrote:
>> You don't need to send that much data, the current implementation of
>> Enumerator uses hGet, which blocks, so just send the server a few bytes and
>> it'll be sitting there waiting for input until it times out (if ever).
>> Open a few hundred of those connections and you're likely to cause the
>> server to run out of FDs.  Of course this is already coded up in tools like
>> slowloris[1] :-)
> Correct me if I'm wrong, but I'm pretty sure changing the implementation to
> something non-blocking like hGetNonBlocking will not fix this. Hooking up an
> iteratee to an enumerator which doesn't block will cause it to loop forever,
> which is arguably worse than simply blocking.
> The best way I can think of to defeat a handle-exhaustion attack is to
> enforce a timeout on HTTP header parsing, using something like
> System.Timeout. This protects against slowloris, since requiring the
> entire header to be parsed within some fixed small period of time
> prevents the socket from being held open via slowly-trickled headers.


In many protocols it would force the attacker to send well-formed requests
though.  I think this is true for many text-based protocols like

The looping can be handled effectively through hWaitForInput.

There are also other reasons for doing non-blocking IO, not least that it
makes developing and manual testing a lot nicer.


Magnus Therning                        (OpenPGP: 0xAB4DFBA4)
magnus@therning.org           Jabber: magnus@therning.org
http://therning.org/magnus         identi.ca|twitter: magthe

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