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Date: Thu, 05 Jan 2012 10:06:51 -0700
From: Kurt Seifried <>
CC: David Hicks <>
Subject: Re: speaking of DoS, openssh and dropbear (CVE-2006-1206)

On 01/05/2012 04:22 AM, David Hicks wrote:
> The question these approaches raise is whether it is advisable to
> reinvent rate limiting in each and every network daemon. Performing rate
> limiting at the system/interface level prevents unwanted and expensive
> context switches to each daemon. Configuration and maintenance is much
> simpler because administrators don't need to learn 50 different ways to
> configure rate limiting for each daemon. There is also less risk for
> bugs to be written into the rate limiting implementation of each daemon.

To a large degree yes, because each daemon is different and the daemon
also knows how bad things are getting, the firewall doesn't. Ideally the
daemons should be auto-tuning and degrading politely to prevent
dying/killing the system/etc. so the admin doesn't have to explicitly
learn how to tune it. THiss problem occurs with firewall limiting
anyways, how many connections per second per C class (or whatever) can I
safely allow to daemon X? What happens if we upgrade/downgrade the
server daemon X runs on? What happens if server X takes additional
duties or is modified in another way that affects the load it can
handle? I vote for daemons that auto-tune intelligently because I am
lazy =).

> On a technical note, rate limiting requires a small amount of memory
> (buckets) to store information about recent connections. For this
> reason, allowing IPv6 rate limiting granularity at the /128 level would
> be inadvisable as an attacker with /64 addresses could quickly exhaust
> the table capacity/available memory. The design of the data structures
> and algorithms for the table need to be very efficient. Taking it down
> another level, a table that is larger than available L1-L3 cache could
> further degrade performance ([4] and [5] discuss hash tables and CPU
> cache).
Again we could do something clever like auto-tune and start
consolidating buckets if the tables start getting too large.


-- Kurt Seifried / Red Hat Security Response Team

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