Date: Sun, 3 Dec 2000 19:28:02 -0500 From: spectre <ai32@drexel.edu> To: Holtor <holtor@yahoo.com> Cc: freebsd-security@freebsd.org Subject: Re: Rate Limiting syn-ack's Message-ID: <20001203192802.A3502@reddog.yi.org> In-Reply-To: <20001203012802.25514.qmail@web116.yahoomail.com>; from holtor@yahoo.com on Sat, Dec 02, 2000 at 05:28:02PM -0800 References: <20001203012802.25514.qmail@web116.yahoomail.com>
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On Sat, Dec 02, 2000 at 05:28:02PM -0800, Holtor wrote:
> Hi all,
>
> Is there anyway I can limit outgoing syn-ack packets
> my computer sends? I had a large syn flood which was
> about 7 mbps incomming. The server also sent 7 mbps
> outgoing to reply to those syn's. How can i stop that
> or somehow rate limit to maybe 500 kbps or 1 mbps?
>
> I'm not able to find an option to do this using ipfw
> and/or dummynet.
>
> Thanks.
>
> Holt
Hello,
I think you should be looking at denying denying incoming
SYN packets instead of denying outoing SYN+ACK.
There's lots of discussion going on about preventing SYN
flooding and the general class thereof that's meant to
consume network resources (look at CERT CA-2000-21).
Basically the question comes down to: how to distinguish the
valid SYNs from the invalid ones. And I for one don't
know of a way to do this.
What you *might* look into is something like:
ipfw add pipe 10 tcp from any to any in setup
ipfw pipe 10 config bw 1Mbit/s queue 150KBytes
or if you could have a service that looks at how much
traffic (SYNs) you are getting and then adds rules
like:
ipfw add pipe 20 tcp from any to any in setup
ipfw pipe 10 config bw 5Mbit/s queue 150Kbytes plr 0.03
If you *must* doing outoing SYN-ACK, then just look
at the first example given, and replace 'in' with
'out', and 'setup' with 'tcpflags syn,ack'.
Again the problem is, how do you limit those 1Mbit/s
incoming SYNs to _valid_ ones. I don't know of any good
way. Perhaps you can look at this as another form of a
bandwidth saturation attack, which, there really is no
defense against without the help of your ISP.
P.S. I thought this off the top of my head, so consule
man ipfw, and note that this doesn't handle
fragments.
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