[cobalt-security] Re: syn_flood dos attack (Nico Meijer)

["Jordan Lowe" <jordan@xxxxxxxxxxxxxxxxx>]

Well, I tried using the iptables stuff- (echo 1 >
/proc/sys/net/ipv4/tcp_syncookies), and I still see the syn stuff, but I
think its working.  Apache hasn't crashed yet, which is great.  Thanks Nico,
I appreciate the info.

Thanks,
Jordan

--

Jordan Lowe
Server Central Network
(888) 875-4804 x255


Date: Tue, 01 Jan 2002 13:29:54 +0100
To: cobalt-security@xxxxxxxxxxxxxxx
From: Nico Meijer <nico.meijer@xxxxxxxxx>
Subject: Re: [cobalt-security] syn_flood dos attack
Reply-To: cobalt-security@xxxxxxxxxxxxxxx

Hi Jordan,

>I'm having a issue on an old raq XTR (yes, the ones that have been
>recalled) with multiple ip addresses attacking port 80 on the server.
>
>
>[root /etc]# netstat -n | grep SYN
>tcp        0      0 64.94.47.100:80         165.247.32.175:42938
SYN_RECV
>tcp        0      0 64.94.47.101:80         165.247.32.175:49098
SYN_RECV
>tcp        0      0 64.94.47.102:80         165.247.32.175:3868
SYN_RECV
>tcp        0      0 64.94.47.103:80         165.247.32.175:65292
SYN_RECV
>tcp        0      0 64.94.47.104:80         165.247.32.175:20280
SYN_RECV
>tcp        0      0 64.94.47.105:80         165.247.32.175:21241
SYN_RECV
>[SNIP]

Are there *many* more?

>Basically the attack goes all the way through each ip on the server
>(64.94.47.0/24) and locks up apache.

Hmmm... This has happened to a machine (non-RaQ) of mine aswell. All IPs
belong to broadband ISPs in either USA or Canada and the IPs are
unreachable (which would indeed indicate a SYN flood with spoofed IPs). On
this machine, the number of connections in SYN_RECV state are hardly ever
more than 20-30, so I can't really call it a 'flood'.

I have a limited number of IPs on that machine and apache keeps running
perfectly.

>Every time I block the attacking ip address on the firewall, the attacker
>find another machine to attack from.

Indeed.

>I know this is a firewall issue, but is there a way to stop this from
>happening on the server side?

Hardly a firewall issue, IIRC; it can be fixed within the kernel. Try this
as root:
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

But check the path first; this is from memory. If it works, add it to
rc.local.

Good luck... Nico


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Message: 2
Date: Tue, 1 Jan 2002 16:27:55 +0000
From: Nick Drage <nickd@xxxxxxxxx>
To: cobalt-security@xxxxxxxxxxxxxxx
Subject: Re: [cobalt-security] syn_flood dos attack
Reply-To: cobalt-security@xxxxxxxxxxxxxxx

On Tue, Jan 01, 2002 at 01:29:54PM +0100, Nico Meijer wrote:
> >I'm having a issue on an old raq XTR (yes, the ones that have been
> >recalled) with multiple ip addresses attacking port 80 on the server.

As has been pointed out elsewhere, do ensure that this is a genuine SYN
flood, rather than an annoyance.

If there isn't an absolute barrage of packets, it could be a broken
router, firewall or proxy server ( especially as its port 80 ) in the
way.  The broken device is sending the SYN to you, you're replying with
an ACK, which at some point is then incorrectly dropped or misrouted on
the way back; so you get a kind of gentle SYN flood effect.

--
Nick Drage - Security Architecture - Demon Internet
"A lonely voice
 Echoing through the wilderness
 Request Timed Out"


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