[cod] Query limiting...

Steven Hartland killing at multiplay.co.uk
Mon Aug 1 19:13:45 EDT 2011

This is cool! I reported just this to TTimo about ETQW not two months ago.

Even with the small number of ETQW servers still running the DoS attacks
we saw where easily saturating a 1Gbps uplink.


----- Original Message ----- 
From: "Ryan C. Gordon" <icculus at icculus.org>
To: <cod at icculus.org>
Sent: Monday, August 01, 2011 9:54 PM
Subject: [cod] Query limiting...

> So we're getting reports of DDoS attacks, where botnets will send 
> infostring queries to COD4 dedicated servers as fast as possible with 
> spoofed addresses. They send a small UDP packet, and the server replies 
> with a larger packet to the faked address. Multiply this by however fast 
> you can stuff UDP packets into the server's incoming packet buffer per 
> frame, times 7500+ public COD4 servers, and you can really bring a 
> victim to its knees with a serious flood of unwanted packets.
> I've got a patch for COD4 for this, and I need admins to test it before 
> I make an official release.
>    http://treefort.icculus.org/cod/cod4-lnxsrv-query-limit-test.tar.bz2
> You'll need a server updated to 1.7 before applying this, because this 
> is only a replacement cod4_lnxded file. The defaults for the new cvars 
> are probably fine, but you can tweak them as you like.
> If you want to see it in action, find your patched server in the in-game 
> server browser, click "Server Info" and keep hitting refresh. If you're 
> doing it faster than the limit, you'll see you don't get a response 
> right away.
> All this info is in README.linux in that tarball, but I'll post it here, 
> too:
> *******
> About query limits:
> There is a class of DDoS attack that can utilize COD4 servers to flood a 
> third party, by spoofing UDP packets so that the game server sends its 
> reply for information to an unsuspecting party, over and over, as fast 
> as it can. Unlike most packets sent by the server, this reply packet 
> does not require a player with a legitimate connection before sending.
> This patch sets up some reasonable defaults to limit the rate at which 
> the server will send these reply packets to a given IP address. It does 
> not throttle legitimate connections in the process.
> The gist is this: If someone sends a query packet, we note their IP 
> address and ignore any further queries for X seconds. If they send a 
> port-unreachable packet (person being spoofed isn't playing the game), 
> we ignore their IP address for Y seconds. This will let normal people 
> play, it prevents people that don't have a legitimate player connection 
> from flooding the server with queries, and it'll stop DDoS attacks 
> against third parties.
> If millions of computers try to wail on a single server, this patch 
> should handle it gracefully (we don't allocate memory when adding IP 
> addresses to the ignore list, we use a hashtable so we don't have to 
> check millions of IP addresses for every query, and we clean out old 
> addresses a little each frame).
> People that are connected to the server don't have their packets 
> ignored; this only limits server responses to packets that don't need a 
> valid player connection (like the infostring, etc). LAN addresses are 
> never limited.
> Admins can tune it (and turn it off completely) with cvars.
> Note that a single IP address using different ports all get lumped into 
> the same ignore list entry. This is to make life difficult for attackers 
> and keep things efficient on the server. If 16 players are all behind 
> the same NAT, they might notice it takes longer for the server to 
> respond to each of them individually at the start. They should run a LAN 
> server anyhow, but gameplay will still perform normally here.
> Server admins have 4 cvars to control this:
> sv_queryIgnoreDebug: set to 1 to log information about attacks. This 
> will write out a lot of logging. Defaults to 0.
> sv_queryIgnoreMegs: Number of megabytes we should use to store the 
> ignore list. Set to 0 to turn off the ignore list (basically disables 
> this patch). 1 megabyte handles about 65000 IP addresses, each megabyte 
> after the first adds about 87000 more. 1 is probably fine unless you're 
> under serious attack, but maybe a server wants to spare 12 megs to block 
> a million IP addresses simultaneously.  :)  Defaults to 1.
> sv_queryIgnoreTime: Number of milliseconds to ignore an IP address's 
> info requests after responding to one of them. Set to 0 to not ignore at 
> all. Defaults to 2000 (2 seconds).
> sv_queryBounceIgnoreTime: Number of milliseconds to ignore an IP 
> address's info requests after a server packet bounced with an ICMP Port 
> Unreachable notice. Set to 0 to not ignore at all. Defaults to 12000 (2 
> _minutes_).
> *******
> Please note that I haven't touched this code since 2008, so even though 
> the changes are relatively localized, don't blast this out to all your 
> servers until you feel it's stable.
> Please give me feedback!
> Thanks,
> --ryan.
> _______________________________________________
> cod mailing list
> cod at icculus.org
> http://icculus.org/mailman/listinfo/cod

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