[cod] Query limiting...

Ryan C. Gordon icculus at icculus.org
Mon Aug 1 16:54:52 EDT 2011

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.


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, 


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 


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!


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