Re: Linux 2.4.27 SECURITY BUG - TCP Local (probable Remote) Denial of Service

["David S. Miller" <davem () davemloft net>]

On Sat, 11 Sep 2004 20:45:43 -0600
"Wolfpaw - Dale Corse" <admin () wolfpaw net> wrote:

> As for it being an application bug - it may be one in Mysql not
> closing the sockets, but it is a Kernel Bug that allows CLOSE_WAIT
> sockets to clog up the connection queues, and cause a DOS conditions
> on other applications (such as Apache). Since most software used for
> denial of service is badly written (intentionally) to exploit the
> holes, the error should be fixed, not blamed on faulty software.

If the application doesn't close it's file descriptors there is
absolutely nothing the kernel can do about it.

It's a resource leak, plain and simple.

> That being said - below is a the proper description, and the code
> used to exploit it. Hope it helps. This version is not the one
> which invokes the CLOSE_WAIT state, but rather the TIME_WAIT one,
> I am not able to publish the source code for the CLOSE_WAIT bug.

There is nothing wrong with creating tons of TIME_WAIT sockets,
they simply time out after 60 seconds (unless hit by a RESET
packet or similar).  This is how TCP works.

> The log however clearly shows that a mysql descriptor is closed, 
> and then used immediately again by the socket call, which causes it 
> never to end up getting closed. Linux apparently has either no 
> timeout for CLOSE_WAIT, or it's a very very long one.. Either way 
> is a bad thing.

Please do us all a favor and learn how TCP works.

CLOSE_WAIT means simply that only one side of the TCP
connection has done a close.  Therefore the other end
stays open until that side closes as well.

There is no way to "time things out" or release the
state.