Date: Wed, 8 Aug 2001 13:15:31 +0800 From: "David Xu" <bsddiy@163.net> To: "Christopher Ellwood" <chris+freebsd-net@silicon.net>, <freebsd-net@freebsd.org> Subject: Re: Problem with Code Red II and HTTP Accept Filtering Message-ID: <004401c11fc9$25a08950$6201a8c0@William> References: <20010807213844.N672-100000@diamond>
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my opinion is don't use accept filter, it can become DOS attack target. sending a big http header and don't complete it, it does not let apache know a connection is already made and there is no timeout counter like which in Apache server. using an accept filter can not get so much benifit. -- David Xu ----- Original Message ----- From: "Christopher Ellwood" <chris+freebsd-net@silicon.net> To: <freebsd-net@freebsd.org> Sent: Wednesday, August 08, 2001 12:42 PM Subject: Problem with Code Red II and HTTP Accept Filtering > The Code Red II worm seems to have a negative impact on FreeBSD machines > with HTTP Accept Filtering enabled either statically in the kernel or via > modules. > > The man page for accf_http states that: > > It prevents the application from receiving the connected descriptor via > accept() until either a full HTTP/1.0 or HTTP/1.1 HEAD or GET request has > been buffered by the kernel. > > What seems to be happening is Code Red II sends its 3.8K malformed > request, but the accept filter doesn't recognize this request as being > completed. So the connection sits in the established state with 3818 > bytes in the Receive Queue as shown in the following netstat: > > Proto Recv-Q Send-Q Local Address Foreign Address (state) > tcp4 3818 0 10.1.1.1.80 64.1.1.1.2932 ESTABLISHED > > If you get enough of these (about 20-30 on a machine with NMBCLUSTERS set > to 1024), your mbuf cluster pool becomes exhausted and network > transactions begin to fail. > > This inadvertent side affect of the Code Red worm suggests that it would > also be relatively easy to launch a denial of service attack against a > machine with HTTP accept filtering. > > This was observed on FreeBSD 4.3-RELEASE machine running both Apache > 1.3.19 and 1.3.20. > > Regards, > > - Christopher Ellwood > Network Security Consultant > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-net" in the body of the message To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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