From owner-freebsd-net Wed Jan 15 6:38:55 2003 Delivered-To: freebsd-net@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 083D837B401 for ; Wed, 15 Jan 2003 06:38:54 -0800 (PST) Received: from mailtoaster1.pipeline.ch (mailtoaster1.pipeline.ch [62.48.0.70]) by mx1.FreeBSD.org (Postfix) with SMTP id AFF1943F6B for ; Wed, 15 Jan 2003 06:38:52 -0800 (PST) (envelope-from oppermann@pipeline.ch) Received: (qmail 90342 invoked from network); 15 Jan 2003 14:37:56 -0000 Received: from unknown (HELO pipeline.ch) ([62.48.0.53]) (envelope-sender ) by mailtoaster1.pipeline.ch (qmail-ldap-1.03) with SMTP for ; 15 Jan 2003 14:37:56 -0000 Message-ID: <3E2571EC.339F829F@pipeline.ch> Date: Wed, 15 Jan 2003 15:36:28 +0100 From: Andre Oppermann X-Mailer: Mozilla 4.76 [en] (Windows NT 5.0; U) X-Accept-Language: en MIME-Version: 1.0 To: "Louis A. Mamakos" Cc: Josh Brooks , freebsd-net@FreeBSD.ORG Subject: Re: ipfw: blocking syn floods - two proposed rules References: <20030114212944.A39623-100000@mail.econolodgetulsa.com> <200301151426.h0FEQS4E027966@whizzo.transsys.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-freebsd-net@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.org "Louis A. Mamakos" wrote: > > > > > My goal is to create an ipfw rule that stops normal syn floods by blocking > > ALL syn packets that have no MSS set. > > > > My understanding is that there is no legitimate packet that is a SYN and > > has no MSS, and further, most of the kiddie tools in existence for syn > > flooding do indeed send syn packets with no MSS. > > Strictly speaking, a TCP stack is not REQUIRED to include an MSS option > on the TCP SYN segment. It's the only time one can be specified, but > if the TCP is happy with the 536 byte default, it needn't bother. > > Even older versions of the 4.3BSD-based TCP/IP stack had this issue, > and didn't include an MSS option if the interface MTU was sufficiently > small. > > In practice, I'm not sure how much of an issue this might be these > days, but you should probably check to see if really see NO legitimate > connections before you really start filtering. In a recent study my diploma students found that out of a dataset of 9 million TCP SYN in real life traffic (Sunsite Switzerland, five popular newspaper sites) approximatly 5% did not have the MSS option set. We did not manage to figure the OS of those SYN packets. -- Andre To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message