From owner-freebsd-questions Tue Oct 9 3:48:39 2001 Delivered-To: freebsd-questions@freebsd.org Received: from pintail.mail.pas.earthlink.net (pintail.mail.pas.earthlink.net [207.217.120.122]) by hub.freebsd.org (Postfix) with ESMTP id 35B9337B406 for ; Tue, 9 Oct 2001 03:48:34 -0700 (PDT) Received: from blossom.cjclark.org (dialup-209.247.139.131.Dial1.SanJose1.Level3.net [209.247.139.131]) by pintail.mail.pas.earthlink.net (EL-8_9_3_3/8.9.3) with ESMTP id DAA12805 for ; Tue, 9 Oct 2001 03:48:33 -0700 (PDT) Received: (from cjc@localhost) by blossom.cjclark.org (8.11.6/8.11.3) id f99AmWB09238 for freebsd-questions@FreeBSD.ORG; Tue, 9 Oct 2001 03:48:32 -0700 (PDT) (envelope-from cjc) Date: Tue, 9 Oct 2001 03:48:32 -0700 From: "Crist J. Clark" To: freebsd-questions@FreeBSD.ORG Subject: Re: Another firewall question - spoofing prevention and syntax Message-ID: <20011009034832.M350@blossom.cjclark.org> Reply-To: cjclark@alum.mit.edu References: <20011008233219.C589@acadia.ne.mediaone.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5i In-Reply-To: <20011008233219.C589@acadia.ne.mediaone.net>; from leblanc+freebsd@smtp.ne.mediaone.net on Mon, Oct 08, 2001 at 11:32:20PM -0400 Sender: owner-freebsd-questions@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG On Mon, Oct 08, 2001 at 11:32:20PM -0400, Louis LeBlanc wrote: > Another firewall question, not for the faint of heart: > > Is the following valid? > > # Refuse incoming packets pretending to be from the external address. > ipfw add deny log all from $IPADDR to any via (null) in > > # Refuse incoming packets claiming to be from a Class A, B or C > private network > ipfw add deny all from $CLASS_A to any via (null) in > ipfw add deny all from $CLASS_B to any via (null) in > ipfw add deny all from $CLASS_C to any via (null) in > > > I can't find any reference to the use of (null) as the interface name > to prevent spoofing, but the tool I use online does this > automagically. > > Any ideas? "(null)" is not a valid interface specification. However, # Refuse incoming packets pretending to be from the external address. ipfw add deny log all from $IPADDR to any in # Refuse incoming packets claiming to be from a Class A, B or C private network ipfw add deny all from $CLASS_A to any in ipfw add deny all from $CLASS_B to any in ipfw add deny all from $CLASS_C to any in Is perfectly vaild. -- Crist J. Clark cjclark@alum.mit.edu cjclark@jhu.edu cjc@freebsd.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message