From owner-freebsd-questions@FreeBSD.ORG Wed Oct 31 13:27:17 2007 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 29ACC16A468 for ; Wed, 31 Oct 2007 13:27:17 +0000 (UTC) (envelope-from nvass@teledomenet.gr) Received: from smtp.teledomenet.gr (smtp.teledomenet.gr [213.142.128.2]) by mx1.freebsd.org (Postfix) with ESMTP id D4C1513C48E for ; Wed, 31 Oct 2007 13:27:16 +0000 (UTC) (envelope-from nvass@teledomenet.gr) Received: by smtp.teledomenet.gr (Postfix, from userid 58) id 3FAD1142236; Wed, 31 Oct 2007 15:20:35 +0200 (EET) X-Spam-Checker-Version: SpamAssassin 3.2.3 (2007-08-08) on smtp.teledomenet.gr X-Spam-Level: X-Spam-Status: No, score=-2.6 required=5.0 tests=BAYES_00 autolearn=ham version=3.2.3 Received: from iris (unknown [192.168.1.71]) by smtp.teledomenet.gr (Postfix) with ESMTP id BA8D514222A; Wed, 31 Oct 2007 15:20:32 +0200 (EET) From: Nikos Vassiliadis To: freebsd-questions@freebsd.org Date: Wed, 31 Oct 2007 15:22:44 +0200 User-Agent: KMail/1.9.7 References: <47285AF7.4060905@boudrand.net> In-Reply-To: <47285AF7.4060905@boudrand.net> X-NCC-RegID: gr.telehouse MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200710311522.45352.nvass@teledomenet.gr> Cc: Eric Boudrand Subject: Re: ICMP and discard oversize frame X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 31 Oct 2007 13:27:17 -0000 On Wednesday 31 October 2007 12:37:43 Eric Boudrand wrote: > Hello, > > I am running a FreeBSD router with two ethernet cards. I have reduced > the MTU to 800 in order to generate ICMP packet "Fragmentation needed > but DF was set". I can find in log : > > rl1: discard oversize frame (ether type 800 flag 3 len 875 > max 814) > > But there is no ICMP sent. By default, is this feature disabled ? How > can I see these ICMP packets generated ? It's discarded before IP processing, the kernel knows nothing about the IPs involved in that packet(well, actually ethernet frame). MTU must be set equally across your link layer... It *will* send a fragmentation needed message if a received packet is to be forwarded to a link with smaller MTU, not big enough for the packet... That's not the same case with the one you've posted. So, long story short, FreeBSD sends fragment. needed messages. But, this is not the correct way to test such a situation. HTH Nikos