From owner-freebsd-net@FreeBSD.ORG Tue Jun 24 19:03:36 2014 Return-Path: Delivered-To: freebsd-net@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 6A80251F for ; Tue, 24 Jun 2014 19:03:36 +0000 (UTC) Received: from zibbi.meraka.csir.co.za (zibbi.meraka.csir.co.za [IPv6:2001:4200:7000:2::1]) by mx1.freebsd.org (Postfix) with ESMTP id 0F91C2D40 for ; Tue, 24 Jun 2014 19:03:36 +0000 (UTC) Received: by zibbi.meraka.csir.co.za (Postfix, from userid 3973) id C8039B843; Tue, 24 Jun 2014 21:03:33 +0200 (SAST) Date: Tue, 24 Jun 2014 21:03:33 +0200 From: John Hay To: Andrea Venturoli Subject: Re: MTU not regrowing? Message-ID: <20140624190333.GA13748@zibbi.meraka.csir.co.za> References: <53A9C6D0.3090900@netfence.it> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53A9C6D0.3090900@netfence.it> User-Agent: Mutt/1.5.21 (2010-09-15) Cc: freebsd-net@freebsd.org X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 24 Jun 2014 19:03:36 -0000 On Tue, Jun 24, 2014 at 08:43:28PM +0200, Andrea Venturoli wrote: > Hello. > > Today I experienced something weird (at least for me) on a 8.4 system: > > _ the system had vlan3 interface, with default MTU (1500 bytes); > _ "ping -D -s 1400 somehost" would work, but "ping -D -s 1500 somehost" > would yield "frag needed and DF set" (forgive me if the message is not > exact, I don't have it anymore); > > _ to make some tests I reduced MTU size with "ifconfig vlan3 mtu 500"; > _ now, of course, "ping -D -s 400 somehost" would work, but "ping -D -s > 500 somehost" would yield "frag needed and DF set"; > > _ then I raised MTU again with "ifconfig vlan3 mtu 1500" (notice > ifconfig would actually report this as "mtu 1500" was shown); > _ however the results were as before, i.e. "ping -D -s 400 somehost" > would work, but "ping -D -s 500 somehost" would yield "frag needed and > DF set"; > > _ no way I could ping with a packet bigger than 500 bytes until I rebooted. > > Is this expected behaviour? Any way to get around this? Do a "route get somehost" and see what mtu is returned. You might be able to delete or tweak that route. John -- John Hay -- jhay@meraka.csir.co.za / jhay@meraka.org.za