From owner-freebsd-net@freebsd.org Mon Mar 23 07:17:17 2020 Return-Path: Delivered-To: freebsd-net@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id DA2A027ABB4 for ; Mon, 23 Mar 2020 07:17:17 +0000 (UTC) (envelope-from vas@sibptus.ru) Received: from admin.sibptus.ru (admin.sibptus.ru [IPv6:2001:19f0:5001:21dc::10]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 48m5L46FYnz4F6q for ; Mon, 23 Mar 2020 07:17:16 +0000 (UTC) (envelope-from vas@sibptus.ru) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=sibptus.ru; s=20181118; h=In-Reply-To:Message-ID:Subject:To:From:Date; bh=fjhJf3e8DgEOZsWpUkopH8f3vZ8BsDrK+dMWbaCP9qY=; b=ac794zKXS+FeZtTOex16iw72kn L1G3DPf/uV/rDhhpbXsrPaBhcuuL8EBkPWvN24eliiXw1XMIWSJO/WPdVI9hsdtucprAA06Dk8AYd w9fZRh878Ah5BLBzPeNlX4nnwnnIvzwCj9bQRXphQBu5i1M/DPZqdiOyWPFhUGZrjQdI=; Received: from vas by admin.sibptus.ru with local (Exim 4.93.0.4 (FreeBSD)) (envelope-from ) id 1jGHKl-000E9c-FL for freebsd-net@freebsd.org; Mon, 23 Mar 2020 14:17:15 +0700 Date: Mon, 23 Mar 2020 14:17:15 +0700 From: Victor Sudakov To: freebsd-net@freebsd.org Subject: Re: IP MTU on gif and gre interfaces (with and without IPSec encryption) Message-ID: <20200323071715.GB54231@admin.sibptus.ru> References: <20200323050012.GA50490@admin.sibptus.ru> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="KsGdsel6WgEHnImy" Content-Disposition: inline In-Reply-To: <20200323050012.GA50490@admin.sibptus.ru> X-PGP-Key: http://admin.sibptus.ru/~vas/ X-PGP-Fingerprint: 10E3 1171 1273 E007 C2E9 3532 0DA4 F259 9B5E C634 X-Rspamd-Queue-Id: 48m5L46FYnz4F6q X-Spamd-Bar: -------- Authentication-Results: mx1.freebsd.org; dkim=pass header.d=sibptus.ru header.s=20181118 header.b=ac794zKX; dmarc=pass (policy=none) header.from=sibptus.ru; spf=pass (mx1.freebsd.org: domain of vas@sibptus.ru designates 2001:19f0:5001:21dc::10 as permitted sender) smtp.mailfrom=vas@sibptus.ru X-Spamd-Result: default: False [-8.46 / 15.00]; ARC_NA(0.00)[]; NEURAL_HAM_MEDIUM(-1.00)[-0.996,0]; R_DKIM_ALLOW(-0.20)[sibptus.ru:s=20181118]; FROM_HAS_DN(0.00)[]; R_SPF_ALLOW(-0.20)[+mx]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.20)[multipart/signed,text/plain]; TO_DN_NONE(0.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000,0]; RCPT_COUNT_ONE(0.00)[1]; IP_SCORE(-3.36)[ip: (-9.89), ipnet: 2001:19f0:5000::/38(-4.94), asn: 20473(-1.94), country: US(-0.05)]; DKIM_TRACE(0.00)[sibptus.ru:+]; DMARC_POLICY_ALLOW(-0.50)[sibptus.ru,none]; SIGNED_PGP(-2.00)[]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+,1:+,2:~]; RCVD_TLS_LAST(0.00)[]; ASN(0.00)[asn:20473, ipnet:2001:19f0:5000::/38, country:US]; RCVD_COUNT_TWO(0.00)[2] X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 23 Mar 2020 07:17:17 -0000 --KsGdsel6WgEHnImy Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Victor Sudakov wrote: >=20 > If the MTU on the external Ethernet interface is 1500, it should be safe > to configure the gif(4) interfaces with mtu=3D1480 and gre(4) > interfaces with mtu=3D1476, correct? >=20 > I've noticed that a newly created gre0 interface has the expected "mtu 14= 76" > value, but a newly created gif0 interface has "mtu 1280", why would the > default be so low? >=20 > A second question. If the gif and gre tunnels will be wrapped in > IPSec transport mode, does it make sense to set the MTU on the > corresponding gif and gre interfaces to some lower value? And to make the matter even more mysterious, if_ipsec(4) which is essentially the same as gif, is created by default with mtu=3D1400 for some reason. --=20 Victor Sudakov, VAS4-RIPE, VAS47-RIPN 2:5005/49@fidonet http://vas.tomsk.ru/ --KsGdsel6WgEHnImy Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJeeGJ7AAoJEA2k8lmbXsY0CyoH/3Rtm1q5B3WuthzkTX0/1+B0 SjHIAaUxUyajiIN7+E4DuqHRvnLoMwM5omnGFgek7hmbJFhpgIzSKuHjmsCNNHX6 BpmeRS2YFlnucq365nr/5yaRJh3VM0DT+K1LPJ9csa8jkl+SMoheEMPGlRQR5lMY 79rvqCYn4lZ3wZ6dAJw0PhY9nEbsXpIp0RuA87Wa0x+zdZg7pcnXEyGdJxc2u2qz 7D4G8Cw8RdN9lcUDeqvUCc7hG1pl9AP6iz7pTo5WGFbmYJ9c+qSYkb8smFcnx04G IdAN4kR4FBrttamrM/UqhuMkAjNCTAmWwmmVtBmETMftrZCkpp5R7KGQfuIi+ro= =PmYG -----END PGP SIGNATURE----- --KsGdsel6WgEHnImy--