From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konstantin Shemyak Subject: IPv4 tunnels: why IP-IP and SIT enforce DF bit, but GRE does not? Date: Thu, 26 Nov 2015 20:28:02 +0200 Message-ID: <56574F32.6070706@shemyak.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit To: "netdev@vger.kernel.org" Return-path: Received: from sub3.mail.dreamhost.com ([69.163.253.7]:46791 "EHLO homiemail-a76.g.dreamhost.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752853AbbKZS2F (ORCPT ); Thu, 26 Nov 2015 13:28:05 -0500 Received: from homiemail-a76.g.dreamhost.com (localhost [127.0.0.1]) by homiemail-a76.g.dreamhost.com (Postfix) with ESMTP id 247CD45807C for ; Thu, 26 Nov 2015 10:28:05 -0800 (PST) Received: from [10.0.0.5] (a91-152-80-240.elisa-laajakaista.fi [91.152.80.240]) (using TLSv1 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) (Authenticated sender: konstantin@shemyak.com) by homiemail-a76.g.dreamhost.com (Postfix) with ESMTPSA id B2D4D45807B for ; Thu, 26 Nov 2015 10:28:04 -0800 (PST) Sender: netdev-owner@vger.kernel.org List-ID: The kernel has taken the decision to always enforce DF bit on IPv4 tunnels, which have fixed (not inherited) TTL (e.g. net/ipv4/ipip.c:ipip_tunnel_ioctl()). Commment by Alexey Kuznetsov in the head of ip_gre.c explains that the reason is attempting to avoid network loops. But the commit c54419321455631 removed this enforcing from GRE tunnels, not changing this behavior for IP-IP (net/ipv4/ipip.c) and SIT (net/ipv6/sit.c). It can be discussed whether such enforcing of DF bit is exactly the desired behavior, but shouldn't it at least be identical across IPv4 tunnels? Konstantin Shemyak