From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Ilya V. Matveychikov" Subject: question: update_pmtu doesn't update dst mtu Date: Thu, 3 Apr 2014 15:57:49 +0400 Message-ID: <533D4CBD.2080903@securitycode.ru> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit To: Return-path: Received: from itna.infosec.ru ([82.198.190.199]:20325 "EHLO itna.infosec.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751929AbaDCL5I (ORCPT ); Thu, 3 Apr 2014 07:57:08 -0400 Sender: netdev-owner@vger.kernel.org List-ID: Hello, Is this a place where I can post questions not the patches? If so, can anybody explain me what the problem I have when trying to update dst PMTU value. So, the scheme is the following: skb->dst rt(pmtu) IN_DEV (MTU 1500) -------> TUNNEL (MTU 1440) -------> OUT_DEV (MTU 1500) I have a simple tunnel_xmit function that handles all the packets that goes trough the tunnel. So, I have an skb with valid skb->dst value (filled earlier in ip_input_route_noref). Next, when encapsulation is done, I needed to get an output route for the packet (rt at the scheme). At this point I know that the output route may have a PMTU value that is different from the OUT_DEV->mtu. So, I'm trying to update the input skb->dst route's PMTU with the update_pmtu function. It seems that all is OK, but when I trying to get the dst mtu value (using dst_mtu(skb_dst(skb))) I always get the value of 1440 (TUNNEL's MTU): tunnel_xmit: ... pmtu = dst_mtu(&rt->dst) - OVERHEAD; // pmtu = 1300, for example skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL, skb, pmtu); // dst_mtu(skb_dst(skb)) still returns 1440 Looking through the code gives me that rt_pmtu is always 0 for the skb->dst entry and ipv4_mtu that called via the dst->ops->mtu() uses dev->mtu :( So, the question is what I missed when trying to dynamically tunning tunnel's input route path MTU? Thanks :)