From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: [RFC] MTU discovery not working over GRE Date: Fri, 4 May 2012 14:04:53 -0700 Message-ID: <20120504140453.452794c4@nehalam.linuxnetplumber.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org To: Herbert Xu , David Miller Return-path: Received: from mail.vyatta.com ([76.74.103.46]:57903 "EHLO mail.vyatta.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932644Ab2EDVE5 (ORCPT ); Fri, 4 May 2012 17:04:57 -0400 Sender: netdev-owner@vger.kernel.org List-ID: When using gretap, I am seeing that Path MTU discovery is not working for packets going out over the tunnel interface. What happens is that the driver correctly identifies the DF bit, and see IPv4 but since skb_dst(skb) is NULL, the icmp_send() ends up doing nothing. The following fixes the problem but does not seem like the correct general solution. IPv6 almost certainly has the same problem. Perhaps we should just set the skb_dst() earlier (before the MTU checks). diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c index 1017460..da57bb0 100644 --- a/net/ipv4/ip_gre.c +++ b/net/ipv4/ip_gre.c @@ -838,8 +838,9 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev if ((old_iph->frag_off&htons(IP_DF)) && mtu < ntohs(old_iph->tot_len)) { + skb_dst_drop(skb); + skb_dst_set(skb, &rt->dst); icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu)); - ip_rt_put(rt); goto tx_error; } }