Linux Ethernet Bridging - May 2007 - [Bridge] [BUG] Dropping fragmented IP packets within VLAN frames on bridge

There is a problem with fragmented IP packet sent within 802.1Q tagged
ethernet frame through bridge. Problem exists when conntrack is enabled
(i.e. nf_conntrack_ipv4 module is loaded). Then, such packets are not
fragmented again (after prior reassembling on bridge device) during
passing it to bridge enslaved NIC. It cause MTU exceeding and as a result
dropping packet.

Problem exists from kernel version 2.6.17 to 2.6.21.3 inclusive.

Below, there is a patch to fix it.

Regards.


--- linux-2.6.21.3.orig/net/bridge/br_netfilter.c	2007-05-25 09:56:15.000000000
+0200
+++ linux-2.6.21.3/net/bridge/br_netfilter.c	2007-05-25 10:11:42.000000000 +0200
@@ -731,7 +731,7 @@
 
 static int br_nf_dev_queue_xmit(struct sk_buff *skb)
 {
-	if (skb->protocol == htons(ETH_P_IP) &&
+	if ((skb->protocol == htons(ETH_P_IP) || IS_VLAN_IP(skb)) &&
 	    skb->len > skb->dev->mtu &&
 	    !skb_is_gso(skb))
 		return ip_fragment(skb, br_dev_queue_push_xmit);

On Fri, 25 May 2007 10:17:50 +0200
Adam Osuchowski <adwol@zonk.pl> wrote:
> There is a problem with fragmented IP packet sent within 802.1Q tagged
> ethernet frame through bridge. Problem exists when conntrack is enabled
> (i.e. nf_conntrack_ipv4 module is loaded). Then, such packets are not
> fragmented again (after prior reassembling on bridge device) during
> passing it to bridge enslaved NIC. It cause MTU exceeding and as a result
> dropping packet.
> 
> Problem exists from kernel version 2.6.17 to 2.6.21.3 inclusive.
> 
> Below, there is a patch to fix it.
> 
> Regards.
> 
> 
> --- linux-2.6.21.3.orig/net/bridge/br_netfilter.c	2007-05-25
09:56:15.000000000 +0200
> +++ linux-2.6.21.3/net/bridge/br_netfilter.c	2007-05-25 10:11:42.000000000
+0200
> @@ -731,7 +731,7 @@
>  
>  static int br_nf_dev_queue_xmit(struct sk_buff *skb)
>  {
> -	if (skb->protocol == htons(ETH_P_IP) &&
> +	if ((skb->protocol == htons(ETH_P_IP) || IS_VLAN_IP(skb)) &&
>  	    skb->len > skb->dev->mtu &&
>  	    !skb_is_gso(skb))
>  		return ip_fragment(skb, br_dev_queue_push_xmit);
It would be better to account for the tag in the length check.
Something like
	if (skb->protocol == htons(ETH_P_IP) &&
	    skb->len > skb->dev->mtu - (IS_VLAN_IP(skb) ? VLAN_HLEN : 0)
&&
	    !skb_is_gso(skb))
		return ip_fragment ...


-- 
Stephen Hemminger <shemminger@linux-foundation.org>

On Fri, 25 May 2007 20:17:05 +0200
Adam Osuchowski <adwol@zonk.pl> wrote:
> Stephen Hemminger wrote:
> > There is a function in br_forward.c that handles VLAN packet length
> > maybe use that
> 
> Do you mean packet_length()? It is up to you, but IMHO it will only slow
> down processing due to extra skb->protocol check.
Your original code was calling ip_fragment for all VLAN tagged frames