Date:	Mon, 03 Jan 2011 19:42:15 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Sebastian J. Bronner" <sebastian.bronner@....de>
Cc:	netdev@...r.kernel.org, Daniel Kraft <daniel.kraft@....de>
Subject: Re: bridge not routing packets via source bridgeport

Le lundi 03 janvier 2011 à 18:52 +0100, Sebastian J. Bronner a écrit :
> Hi all,
> 
> we recently upgraded from 2.6.32.25 to 2.6.35.24 and discovered that our
> virtual machines can no longer access their own external IP addresses.
> Testing revealed that 2.6.34 was the last version not to have the
> problem. 2.6.36 still had it. But on to the details.
> 
> Our setup:
> 
> We use KVM to virtualise our guests. The physical machines (nodes) act
> as One-to-One NAT routers to the virtual machines. The virtual machines
> are connected via virtio interfaces in a bridge.
> 
> Since the virtual machines only know about their RFC-1918 addresses, any
> request they make to their NATed global addresses requires a trip
> through the node's netfilter to perform the needed SNAT and DNAT operations.
> 
> Take the following setup:
> 
>    {internet}
>        |
>      (eth0)       <- 1.1.1.254, proxy_arp=1
>        |
>      [node]       <- ip_forward=1, routes*, nat**
>        |
>     (virbr1)      <- 10.0.0.1
>     /      \
> (vnet0)     |
>    |     (vnet1)
> (veth0)     |     <- 10.0.0.2
>    |     (veth0)  <- 10.0.0.3
>  [vm1]      |
>           [vm2]
> 
> * The static routes on the node for the vms mentioned above are as follows:
> # ip r
> 1.1.1.2 dev virbr1 scope link
> 1.1.1.3 dev virbr1 scope link
> 
> ** The NAT rules are set up as follows (in reality, they're a bit more
> complicated - but this suffices to illustrate the problem at hand):
> # iptables-save -t nat
> -A PREROUTING -d 1.1.1.2 -j DNAT --to-destination 10.0.0.2
> -A PREROUTING -d 1.1.1.3 -j DNAT --to-destination 10.0.0.3
> -A POSTROUTING -s 10.0.0.2 -j SNAT --to-source 1.1.1.2
> -A POSTROUTING -s 10.0.0.3 -j SNAT --to-source 1.1.1.3
> 
> This means that 1.1.1.2 maps to 10.0.0.2 (vm1) and
>                 1.1.1.3 maps to 10.0.0.3 (vm2).
> 
> Assuming ssh is running on both vms, running 'nc -v 1.1.1.3 22' from vm1
> gets me ssh's introductory message.
> 
> Assuming, no service is running on port 23, running 'nc -v 1.1.1.3 23'
> from vm1 gets me 'Connection refused'.
> 
> That's all fine and exactly as it should be. The vms are accessible from
> the internet as well, and can access the internet.
> 
> If, however, i run 'nc -v 1.1.1.2 22' from vm1 (or any port for that
> matter), I get a timeout!
> 
> Running tcpdump on all the involved interfaces showed me that the
> packets successfully traverse veth0 and vnet0 and appear to get lost
> upon reaching virbr1.
> 
> So, then I decided to set up a packet trace with iptables:
> [on the node]
> # modprobe ipt_LOG
> # iptables -t raw -A PREROUTING -p tcp --dport 4577 -j TRACE
> # tail -f /var/log/messages | grep TRACE
> [on vm1]
> # nc -v 1.1.1.2 4577
> 
> The results were very interesting, if somewhat dumbfounding. They are
> attached for easier perusal. The gist of it is that the packet in
> question disappears without a trace after going through the DNAT rule in
> the PREROUTING chain of the NAT table. This can be seen happening three
> times in vm1-to-1.1.1.2.txt in three and six second intervals (retries).
> 
> For comparison, I have also included a trace of a successful packet
> traversal that ends in a 'Connection refused'. It is in vm1-to-1.1.1.3.txt.
> 
> As a last note, I should add that the problem isn't related to the IP
> address. I eliminated that by putting two RFC-1918 IPs on vm1 and
> mapping two IPs to it, then running nc on one IP, while the other one
> was being used as the source IP.
> 
> The problem appears to be that packets can't be routed out the same
> bridgeport that they arrived from.
> 
> I hope this all makes sense and that you can reproduce the problem. One
> virtual machine will suffise to see the problem at work.
> 
> Feel free to contact me if you need more information or have suggestions
> for me.
> 
> Cheers,
> Sebastian Bronner
> 
> P.S.: The IP addresses are faked. I used vim to replace all instances of
> the real IPs with the fake ones used in this e-mail consistently.

random guess: maybe rp_filter hits you ?


With 2.6.36, a new SNMP counter was added, 
"netstat -s | grep IPReversePathFilter"



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