> On Feb 6, 2017, at 9:07 AM, Pravin Shelar <pshe...@ovn.org> wrote:
>
> On Thu, Feb 2, 2017 at 5:10 PM, Jarno Rajahalme <ja...@ovn.org> wrote:
>> When looking for an existing conntrack entry, the packet 5-tuple
>> must be inverted if NAT has already been applied, as the current
>> packet headers do not match any conntrack tuple. For
>> example, if a packet from private address X to a public address B is
>> source-NATted to A, the conntrack entry will have the following tuples
>> (ignoring the protocol and port numbers) after the conntrack entry is
>> committed:
>>
>> Original direction tuple: (X,B)
>> Reply direction tuple: (B,A)
>>
>> Now, if a reply packet is already transformed back to the private
>> address space (e.g., with a CT(nat) action), the tuple corresponding
>> to the current packet headers is:
>>
>> Current packet tuple: (B,X)
>>
>> This does not match either of the conntrack tuples above. Normally
>> this does not matter, as the conntrack lookup was already done using
>> the tuple (B,A), but if the current packet does not match any flow in
>> the OVS datapath, the packet is sent to userspace via an upcall,
>> during which the packet's skb is freed, and the conntrack entry
>> pointer in the skb is lost. When the packet is reintroduced to the
>> datapath, any further conntrack action will need to perform a new
>> conntrack lookup to find the entry again. Prior to this patch this
>> second lookup failed for NATted packets. The datapath flow setup
>> corresponding to the upcall can succeed, however, allowing all further
>> packets in the reply direction to re-use the conntrack entry pointer
>> in the skb, so typically the lookup failure only causes a packet drop.
>>
>> The solution is to invert the tuple derived from the current packet
>> headers in case the conntrack state stored in the packet metadata
>> indicates that the packet has been transformed by NAT:
>>
>> Inverted tuple: (X,B)
>>
>> With this the conntrack entry can be found, matching the original
>> direction tuple.
>>
>> This same logic also works for the original direction packets:
>>
>> Current packet tuple (after NAT): (A,B)
>> Inverted tuple: (B,A)
>>
>> While the current packet tuple (A,B) does not match either of the
>> conntrack tuples, the inverted one (B,A) does match the reply
>> direction tuple.
>>
>> Since the inverted tuple matches the reverse direction tuple the
>> direction of the packet must be reversed as well.
>>
>> Fixes: 05752523e565 ("openvswitch: Interface with NAT.")
>> Signed-off-by: Jarno Rajahalme <ja...@ovn.org>
>
> I could not apply this patch series to net-next branch. But it does
> applies to net, which branch are you targeting it for?
The patches were against net-next, but there likely was a merge from netfilter
around the time of me sending the email out causing the difficulty. Will
address all comments, rebase and post a v2 later today.
Jarno
