[ovs-dev] [PATCH] ofproto-dpif-trace: Improve NAT tracing.

Ben Pfaff blp at ovn.org
Mon Jan 6 22:11:25 UTC 2020

On Fri, Jan 03, 2020 at 05:27:31PM +0100, Dumitru Ceara wrote:
> When ofproto/trace detects a recirc action it resumes execution at the
> specified next table. However, if the ct action performs SNAT/DNAT,
> e.g., ct(commit,nat(src=,table=42), the src/dst IPs and
> ports in the oftrace_recirc_node->flow field are not updated. This leads
> to misleading outputs from ofproto/trace as real packets would actually
> first get NATed and might match different flows when recirculated.
> Assume the first IP/port from the NAT src/dst action will be used by
> conntrack for the translation and update the oftrace_recirc_node->flow
> accordingly. This is not entirely correct as conntrack might choose a
> different IP/port but the result is more realistic than before.
> This fix covers new connections. However, for reply traffic that executes
> actions of the form ct(nat, table=42) we still don't update the flow as
> we don't have any information about conntrack state when tracing.
> Signed-off-by: Dumitru Ceara <dceara at redhat.com>

This is a great idea.

I have an idea for further improvement.  Currently this patch is quiet:
it doesn't say what's going on.  It would be better if it said that it
was replacing the source and/or destination addresses, and by what and
why.  I think that would be easiest done in ofproto_trace() itself near
the existing code that does it already for ct_state.  I think we could
just save the ofn pointer in the recirc node and move the replacement
code to ofproto_trace().

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