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Re: [lwip-users] Receiving UDP (DNS) over PPPoS, source and destination

From: Sylvain Rochet
Subject: Re: [lwip-users] Receiving UDP (DNS) over PPPoS, source and destination ports same?
Date: Fri, 14 Feb 2020 13:21:13 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

Hi Andrew,

On Thu, Feb 13, 2020 at 09:29:39PM -0800, Andrew Pullin wrote:
> I am having a little issue with lwip on ESP32 here.
> I am using a PPP (over serial) link to get two IP stacks to communicate. And
> then later doing some source routing and other stuff, but this issue
> manifests far before that, upon reception of a packet from the PPP netif:
> UDP packets are showing up with the source and destination port the same.
> Early in `ip4_input`, I added a check for IP_PROTO_UDP to grab the UDP
> header and then read out the src and dest ports, and I am seeing that both
> are the same for PPP input. e.g. for a DNS request, I see both are 53.
> This seems incorrect. Or maybe there is something I am overlooking?

Nothing prevent a DNS request to use the same port as source and 
destination. It's bad for security (DNS poisoning) but it is technically 
correct to do so.

A correct NAT implementation should randomise the source port if there 
is a conflict with a previously established tracked session (same dest, 
same source port, from two differents NATed hosts).


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