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). Sylvain
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