[lwip-users] Receiving UDP (DNS) over PPPoS, source and destination ports same?

[Andrew Pullin]

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?

Wireshark reflect the same result, src=dest=53.
I am also seeing mDNS UDP packets with src=dest=5353, but that appears 
to be correct per 6762.

I wanted to get the expert's input before going to the level of sniffing 
the UART transmission and manually disassembling the actual PPP frame to 
see what is possibly going on ...

A bug in the sending side of the PPP is not out of the question, but 
that is a "professional" network stack there, so it seems unlikely.

Any ideas?

Thanks,
Andrew Pullin