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

From: Andrew Pullin
Subject: [lwip-users] Receiving UDP (DNS) over PPPoS, source and destination ports same?
Date: Thu, 13 Feb 2020 21:29:39 -0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.4.2

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?

Andrew Pullin

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