Spurious MDNS collision detection (IDFGH-4263)

[djphoenix]

Environment

• Development Kit: none
• Module or chip used: ESP32-WROOM-32
• IDF version: v4.0.2
• Build System: Make
• Compiler version: esp-2019r2
• Operating System: macOS
• Using an IDE?: No
• Power Supply: USB

Problem Description

I have faced an issue with spurious MDNS TXT collision detection, that cause an infinite MDNS query/answer loop when router retransmits MDNS message to ESP module. E.g.

• ESP sends multicast response with TXT record
• ESP receives multicast response with TXT record (actually from itself)
• ESP validates response against service description ** FAIL HERE **
• ESP sends multicast request (probe) for its service name with ANY type
• ESP receives multicast request with its service name
• Looped to step 1

Steps to reproduce

Register MDNS service with multiple TXT entries

Point to code

https://github.com/espressif/esp-idf/blob/v4.0.2/components/mdns/mdns.c#L2108

It counts length of first record regardless to current linked-list foreach-loop record.
Also it counts data_len from 1 instead of 0.
Seems like master and ESP8266_RTOS_SDK have same issues.

[Alvin1Zhang]

Thanks for reporting, we will look into.

[lhespress]

@djphoenix

It is much easier for us if you could provide small test code on top of esp-idf/examples/protocols/mdns.

[AndreyKarpov]

P.S. Continuation of the story: https://www.viva64.com/en/b/0790/ :)