This seems to be a bug for framework-arduinioespressif8266 since 2.20402.3:
When using espconn_mdns_set_hostname linking fails with undefined references.
This is due to lack of -llwip_gcc in the linking stage.
Where can I submit a bug report for that?
lwip_gcc is added automatically only for lwIP “v1.4 Higher Bandwidth”. See Arduino/boards.txt at 2.4.2 · esp8266/Arduino · GitHub
You can switch between lwIP variants with Espressif 8266 — PlatformIO latest documentation
So are you implying this is not a bug? This used to work for older version, and I am unable to find any relation or requirement between “high bandwidth” and MDNS… MDNS seems just to be located in that library, and I am unable to see a reason why the “low bandwidth” variant shouldn’t use that library and provide MDNS…
Also, linking is successful, when supplying -llwip instead of -llwip_gcc. So either the MDNS symbols are in the wrong library or the dependencies between MDNS and lwip are not resolved correctly…