lwIP is a lightweight embedded TCP/IP stack used as a network target by EtcPal. To use EtcPal with lwIP, you must set the ETCPAL_NET_TARGET variable to lwip.

set(ETCPAL_NET_TARGET lwip CACHE STRING "The network target for EtcPal")

Additionally, you must provide a target called lwIP to help EtcPal find the lwIP headers. If you are not building lwIP from CMake (e.g. if your project is not a CMake project), this can simply be an interface library:

add_library(lwIP INTERFACE)

target_include_directories(lwIP INTERFACE [lwip/include/paths...])

If you are building lwIP with CMake, make sure your lwIP library target is called lwIP exactly and has its include paths set using PUBLIC or INTERFACE.