A version of the ESP-IDF lwIP library with NAT support.
To use it just replace the lwIP library from the lwIP component of the ESP-IDF with this one. To do this change to the esp-idf/components/lwip directory and rename or delete the existing lwip folder. Then clone this repository and either move it to esp-idf/components/lwip and rename it to lwip or create a symlink.
The NAPT functionality can be enabled with the IP_NAPT and IP_FORWARD options in esp-lwip/src/include/lwip/opt.h.
In order for the NAT feature to work the option "Enable copy between Layer2 and Layer3 packets" must be enabled in the ESP-IDF project configuration.
An ESP-IDF project can be configured by calling make menuconfig. In the configuration go to Component config -> LWIP.
In the code of your firmware enable NAPT for the softAP interface by calling the ip_napt_enable function from lwipt_napt.h header file.
Additionally, you may want to change the DNS server ip address that is offered by the DHCP server of the softAP. By default, the DHCP server will use the ip address of the softAP as DNS server ip address.
To set a different ip address use the dhcps_dns_setserver function defined in the esp-idf/components/lwip/include/apps/dhcpserver/dhcpserver.h header file.
Example esp-idf project where NAT is enabled and a custom DNS server ip is set: https://github.com/jonask1337/esp-idf-nat-example
Tested with version v3.3 of the ESP-IDF: https://github.com/espressif/esp-idf/tree/release/v3.3
It is based on the NAT implementation from this ESP8266 lwIP library: https://github.com/martin-ger/esp-open-lwip