Summary

This PR broadens IPv6 support, enabling it by default on supported platforms.

It was originally part of #16459 but I felt it should be tested separately.

This turns on some of the missing pieces to ensure auto detection to work correctly, similar to mdns in ipv4.
Part of this was consolidating changes from #15422 into the common settings for all ports.

With the work on usb-networking I've found ipv6 very helpful for local devices to not need to worry about dhcp configuration and discovery, ipv6 can "just work" without ip address collisions from multiple devices.

Core IPv6 Features

LWIP IPv6 Configuration:

• Consolidates more of the common lwip configuration.
• Enables LWIP_IPV6 support by default in common LWIP configuration
• Configures IPv6 autoconfiguration (LWIP_IPV6_AUTOCONFIG)
• Enables IPv6 multicast listener discovery (LWIP_IPV6_MLD)
• Supports multicast ping responses for neighbor discovery

Ethernet Driver IPv6 Support:

• STM32 ethernet driver: IPv6 packet handling and multicast support
• i.MX RT ethernet driver: IPv6 packet handling and multicast support
• ESP-Hosted WiFi driver: IPv6 support for ESP-hosted network interface

Implementation Pattern:
Each network driver follows a consistent IPv6 integration pattern:

• Include lwip/ethip6.h when LWIP_IPV6 is enabled
• Set output_ip6 to ethip6_output for IPv6 packet handling
• Add NETIF_FLAG_MLD6 flag for IPv6 multicast support
• Create IPv6 link-local address after interface initialization

Files Modified

Core LWIP Configuration:

• extmod/lwip-include/lwipopts_common.h: Enable IPv6 by default with autoconfig

Network Drivers:

• ports/stm32/eth.c: Add IPv6 support to STM32 ethernet driver
• ports/mimxrt/eth.c: Add IPv6 support to i.MX RT ethernet driver
• drivers/esp-hosted/esp_hosted_netif.c: Add IPv6 support to ESP-hosted driver

Port Configuration Cleanup:

• ports/renesas-ra/lwip_inc/lwipopts.h: Remove port-specific IPv6 disable
• ports/mimxrt/lwip_inc/lwipopts.h: Remove port-specific IPv6 disable

Test Coverage

New IPv6-Specific Tests:

• tests/extmod/socket_ipv6_basic.py: Basic IPv6 socket operations and binding
• tests/multi_net/tcp_data_ipv6.py: IPv6 TCP client-server communication
• tests/multi_net/udp_data_ipv6.py: IPv6 UDP client-server communication
• tests/net_inet/getaddrinfo_ipv6.py: IPv6 DNS resolution and dual-stack support

All tests include graceful fallback when IPv6 support is not available.

Testing

Unix Port Testing (Completed):

• ✅ Basic IPv6 socket creation and binding operations
• ✅ IPv6 TCP client-server data transfer via multi-instance tests
• ✅ IPv6 UDP client-server communication with packet loss tolerance
• ✅ IPv6 DNS resolution including dual-stack and domain name resolution
• ✅ All tests pass on Unix port with full IPv6 network stack

On-Device Testing (Pending):

• STM32 boards with ethernet: IPv6 functionality verification needed
• i.MX RT boards with ethernet: IPv6 functionality verification needed
• ESP32/ESP8266 with ESP-hosted: IPv6 WiFi connectivity verification needed

Test Commands:

# Basic socket tests
./tests/run-tests.py tests/extmod/socket_ipv6_basic.py

# Multi-instance communication tests  
./tests/run-multitests.py tests/multi_net/tcp_data_ipv6.py
./tests/run-multitests.py tests/multi_net/udp_data_ipv6.py

# DNS resolution tests
./tests/run-tests.py tests/net_inet/getaddrinfo_ipv6.py

Trade-offs and Alternatives

Benefits:

• Enables modern IPv6 networking capabilities
• Maintains full backward compatibility with existing IPv4 code
• Consistent implementation across multiple hardware platforms
• Comprehensive test coverage for validation

Code Size Impact:

• Minor increase in code size due to IPv6 protocol stack inclusion
• Only affects builds with networking enabled
• Can be disabled by setting LWIP_IPV6=0 if needed

Alternative Approaches Considered:

• Port-by-port IPv6 enablement: Rejected in favor of unified approach
• Optional IPv6 support: Current implementation provides good defaults while allowing override

The implementation provides a solid foundation for IPv6 networking across the MicroPython ecosystem with minimal disruption to existing functionality.