Note

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lwIP 2.2.1 Usage Guide

Version Support

The RT-Smart network component supports both lwIP 2.1.2 and lwIP 2.2.1. The current default version remains lwIP 2.1.2; when 2.2.1 is needed, it should be explicitly selected in the build configuration. Both versions share upper-layer components such as RT-Thread netdev, SAL socket, WLAN, DHCP server, and IPv4 NAT, so applications usually do not need to bypass SAL and depend directly on a specific lwIP version.

Note

lwIP 2.2.1 and IPv4 NAT are two independent options. NAT supports both the current 2.1.2 and 2.2.1, and there is no need to force a network stack version switch to use NAT.

Selecting lwIP 2.2.1

First, load the target board configuration in the SDK root directory, and then open the configuration interface:

make <board>_defconfig
make menuconfig

Enter light weight TCP/IP stack and select lwIP v2.2.1 under lwIP version. You can also search for RT_USING_LWIP221 in the configuration interface. After saving, the configuration should contain:

CONFIG_RT_USING_LWIP=y
# CONFIG_RT_USING_LWIP212 is not set
CONFIG_RT_USING_LWIP221=y

Then rebuild the firmware using the normal process. lwIP version selection is mutually exclusive; do not write both CONFIG_RT_USING_LWIP212=y and CONFIG_RT_USING_LWIP221=y at the same time.

RT-Smart Adaptation

The lwIP 2.2.1 in the SDK is based on STABLE-2_2_1_RELEASE, and the following RT-Smart adaptations have been completed:

• SAL socket and POSIX poll use the netconn socket state layout from lwIP 2.2.x;

• netdev synchronizes changes to IPv4 addresses, gateways, masks, administrative status, link status, and IPv6 addresses;

• DHCP uses the RFC 5227 address conflict detection from lwIP 2.2;

• Memory allocation now uses the lwIP 2.2 MEM_CUSTOM_ALLOCATOR interface to connect to the RT-Thread heap;

• The global DNS interface is retained, and DHCPv4, DHCPv6, RDNSS, PPP, and manual DNS configurations are synchronized according to netdev; only the default interface updates the global resolver;

• SO_SNDBUF returns the fixed TCP_SND_BUF capacity and supports querying TCP_MAXSEG.

Applications can continue using POSIX sockets or the SAL interface. Code that directly includes private lwIP headers or accesses internal fields of struct netconn or struct lwip_sock is not cross-version compatible and should be changed to use public APIs before switching to 2.2.1.

Runtime Verification

The startup log directly prints the actual version. When the selection succeeds, output similar to the following can be seen:

lwIP-2.2.1 initialized!
Socket Abstraction Layer initialize success, lwIP 2.2.1.

Then use an actually existing network interface to verify addresses, the default route, DNS, and basic socket communication:

ifconfig
ping <gateway-address>

Interface names in multi-NIC systems are dynamically assigned according to the probing order. First run wifi devices or query interfaces through NetMgmt; do not hard-code wlan0, wlan0ap, or eth0 in applications. For the default route selection rules, see NetMgmt Examples.

Checklist When Switching Versions

1. Clean and rebuild kernel components and applications that depend on lwIP headers to avoid mixing object files from old versions.

2. Confirm that the SAL initialization log shows 2.2.1; checking defconfig alone cannot prove that the runtime image has been updated.

3. Perform regression testing on applications that use poll, non-blocking connect, SO_SNDBUF, or TCP_MAXSEG.

4. In multi-NIC scenarios, simultaneously verify addresses, gateways, DNS, link switching, and the default route.

5. When SoftAP sharing of an upstream network is needed, continue to configure and verify it according to IPv4 NAT Usage Guide.

Source Locations

• src/rtsmart/rtsmart/kernel/rt-thread/components/net/lwip-2.2.1/

• src/rtsmart/rtsmart/kernel/rt-thread/components/net/sal_socket/

• src/rtsmart/rtsmart/kernel/rt-thread/components/net/netdev/