lwip-tap is an application of "lwIP - A Lightweight TCP/IP Stack" which is a small, user-space implementation of the TCP/IP protocol stack. Using lwip-tap, you can easily plug an instance of lwIP to Linux Bridge or Open vSwitch through TAP devices. lwip-tap supports multiple network interfaces, so that it also works as a router. A wide range of different network topologies can be built flexibly.

As of now, lwip-tap has been developed only in Linux, more precisely Arch Linux with latest updates. The original tapif driver available in the lwip-contrib, however, includes code chunks for other environments as well. Although lwip-tap is fully supported only in Linux right now, it should work on any environment that supports TAP/TUN-like devices if the tapif driver is updated.

Clone the git repository from GitHub and initialize submodules,

$ git clone git://github.com/takayuki/lwip-tap.git $ cd lwip-tap $ git submodule init $ git submodule update

CMake is required to build lwip-tap. A compile-time option to enable debugging facilities lwIP provides is available. See "8. Debugging" below.

cmake -B build -S. -GNinja
cmake --build build

3. Example: Running lwip-tap with a transient TAP device

The quickest, but less flexible way to run lwip-tap is, having it create a transient TAP device for you,

In this example, lwip-tap (1) creates a transient TAP device, (2) assign 172.16.0.1 to the tap, (3) adds a route 172.16.0.1/255.255.255.0 to the tap, (4) links an instance of lwIP to the tap, and (5) assign 172.16.0.2 to the interface of the lwIP. To check if the lwIP is working,

4. Example: Attach lwip-tap to a persistent TAP device

For more flexible network settings, all the steps mentioned above can be made separately using persistent TAP devices,

5. Example: Use DHCP with a persistent TAP device

If you omit an address to an instance, lwip-tap automatically uses DHCP.

6. Example: Run lwip-tap with Open vSwitch

Add br0 (172.16.0.1),

Add tap0 to br0,

Run a lwip-tap on tap0 (172.16.0.2),

7. Example: Run lwip-tap as a router

Add br0 (172.16.0.1),

Add br1 (without IP address),

Add tap0 to br0,

Add tap1 and tap2 to br1,

Run one lwip-tap on tap2 (172.16.1.2),

  -i name=tap2,addr=172.16.1.2,netmask=255.255.255.0,gw=172.16.1.1

Run another lwip-tap on tap0 (172.16.0.2) and tap1 (172.16.1.1), which is a router between 172.16.0.2/24 and 172.16.1.1/24,

  -i name=tap0,addr=172.16.0.2,netmask=255.255.255.0,gw=172.16.0.1 \
  -i name=tap1,addr=172.16.1.1,netmask=255.255.255.0

8. Build-in Applications

lwip-tap has currently three built-in applications which has been integrated from lwip-contrib for debugging purposes. All applications are disabled by default. Use command-line options to enable those services.

• echo (port 7) in TCP/UDP (-E)
• chargen (port 19) in TCP (-C)
• http (port 80) in TCP (-H)

9. Debugging

lwIP has a compile-time option for debugging which is disable by default. To enable it, you need to rebuild lwip-tap giving an option to configure,

$ configure --enable-debug $ make

which compiles all object files with LWIP_DEBUG set to 1 and adds another option -d to lwip-tap.