l2-sockets
A small hands-on experiment with L2 layer socket programming.
1. Simple veth pair.
Let's start with a simple veth pair.
Create the sockets
Use ip link to create virtual sockets to play with:
sudo make links
This creates a veth pair of interfaces. The thing about the veth pair is that bytes going into one come out of the other.
Use sudo make clean when you are down to remove the virtual interfaces.
Start a L2 sniffer
Run:
sudo ./l2.py sniff
to start a L2 sniffer that only looks for the specific header 0x6001. This is a randomly chosen number that is not used in any common protocols.
Inject packets
Run:
sudo ./l2.py inject my-veth0
The Sniffer should capture this coming in.
Note:
- Trt to inject into
my-veth1; the sniffer will capture it on the other interface. - Try to inject into
lo.
2. Veth bridged.
Create the sockets
Run:
sudo make -f ./bridge.Makefile links
Creates 1 bridge and 2 veth pairs and connects one ends of each pair to the bridge.
As before, use sudo make clean when you are down to remove the virtual interfaces.
Running sniffer and injector
If you run the sniffer and injector as before, you will notice that the packet does not make it through. Why is this?
This is because that the bridge is "switching" and only forwarding "packets" when the MAC destination is findable.
Run:
sudo make -f ./bridge.Makefile show
Note the MAC address of my-veth1.
So, let's run the injector differently this way:
sudo ./l2.py inject my-veth0 --dest-mac=<mac of my-veth1>
Now, the L2 packets are generated with MACs that make sense and get delivered correctly.