This is a sample application where we rely on ebpf to set the next hop for egress traffic based on the source address of a given packet.

Under the validate folder there's a docker-compose setup which will start the following setup:

             │
             │
             │
             │10.111.220.11
┌────────────┴─────┐       ┌──────────────────┐        ┌────────────────┐
│                  │       │                  │        │                │
│                  │       │                  │        │                │
│                  │       │                  │        │                │
│                  │       │                  │        │                │
│                  ├───────┤                  ├────────┤                │
│                  │   ▲   │                  │    ▲   │                │
│                  │   │   │                  │    │   │                │
│                  │   │   │                  │    │   │                │
│ server           │   │   │ router           │    │   │ client         │
└──────────────────┘   │   └──────────────────┘    │   └────────────────┘

                10.111.221.0/24             10.111.222.0/24

The server's default gateway is via the 10.111.20.11 interface.

The 192.168.1.5 address is added to the server's lo interface.

Run a listener on the server, such as nc -l 30100 and try to reach the listener from the client, and then try to reach the server by running nc 192.168.1.5 on the client.

The traffic coming from the client container can reach the server because of the routes added on the router container.

When the server replies, there is no routing in place to allow the traffic reach the router, so the traffic would exit via the default gateway, leading to asymmetric routing (and in this case, to the reply to be lost).

Running the binary will add an ebpf program to the default gateway interface that leverages the bpf_redirect_neigh helper to redirect the traffic via another interface.

The poc leverages the go ebpf library and is split in two functions:

func attachFilter(attachTo string, program *ebpf.Program) error {

which is used to attach the bpf program to the given interface.

and

func enableRedirect(src, nextHop net.IP, interfaceName string, ebpfMap *ebpf.Map) error {

were we expose the a way to set a new nexthop / interface based on the source address of the traffic (in this example we pass 192.168.1.5 as the src ip, and eth1 and 10.111.221.21 as the interface / next hop).