Use native XDP with veth interfaces

When Ella Core's N3 or N6 interface is a veth pair and XDP is set to native mode, you must attach a minimal XDP program to the peer side of the veth. Without it, downlink traffic will silently fail.

For an explanation of why this is needed, see XDP redirect on veth pairs.

1. Install prerequisites

Install the BPF toolchain needed to compile the XDP program:

sudo apt update
sudo apt install -y clang llvm libbpf-dev linux-headers-$(uname -r)

2. Create the zero entrypoint program

Create a file called zero_entrypoint.c with the following content:

#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>

SEC("xdp")
int zero_entrypoint(struct xdp_md *ctx)
{
    return XDP_PASS;
}

char _license[] SEC("license") = "GPL";

3. Compile the program

clang -O2 -g -Wall -target bpf -c zero_entrypoint.c -o zero_entrypoint.o

4. Attach the program to the peer veth

Attach the compiled program to the peer veth interface. If the peer is in a network namespace (e.g. n3ns), use ip netns exec:

sudo ip netns exec n3ns ip link set dev n3-ran-veth xdpgeneric off
sudo ip netns exec n3ns ip link set dev n3-ran-veth xdp obj zero_entrypoint.o sec xdp

If the peer is not in a namespace:

sudo ip link set dev <peer-veth-name> xdp obj zero_entrypoint.o sec xdp

5. Verify

Confirm the program is attached:

sudo ip netns exec n3ns ip link show n3-ran-veth

You should see prog/xdp in the output.

6. Configure Ella Core for native XDP

Set the XDP attach mode to native in the Ella Core configuration file:

xdp:
  attach-mode: "native"

Restart Ella Core for the change to take effect.