Make it clear that this XDP program depend on the network
stack to do the ARP resolution. This is connected with the
BPF_FIB_LKUP_RET_NO_NEIGH return code from bpf_fib_lookup().
Another common mistake (seen via XDP-tutorial) is that users
don't realize that sysctl net.ipv{4,6}.conf.all.forwarding
setting is honored by bpf_fib_lookup.
Reported-by: Anton Protopopov <a.s.protopo...@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <bro...@redhat.com>
---
samples/bpf/xdp_fwd_kern.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)
diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
index 4a5ad381ed2a..df11163454e7 100644
--- a/samples/bpf/xdp_fwd_kern.c
+++ b/samples/bpf/xdp_fwd_kern.c
@@ -103,8 +103,23 @@ static __always_inline int xdp_fwd_flags(struct xdp_md
*ctx, u32 flags)
fib_params.ifindex = ctx->ingress_ifindex;
rc = bpf_fib_lookup(ctx, &fib_params, sizeof(fib_params), flags);
-
- if (rc == 0) {
+ /*
+ * Some rc (return codes) from bpf_fib_lookup() are important,
+ * to understand how this XDP-prog interacts with network stack.
+ *
+ * BPF_FIB_LKUP_RET_NO_NEIGH:
+ * Even if route lookup was a success, then the MAC-addresses are also
+ * needed. This is obtained from arp/neighbour table, but if table is
+ * (still) empty then BPF_FIB_LKUP_RET_NO_NEIGH is returned. To avoid
+ * doing ARP lookup directly from XDP, then send packet to normal
+ * network stack via XDP_PASS and expect it will do ARP resolution.
+ *
+ * BPF_FIB_LKUP_RET_FWD_DISABLED:
+ * The bpf_fib_lookup respect sysctl net.ipv{4,6}.conf.all.forwarding
+ * setting, and will return BPF_FIB_LKUP_RET_FWD_DISABLED if not
+ * enabled this on ingress device.
+ */
+ if (rc == BPF_FIB_LKUP_RET_SUCCESS) {
int *val;
/* Verify egress index has been configured as TX-port.
