FAQ

Q: I am not seeing any traffic on the socket. What am I doing wrong?

A: When a netdev of a physical NIC is initialized, Linux usually

allocates one Rx and Tx queue pair per core. So on a 8 core system,

queue ids 0 to 7 will be allocated, one per core. In the AF_XDP

bind call or the xsk_socket__create libbpf function call, you

specify a specific queue id to bind to and it is only the traffic

towards that queue you are going to get on you socket. So in the

example above, if you bind to queue 0, you are NOT going to get any

traffic that is distributed to queues 1 through 7. If you are

lucky, you will see the traffic, but usually it will end up on one

of the queues you have not bound to.

There are a number of ways to solve the problem of getting the

traffic you want to the queue id you bound to. If you want to see

all the traffic, you can force the netdev to only have 1 queue, queue

id 0, and then bind to queue 0. You can use ethtool to do this::

sudo ethtool -L <interface> combined 1

If you want to only see part of the traffic, you can program the

NIC through ethtool to filter out your traffic to a single queue id

that you can bind your XDP socket to. Here is one example in which

UDP traffic to and from port 4242 are sent to queue 2::

sudo ethtool -N <interface> rx-flow-hash udp4 fn

sudo ethtool -N <interface> flow-type udp4 src-port 4242 dst-port \

4242 action 2

A number of other ways are possible all up to the capabilitites of

the NIC you have.