I'm trying to transmit udp packets via xdp (receives work just fine) but the packets are dropped before udp processing.
Kernel: Linux fedora 6.5.12-300.fc39.x86_64 #1 SMP PREEMPT_DYNAMIC Mon Nov 20 22:44:24 UTC 2023 x86_64 GNU/Linux
PRETTY_NAME="Fedora Linux 39 (Workstation Edition)"
My xdp program and socket are bound to localhost using xdpgeneric. I see the transmitted packets in tcpdump and via bpftrace I see that ip_rcv is invoked for the packets. ip_rcv_finish however is not invoked as determined by attach a kfunc via bpftrace. On kfree_skb I see the reason for the drop is reason not specified. Examinig the packet in tcpdump I see no errors with the checksums or packet lengths and ports. Listeners for the corresponding udp ports never receieve the packets.This is how I create my socket
cfg.rx_size = XSKQueues::NUM_READ_DESC;
cfg.tx_size = XSKQueues::NUM_WRITE_DESC;
cfg.libxdp_flags = XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD;
cfg.xdp_flags = XDP_FLAGS_SKB_MODE;
cfg.bind_flags = XDP_USE_NEED_WAKEUP | XDP_COPY;
if (xsk_socket__create(&socket, iface.c_str(), QUEUE, umem.umem,
>! &qs.rxQ, &qs.txQ, &cfg)) {
perror("XSK: ");
exit(EXIT_FAILURE);
>! }`
What could be the issue?
I have tried running the AF_XDP-example for tx only from here https://github.com/xdp-project/bpf-examples/blob/master/AF_XDP-example/xdpsock.c. However I observe the same behavior. I have tried sending the packet via nc and copied the exact packet contents when sending via xdp. I make sure to invoke a sendto to kick the napi thread and I see the following kernel stack trace indicating that the packet is being picked up by the stack
common_stacktrace:
__netif_receive_skb_one_core+0x3c/0xa0
process_backlog+0x85/0x120
__napi_poll+0x28/0x1b0
net_rx_action+0x2a4/0x380
__do_softirq+0xd1/0x2c8
do_softirq.part.0+0x3d/0x60
__local_bh_enable_ip+0x68/0x70
__dev_direct_xmit+0x152/0x210
__xsk_generic_xmit+0x3e4/0x710
xsk_sendmsg+0x12f/0x1f0
__sys_sendto+0x1d6/0x1e0
__x64_sys_sendto+0x24/0x30
do_syscall_64+0x5d/0x90
entry_SYSCALL_64_after_hwframe+0x6e/0xd8
Data captured by tcpdump of the outgoing packet
21:54:47.008430 IP 127.0.0.1.search-agent > 127.0.0.1.9012: UDP, length 51
0x0000: 4500 004f 1400 0000 ff11 a99b 7f00 0001 E..O............
0x0010: 7f00 0001 04d2 2334 003b 0000 0200 0000 ......#4.;......
0x0020: 0000 7b5e 8fdb 0171 9d17 7600 0000 0000 ..{^...q..v.....
0x0030: 0000 9856 8fdb 0171 9d17 1400 0000 0000 ...V...q........
0x0040: 0000 80fe 1154 0200 0000 0100 0000 00 .....T.........
Are there known issues with transmitting on the loopback interface?