Packet Filtering with AF_XDP in Forwarded Traffic

[HAKAN SARIMAN <hakansariman@std.iyte.edu.tr>]

Hi folks,

I'm currently working on setting up a packet filter using AF_XDP on my Linux server to manage network traffic  in userspace.
I tried to work with xdp-tutorial/AF_XDP example,
I've attached a simple XDP program that redirects to the traffic by using xsks_map on enp8s0.
You can find the XDP code below. 
To test my setup, I'm trying to block a specific IPv4 address.
Although I can see the traffic on userspace, I couldn't block any traffic. 
You can find the userspace code below.

I think there is something I am missing, can you help me what I am doing wrong?

Here's a brief overview of my current setup:

Network Interfaces: enp1s0 (Egress Interface),  ens8s0 (Local Network Interface)

Current iptables Rules:
-A FORWARD -i enp1s0 -o ens8s0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i ens8s0 -o enp1s0 -j ACCEPT

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
23461 1790K MASQUERADE  0    --  *      enp1s0  0.0.0.0/0            0.0.0.0/0



XDP Code:
-----
/* SPDX-License-Identifier: GPL-2.0 */

#include <linux/bpf.h>

#include <bpf/bpf_helpers.h>

struct {
 __uint(type, BPF_MAP_TYPE_XSKMAP);
 __type(key, __u32);
 __type(value, __u32);
 __uint(max_entries, 64);
} xsks_map SEC(".maps");

struct {
 __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
 __type(key, __u32);
 __type(value, __u32);
 __uint(max_entries, 64);
} xdp_stats_map SEC(".maps");

SEC("xdp")
int xdp_sock_prog(struct xdp_md *ctx)
{
    int index = ctx->rx_queue_index;
    __u32 *pkt_count;

    pkt_count = bpf_map_lookup_elem(&xdp_stats_map, &index);
    if (pkt_count) {

        /* We pass every other packet */
        if ((*pkt_count)++ & 1)
            return XDP_PASS;
    }

    /* A set entry here means that the correspnding queue_id
     * has an active AF_XDP socket bound to it. */
    if (bpf_map_lookup_elem(&xsks_map, &index))
        return bpf_redirect_map(&xsks_map, index, 0);

    return XDP_PASS;
}

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


----
Userspace code (rest of the code is identical with xdp-tutorial/AF_XDP [https://github.com/xdp-project/xdp-tutorial/blob/master/advanced03-AF_XDP/af_xdp_user.c]):

static bool process_packet(struct xsk_socket_info *xsk,
			   uint64_t addr, uint32_t len)
{
	uint8_t *pkt = xsk_umem__get_data(xsk->umem->buffer, addr);

		struct ethhdr *eth = (struct ethhdr *) pkt;
		struct iphdr *ip = (struct iphdr *) (eth + 1);
		struct in_addr tmp_ip;

		tmp_ip.s_addr = ip->saddr;
		char src_ip_str[INET_ADDRSTRLEN];
		inet_ntop(AF_INET, &tmp_ip, src_ip_str, INET_ADDRSTRLEN);

		tmp_ip.s_addr = ip->daddr;
		char dest_ip_str[INET_ADDRSTRLEN];
		inet_ntop(AF_INET, &tmp_ip, dest_ip_str, INET_ADDRSTRLEN);

		printf("saddr: %s, daddr: %s\n", src_ip_str, dest_ip_str);

                // trying to block an ip 
		if (strcmp(src_ip_str, "x.y.z.t") == 0 || strcmp(dest_ip_str, "x.y.z.t") == 0) {
			return false;
    	}

	int ret;
	uint32_t tx_idx = 0;
	ret = xsk_ring_prod__reserve(&xsk->tx, 1, &tx_idx);
	if (ret != 1) {
		/* No more transmit slots, drop the packet */
		return false;
	}

	xsk_ring_prod__tx_desc(&xsk->tx, tx_idx)->addr = addr;
	xsk_ring_prod__tx_desc(&xsk->tx, tx_idx)->len = len;
	xsk_ring_prod__submit(&xsk->tx, 1);
	xsk->outstanding_tx++;

	return true;
}

...

/* Process received packets */
	for (i = 0; i < rcvd; i++) {
		uint64_t addr = xsk_ring_cons__rx_desc(&xsk->rx, idx_rx)->addr;
		uint32_t len = xsk_ring_cons__rx_desc(&xsk->rx, idx_rx++)->len;

		if (!process_packet(xsk, addr, len))
			xsk_free_umem_frame(xsk, addr);
        }