c++ - 使用 libnetfilter_queue 的简单程序无法正常工作

Question

我正在努力学习如何使用 libnetfilter_queue。我已经编译了库提供的示例。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>
#include <linux/types.h>
#include <linux/netfilter.h>        /* for NF_ACCEPT */

#include <libnetfilter_queue/libnetfilter_queue.h>

/* returns packet id */
static u_int32_t print_pkt (struct nfq_data *tb)
{
    int id = 0;
    struct nfqnl_msg_packet_hdr *ph;
    struct nfqnl_msg_packet_hw *hwph;
    u_int32_t mark,ifi; 
    int ret;
    char *data;

    ph = nfq_get_msg_packet_hdr(tb);
    if (ph) {
        id = ntohl(ph->packet_id);
        printf("hw_protocol=0x%04x hook=%u id=%u ",
            ntohs(ph->hw_protocol), ph->hook, id);
    }

    hwph = nfq_get_packet_hw(tb);
    if (hwph) {
        int i, hlen = ntohs(hwph->hw_addrlen);

        printf("hw_src_addr=");
        for (i = 0; i < hlen-1; i++)
            printf("%02x:", hwph->hw_addr[i]);
        printf("%02x ", hwph->hw_addr[hlen-1]);
    }

    mark = nfq_get_nfmark(tb);
    if (mark)
        printf("mark=%u ", mark);

    ifi = nfq_get_indev(tb);
    if (ifi)
        printf("indev=%u ", ifi);

    ifi = nfq_get_outdev(tb);
    if (ifi)
        printf("outdev=%u ", ifi);
    ifi = nfq_get_physindev(tb);
    if (ifi)
        printf("physindev=%u ", ifi);

    ifi = nfq_get_physoutdev(tb);
    if (ifi)
        printf("physoutdev=%u ", ifi);

    ret = nfq_get_payload(tb, &data);
    if (ret >= 0)
        printf("payload_len=%d ", ret);

    fputc('\n', stdout);

    return id;
}


static int cb(struct nfq_q_handle *qh, struct nfgenmsg *nfmsg,
          struct nfq_data *nfa, void *data)
{
    u_int32_t id = print_pkt(nfa);
    printf("entering callback\n");
    return nfq_set_verdict(qh, id, NF_ACCEPT, 0, NULL);
}

int main(int argc, char **argv)
{
    struct nfq_handle *h;
    struct nfq_q_handle *qh;
    int fd;
    int rv;
    char buf[4096] __attribute__ ((aligned));

    printf("opening library handle\n");
    h = nfq_open();
    if (!h) {
        fprintf(stderr, "error during nfq_open()\n");
        exit(1);
    }

    printf("unbinding existing nf_queue handler for AF_INET (if any)\n");
    if (nfq_unbind_pf(h, AF_INET) < 0) {
        fprintf(stderr, "error during nfq_unbind_pf()\n");
        exit(1);
    }

    printf("binding nfnetlink_queue as nf_queue handler for AF_INET\n");
    if (nfq_bind_pf(h, AF_INET) < 0) {
        fprintf(stderr, "error during nfq_bind_pf()\n");
        exit(1);
    }

    printf("binding this socket to queue '0'\n");
    qh = nfq_create_queue(h, 1, &cb, NULL);
    if (!qh) {
        fprintf(stderr, "error during nfq_create_queue()\n");
        exit(1);
    }

    printf("setting copy_packet mode\n");
    if (nfq_set_mode(qh, NFQNL_COPY_PACKET, 0xffff) < 0) {
        fprintf(stderr, "can't set packet_copy mode\n");
        exit(1);
    }

    fd = nfq_fd(h);

    while ((rv = recv(fd, buf, sizeof(buf), 0)) && rv >= 0) {
        printf("pkt received\n");
        nfq_handle_packet(h, buf, rv);
    }

    printf("unbinding from queue 0\n");
    nfq_destroy_queue(qh);

#ifdef INSANE
    /* normally, applications SHOULD NOT issue this command, since
     * it detaches other programs/sockets from AF_INET, too ! */
    printf("unbinding from AF_INET\n");
    nfq_unbind_pf(h, AF_INET);
#endif

    printf("closing library handle\n");
    nfq_close(h);

    exit(0);
}

启动可执行文件它没有收到任何东西。 我也关注了这个thread并尝试使用 netcat 生成流量，但我还是没有看到任何数据包 也许我缺少与过滤器队列真正交互的东西？

Answer 1

是的，你有。如果您的防火墙允许数据包通过规则，则不会发生任何事情，数据包也不会离开内核空间。但是如果你设置一些规则将数据包发送到用户空间，你的程序就会工作。

iptables -A INPUT -i eth0 -j NFQUEUE --queue-num 0

此行会将带有 --in-interface eth0 的 INPUT 中的所有数据包发送到用户空间。