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。