c - Linux 内核模块 : How to reinject packets the kernel considers as NF_STOLEN?

Question

晚上好。发帖到这个网站对我来说是新的，但我一直是一个感恩的读者，从这个论坛学到了很多东西，现在已经有一段时间了。这是我第一次遇到自己无法解决的问题，也无法借助 Stackoverflow 上已有的条目或互联网提供的任何其他资源。

我希望你介意再次帮助我(而且，从现在开始，我也可以帮助别人，因为我觉得我已经成长到可以开始成为这里的写作成员的地步了)。

问题:

我正在研究内核模块。它的目的是使用 PRE_ROUTING netfilter hook 从内核窃取具有特定源 IP 的传入数据包。它只对 TCP 数据包感兴趣。

现在，钩子(Hook) ony 通过 dev_queue_xmit() 将数据包重新注入(inject)正常的内核数据包处理例程，并将数据包的 NF_STOLEN 返回给内核。来自其他源地址的数据包不会被重新注入(inject)，而是通过为它们返回 NF_ACCEPT 而不是 NF_STOLEN 来忽略。

内核模块还存储每个被盗数据包的 TCP seq 编号，以确定来自上述 IP 的传入数据包是新的，还是已经修改并通过 dev_queue_xmit() 重新注入(inject)，因为这些数据包再次遍历 hook .

还有什么工作:

1. 模块被加载
2. Hook 已注册
3. Hook 为每个数据包调用。
4. Hook可以判断数据包的SRC IP是不是我要找的IP。
5. Hook 为具有其他源地址的数据包返回 NF_ACCEPT
6. 带有源地址的数据包被重新注入(inject)，同时为它们返回 NF_STOLEN
7. 重新注入(inject)的数据包再次遍历钩子(Hook)并被忽略

问题

当我在加载模块后用浏览器访问 IP 时，我的 IP 堆栈似乎崩溃了。我无法再 ping 通任何地址。该模块记录它遇到来自相关 IP 的数据包，并且它重新排队它们并且之后它发现了一个已知的数据包(所以一切看起来都很好)，但仍然:没有正确连接到该站点/任何其他地址。

钩子(Hook)代码如下:

static unsigned int hook(unsigned int hooknum, struct sk_buff *skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *))
{
    struct iphdr *iph;
    struct tcphdr *tcph;
    unsigned int i;

    if(!skb)
        return NF_ACCEPT;

    iph = (struct iphdr *)skb_network_header(skb);
    if(!iph || !(iph->saddr) || iph->saddr != *(unsigned int*)suspicious_ip)
        return NF_ACCEPT;

    tcph = (struct tcphdr *)skb_transport_header(skb);
    for(i=0; i < number_of_known_packets; i++)
    {
        if(tcph->seq == *(already_known_packets+i))
        {
            debug("Already known packet");
            return NF_ACCEPT;
        }
    }
    debug("New packet");
    printk("seq: %u\n", tcph->seq);

    if((number_of_known_packets + 1) * 4 >= memory_allocated_for_known_packets) 
        imba_realloc(500*4);

    *(already_known_packets+number_of_known_packets++) = tcph->seq; 
    debug("Requeuing packet");

    // once the requeuing is working proper, I want to manipulate the payload as well
    printk("Result: %i", dev_queue_xmit(skb));
    return NF_STOLEN;


}

钩子(Hook)是如何注册的:

static struct nf_hook_ops nfho;

int init_module(void)
{
    debug("module loaded");

    already_known_packets = kmalloc(memory_allocated_for_known_packets, GFP_KERNEL);
    debug("initial memory allocated");

    nfho.hook = hook;
    nfho.hooknum = NF_INET_PRE_ROUTING;
    nfho.pf = PF_INET;
    nfho.priority = 1;

    nf_register_hook(&nfho);

    debug("hook registered");

    return 0;
}

系统日志:

Sep 21 13:11:43 linux kernel: [ 3298.937902] [PACKET PROXY] module loaded
Sep 21 13:11:43 linux kernel: [ 3298.937907] [PACKET PROXY] initial memory allocated
Sep 21 13:11:43 linux kernel: [ 3298.937931] [PACKET PROXY] hook registered
Sep 21 13:11:49 linux kernel: [ 3305.415404] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.415410] seq: 1538346824
Sep 21 13:11:49 linux kernel: [ 3305.415412] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.415430] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.415440] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.415441] seq: 618234741
Sep 21 13:11:49 linux kernel: [ 3305.415442] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.415447] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.421440] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.421452] seq: 2129598066
Sep 21 13:11:49 linux kernel: [ 3305.421458] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.421477] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.427449] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.427456] seq: 2327127721
Sep 21 13:11:49 linux kernel: [ 3305.427458] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.427466] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.427470] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.427471] seq: 1333567182
Sep 21 13:11:49 linux kernel: [ 3305.427473] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.427476] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.427494] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.427502] seq: 2650236943
Sep 21 13:11:49 linux kernel: [ 3305.427506] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.427514] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.427522] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.427533] seq: 444387468
Sep 21 13:11:49 linux kernel: [ 3305.427534] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.427539] Result: 0
Sep 21 13:11:49 linux kernel: [ 3305.427544] [PACKET PROXY] New packet
Sep 21 13:11:49 linux kernel: [ 3305.427545] seq: 1405773113
Sep 21 13:11:49 linux kernel: [ 3305.427547] [PACKET PROXY] Requeuing packet
Sep 21 13:11:49 linux kernel: [ 3305.427550] Result: 0
Sep 21 13:11:50 linux kernel: [ 3306.413448] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.413641] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.414153] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.414989] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.415102] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.417880] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.418065] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.418134] [PACKET PROXY] Already known PACKET
Sep 21 13:11:50 linux kernel: [ 3306.433788] [PACKET PROXY] New packet
Sep 21 13:11:50 linux kernel: [ 3306.433812] seq: 2146375282
Sep 21 13:11:50 linux kernel: [ 3306.433816] [PACKET PROXY] Requeuing packet
Sep 21 13:11:50 linux kernel: [ 3306.433850] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.441424] [PACKET PROXY] Already known PACKET
Sep 21 13:11:51 linux kernel: [ 3306.441587] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.441596] seq: 3958642290
Sep 21 13:11:51 linux kernel: [ 3306.441610] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.441634] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.441646] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.441648] seq: 1476007538
Sep 21 13:11:51 linux kernel: [ 3306.441652] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.441660] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.443131] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.443139] seq: 3288274546
Sep 21 13:11:51 linux kernel: [ 3306.443148] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.443194] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.443226] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.443231] seq: 788862834
Sep 21 13:11:51 linux kernel: [ 3306.443241] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.443258] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.443276] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.443278] seq: 2601129842
Sep 21 13:11:51 linux kernel: [ 3306.443281] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.443286] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.443294] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.443295] seq: 2131695474
Sep 21 13:11:51 linux kernel: [ 3306.443299] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.443305] Result: 0
Sep 21 13:11:51 linux kernel: [ 3306.443313] [PACKET PROXY] New packet
Sep 21 13:11:51 linux kernel: [ 3306.443314] seq: 3943962482
Sep 21 13:11:51 linux kernel: [ 3306.443317] [PACKET PROXY] Requeuing packet
Sep 21 13:11:51 linux kernel: [ 3306.443320] Result: 0
Sep 21 13:11:57 linux kernel: [ 3312.685399] [PACKET PROXY] New packet
Sep 21 13:11:57 linux kernel: [ 3312.685425] seq: 2667014159
Sep 21 13:11:57 linux kernel: [ 3312.685430] [PACKET PROXY] Requeuing packet
Sep 21 13:11:57 linux kernel: [ 3312.685463] Result: 0

Answer 1

对于我想要实现的目标，我找到了一个更简单的解决方案。不需要自定义内核模块的解决方案。

此外，经过更多研究，NF_STOLEN 数据包不能简单地“重新注入(inject)”。不过，要修改数据包，甚至不需要返回 NF_STOLEN。

可以只更改有效负载、调整校验和然后返回 NF_ACCEPT，因为您在钩子(Hook)中访问的 sk_buffer 将在进一步处理数据包时被内核重用。