C - Linux - 内核模块 - TCP头

Question

我正在尝试创建linux内核模块，它将检查传入的数据包。 目前，我正在提取数据包的TCP头并读取源和目标端口 - >但是我得到的值不正确。 我有钩功能：

unsigned int hook_func(unsigned int hooknum, struct sk_buff *skb, 
                       const struct net_device *in, 
                       const struct net_device *out, 
                       int (*okfn)(struct sk_buff *)) 
{
    struct iphdr *ipp = (struct iphdr *)skb_network_header(skb);
    struct tcphdr *hdr;
    /* Using this to filter data from another machine */
    unsigned long ok_ip = 2396891328;

    /* Some problem, empty network packet. Stop it now. */
    if (!skb)
        return NF_ACCEPT;

    /* Just to track only packets coming from 1 IP */
    if (ipp->saddr != ok_ip)
        return NF_ACCEPT;

    /* Incomming packet is TCP */
    if (ipp->protocol == IPPROTO_TCP) {
        hdr = (struct tcphdr *) skb_transport_header(skb);
        printk(" TCP ports: source: %d, dest: %d .\n", ntohs(hdr->source), 
                                                       ntohs(hdr->dest));
    }
}

现在，当我尝试telnet端口21 （我没有收听）：

[ 4252.961912]  TCP ports: source: 17664, dest: 52 .
[ 4253.453978]  TCP ports: source: 17664, dest: 52 .
[ 4253.953204]  TCP ports: source: 17664, dest: 48 .

当我telnet端口22 - SSH deamon在那里听：

[ 4299.239940]  TCP ports: source: 17664, dest: 52 .
[ 4299.240527]  TCP ports: source: 17664, dest: 40 .
[ 4299.552566]  TCP ports: source: 17664, dest: 40 .

从输出可见我得到了非常奇怪的结果，任何人都知道问题来自哪里？ 当我编译模块时，我没有错误/警告。 内核版本（标题）：3.7.10。 不使用SELinux或类似的。

Answer 1

我遇到了为网络类编写小型防火墙的同样问题，我刚刚发现了我遇到的问题。 我正在强制转换tcp标头。 尝试转换为tcp然后访问端口。

这是它的代码片段

struct iphdr *ip_header;       // ip header struct
struct tcphdr *tcp_header;     // tcp header struct
struct udphdr *udp_header;     // udp header struct
struct sk_buff *sock_buff;

unsigned int sport ,
             dport;


sock_buff = skb;

if (!sock_buff)
    return NF_ACCEPT;

ip_header = (struct iphdr *)skb_network_header(sock_buff);
if (!ip_header)
    return NF_ACCEPT;


//if TCP PACKET
if(ip_header->protocol==IPPROTO_TCP)
{
    //tcp_header = (struct tcphdr *)skb_transport_header(sock_buff); //doing the cast this way gave me the same problem

    tcp_header= (struct tcphdr *)((__u32 *)ip_header+ ip_header->ihl); //this fixed the problem

    sport = htons((unsigned short int) tcp_header->source); //sport now has the source port
    dport = htons((unsigned short int) tcp_header->dest);   //dport now has the dest port
}

Answer 2

要从套接字缓冲区（ skb ）获取IP头或TCP头，只需应用函数ip_hdr(skb)和tcp_hdr(skb) 。