C – Linux – 内核模块 – TCP标头

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function：

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时 （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或类似产品。

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。 我错误地将tcp标头强制转换。 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) 。