Question

在接收数据包时遇到一些问题。我可以接收和读取传入的数据包，但我想我不会与任何主机握手。我只想在收到答案的情况下将数据包发送到具有开放端口的远程计算机，以查看TTL（生存时间）和窗口大小。有谁知道错误在哪里？ ~~（我对C编程知之甚少）~~

CODE：

#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <net/ethernet.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>

struct pseudohdr {
    u_int32_t src_addr;
    u_int32_t dst_addr;
    u_int8_t padding;
    u_int8_t proto;
    u_int16_t length;
};

struct data_4_checksum {
    struct pseudohdr pshd;
    struct tcphdr tcphdr;
    char payload[1024];
};
unsigned short comp_chksum(unsigned short *addr, int len) {
    long sum = 0;

    while (len > 1) {
        sum += *(addr++);
        len -= 2;
    }

    if (len > 0)
        sum += *addr;

    while (sum >> 16)
        sum = ((sum & 0xffff) + (sum >> 16));

    sum = ~sum;

    return ((u_short) sum);

}

int main(int argc, char *argv[]) {

    int sock, bytes, on = 1;
    char buffer[1024];
    struct iphdr *ip;
    struct tcphdr *tcp;
    struct sockaddr_in to;
    struct pseudohdr pseudoheader;
    struct data_4_checksum tcp_chk_construct;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s ", argv[0]);
        fprintf(stderr, "<dest-addr>\n");
        return 1;
    }

    sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
    if (sock == -1) {
        perror("socket() failed");
        return 1;
    }else{
        printf("socket() ok\n");
    }

    if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) == -1) {
        perror("setsockopt() failed");
        return 2;
    }else{
        printf("setsockopt() ok\n");
    }

    ip = (struct iphdr*) buffer;
    tcp = (struct tcphdr*) (buffer + sizeof(struct tcphdr));

    int iphdrlen = sizeof(struct iphdr);
    int tcphdrlen = sizeof(struct tcphdr);
    int datalen = 0;
    printf("Typecasting ok\n");

    ip->frag_off = 0;
    ip->version = 4;
    ip->ihl = 5;
    ip->tot_len = htons(iphdrlen + tcphdrlen);
    ip->id = 0;
    ip->ttl = 40;
    ip->protocol = IPPROTO_TCP;
    ip->saddr = inet_addr("192.168.165.135");
    ip->daddr = inet_addr(argv[1]);
    ip->check = 0;

    tcp->source     = htons(12345);
    tcp->dest       = htons(80);
    tcp->seq        = random();
    tcp->doff       = 5;
    tcp->ack        = 0;
    tcp->psh        = 0;
    tcp->rst        = 0;
    tcp->urg        = 0;
    tcp->syn        = 1;
    tcp->fin        = 0;
    tcp->window     = htons(65535);

    pseudoheader.src_addr = ip->saddr;
    pseudoheader.dst_addr = ip->daddr;
    pseudoheader.padding = 0;
    pseudoheader.proto = ip->protocol;
    pseudoheader.length = htons(tcphdrlen + datalen);

    tcp_chk_construct.pshd = pseudoheader;
    tcp_chk_construct.tcphdr = *tcp;

    int checksum = comp_chksum((unsigned short*) &tcp_chk_construct,
            sizeof(struct pseudohdr) + tcphdrlen + datalen);

    tcp->check = checksum;

    printf("TCP Checksum: %i\n", checksum);
    printf("Destination : %i\n", ntohs(tcp->dest));
    printf("Source: %i\n", ntohs(tcp->source));

    to.sin_addr.s_addr = ip->daddr;
    to.sin_family = AF_INET;
    to.sin_port = tcp->dest;

    bytes = sendto(sock, buffer, ntohs(ip->tot_len), 0, (struct sockaddr*) &to,
            sizeof(to));

    if (bytes == -1) {
        perror("sendto() failed");
        return 1;
    }

    recv(sock, buffer, sizeof(buffer), 0);
    printf("TTL= %d\n", ip->ttl);
    printf("Window= %d\n", tcp->window);
    printf("ACK= %d\n", tcp->ack);
    printf("%s:%d\t --> \t%s:%d \tSeq: %d \tAck: %d\n",
                    inet_ntoa(*(struct in_addr*) &ip->saddr), ntohs(tcp->source),
                    inet_ntoa(*(struct in_addr *) &ip->daddr), ntohs(tcp->dest),
                    ntohl(tcp->seq), ntohl(tcp->ack_seq));

    return 0;
}

Answer 1

您正在buffer接收和存储数据包，但是您正在打算来自ip和tcp的数据而不解析该缓冲区。您应该在收到之后解析来自buffer 的数据包，然后在打印之前解析。

您的代码假定所有数据包都是TCP，但实际情况并非如此。 RAW套接字仅支持第3层协议（IP，ICMP等）。换句话说，在创建RAW套接字时使用IPPROTO_TCP会产生误导。坚持IPPROTO_IP，并为您关注的每个协议（TCP，UDP等）的代码添加必要的条件。这恰好是有效的，因为Linux内核验证协议号，并回退到IPPROTO_IP。但是，这可能不适用于其他系统。

检查您的网络通信是否使用了正确的字节顺序。网络字节顺序是Big-Endian，而主机字节顺序取决于您的体系结构，因此您可能需要来回转换多字节字段。

您的tcp->seq可能包含无效值，因为TCP仅接受最多65535的值，而random()则返回0到RAND_MAX（0x7fffffff）的值。试试tcp->seq = htonl(random() % 65535);

您对TCP标头的偏移计算不正确。它应该是sizeof(struct iphdr)而不是sizeof(struct tcphdr)。

Answer 2

ip = (struct iphdr*) buffer;
tcp = (struct tcphdr*) (buffer + sizeof(struct tcphdr));

这里要获取buffer中tcp标头的数组索引，您需要将sizeof(struct iphdr)添加到buffer，如下所述。

ip = (struct iphdr*) buffer;
tcp = (struct tcphdr*) (buffer + sizeof(struct iphdr));

Raw Socket Linux发送/接收数据包

2 个答案: