Linux C++ 发送DHCP报文获取DHCP信息

发送DHCP报文获取DHCP信息的方法如下：

1. 创建原始套接字：

int sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
if (sock < 0) {
    perror("socket");
    exit(1);
}

2. 设置套接字选项：

int on = 1;
if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) < 0) {
    perror("setsockopt");
    exit(1);
}

3. 构造DHCP报文：

struct dhcp_packet {
    uint8_t  op;           /* op: Message opcode/type */
    uint8_t  htype;        /* htype: Hardware addr type */
    uint8_t  hlen;         /* hlen: Hardware addr length */
    uint8_t  hops;         /* hops: Number of relay agent hops from client */
    uint32_t xid;          /* xid: Transaction ID */
    uint16_t secs;         /* secs: Seconds since client started trying to boot */
    uint16_t flags;        /* flags: Flag bits */
    struct in_addr ciaddr; /* ciaddr: Client IP address (if already in use) */
    struct in_addr yiaddr; /* yiaddr: Your (client) IP address */
    struct in_addr siaddr; /* siaddr: Next server to use in bootstrap */
    struct in_addr giaddr; /* giaddr: Relay agent IP address */
    uint8_t  chaddr[16];   /* chaddr: Client hardware address */
    uint8_t  sname[64];    /* sname: Server host name */
    uint8_t  file[128];    /* file: Boot file name */
    uint8_t  options[312]; /* options: Optional parameters (actual length dependent on MTU). */
} dhcp_packet;

memset(&dhcp_packet, 0, sizeof(dhcp_packet));
dhcp_packet.op = BOOTREQUEST;
dhcp_packet.htype = ARPHRD_ETHER;
dhcp_packet.hlen = ETH_ALEN;
dhcp_packet.xid = htonl(random());
memcpy(dhcp_packet.chaddr, mac_address, ETH_ALEN);

在构造DHCP报文时，需要填写一些固定的字段，如op、htype、hlen、xid、chaddr等，还需要填写一些可选参数，如sname、file、options等。其中，mac_address是客户端的MAC地址。

4. 发送DHCP报文：

struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_port = htons(67);
addr.sin_addr.s_addr = INADDR_BROADCAST;
if (sendto(sock, &dhcp_packet, sizeof(dhcp_packet), 0, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
    perror("sendto");
    exit(1);
}

在发送DHCP报文时，需要指定目的地地址为广播地址，即INADDR_BROADCAST。

5. 接收DHCP响应报文：

struct sockaddr_in from;
socklen_t fromlen = sizeof(from);
char buf[1024];
ssize_t n = recvfrom(sock, buf, sizeof(buf), 0, (struct sockaddr*)&from, &fromlen);
if (n < 0) {
    perror("recvfrom");
    exit(1);
}

在接收DHCP响应报文时，需要设置接收缓冲区大小，一般建议设置为1024字节。收到响应报文后，可以解析报文中的各个字段，如yiaddr、siaddr、giaddr等，从而获取DHCP服务器分配的IP地址等信息。

以上就是发送DHCP报文获取DHCP信息的基本步骤。注意，发送DHCP报文需要root权限才能执行。