发送raw tcp包

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struct iphdr {
	uint8_t ihl :4,
		 version:4;
	uint8_t tos;
	uint16_t tot_len;
	uint16_t id;
	uint16_t frag_off;
	uint8_t ttl;
	uint8_t protocol;
	uint16_t check;
	int32_t saddr;
	int32_t daddr;
};

struct tcphdr {
	uint16_t source;
	uint16_t dest;
	uint32_t seq;
	uint32_t ack_seq;
	uint16_t res1:4,
		doff:4,
		fin:1,
		syn:1,
		rst:1,
		psh:1,
		ack:1,
		urg:1,
		ece:1,
		cwr:1;
	uint16_t window;
	uint16_t check;
	uint16_t urg_ptr;
};

struct pseudo_header
{
    u_int32_t source_address;
    u_int32_t dest_address;
    u_int8_t placeholder;
    u_int8_t protocol;
    u_int16_t tcp_length;
};

static uint16_t checksum(uint8_t* buf, size_t size)
{    
    int res = 0;
    uint16_t* ptr = (uint16_t*)buf;
    
    for (int i = 0; i < size / 2; i++) {
        res += ptr[i];
    }
    
    if (size & 1) {
        res += buf[size - 1];
    }
    
    res = (res >> 16) + (res & 0xFFFF);
    res += (res >> 16);
    
    return ~res; 
}

static int send_rst(std::string ip, uint16_t port)
{
    int fd;
    uint8_t buf[128];
    uint8_t psb[128];
    struct pseudo_header psh;
    struct iphdr* iph = (struct iphdr*)buf;
    struct tcphdr* tcph = (struct tcphdr*)(buf + sizeof(struct iphdr));
    struct sockaddr_in dest;
    int val;

    fd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
    if (fd < 0) {
        printf("socket open failed\n");
        return -1;
    }
    
    val = 1;
    setsockopt(fd, IPPROTO_IP, IP_HDRINCL, &val, sizeof(val));
    
    memset(buf, 0, sizeof(buf));

    iph->ihl = sizeof(struct iphdr) >> 2;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = htons(sizeof(struct iphdr) + sizeof(struct tcphdr));
    iph->id = htonl(54321);
    iph->frag_off = 0;
    iph->ttl = 255;
    iph->protocol = IPPROTO_TCP;
    iph->saddr = inet_addr("192.168.1.1");
    iph->daddr = inet_addr(ip.c_str());
    iph->check = 0;
    iph->check = checksum((uint8_t*)iph, sizeof(struct iphdr));

    tcph->source = htons(7000);
    tcph->dest = htons(port);
    tcph->seq = 123456789;
    tcph->ack_seq = 0;
    tcph->doff = sizeof(struct tcphdr) >> 2;
    tcph->rst = 1;
    tcph->window = htons(5840);
    tcph->check = 0;
    tcph->urg_ptr = 0;
    
    psh.source_address = iph->saddr;
    psh.dest_address = iph->daddr;
    psh.placeholder = 0;
    psh.protocol = IPPROTO_TCP;
    psh.tcp_length = htons(sizeof(struct tcphdr));
    
    memcpy(psb, &psh, sizeof(struct pseudo_header));
    memcpy(psb + sizeof(struct pseudo_header), tcph, sizeof(struct tcphdr));
    
    tcph->check = checksum(psb, sizeof(struct pseudo_header) + sizeof(struct tcphdr));
     
    dest.sin_family = AF_INET;
    dest.sin_port = tcph->dest;
    dest.sin_addr.s_addr = iph->daddr;

    if (sendto(fd, buf, sizeof(struct iphdr) + sizeof(struct tcphdr), 0, (struct sockaddr*)&dest, sizeof(dest)) < 0) {
        printf("send failed: %s\n", strerror(errno));
    }

    close(fd);
    return 0;
}