柚子快報(bào)邀請(qǐng)碼778899分享：網(wǎng)絡(luò) DNS和UDP編程的實(shí)現(xiàn)

http://yzkb.51969.com/

1.DNS的報(bào)文頭部格式

2.wireshark的顯示

在抓以太網(wǎng)包之后，通過(guò)篩選DNS并停止抓包，雙擊一個(gè)包，有：

圖1中從上到下，依次是物理層，鏈路層，網(wǎng)絡(luò)層，傳輸層和應(yīng)用層。

Domain Name System和圖一對(duì)應(yīng)，并且request包和對(duì)應(yīng)的response包的Transcation ID是一樣的。

In表示internat，代號(hào)為1 。

3.c代碼實(shí)現(xiàn)

Query? name的格式：

在查找域名時(shí)，使用類似哈夫曼樹的方式。以0為根，然后查找w，然后w，然后w。以此類推。

定義頭部和Query數(shù)據(jù)結(jié)構(gòu)：

typedef struct DNS_header {

?? ?unsigned short ID;

?? ?unsigned short Flags;

?? ?unsigned short Questions;

?? ?unsigned short Answer;

?? ?unsigned short Authority;

?? ?unsigned short Addtional;

};

typedef struct Query {

?? ?int length;

?? ?unsigned short Type;

?? ?unsigned short Class;

char* name;

};

這里由于name長(zhǎng)度不確定，所以使用length和char*name；

創(chuàng)建header：

int Create_DNS_Header(DNS_header* header) {

if (!header)return -1;

memset(header, 0, sizeof(DNS_header));

srandom(time(NULL));

header->ID = random();

header->Questions = 1;

header->Flags = htons(0X0100);

return 0;

}

注意： ?? ?srandom(time(NULL));

? ? ?header->ID = random();

不是線程安全的，如果?srandom(time(NULL));后其他線程使用random()，則會(huì)重新在隨機(jī)種子中隨機(jī)。

?? ?header->Flags = htons(0X0100); 把大端和小端的存儲(chǔ)方式統(tǒng)一成網(wǎng)絡(luò)傳輸格式。

創(chuàng)建Query：

int Create_DNS_Query(Query* q, char* name) {

if (!q || !name)return -1;

memset(q, 0, sizeof(Query));

q->Type = htons(1);

q->Class = htons(1);

q->length = strlen(name) + 2;

char split = '.';

q->name = NULL;

char* token;

char* namedup = strdup(name);

token = strtok(name, split);

char* token_name = q->name;

while (token) {

size_t size = strlen(token);

*token_name = size;

token_name++;

strncpy(token_name, token, size+1);

token_name += size;

token = strtok(NULL, split);

}

free(namedup);

return 0;

}

strncpy后的size+1以及q-

注意：

? ? ? ? strtok會(huì)把結(jié)果保存，不是一個(gè)線程安全的函數(shù)。

所有的代碼（cpp）：

#include

#include

#include

#include

#include

#include

#include

#include

#include

#include

#define DNS_HOST 0x01

#define DNS_CNAME 0x05

#define DNSPORT 53

#define DNSIP "114.114.114.114"

typedef struct DNS_Header {

unsigned short ID;

unsigned short Flags;

unsigned short Questions;

unsigned short Answer;

unsigned short Authority;

unsigned short Addtional;

};

struct dns_item {

char* domain;

char* ip;

};

typedef struct DNS_Query {

int length;

unsigned short Type;

unsigned short Class;

char* name;

};

int Create_DNS_Header(DNS_Header* header) {

if (!header)return -1;

memset(header, 0, sizeof(DNS_Header));

srandom(time(NULL));

header->ID = random();

header->Questions = htons(1);

header->Flags = htons(0X0100);

return 0;

}

int Create_DNS_Query(DNS_Query* q, char* name) {

if (!q || !name)return -1;

memset(q, 0, sizeof(DNS_Query));

q->Type = htons(1);

q->Class = htons(1);

q->length = strlen(name) + 2;

char split[2] = ".";

q->name = (char*)malloc(strlen(name) + 2);;

char* token;

char* namedup = strdup(name);

token = strtok(name, split);

char* token_name = q->name;

while (token) {

size_t size = strlen(token);

*token_name = size;

token_name++;

strncpy(token_name, token, size+1);

token_name += size;

token = strtok(NULL, split);

}

free(namedup);

return 0;

}

int Bind_Header_Query(DNS_Query* query, DNS_Header* header, char* request) {

size_t header_size = sizeof(DNS_Header);

memcpy(request, header, header_size);

size_t offset = header_size;

memcpy(request+ offset, query->name, query->length);

offset += query->length;

memcpy(request+ offset, &query->Type, sizeof(query->Type));

offset += sizeof(query->Type);

memcpy(request+ offset, &query->Class, sizeof(query->Class));

offset += sizeof(query->Class);

return offset;

}

static int is_pointer(int in) {

return ((in & 0xC0) == 0xC0);

}

static void dns_parse_name(unsigned char* chunk, unsigned char* ptr, char* out, int* len) {

int flag = 0, n = 0, alen = 0;

char* pos = out + (*len);

while (1) {

flag = (int)ptr[0];

if (flag == 0) break;

if (is_pointer(flag)) {

n = (int)ptr[1];

ptr = chunk + n;

dns_parse_name(chunk, ptr, out, len);

break;

}

else {

ptr++;

memcpy(pos, ptr, flag);

pos += flag;

ptr += flag;

*len += flag;

if ((int)ptr[0] != 0) {

memcpy(pos, ".", 1);

pos += 1;

(*len) += 1;

}

}

}

}

static int dns_parse_response(char* buffer, struct dns_item** domains) {

int i = 0;

unsigned char* ptr = (unsigned char*)buffer;

ptr += 4;

int querys = ntohs(*(unsigned short*)ptr);

ptr += 2;

int answers = ntohs(*(unsigned short*)ptr);

ptr += 6;

for (i = 0; i < querys; i++) {

while (1) {

int flag = (int)ptr[0];

ptr += (flag + 1);

if (flag == 0) break;

}

ptr += 4;

}

char cname[128], aname[128], ip[20], netip[4];

int len, type, ttl, datalen;

int cnt = 0;

struct dns_item* list = (struct dns_item*)calloc(answers, sizeof(struct dns_item));

if (list == NULL) {

return -1;

}

for (i = 0; i < answers; i++) {

bzero(aname, sizeof(aname));

len = 0;

dns_parse_name((unsigned char*)buffer, ptr, aname, &len);

ptr += 2;

type = htons(*(unsigned short*)ptr);

ptr += 4;

ttl = htons(*(unsigned short*)ptr);

ptr += 4;

datalen = ntohs(*(unsigned short*)ptr);

ptr += 2;

if (type == DNS_CNAME) {

bzero(cname, sizeof(cname));

len = 0;

dns_parse_name((unsigned char*)buffer, ptr, cname, &len);

ptr += datalen;

}

else if (type == DNS_HOST) {

bzero(ip, sizeof(ip));

if (datalen == 4) {

memcpy(netip, ptr, datalen);

inet_ntop(AF_INET, netip, ip, sizeof(struct sockaddr));

printf("%s has address %s\n", aname, ip);

printf("\tTime to live: %d minutes , %d seconds\n", ttl / 60, ttl % 60);

list[cnt].domain = (char*)calloc(strlen(aname) + 1, 1);

memcpy(list[cnt].domain, aname, strlen(aname));

list[cnt].ip = (char*)calloc(strlen(ip) + 1, 1);

memcpy(list[cnt].ip, ip, strlen(ip));

cnt++;

}

ptr += datalen;

}

}

*domains = list;

ptr += 2;

return cnt;

}

int Send_Client_Commit(char* domain) {

int out = socket(AF_INET, SOCK_DGRAM, 0);

if (out < 0) {

printf("Socket error: %s\n", strerror(errno));

return -1;

}

sockaddr_in skaddr = { 0 };

skaddr.sin_family = AF_INET;

skaddr.sin_port = htons(DNSPORT);

skaddr.sin_addr.s_addr = inet_addr(DNSIP);

DNS_Query query = { 0 };

DNS_Header header = { 0 };

int ret = Create_DNS_Query(&query, domain);

if (ret) {

return -2;

}

ret = Create_DNS_Header(&header);

if (ret) {

return -3;

}

connect(out, (struct sockaddr*)&skaddr, sizeof(sockaddr_in));

char request[1024] = { 0 };

int length = Bind_Header_Query(&query, &header, request);

if (length<0) {

return -4;

}

int size = sizeof(sockaddr);

ret = sendto(out, request, length, 0, (struct sockaddr*)&skaddr, sizeof(struct sockaddr));

if (ret==-1) {

return -5;

}

char response[1024] = { 0 };

sockaddr_in add;

size_t addr_len = sizeof(struct sockaddr_in);

ret = recvfrom(out, response, sizeof(response), 0, (struct sockaddr*)&add, (socklen_t*)&addr_len);

struct dns_item* dns_domain = NULL;

dns_parse_response(response, &dns_domain);

free(dns_domain);

return 0;

}

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

if (argc < 2) {

printf("input error");

return -1; }

Send_Client_Commit(argv[1]);

return 0;

}

柚子快報(bào)邀請(qǐng)碼778899分享：網(wǎng)絡(luò) DNS和UDP編程的實(shí)現(xiàn)

http://yzkb.51969.com/

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