Copying a struct with dynamic array into a buffer in c - c

Question

I want to manually build a dns query to the dns sever using socket APIs (it's part of the project and can not be changed). So I want to copy a struct like:
typedef struct {
uint16_t dns_id; /* identification number */
uint16_t dns_flags; /* DNS flags */
uint16_t dns_qdc; /* number of question entries */
uint16_t dns_anc; /* number of answer entries */
uint16_t dns_nsc; /* number of authority entries */
uint16_t dns_arc; /* number of resource entries */
unsigned char *host;
unsigned short qtype;
unsigned short qclass;
} DNS_QUERY;
DNS_QUERY* dns_query = (DNS_QUERY *) malloc(sizeof(DNS_QUERY));
into a buffer like:
char* dns_buf = malloc(500);
but when I do that, nothing has been copied into the buffer. When I input different strings to the *host element, the sizeof(dns_query) stays the same. When I use
memcpy(dns_buf, dns_query, sizeof(DNS_QUERY));
it gives output like:
dns_buf:
strlen(dns_buf)= 0
Also when I print it using for loop like:
for (i = 0 ; i<strlen(dns_buf) ; i++){
printf("%c", dns_buf[i]);
}
it doesn't output anything
When I use pointers instead of memcpy, like:
dns_buf = dns_query;
it gives the same output. Could anybody instruct me on what I should do?
This is the whole code:
int udp_connect(char *hostname, char *dns_name, char buf[]){
int sockfd;
int num_bytes;
struct sockaddr_in servaddr;
char str[INET_ADDRSTRLEN];
struct hostent *hptr; /* For gethostbyname() */
char **pptr; /* For inet_ntop() */
char* dns_buf, recv_buf = NULL;
int len = 0;
int i = 0;
static unsigned short id = 0; /* For the query ID */
/* Allocating memory for structs */
DNS_QUERY* dns_query = malloc(sizeof(DNS_QUERY));
/* QUESTION *dns_question = malloc(sizeof(QUESTION));*/
recv_buf = malloc(MAXDATASIZE);
if((hptr = gethostbyname(dns_name)) == NULL){
perror("Error in gethostbyname()\n");
exit(1);
}
if ((pptr = hptr->h_addr_list) != NULL) { /* (hptr->h_addrtype == AF_INET) && */
printf("The address is: %s\n", inet_ntop(hptr->h_addrtype, *pptr, str, sizeof(str)));
} else {
perror("Error in inet_ntop() \n");
}
memset(&servaddr, 0, sizeof (servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(DNS_PORT);
inet_pton(AF_INET, str, &servaddr.sin_addr);
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1){
perror("Error while making a socket");
return -1;
}
/* Setting hostname into the name segment of the query */
ChangetoDnsNameFormat(hostname);
dns_query->host = hostname;
printf("\ndns_query->host: %s\n", dns_query->host);
/* memcpy(dns_buf, dns_query, sizeof(DNS_QUERY)); */
/* setting up the header */
dns_query->dns_id = htons(id); /* id */
dns_query->dns_flags = htons(0x10); /* 0000000100000000 recursion is desired*/
dns_query->dns_qdc = htons(0x01); /* We have one question */
dns_query->dns_anc = htons(0x00);
dns_query->dns_nsc = htons(0x00);
dns_query->dns_arc = htons(0x00);
dns_query->qtype = htons(1); /* For IPv4 */
dns_query->qclass = htons(1); /* For internet */
len = sizeof(DNS_QUERY) + strlen(hostname); /* Calculating the length to use it in sendto() */
dns_buf = malloc(len);
dns_buf = dns_query;
/* memcpy(dns_buf, dns_query, sizeof(DNS_QUERY)); */
for (i = 0 ; i<sizeof(DNS_QUERY) ; i++){
printf("%02X", dns_buf[i]);
}
/* to check if the same as dns_query, use the following print also */
for (i = 0 ; i<sizeof(DNS_QUERY) ; i++){
printf("%02X", dns_query[i]);
}
print("\n");
/* Sending the datagram to the dns server */
printf("\n--------------------\nSending datagram: \n%s\n", dns_buf);
if ((num_bytes = sendto(sockfd, dns_buf, len, 0, (struct sockaddr *) &servaddr, sizeof(servaddr))) == -1){
perror("Error in sendto");
exit(1);
}
/* Receiving the datagram from the dns server */
printf("Receiving datagram...\n");
num_bytes = recvfrom(sockfd, recv_buf, MAXDATASIZE, 0, NULL, NULL);
printf("Received %i bytes of datagram...\n", num_bytes);
printf("dns_buf in udp connect: %s\n", recv_buf);
id = id + 1;
free(dns_query);
freeaddrinfo((struct addrinfo *) &servaddr);
close(sockfd);
return sockfd;
}
/******************************************************************************/
void ChangetoDnsNameFormat(char *hostname)
{
int walker=0;
int i;
int counter = 0;
char tmp[40];
strcat(hostname, "$"); /* For having a $ at the end of the dns name format */
for(i=0 ; i< (int) strlen((char*)hostname) ; i++)
{
if(hostname[i]=='.')
{
tmp[walker] = (char) counter + 48;
/*printf("%s\n", tmp);*/
for( ; walker < i ; walker++)
{
tmp[walker + 1] = hostname[walker];
}
walker++;
counter = -1;
}else if(hostname[i]=='$'){
tmp[walker] = (char) counter + 47;
/*printf("%s\n", tmp);*/
for( ; walker < i ; walker++)
{
tmp[walker + 1] = hostname[walker];
}
walker++;
counter = -1;
}
counter++;
}
walker--;
tmp[walker] = '\0'; /* Terminate the string */
strcat(tmp, "0"); /* For having a 0 at the end of the dns name format */
strcpy(hostname, tmp);
}

Answer 1

The struct doesn't really contain the array of character host but rather a pointer to the first element.
This means that, although you are copying the whole structure, you just copy the pointer to the data contained in host (which then points to the same string). What you shall do is something like:
DNS_QUERY *dns_buf = malloc(sizeof(DNS_QUERY));
memcpy(dns_buf, dns_query, sizeof(DNS_QUERY));
dns_buf.host = "yourmodifiedhost";

Answer 2

dns_buf is not a string (which finish with null charcter).
it's a pointer (as char *) to a memory space and the memory could contains 0 as values at the beginning so that's why when you use strlen(dns_buf) you get it = to 0 that means the fisrt element in the dns_buf is 0
to see the dns_buf content you can use:
for (i = 0 ; i<sizeof(DNS_QUERY) ; i++){
printf("%02X", dns_buf[i]);
}
print("\n");
// to check if the same as dns_query, use the following print also
for (i = 0 ; i<sizeof(DNS_QUERY) ; i++){
printf("%02X", dns_query[i]);
}
print("\n");