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#include <winsock2.h>
#include <iphlpapi.h>
#include <string.h>
#include <windows.h>
#include <string.h>
#define MALLOC(x) HeapAlloc(GetProcessHeap(), 0, (x))
#define FREE(x) HeapFree(GetProcessHeap(), 0, (x))
// Fills `dnsAddresses` with the DNS addresses found, up to `bufferLen`.
// Returns NO_ERROR (0x0) in case the operation succeeds, otherwise a non-zero
// error code. See: https://msdn.microsoft.com/en-us/library/windows/desktop/ms681382(v=vs.85).aspx
DWORD getWindowsDefDnsServers(char* dnsAddresses, size_t bufferLen) {
FIXED_INFO *pFixedInfo;
ULONG ulOutBufLen;
DWORD dwRetVal;
if (bufferLen <= 0) return ERROR_NOT_ENOUGH_MEMORY;
pFixedInfo = (FIXED_INFO *) MALLOC(sizeof (FIXED_INFO));
if (pFixedInfo == NULL)
return ERROR_NOT_ENOUGH_MEMORY;
ulOutBufLen = sizeof (FIXED_INFO);
// Make an initial call to GetAdaptersInfo to get the necessary size into the
// ulOutBufLen variable
if (GetNetworkParams(pFixedInfo, &ulOutBufLen) == ERROR_BUFFER_OVERFLOW) {
FREE(pFixedInfo);
pFixedInfo = (FIXED_INFO *) MALLOC(ulOutBufLen);
if (pFixedInfo == NULL)
return ERROR_NOT_ENOUGH_MEMORY;
}
dwRetVal = GetNetworkParams(pFixedInfo, &ulOutBufLen);
if (dwRetVal == NO_ERROR) {
int offset = 0;
int spaceAvailable = bufferLen;
IP_ADDR_STRING* head = &pFixedInfo->DnsServerList;
int count = 0;
while (head != NULL) {
int ipLen = strlen(head->IpAddress.String);
int copySize = ipLen + 1;
spaceAvailable -= copySize;
if (spaceAvailable >= 0) {
// Write the separator.
// The string is already terminated due to the call to
// strcpy_s, which copies the null terminator.
if (count != 0) dnsAddresses[offset - 1] = ',';
// Copy the IP address, including the null terminator.
strcpy_s(dnsAddresses + offset, copySize, head->IpAddress.String);
if (spaceAvailable == 0) break;
} else
break;
offset += copySize;
count++;
head = head->Next;
}
}
if (pFixedInfo) FREE(pFixedInfo);
return dwRetVal;
}
/*
// Test with 'gcc -o dnsServer -Wall -Werror -pedantic -liphlpapi -Iinclude dns.c' on a
// Windows machine.
int main(){
printf(getWindowsDefDnsServers()->dnsAddresses);
return 0;
}*/
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