How to Clear/Flush the DNS Cache in Win32 API's

user_2743345 0
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I am looking for a way to programmatically clear/flush the local win32 dns cache (Equivalent of calling "ipconfig /flushdns").

There were ways to do this with a hidden API in winsock.dll but winsock.dll is no longer part of Windows and as such this method will no longer work.

Does anyone know how this should be done now?

dns winapi
1 Answers
  • Checked ipconfig.exe's dependencies using Dependency Walker
  • Found dnsapi.dll among them
  • Checked its exported functions, and found DnsFlushResolverCache
  • Shallowly browsed the web, and found its signature (only found references like this on official site: [MS.Docs]: Windows 8 API Sets), meaning it's not public, so software relying on it, is not robust)
  • Created a small test program

main00.c:

#include <stdio.h>
#include <Windows.h>

typedef BOOL (WINAPI *DnsFlushResolverCacheFuncPtr)();


int main() {
    HMODULE dnsapi = LoadLibrary("dnsapi.dll");
    if (dnsapi == NULL) {
        printf("Failed loading module: %d\n", GetLastError());
        return -1;
    }
    DnsFlushResolverCacheFuncPtr DnsFlushResolverCache = (DnsFlushResolverCacheFuncPtr)GetProcAddress(dnsapi, "DnsFlushResolverCache");
    if (DnsFlushResolverCache == NULL) {
        printf("Failed loading function: %d\n", GetLastError());
        FreeLibrary(dnsapi);
        return -2;
    }
    BOOL result = DnsFlushResolverCache();
    if (result) {
        printf("DnsFlushResolverCache succeeded\n");
    } else {
        printf("DnsFlushResolverCache succeeded: %d\n", GetLastError());
    }
    FreeLibrary(dnsapi);
    return 0;
}

Output:

e:\Work\Dev\StackOverflow\q052007372>"c:\Install\x86\Microsoft\Visual Studio Community\2015\vc\vcvarsall.bat" x64

e:\Work\Dev\StackOverflow\q052007372>dir /b
dnsapi_func_list.txt
main00.c

e:\Work\Dev\StackOverflow\q052007372>cl /nologo main00.c  /link /OUT:main00.exe
main00.c

e:\Work\Dev\StackOverflow\q052007372>dir /b
dnsapi_func_list.txt
main00.c
main00.exe
main00.obj

e:\Work\Dev\StackOverflow\q052007372>main00.exe
DnsFlushResolverCache succeeded

Note: Even if the function call completed successfully, I am not sure how to check whether it did what it's supposed to do (or better: what its name suggests it should do, which seems to be what you need).
Let me know how it works.


Update #0

Thank you for the info @TimJohnson!! I was too in a rush to look at ipconfig /? ([MS.Docs]: ipconfig) output (which I had in another cmd window :d ) and notice the option :) .
It does work (the cache is heavily updated, and I can see differences before and after running the program) !!!

user_47885460
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