如何清除System.Net客户端DNS缓存？

I'm using the .NET WebRequest while changing my HOSTS file. I'm observing that System.Net doesn't honor those changes - how can I make it do so?

我在更改我的HOSTS文件时使用的是.NET WebRequest。我观察到System.Net不尊重这些变化 - 我怎么能这样做呢？

I have a number of servers load-balanced behind a single hostname, let's say 'example.com'. I want to target several of them individually, so my program will hard-code the machine-specific IP address in my HOSTS file before sending a request to example.com:

我在一个主机名后面有一些负载均衡的服务器，比方说'example.com'。我想单独定位其中几个，所以我的程序会在向example.com发送请求之前在我的HOSTS文件中硬编码机器特定的IP地址：

163.56.0.34  example.com

For the first server and first request, this works fine. Then my program changes the HOSTS file again:

对于第一个服务器和第一个请求，这很好。然后我的程序再次更改HOSTS文件：

163.56.0.48  example.com

And I create a new HttpWebRequest. When I send this one off, I can observe in NETMON that it goes to the first IP address (163.56.0.34) instead of the expected second one.

我创建了一个新的HttpWebRequest。当我发送这个时，我可以在NETMON中观察到它转到第一个IP地址（163.56.0.34）而不是预期的第二个IP地址。

Using breakpoints and debug traces, I've verified that the correct value does get written to the HOSTS file each time. When I attempt to access example.com from a browser or other program, it does honor the HOSTS file and go to the second IP address.

使用断点和调试跟踪，我已经验证每次都将正确的值写入HOSTS文件。当我尝试从浏览器或其他程序访问example.com时，它确实尊重HOSTS文件并转到第二个IP地址。

Using NETMON I've verified that requests are going directly to the IP address shown; there is no HTTP proxy.

使用NETMON我已经验证了请求是直接进入显示的IP地址;没有HTTP代理。

Since everything else is honoring the changed HOSTS file, I strongly suspect that the System.Net infrastructure has cached the DNS host-IP association for example.com. However, I can find no reference to this caching, and know of no way to flush it or turn it off.

由于其他一切都是为了兑现已更改的HOSTS文件，因此我强烈怀疑System.Net基础结构已缓存example.com的DNS主机-IP关联。但是，我找不到对此缓存的引用，并且知道无法刷新或关闭它。

I would welcome instructions for dealing with the cache, suggestions for what else might be causing these symptoms, or other proposed diagnostic steps that might be useful.

我欢迎处理缓存的说明，可能导致这些症状的其他建议，或其他可能有用的诊断步骤。

4 个解决方案

#1

I finally dug up the obscure command from MSDN that fixes this:

我终于从MSDN中挖出了修复此命令的obscure命令：

ServicePointManager.DnsRefreshTimeout = 0;

As I unwound all the weird things I'd tried previously, I discovered one other setting that I need along with the one above; on the request object, turn off keep-alive:

当我解开之前尝试的所有奇怪的东西时，我发现了另外一个我需要的设置和上面的设置;在请求对象上，关闭keep-alive：

request.KeepAlive = false;

#2

If you want to keep the DnsRefreshTimeout > 0 then you can update the cache by making a call to:

如果要保持DnsRefreshTimeout> 0，则可以通过调用以下内容来更新缓存：

System.Net.Dns.GetHostEntry("example.com");

#3

you could use System.Diagnostics.Process to launch ipconfig /flushdns

您可以使用System.Diagnostics.Process启动ipconfig / flushdns

#4

A late answer to be sure, but I found this solution worked better than the accepted answer. In my scenario I am testing SQL connections, and I couldn't apply the existing answer since I have no request.KeepAlive to set.

一个迟到的答案可以肯定，但我发现这个解决方案比接受的答案更好。在我的场景中，我正在测试SQL连接，并且我无法应用现有的答案，因为我没有请求。要设置要保持活动状态。

This page by Brian Mancini ("derp turkey") details his adventure in clearing the DNS cache. Full props to him, I'm just adding his solution here:

本页由Brian Mancini（“derp turkey”）详细介绍了他清除DNS缓存的冒险经历。完全道具给他，我只是在这里添加他的解决方案：

public class DnsUtils  
{        
    [DllImport("dnsapi.dll", EntryPoint="DnsFlushResolverCache")]
    static extern UInt32 DnsFlushResolverCache();

    [DllImport("dnsapi.dll", EntryPoint = "DnsFlushResolverCacheEntry_A")]
    public static extern int DnsFlushResolverCacheEntry(string hostName);

    public static void FlushCache()
    {
        DnsFlushResolverCache();
    }

    public static void FlushCache(string hostName)
    {
        DnsFlushResolverCacheEntry(hostName);
    }
}

#1