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Can't enter certain websites from China (DNS Cache-Poisoning)


Sometimes, although you are connected on a VPN, you may not be able to access certain blocked sites like YouTube or Facebook in China. The reason for this is most likely DNS Cache-Poisoning. As you may know every Hostname (e.g. youtube.com) leads to a specific IP-Address. As IPs are really hard to remember for humans your computer always asks a Nameserver which IP it should connect to. An often used analogy to explain the Domain Name System is that it serves as the "phone book" for the Internet by translating human-friendly computer hostnames into IP addresses.

Most DNS clients cache the results of name resolution requests. This means that the DNS client will temporarily store the website names that you have been visiting lately. This process speeds up name resolution if multiple lookups are done to the same address, which is common when browsing the web. If you visit Google.com a second time, the DNS client will already have the name mapped and will therefore retrieve the mapping quickly.

Sometimes a bad DNS entry will be cached and you will need to either flush the DNS cache to get rid of it, or you will want to delete the cache manually. Below is a comprehensive step-by-step process to flush the DNS.

How to Flush DNS in Microsoft Windows

In Microsoft Windows, you can use the command ipconfig /flushdns to flush the DNS resolver cache. Open the command prompt and type the following:

C:>ipconfig /flushdns
Windows IP Configuration
Successfully flushed the DNS Resolver Cache.

The above command will completely flush the DNS, deleting any incorrect entries too. You can also use the command ipconfig /displaydns to view the DNS resolver cache.

Turning Off DNS Caching with Microsoft Windows

If you experience frequent issues with DNS caching with Microsoft Windows, you can disable the client-side DNS caching with either of these two commands:

net stop dnscache 
sc servername stop dnscache

This will disable DNS caching until the next reboot. To make the change permanent, use the Service Controller tool or the Services tool to set the DNS Client service startup type to Disabled. You can permanently disable DNS Client by following the below steps:

·Go to Start and click on Run.

·Type Services.msc in the Run command box.

·A window listing all the services will popup. Search for a service called DNS Client.

·Double click on the listed DNS Client service and click Stop. Similarly, you can restart it by clicking Start.



Is the information incorrect or does it contradict what we say somewhere else at the site? These FAQs are a work in progress and unfortunately as a result our product updates and network changes we can't always get it right instantly in all the FAQs. Please let us know if you see something incorrect and we'll do everything we can to straighten out the misleading bits and pieces! None of these FAQs answer your question? Search our Knowledgebase or Ask Support.





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