Setting the Machine Key
Sometimes when using ASP.NET enabled website you may get the following error
Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
There are several ways to get around this problem:
1) Host your site on a server that never restarts or recycles! Obviously, this is impossible!
2) Disable ViewstateMac by putting this “enableViewStateMac="false"” in your web.config. This approach is not 100% secure.
3) Configure ASP.NET to not use Auto-Generated Key but rather a predefined key. This is the preferred method.
The third option is the most secure and the best one and we suggest to use this. There is a free Machine Key generator tool available at the following link
In order to generate the key using this tool Leave the defaults in place, validation key length 64, encryption key length 24, type sha1. Then Click Generate to create a random machine key for you and copy the result.
Now open the web.config file from your site. Find around line which shows <system.web> And paste in the <machineKey ... /> on the next line. Save that file.