Consistent Redirects and Session State

One thing I learnt from experience to check when getting unexpected behaviour with sessions in ASP.Net is the redirects. I have worked on sites where the HTTP and HTTPS redirects are inconsistent and this can cause issues.
 
One of the more serious cases experiences was were a 'www' redirect was missing and redirect was being used to transfer to a third party site. In this case the user started a session using the address at http://websitedomain... and then transferred to the third party site.
 
When the user was redirected to the full web address http://www.websitedomain... a new session was started by IIS and the user lost their session and all information associated with it. This severely affected the user experience on the site.
 
Many issues with lost sessions are difficult to diagnose due to the symptoms being misleading. Fortunately the fix is quick and easy , simply specifying the redirects in the web.config for the site ensured the consistency of URL for all sessions on the site.
 
These rules can be put into IIS but these can be lost in some cases when the web.config is overwritten on a deployment. In the rules are in the web.config then there is the benefit of versioning in source control and consistency across web-farms.
 
 

Here is an example web.config entry for http to https and non-www to www redirects;

   <rewrite>
      <rules>
        <rule name="Redirect landing request to trailing slash" stopProcessing="true">
          <match url="landing\/(.*[^\/])$" />
          <action type="Redirect" url="{R:0}/" redirectType="Permanent" />
          <conditions>
            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" />
          </conditions>
        </rule>
        <rule name="Redirect to HTTPS" enabled="true" stopProcessing="true">
          <match url=".*" />
          <conditions logicalGrouping="MatchAll" trackAllCaptures="false">
            <add input="{HTTPS}" pattern="^OFF$" />
          </conditions>
          <action type="Redirect" url="https://{HTTP_HOST}/{R:0}" redirectType="Permanent" />
        </rule>
        <rule name="redirect non www to www" stopProcessing="true">
          <match url=".*" />
          <conditions  logicalGrouping="MatchAll" trackAllCaptures="false">
            <add input="{HTTP_HOST}" pattern="^domain.com$" />
          </conditions>
          <action type="Redirect" url="https://www.domain.com/{R:0}" />
        </rule>
      </rules>
    </rewrite>

ASP.NET Web Forms - Intermittent View State Error


On a web forms application on load balanced environment I recently had to investigate some errors for invalid viewstate. It was an intermittent error which only occurred when someone important used the site.

This became high profile quickly and very annoying, it has been a long time since I had to deal with view state thanks to MVC and it was difficult to reproduce. Eventually we found that clearing all caches and browser history allowed the error to be recreated.

The error message was a value cannot be null error;

System.Web.HttpException: Value cannot be null.
Parameter name: inputString [ArgumentNullException: Value cannot be null.Parameter name: inputString]
at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString, Purpose purpose)
at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter2 formatter, String serializedState, Purpose purpose)
at System.Web.UI.HiddenFieldPageStatePersister.Load()
[ViewStateException: Invalid viewstate. Client IP: 86.131.46.52 Port: 49531 Referer: https://*********.com/checkout/payment Path: *******.aspx User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko ViewState: ] [HttpException: The state information is invalid for this page and might be corrupted.]

The machinekey was in the web.config for both servers and the load balancing set up correctly. Looking at New Relic there was more viewstate errors which didn''t hit the application so were not logged.

The most likely solution found when googling was to move the machine key to the machine.config file at the server level C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config

The site web.config had the machine key removed and there is a web.config in the same directory as the machine.config , in one server this had the machine key and on another it didn''t. So it was removed from all.

This url has some useful information about the scope of the configuration files


ASP.NET 4.5 introduced cryptographic improvements, when the configuration was set wrong these were picked up and the following error was received.

System.Configuration.ConfigurationErrorsException: When using <machineKey compatibilityMode="Framework45" /> or the MachineKey.Protect and MachineKey.Unprotect APIs, the ''validation'' attribute must be one of these values: SHA1, HMACSHA256, HMACSHA384, HMACSHA512, or alg:[KeyedHashAlgorithm]. (C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config\web.config line 31)

Explicitly setting the validation key'' validation="SHA1" '' to ensure the correct encryption  setting  was picked up solved this.

There is more information about the improvements here;


But after all that reconfiguration  the error popped up once more in New Relic!

After some more investigation we found that the viewstate had been moved to the bottom of the page for SEO, on the page receiving the error it was possible to submit the page before it had completed loading and then error then occured. Moving the viewstate back to the top seems to have cured it but we are keeping a close eye on New Relic

Session State being lost in Web Farm

Investigating and issue with session data being lost on a load balance ASP.Net website the other day, users we''re losing items from a basket when bouncing between servers.

These issues have always been difficult to investigate but it turned out that when setting up the web servers there was multiple sites in IIS. These has been created in different order so had ended up with different site ids. A simple error but the symptoms were dramatic for users and difficult to recreate and trace.

This article details the issues https://support.microsoft.com/en-us/kb/325056 , although I did not have to use the described resolution.

I found that I could just change the site id through IIS, if a another site had the required id I renamed that until both web servers were is sync.

In IIS select the site and advanced settings; the edit the id to the required value




Sql Server Management Studio Freezing On Start Up

For a few weeks I had a really annoying issue with Sql Server Management Studio freezing on start up, this happened a few times at crucial moments and caused me delays.

I tried several re-installs and repairs and even the 2016 RC version, but it would just keep happening randomly.

Until I found his blog and following the second option to clear the profile information.


An bingo it never happened again, turns out the profile data is not removed on a uninstall.