A solution to WinRM in a NLB cluster…

I’ve written a couple of posts discussing the remoting options for PowerShell:
• fan-out model – Windows Remote Management service (WinRM)
• fan-in model – IIS hosted PowerShell endpoint (using the IIS WinRM extension)

When running load balanced WCF services in IIS that are secured using Windows Authentication, the web applications are mapped to app pools that use a domain account. This is required by kerberos to ensure that the encrypted messages can be decoded using a common set of credentials. By default, the HTTP SPN would be registered against the machine account, however this is changed to map to the domain account. This broke WinRM which is also an HTTP endpoint but runs as the network service, therefore the kerberos authentication failed because it is expected to be running under the domain account.

PowerShell supports two machine name formats, when setting the Invoke-Command -ComputerName parameter: the NETBIOS name and the fully qualified domain name (FQDN). To be able to call the WinRM service and authenticate using kerberos, you need to use the machine name format that is not used in the SPN. For example, if

HTTP/myserver.domain.com

is the SPN registered against the domain account used by the application pools, then

PS>icm -ComputerName myserver.domain.com -scriptblock { ‘foo’}

will fail, however

PS>icm -ComputerName myserver -ScriptBlock {‘foo’}

will succeed. It works because the SPN must be an exact match for the machine name used (though case insensitive on Windows). If HTTP/myserver was registered, the command would fail. [I tried using the IP address too but PowerShell reports an error saying it does not support that scenario unless the IP address is in the TrustedHosts list]. This is still a little ‘magic’ and the better way to do this is to enable CredSSP in PowerShell.

This discovery removes the need to use the fan-in model, which we’ve found to be more problematic than the WinRM Windows Service:
• Cannot use the IIS:/AppPools/ path, returns no results
• Cannot use IIS:/Sites/, throws a COM exception
• AppPool identity must have ‘Generate Security Audit’ right on the machine
• Intermittent failures with the Windows Process Activation

Another recent discovery is around the effect of the NETBIOS name with IE zone security. If a resource is consider to be outside the local intranet or trusted sites zone, then kerberos does not work – the ticket is not issued. Therefore using the FQDN requires the domain to be added to the local intranet zone sites. The use of the NETBIOS name however is considered to be within the local intranet zone and therefore no amendment to the zones are required.

One last tangential gotcha… it is possible to extend the probe path that IIS uses when looking for assemblies beyond the standard bin directory.

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
        <probing privatePath="bin;SharedBin" />
    </assemblyBinding>
</runtime>

However files not in the normal bin directory are not shadow copied, therefore you can get file locking that you don’t expect when updating the files – in the case above, the SharedBin.

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