Tag Archives: kerberos

Chrome almost supports SSO in Windows Kerberos environments

I was pleasantly surprised to find that Google Chrome has support for SSO and the Negotiate algorithm. Indeed it also has support for NTLM. So why the need for this post? I think the implementation could do with a little refinement.

Here’s my assumption. Credential delegation in a Kerberos environment is managed by the Kerberos system and its configuration, clients should not attempt to interfere with it. However, Google Chrome disallows ticket forwarding by default, effectively preventing delegation (constrained or otherwise). You can change this with an option on the command line but that means you have to know the option exists and have to plan to change it for every user of your web site. Seems the wrong way round to me. This default means that, out of the box, most web sites of any complexity will not operate as per their intended design.

Secondly, the default SPN behaviour is incorrect for Windows platforms. The Kerberos specification does not say much about SPNs, but they do at least have several parts: the service type, the host and port, and optionally an additional service identifier. Including the port is standard, but Chrome doesn’t do this by default. Secondly, the Chrome default behaviour is to resolve DNS CNAME records to A records and use this for the host part. I can’t fault Google for this approach but it does differ from the widely documented Windows approach of using SPNs for the host header (i.e. before CNAME resolution). (As an aside, note that if you take that approach then why shouldn’t you use the IPv4 address, or the IPv6 address, and what if the machine is multi-homed?). It also interferes with the ability of a host to provide multiple independent services because with the Google approach they all have identical SPNs. In Chrome’s defense, these options can also be controlled via the command line.

Finally, note that NTLMv2 is only available on Windows platforms. Chrome supports NTLMv1 on other platforms but that is horrendously insecure! This is not intended as a negative comment on Chrome, just something to be aware of.

It is great to see other browsers finally supporting SSO, Negotiate, NTLM and Kerberos. I just hope that interoperability is considered a desirable end goal. Without it these are just more competing proprietary solutions, and that would be a shame.

Material about Google Chrome was taken from here: HTTP authentication [The Chromium Projects]. See my recent post about Kerberos in Windows for links to supporting Windows implementation materials.

Ten tips for using and configuring Kerberos authentication on Windows

Lately, I’ve been having some fun with Kerberos in Windows/Active Directory. Fun might not be the best way to describe it, but I thought I’d spend a few moments capturing some of what I’ve learnt in the past few days.

Tip 1. Debugging Kerberos issues is very hard. I recommend that you don’t change anything without making a note of what you did and also what side-effects it might cause. Also keep track of whether you restarted any services or servers, whether you emptied any caches, etc. Otherwise, you might not be able to interpret your results.

Tip 2. Premature success is evil. If you’ve changed something and you are testing whether the configuration is working, you had better make sure that your test results aren’t due to the previous behaviour being cached. This is much worse than having a step that fails. So, if you think something is working, test thoroughly before moving on to the next step or declaring victory!

Tip 3. Using custom service accounts is a common trouble spot. In theory, a correctly configured service account should work just like a computer account. My experience is that sometimes they don’t. At the time of writing, I don’t know why not. Everything I can think of has been checked. One major consideration is the distinction between kernel-mode and user-mode code execution.

Tip 4. Capture network traces. It can be useful to see whether a Kerberos negotiation actually takes place, or if the client abandons Kerberos in favour of NTLM authentication. Sometimes, this can be caused by the Kerberos token cache on the client machine answering the request. This may be fine, or it may have an old configuration cached. Execute klist purge using an elevated administrator account. Both WireShark and NetMonitor are good tools for this. Use your preferred tool but make sure you learn how to use it effectively. Both tools can help you identify communication sessions or filter the trace to a set of protocols or addresses.

Tip 5. Make sure your DNS configuration is correct. I’ve often seen Windows clients set to load-balance between public and corporate DNS servers. This is an incorrect configuration. The Windows DNS client only uses the alternate server if the former cannot respond to a query. It is assumed that both would provide identical results. In a recent case, I saw a public DNS providing records for a the DC’s own test domain that wasn’t intended to be public (because there was a real public registration for the FQDN). Use .local domains unless you need Apple Mac integration (the Rendezvous service had problems with this in the past). The DNS specification lists .local addresses as private registrations. This is the DNS equivalent of private IP ranges. Note that Windows clients use DNS to identify the appropriate Kerberos servers.

Tip 6. Don’t just restart application pools in IIS. Restarting an application pool is a quick way of restarting a web-site. However, it is flawed. Restarting an application pool does not restart the entire user-mode stack. In particular, you need to pay attention to Windows Activation Services (WAS). Make sure this service is restarted when testing. Don’t forget klist purge, either.

Tip 7. Check your SPNs whenever a configuration is changed. In some cases, I believe, IIS configures SPNs for you. However, sometimes these can become out of sync. So check. Use setspn.exe -L [accountname] to review.

Tip 8. Check your Allowed-To-Delegate-To configuration. In Windows 2008 R2, these views in Active Directory Users & Computers show you whether the account supports delegation, whether it is constrained and whether any protocol can be used.

Tip 9. Know your abbrebiations! If you don’t know the abbreviations, you can’t search effectively. S4U (the ‘Services for User’ Kerberos extension) is ‘Protocol Transition’. S4U2proxy (the ‘Services for User to Proxy’ Kerberos extension) is ‘Constrained Delegation’, also look for blog entries with the incorrect S4Uproxy abbreviation, missing the numeral ‘2’).

Tip 10. Don’t forget the rest. Unfortunately, ten tips isn’t enough to cover all the things you need to be aware of. Here are a few of the other things to consider:

  • Account option ‘Do not require Kerberos preauthentication’. You shouldn’t need to use this in a Windows environment. Kerberos protocol errors referring to KRB5KDC_ERR_PREAUTH_REQUIRED can usually be ignored. You should see a normal Kerberos negotiation following. Kerberos pre-authentication is used to validate the calling user’s identity.
  • Account option ‘This account is sensitive and cannot be delegated’. This will prevent delegation. It can be configured on service accounts, unless the service account needs to act as itself on a delegated service. If you are using impersonation, you may want this enabled because it will help to avoid false-positives.
  • IIS 7.5 authentication. There are new options to specify the protocols and other behaviours for Windows authentication. Make sure you review them. There is more information in the links below.
  • Try to test several different approaches. You may find that delegation to a file share is working but delegation to a web server is not. Don’t just follow one path. If things are working correctly then both approaches should work easily.
  • Windows servers use IPsec between servers and especially between domain controllers. I have no idea whether this can affect the success or failure of Kerberos interactions when running as a user account.
  • This is not a definitive guide! Sorry, but you are going to have to investigate and try things out. I recommend that you build an entirely clean, virtual environment to test your configuration. Also, try not to use it as an experimentation platform. Assume it is production and script or document everything. You need it to be reproducible.

References:

Finally, don’t forget that Kerberos relies on near-synchronisation of computer clocks. See my previous post Windows: The Windows Time Service.

Thanks for this article have to also go to several Microsoft engineers who have helped me to understand more about the implementation of Kerberos on Windows. You know who you are!