How Kerberos Works

What happens at the windows logon screen between the time you press enter after entering your credentials and ‘Loading Your Personal Settings’ message appears

It hardly takes a second for your password to be accepted at the logon screen but what goes on behind the scenes to log you in on to your workstation in a domain environment will take much more than a second to explain.

My primary area of expertise is Active Directory but if you have read my previous blogs then you would know that I haven’t written anything on my favourite subject till now. That’s because so much has been written on AD that there is nothing new which I can write about.  Moreover, I don’t like to like to write a blog just to update my blog site. In order to write something, I need a subject which is not discussed on the web as much as other stuff is. This is because I want to contribute to the IT Infrastructure community in general and the Windows folks in particular through my blog and writing on subjects which have been written about zillions of times by great authors would not contribute anything to our community. One of the examples could be ABE (Access Based Enumeration) there is not too much about ABE on the internet apart from the Microsoft website and that came as a motivation for me to write about ABE. But, I always wanted to write about AD and now I have found something related to AD, which does not have too much written about it. It’s Kerberos, the preferred authentication protocol in Active Directory environments. This also gives me the opportunity to explain the behind the scenes action during a logon process.

Kerberos replaces LM, NTLM and NTLMV2 which were used in the pre Windows 2000 era (and are still used in some cases, we will come to that later). Massachusetts Institute of Technology (MIT) developed Kerberos to protect network services provided by Project Athena; Project Athena was a joint project of MIT, Digital Equipment Corporation, and IBM (my current employer) to produce a campus-wide distributed computing environment for educational use. It was launched in 1983, and research and development ran until June 30, 1991, eight years after it began. As of 2010, Athena is still in production use at MIT. Project Athena was important in the early history of desktop and distributed computing. It created the X Window System, Kerberos, and Zephyr Notification Service. It influenced the development of thin computing, LDAP, Active Directory, and instant messaging.

The name Kerberos comes from Greek mythology; it is the three-headed dog that guarded the entrance to Hades (hell), according to Hindu mythology, Lord Yama (God of Death) has a dog named ‘sarvara’, which sounds similar to kerberos. Why was this name chosen, remains a mystery to me.

Now, let’s get to know this dog better, Kerberos sees users (which are usually the client) as UPNs (User Principal Names) and services as SPNs (Service Principal Names), Your AD logon name – the one that looks like an email address (e.g., – is your UPN, Kerberos “introduces” UPNs to SPNs by giving a UPN a “ticket” to the SPN’s service. Let’s try to understand the user logon process through an example.  Let’s call our user OM, OM comes to office in the morning and starts his workstation. At the login screen he enters his username and password. At this point, his workstation sends a pre-authenticator to the Authentication Service (AS) of his local KDC (Key Distribution Centre), the KDC is better known as the domain controller, or we should say KDC is one of the roles of a domain controller. The KDC has two components, the Authentication Service or AS and the Ticket Granting Service or TGS. The pre-authenticator contains the current date and time in YYYYMMDDHHMMSSZ format, the Z in the end denotes that the date and time is in universal (zulu) time. This info is encrypted with OM’s password. Upon receiving the pre-authenticator, the AS decrypts the pre-authenticator using OM’s password, which it already has. If the AS is unable to decrypt the pre-authenticator then it means that the user entered the wrong password as that doesn’t match with the user’s password in the domain controller and hence the user receives a message saying his/her password is wrong. If the AS is able to decrypt the pre-authenticator then it compares the date and time inside with the DC’s own date and time, if the difference is not more than 5 minutes (default value, but can be changed) then AS sends the user a TGT (Ticket Granting Ticket). This is the reason that all domain joined machines need to have a time which is not different from the DC’s time by more or less than five minutes. The TGT is valid for 10 hours (default value but can be changed), This TGT contains a temporary password for the user and that is encrypted by the password of the krbtgt user account’s password. Krbtgt account is created by default when the DC is first installed. The user does not need to decrypt the TGT and hence it doesn’t need to know the krbtgt user’s password. As you might have noticed that the user has still not logged on to his workstation, that’s because the authentication process is yet to be completed. The next step in this process starts when the user sends the newly acquired TGT to the TGS, TGS is as we discussed earlier the second component of KDC, TGS decrypts the TGT and that confirms that OM is indeed OM and not an impersonator and then assigns OM a Service Ticket (ST) to his workstation. The service tickets generally contain time start, total lifetime, security token etc. This service ticket is encrypted with the workstation’s computer account password in AD. OM presents this ST to his workstation upon which his workstation decrypts it, as it has it’s own password and then allows OM to login. At this point OM sees the “Loading Your Personal Settings” message, then his profile gets loaded/created and then he is ready to work on his workstation. Now, to better understand Kerberos, let’s take this example a bit further and see what happens when OM tries to access a file server. When the user tries to access any particular service, like the file server or print server, it needs to authenticate itself to that particular server or service. This authentication process starts by the user sending his/her TGT to the TGS and upon verifying it, the TGS assigns a service ticket for the file server to OM, this ST is again encrypted with the password of the computer account of file server. OM presents this ST to the file server, file server decrypts it and then allows OM to view the list of shared folders on that file server. Whether OM is able to enter any or all of those shares depends on whether OM has the required NTFS and share permissions to those shares. This emphasises the fact that Kerberos is used for authentication and not for authorization.

Now, that we have understood how Kerberos works, let’s get to know, why is it considered so cool and better than NTLM. There are several reasons; we will discuss the two most important ones, one of them being that if you are using Kerberos then the user’s actual password is sent over the network just once in a day (or 10 hours to be exact). For the rest of the day the user uses his Ticket Granting Ticket (TGT) to authenticate for the various services that it might need. Second, would be the advanced encryption techniques available for Kerberos. If you are using Windows Server 2008 or above you can opt to use AES (Advanced Encryption Standard) which is one of the best generally available encryption techniques and it hasn’t been hacked yet. For, older versions of Windows you can use RC4 HMAC which isn’t a bad encryption technology either. Microsoft had to opt for a comparatively lesser encryption technology for older versions of Windows because till the late nineties it was unlawful in the United States to export software which used encryption technology beyond a certain level. LM and NTLM were ridiculously easy to hack through replay/mirror attacks, NTLMV2 was much better and Kerberos is extremely difficult or even impossible to hack.

It’s time to look at the scenarios where Kerberos is NOT used for authentication. The first scenario would be when you use an IP address in a UNC path. In this case Kerberos is not used because Kerberos needs SPNs and SPNs need DNS names. The second would be trying to connect to a computer which is in a workgroup. The third would be trying to connect to a pre windows 2000 computer. The most interesting scenario is when the domain controller is inundated with logon requests, it starts to login users with the previous authentication protocols to avoid the extra work that it needs to do with using Kerberos. I think this is one of the contributing reasons why Microsoft recommends the maximum hardware resources usage for domain controllers to be at 30%. But, How would you know whether you have been logged in with Kerberos or something else?? There are several indicators. If you are not logged in with Kerberos then you wouldn’t be able to add machines to domain, you won’t get any group policies etc. but the simplest way to find that out would be to run the command klist. Klist allows us to see the tickets that we currently have. It comes by default with Windows 7, Windows Server 2008 and later and can be installed on the previous versions. It’s part of the Windows Server 2003 resource kit. If you are not logged in with Kerberos then there won’t be any tickets.

This was the story behind what all happens within the blink of an eye during the logon process. I hope you liked what I had to share with you as my first blog on my favourite subject – Active Directory.

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IT in Retail

How it’s different from other Sectors

It’s been two month’s since my last blog, in these two months, I celebrated my birthday, got hospitalized for the first time in my life and also switched my job (quite an eventful period, huh?). Now, I work for the BIG BLUE – IBM. Obviously, It’s one of the largest organizations anyone can ever work for and I hope to enjoy my time here.

Since, I have just switched jobs let me talk about the experiance with my previous organization, It’s also going to be my first non technical blog. My previous employer was a retail organization, in fact the second largest retail company in the country. During my tenure of just under 3 years, I experienced how IT in retail is very different from most other sectors. 

In Retail an incident of server down impacts cash flow directly, this is unlike most other sectors where such an incident would only have an indirect impact on the business. As a result escalations are fast, mounting more pressure on the IT professional. It demands quick and correct decision making, you can’t take your time in order to arrive at the right conclusion, you need to be fast and you need to be accurate all the time. There is no room for error. You might say that pressure is there in every sector and not just retail. Well, yes, it’s true but most of that pressure is artificial and usually the source is an over excited delivery manager. Such man made pressures are normally created for vested interests rather than any real urgency for solving the problem at hand. In Retail, the pressure is REAL, as real as it can get because the company is actually loosing money by the minute, you cannot compare that with anything else.

The number of stores is normally very high in any retail organization, usually in hundreds, and you can’t have an IT superhero at all your stores. Thus, arises the need to keep things as central as possible and whatever you have to keep at the store level, it should be as simple and tamper proof as possible, Architecture Design and Implementation are the two key phases to achieve that. Since, critical IT equipments like Servers and Routers are placed at the store, IT security and monitoring requirements go beyond just meeting compliance norms. I can go on and on like this but in short what I have learned is that IT Infrastructure requirements for an Retail Organization are very special in terms of Design, Operations Management, Incident Management and Hardware and Software requirements. You need a sturdy and stable IT Infra setup to run a retail organization, this may be true for any sector but more so in Retail.

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The “SERVER” and “WORKSTATION” services

Why are they named so??

The file and printer sharing service in Windows is called the “SERVER” service but every other server service has a name which explains what it serves, like DNS Service or the DHCP service then why does the file and printer sharing service call itself as just SERVER as if it were an all encompassing service and all the other server based services are dependent on it? Although other services are not dependent on it but it’s name gives that illusion. Here’s the actual reason……….

In the olden days when Microsoft started writing network software there was just one thing that the OSs served – File and Printer Sharing. So, any server meant file and printer sharing server and hence the name was given to the service which provided it. Over the years nobody bothered to change the name and it just remained that way. That also explains the name “WORKSTATION” given to the client service for file and print server.

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Hiding Folders with Access Based Enumeration (ABE)

Invisible Folders

We have a heavily used Windows File Server which acts as the only way to share files within a department and with other departments in the corporate head office. The folders are all secure with NTFS permissions wherein every user is authorized to access the folders which he/she needs to but would be greeted with an “Access Denied” message if the user tried to access any other folder. Pretty good security by any standards but we started facing a peculiar problem which was not related to technology at all. Some users started requesting rights for folders which they had no business to access, we have a proper authorization policy in place which says that the HOD of a particular department needs to authorize a person wanting to access any folder belonging to that department. But still such requests led to sticky situations when very senior people in the organization requested such rights as it was difficult to tell them to follow the proper process when it’s more than clear that they don’t want to go the proper way. This led to a thought that if users couldn’t see the folders which they had no access to then they won’t ask for it and this problem could be minimized if not eliminated completely. It sounded very cool but I had never heard about any particular way to do it in Windows although I knew that the novell guys could do it with their OS.

That’s when ABE came to the rescue which was released with Microsoft Windows Server 2003 SP1. ABE is a feature of the SERVER service, the windows service that provides file and printer sharing. It works by modifying a feature of the server service called “enumeration”, which basically means how the server service answers the question, “What files and folders exist in  a given share?” Windows Explorer does the same thing when you open a folder. That flashlight which shines for a second sometimes when you open a share is the way explorer entertains you while it enumerates the folder contents.

ABE only works on Windows Server 2003 SP1 and higher, it’s not enabled by default and needs another small program to be turned ON or OFF. To be able to see a file/folder when ABE is turned ON, a user will need at least READ permissions on it. ABE works on both files and folders but only when accessed through a share, when someone directly logs on to the computer ABE has no affect on what the user is able to see.  Even through a share all files/folders, irrespective of their NTFS permissions, will be visible to anobody who is a member of the administrator’s group on the server hosting the share. This means, if you are an administrator, everything will be visible to you even if you don’t have even READ rights on some or any of the files/folders. This can be particularly tricky while you are testing the affects of ABE after enabling it, you might believe that ABE is not working because you can still see all the files/folders.  The tool used to enable/disable ABE is called abeui.msi and is downloadable from, it’s installation is pretty simple and once installed you can turn ABE ON or OFF for all shares or only some particular shares. Although you can do the same during the installation as well when the following dialog box is presented to you.

A new tab is added to the properties dialog box, of every share on the server on which ABEUI is installed, by the name “Access-Based Enumeration” as shown in the image given below.


Checking the first check box enables ABE for that particular share and checking the next one enables ABE for all the shares on that particular server. There is also a CLI for doing the same but since I’m not a big CLI fan, I have never tried that out myself. The installation of ABEUI is necessary even to get the CLI for ABE.

In Windows Server 2008, ABE comes pre-installed and pre-enabled. You can however, disable it for some or all shares if you need to.

If nothing then at least enabling ABE on your shares will reduce the temptation of your users to play internal hacker. And yes I enabled ABE on my File Server and it resulted in reduction of the problem that it was intended to reduce but then as luck would have it, the management decided to give at least READ access of all the folders on the file server to every user in the office 😉

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Windows 7 Supports RAID1

Disk Mirroring for Desktops, umm interesting!!

I just found out that Windows 7 Supports RAID 1 or Disk Mirroring as it’s commonly known as. It’s the first MS Client OS to support RAID1. For me it’s interesting to see RAID for desktops as untill recently it was something so niche that it was only available for servers. It’s amusing how niche tech becomes common place so fast.

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Notebook Entry: Leftover of Conficker/Downadup/Kido Worm

Another Leftover Story

Talking about virus leftovers – I recently noticed a leftover of the Conficker worm which is used by the virus to reinfect the systems, they are scheduled tasks on windows systems. I have only noticed them on Server OSs, they are named like ‘At’ and then any random numbers, usually starting with 1 like At1, At2……….At15, At4098 etc. As files they are named as At1.job…..At4753.job etc. as .job is the extension for secheduled tasks.

These tasks are used to run the infected dlls at a particular hour in a day, so, if a server has got 10 such jobs then it means that an attempt to resurrect the kido virus is made 10 times a day. These jobs try to run dlls followed by some random characters, one example would be “rundll32.exe ecaosp.hwo,cpmjb”.

Our antivirus (Kaspersky) doesn’t detect them – mainly because they are not viruses themselves but just a tool to re-enable the virus. Mcafee does detect them and cleans them up. The manual cleanup process for these jobs is well documented at . If a system is infected with conficker the jobs are said to reappear after a few hours upon manual deletion but I didn’t see this behaviour on my servers may be because the servers were all cleaned up and only the jobs remained as  –     the leftovers.

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No “Folder Options” under the ‘Tools’ menu

No “Folder Options” under the ‘Tools’ menu – The common leftover after a virus infection

The “Folder Options” from the ‘Tools’ menu of the windows explorer disappears mainly due to a virus infection. Brontok has been seen as the main culprit behind this apart from many other viruses which do this. Apart from removing the folder options such viruses also disable booting into safe mode.

There are many tools available which have a cure to these problems or at least claim to have a cure, like the RRT Tool etc. but in my experience there isn’t any single tool which works every time and on all PCs. Also, this problem may seem to be trivial for a seasoned administrator but in a large and complex environment it can really cause some pain.

Sometime back, after successfully tackling another zero day attack (have faced three so far), I found that the virus in question (later named as sality.z) has had the same affects on the infected PCs i.e; no folder options, no safe boot etc. which didn’t go away even though the computers were now completely disinfected. This time I didn’t want to use any random (read ‘unreliable’) tool as a workaround because the affected PCs were in geographically disparate locations and I wanted something 100% reliable and something which could be pushed centrally to all the infected PCs.

That’s when my good friend Mr. Murali Murugesan suggested using the Kido Killer tool from Kaspersky. Murali handles Kaspersky support in India. My current organization is the largest client (in terms of number of licenses) of Kaspersky in India. I have to say that this is the most reliable tool, I have ever used for this very common problem. Kido is the name given by Kaspersky to the now famous conficker/downadup virus. The main job for this tool, which is updated frequently, is to fight the kido/conficker/downadup virus but by using some of the available command line switches, one can easily use it as the most potent tool against the “No Folder Options” problem.

This tool can be found here for download.

After downloading the zip file, extract the contents (kk.exe file) to a folder, if that folder is stored in D:\Kido then open the command prompt and type “D:\kido\kk.exe –x –a –j –y” without the double quotes and press enter. If you are not able to open the command prompt, you can create a batch file (type the same command as above in a notepad and save it with .bat extension instead of the usual .txt extension) and run it on your PC. The tool runs, scans for the kido virus, re-enables the lost settings in the registry and then disappears. Folder Options and safe boot should be back after a quick reboot.

However, I used an automated way of doing the same (I’m an automation freak, you know) by pushing the kk.exe file with the said command line switches to the infected PCs through Kaspersky’s administration console. You can do the same by using any piece of software which can remotely execute files and takes command line switches as an option or you could use theKaspersky administration kitas that is also free to use.

Here’s the complete list of command line switches which can be used with Kido Killer

-p <Scan path> – scan a defined folder
-f – scan hard disks
-n – scan network disks
-r – scan flash drives, scan removable hard disks connected via USB and Fire Wire
-y – end program without pressing any key
-s – silent mode (without a black window)
-l <file_name> – write info into a log
-v – extended log maintenance (the switch -v works only if the -l switch is entered in the command prompt)
-j – restore the registry branch SafeBoot (if the registry branch is deleted, computer cannot boot in safe mode)
-z – restore the services 

  • Background Intelligent Transfer Service (BITS), 
  • Windows Automatic Update Service (wuauserv),
  • Error Reporting Service (ERSvc/WerSvc)
  • Windows Defender (WinDefend),
  • Windows Security Center Service (wscsvc).

-t – registry clean up from the services that remain after removing the network worm using Kaspersky Lab’s       products.
-x – restore display of hidden system files
-m – monitoring mode to protect the system from getting infected
-a – disable auto start from all drives


Please do share your experiences with the kk.exe tool by commenting on this blog post.


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