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Kerberos: The Definitive Guide
I got started using Kerberos many moons ago, at my university. This is probably how many people got to know about it. While I didn't use it very much, it's there that I learned the basics and experimented a bit with Kerberos. Interest in it took off after Microsoft incorporated Kerberos authentication mechanisms into Windows 2000. Suddenly it wasn't such arcane knowledge.
Two open source Kerberos implementations exist, the MIT reference implementation, and the Heimdal Kerberos implementation. Even then, there are two main versions which you can find, Kerberos IV and Kerberos V. Kerberos IV went away for most environments with the passing of the Y2K mark, but some legacy apps need support. So, you still have to deal with it on occasion.
In writing Secure Architectures with OpenBSD, I got a lot more intimate with Kerberos, and even set up a decently sized realm in my house. Hence, I got to experience the turmoil of setup and debugging. A book like Kerberos: The Definitive Guide (K:TDG) would have been very welcome. Instead, I slogged my way through it, and got it to work for the most part.
K:TDG will help you set up your Kerberos world by introducing you to the complex subject, terminology, and the pieces. Once you learn the basics, you recognize that a simple realm is actually somewhat easy to set up. The author, Jason Garman, uses a mixed Mac OS X, UNIX, and Windows environment, focusing on UNIX most of the time. The bulk of the examples deal with MIT Kerberos 5 version 1.3 (krb5-1.3) but should work for most versions. Some attention is given to the Heimdal implementation (which is integrated with BSD, for example), and for the most part you'll be OK. Windows examples are also pretty copious but always come second. If you're comfortable with UNIX, you'll easily be able to translate these into Windows examples to help bridge the Windows gaps.
Chapter 1 is an obligatory Introduction, a short chapter that introduces the key concepts of Kerberos and what the book will cover. A very quick comparison of Kerberos to DCE, SESAME, and earlier versions of Kerberos is given. This chapter serves as a nice selling point for the book, it's the type of thing you'd flip through in the book store to decide if you should buy the book or not.
Chapter 2 is a decent overview for the new user of Kerberos to the system and how it works. Kerberos is placed into its role in a AAA infrastructure - authentication, authorization, and accounting - as well as some caveats that are commonly made. You'll learn about core Kerberos features like tickets, realms, principles, instances, ticket granting tickets, and the ticket cache. A decent overview for practical purposes is given, but you will definitely want another resource if you're interested in diving headlong into Kerberos.
These pieces come together in Chapter 3, where the actual protocols are described. They're laid out for a non-cryptographer, so go elsewhere if you want to learn the real formal material behind the system. Understanding the protocols is important to understanding the service as a whole. For someone new to Kerberos, you'll probably want to spend a little more time reading this to get oriented in the Kerberos world. The chapter doesn't mess around too much and delivers a fair treatment of the material.
Chapter 4 is the meat of the book's material, setting up your implementation. It all starts with the KDC (key distribution center) and realm initialization. Again, the bulk of the treatment is on the MIT implementation on UNIX, with the Heimdal and then Windows sections following next. Slave KDCs are also introduced, which is useful for large environments. An OS X server is missing, but Kerberos clients for all three (UNIX, Windows and OS X) is given. The role of DNS is also explained well, a useful touch that's missing in some Kerberos documents I've used in the past. This chapter will get you started, and with some of the supplied documentation you should be up and running in no time.
Chapter 5 is devoted to troubleshooting, an all too familiar task for a new Kerberos administrator. Common problems, their diagnosis, and resolution are discussed. I like the presentation of this chapter and think it will be useful for most real-world situations you'll encounter.
Security concerns with Kerberos are covered in Chapter 6, which discusses concrete and abstract attacks on the Kerberos scheme. Since all of the security in Kerberos resides in your KDC hosts, obviously this covers some of the material. However, the clients can exposes your Kerberos realm to attacks, as well, and how to circumvent these problems is covered. A decent and practical chapter, and covered on both UNIX and Windows.
In Chapter 7 a number of Kerberos enabled applications are discussed. After all, you can do more than just log on locally with Kerberos, you can use remote login programs like SSH, remote access scenarios like printing, and even control X via Kerberos. While not every application that I would have liked was covered, the treatment was fair and should get you started with a number of Kerberos enabled tools in your new realm.
A strong selling point of the book is given in Chapter 8, titled Advanced Topics. Three main topics are discussed. The first is cross-realm authentication, where you have more than one separate Kerberos realm on your network but you want to have users switch between the two without creating accounts in the other. This can get tricky, and the book does a decent job of introducing it, but it's not as complete as it could be. The second main topic in this chapter is Kerberos 4 and 5 interoperability, which is relatively straightforward. Most Kerberos 5 implementations come with tools to process Kerberos 4 ticket scenarios to handle legacy applications. And finally, a really valuable section covers UNIX and Windows Kerberos interoperability, a hairy issue. Again, incomplete but strong enough that you should be able to get it working with some elbow grease. This is probably the most valuable chapter of the book, which does a decent job at the introductory level, but you'll be left to tie up a few loose ends on your own.
An obligatory case study is given in Chapter 9, where you can see a number of configuration samples and even a mixed Windows-UNIX environment. Not terribly useful when compared to chapters 4 and 8, but overall worthwhile. It may answer some of your questions, even. Chapter 10 wraps up the book with looking at Kerberos futures, which isn't all that useful, honestly. What gets more useful is the appendix, which gives an administration reference. Lots of commands are given for MIT, Heimdal and even for Windows, so you can quickly jump there to refresh your memory on a topic.
Overall this book is recommended if you need a place to start working on Kerberos, especially in a mixed environment. The MIT and Heimdal documents are a fair place to start for a UNIX only Kerberos realm, but if you find they aren't enough, this is probably the right book for you. The book's main strength is that it covers Kerberos on the three main platforms in use (Windows, OS X, and UNIX), although it could provide a deeper treatment to the mixed environment than it gives. Still, you should be able to use this as a starting point, and it's probably the best treatment I've seen so far on Kerberos setup and administration.
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When are people going to realise that the problem with single sign on isn't a technical one....
Too bad there seems to be no coverage of AFS. I'd love to see a book documenting using Kerberos V with AFS.
File that one in the "When your only tool is a hammer everything looks like a nail" folder.
Hey, Windows users, there is no such thing as "forward" slash, there is only slash and backslash.
I should not feed the trolls.... But oh well....
OpenSSH is great for some things, but it does NOT do remote authentication for services, nor does it authenticate the service to the client. So it does not scale well.
Kerberos scales better and is more secure than OpenSSH for most things, I think. You can even use it along with telnet to provide a single-sign on remote shell with an encrypted session (without the key management issues of OpenSSH). Of course you can also use OpenSSH with Kerberos (requires a patch I think) but then that negates your point.
Also, it is *easy* to integrate Win2k and XP with Kerberos.
LedgerSMB: Open source Accounting/ERP
I made a list of corrections that I sent to both O'Reilly and the author which were ignored.
Perhaps the corrections you sent had already been caught by someone else (at O'Reilly or otherwise), only too late to fix before this publication, and will be corrected in any future publications. Granted I suppose a note of thanks from the author would have been nice, but what did you expect, Hallmark?
Uh, Win2k's Kerberos implementation is suprisingly standards-compliant. I've had NO issues whatsoever getting my MIT Krb5 systems taking to Win2k and Win2k3 KDCs. Microsoft even has a simple step-by-step guide on how to export the account data to a Kerberos keytab. If your krb5.conf file is set up properly, it's less than 5 minutes to set up the user account, export the keytab and import it onto your system.
Try harder.
If you store the keys you need to authenticate at other services in the
Not really. It authenticates you to *a user account* on the *host system.* This is very different from authenticating to, say, and LDAP service, an HTTPS application, or a database manager. OpenSSH cannot readily do this. Sure you can use it for mining expeditions (lots of tunnels) but this does not approach problem in a reasonable manner.
Kerberos isn't particularly impressive at scaling, what with all those secret keys flying about.
It seems to scale better than the alternatives. I assume that this is why Novell, IBM, and even Microsoft use it for their larger-scale AAA systems. The reason is simply central management. You could try to hack something together using LDAP and SSH but the same issues exist regarding the fact that it doesn't really have a way to authenticate you to a centrally managed account from an application server's perspective.
Really, the only thing SSH doesn't do is authenticate the remote service to the extent that Kerberos would, although really the only benefit Kerberos brings is that it prevents some MITM attacks - it's still based on remote systems (working together) "proving" who they are by knowing a shared secret (your password).
No, actually, it protects you against modified replay attacks. MITM is a problem which can occur with SSH, SSL, and even Kerberos on the first time a key for a PKI server is received. This can be a X509 CA, and SSH server, or a Kerberos KDC.
OTOH, the fact that each ticket is signed and timestamped (and cached against playback), and each service (including the KDC) is authenticated to the client, there is no way to forge or replay the ticket unless the keys are compromised.
I do agree with your points regarding PKI but I am not really sure that there is a secure way to do this. It is a difficult problem (i.e. login requires the key which either must be cashed and then decrypted with a passphrase or be carried on the person of the user). This has its own dangers and is far from being perfect.
LedgerSMB: Open source Accounting/ERP