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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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Large corporations and educational institutions benefit greatly from single-sign-on ... Consider a college - When you have 10 computer labs with 4-5 operating systems and N SANs all mounting common home directories, the ability to log in to all of them with the same username/password saves a LOT of support headaches. We used to implment (S)LDAP, which worked great for everything but the Win2k boxes - SSO for OSX+Linux with NFS mounted homes actually made a lot of people happy.
Too bad there seems to be no coverage of AFS. I'd love to see a book documenting using Kerberos V with AFS.
No article about Kerberos would be complete without a link to one of the more interesting introductions out there:
Designing an Authentication System: a Dialogue in Four Scenes
I've read the majority of this book and overall it's pretty good. However, even considering that this is a first edition book there are quite a few mistakes (mostly editing... grammar, spelling, etc).
I made a list of corrections that I sent to both O'Reilly and the author which were ignored. I think O'Reilly is getting too arrogant and it's going to hurt their reputation.
You started to say something interesting and useful, then your attention wandered. Inquiring minds want to know: if the problem with SSO isn't technical, what is it?
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.
Well, duh! Even my grandmother uses Kerberos to solve her single sign-on dilemma!
Actual answer:
Kerberos is an authentication protocol. You have a client, a server, and a kerberos server. The kerberos server itself has three parts, the key distribution center, the authentication server and the ticket granting server. This is a symmetric encryption system: no public or private keys, just private keys.
Before anything happens, both Client and Server share their cryptographic keys with the Key Distribution Center. This setup is required for kerberos to work. Kerberos doesn't work if you can't set things up beforehand.
When it's authentication time, Client goes to Ticket Grantor and says, "I want to talk to Server, and here's my key." Ticket Grantor asks Server, "Client wants to talk to you. Is that okay?" Server says it's okay, so the Ticket Grantor sends a ticket-granding ticket (encrypted with Ticket Grantor's key, so only TG can read it) and a session key (encrypted with the Client's key, so only Client can read it) to Client. Note that at this point we haven't authenticated Client -- we've just checked that Client is authorized to talk to Server.
Client unencrypts the session key using its own key. If Client really is who it says it is, the unencrypted key will be correct. Client goes to Ticket Grantor with the ticket-granting ticket and the session key and says, "Look, I can do it! It's me! Gimme a real ticket already so I can talk to the server." Ticket Grantor says "Ok" and does gives the Client a ticket encrypted with the Server's key and a new session key encrypted with the Client's key.
The Client decrypts the session key: now it knows how to handle talking to the Server. Then it sends the Server the ticket. If the Server is who the kerberos server thinks it is, it will be able to decrypt the ticket and establish a session with the client.
It's more complicated than that, but I think this covers it. Does that help? I expect if I have erred I will be corrected forthwith, as nothing gets the right answer faster than posting the wrong one.
What I say does not represent the views of my employers, my friends, my cats, or myself.
It's suprisingly easy to do. I've done it for a couple of organizations and at the house. Simplifies my ssh logons considerably. Microsoft was suprisingly strict with its standards compliance on this one.
Microsft Krb5 interop guide
I'm a Kerberos fan. I wrote the Kerberos5 chapter of the FreeBSD Handbook (and I have a re-write mostly completed) and I worked with quite a few Realms over the past few years.
I've read Kerberos: TDG several times now, and I've tried to find the answers to obscure problems often in it -- usually, without success. I think it should have been named Kerberos: An introduction because it isn't a Definitive Guide. Look at the page count alone: it's a slim, slim book. An in spite of being slim it tends to be a repititious. Not a good sign for something trying to living up the Definitive Guide tag.
It also misses quite a few topics that would be great to see covered in a second edition:
I liked the book. I'll take it over not having an O'Reilly Kerberos book any day. But I look forward to a revised second edition ;-)
"The purpose of argument is to change the nature of truth." -- Bene Gesserit Precept
Kerberos isn't designed for the Internet. You need to set it up explicitly for each trust domain on each client computer. So users can't just pull out their personal laptops and use them to log in...
/etc/krb5.keytab). A Kerberos client can run anywhere and does not require a prior host connection with the realm at all. You *can*, in fact, do exactly what you describe here as impossible: connect your laptop to a network and type "kinit user@REALM" to get credentials, then use a kerberized application such as OpenSSH. The Kerberos software can find a KDC for "REALM" from the DNS (assuming the appropriate rr's are available). Note that this is secure despite the insecurity of the DNS in general, since Kerberos is a shared-secret system: since you share a secret with the KDC (essentially your password), you can validate the KDC's response.
I'm afraid your entire point here is just technically false. Kerberos only requires host setup with if the host is a server; that is, to run a kerberized service on it, you need to establish a shared key for the service principal with the Kerberos system (KDC) and store it on the host where the corresponding server can find it (e.g.