PDF Kerberos: The Definitive Guide

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Obviously this file must be protected from general read access, since anyone who can read it can impersonate the service. Obtaining credentials with kinit Let's use an example to get a practical look at Kerberos. Suppose you are on a Unix host spot in the realm FIDO, and you want to use kerberized telnet to log into another host, rover. If you had wanted a different principal, you could have supplied it as an argument to kinit. Note that the TGT has a validity period: it expires in 10 hours. After that, you must do another kinit to reauthenticate yourself. Connected to rover It succeeds: the remote side accepts your Kerberos identification, and allows you to log in without providing a password.

This principal is used for remote login and command execution services on the host rover, such as kerberized telnet , rlogin , rsh , etc.

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Authorization So far we've taken care of authentication, but what about authorization? You could have changed that with telnet -l username. Using this default rule implies that the system administrators are managing the correspondence between operating system OS usernames and Kerberos principals. Note that authentication was still successful: the telnet server accepted you as res FIDO.

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The authorization decision failed, though: Kerberos decided that the principal res FIDO was not allowed to access the account bob rover. He would also have to place his own principal in there, since if a. The programs will likely refer to Kerberos shared libraries that must be present for the programs to run. Once running with Kerberos support, SSH1 operates essentially as we described for kerberized telnet ; Figure illustrates the process.


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Kerberos password authentication If Kerberos authentication is enabled in the SSH server, password authentication changes in behavior. Passwords are now validated by Kerberos instead of the host operating system. This behavior is usually desired in a fully kerberized environment, where local passwords might not be usable at all. In a mixed environment, however, it may be useful to have SSH fall back on the operating system OS password if Kerberos validation fails.

Another feature of kerberized password authentication is that sshd stores your TGT upon login, so you don't need to run kinit and retype your password to get Kerberos credentials on the remote host. Kerberos and NAT SSH is frequently used across firewalls, and these days such a boundary often includes network address translation. Unfortunately, Kerberos has a serious problem with NAT. Kerberos tickets usually include a list of IP addresses from which they are allowed to be used; that is, the client presenting the ticket must be transmitting from one of those addresses.

Host spot 's IP address is If spot has multiple network interfaces or addresses, they are listed here as well. When you obtain subsequent service tickets based on this TGT, they are also limited to the same set of addresses. The SSH server, however, sees your NAT'd address as the source of the connection, notes that this doesn't match the address encoded in the ticket, and refuses authentication. In this case, ssh -v reports: Trying Kerberos V5 authentication.

Kerberos V5: failure on credentials Incorrect net address. Figure illustrates this problem. It has no good solution at the moment. One workaround is the undocumented kinit - A switch, which causes kinit to request a ticket with no addresses in it at all. This trick decreases security, because a stolen ticket cache can then easily be used from anywhere, but it gets around the problem.

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Cross-realm authentication Kerberos realms are distinct collections of principals under separate administrative control. For instance, you might have two departments, Sales and Engineering, that don't trust each other just for the sake of example, of course. The Sales people don't want any of those weird Engineers to be able to create accounts in their space, and Engineering certainly doesn't want any sales-droids mucking about with their logins. The catch is, of course, that Sales and Engineering do need to work together. Sales guys need to log into Engineering machines to try out new products, and Engineering needs access to Sales desktops to fix the problems they constantly get into.

Suppose Erin the engineer needs Sam from Sales to access her account on an Engineering machine, erin bulwark. It's also cumbersome, since Sam would have to do another kinit each time he wants to access resources in a different realm. The solution to this problem is called cross-realm authentication. Then the administrators of the two realms establish a shared secret key between them, called a cross-realm key. The key is realized as a common key for two specially named principals, one in each realm. With one cross-realm key in place, authentication in one realm provides a verifiable identity in the other realm as well.

If the trust is symmetric -- that is, if each realm should trust the other -- then two cross-realm keys are needed, one for each direction. Hierarchical Realms in Kerberos-5 For a large number of realms, the system as described quickly becomes unwieldy. If you want cross-realm trust between all of them, you must manually establish cross-realm keys for each pair of realms. Kerberos-5 supports hierarchical realms to address this problem. COM, if Kerberos doesn't find a direct cross-realm key, it attempts to navigate up and then down the realm hierarchy, following a chain of cross-realm relations to the target realm.

This allows for scalable, complete, cross-realm relationships among a large collection of realms. That's the basic idea. However, when SSH enters the picture, cross-realm authentication can fail due to a confusing catch. Suppose Sam uses bulwark so often that he's given an account there.


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  • But, it doesn't work. Even with everything set up correctly so that cross-realm kerberized telnet works, SSH Kerberos authentication still fails for him. Even more mysteriously, every other form of authentication starts failing as well. Sam had public-key authentication set up and working before, and you'd expect it to try Kerberos, fail, then try public-key and succeed.

    But all the public-key attempts fail, too. Trying Kerberos V5 authentication.


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    • Kerberos V5 authentication failed. Connection to authentication agent opened. Doing password authentication.

      Kerberos

      And in fact, the quick fix for this problem is for Sam to specify his username explicitly, with ssh -l sam bulwark , even though this seems redundant. The reason for this odd problem is a Kerberos-5 feature that SSH employs, called aname lname mapping authentication name to local name mapping. Kerberos can be used with a variety of operating systems, some of whose notions of a username don't correspond easily with Kerberos principal names.

      Perhaps the usernames allow characters that are illegal in principal names, or there are multiple operating systems with conflicting username syntaxes. Or perhaps when you merge two existing networks, you find username conflicts among existing accounts, so that the principal res REALM must translate to the account res on some systems, but rsilverman on others. The Kerberos-5 designers thought it would be good if Kerberos could automate handling this problem itself, and so they included the aname lname facility for translating principals to the correct local account names in various contexts.

      SSH1 uses aname lname.

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      When doing Kerberos authentication, the SSH1 client supplies the principal name as the target account name by default, rather than the current local account name that is, it behaves as if Sam had typed ssh -l sam SALES bulwark. The server in turn applies the aname lname mapping to this, to turn it into a local account name. So the aname lname mapping fails, and sshd goes ahead with using "sam SALES" as the local account name.

      Since there is no account with that name, every form of authentication is guaranteed to fail. It's almost entirely undocumented and includes references to utilities and files that don't appear to exist. However, we did manage to uncover enough information to give one example solution. TGT forwarding Recall that Kerberos tickets are normally issued to be usable only from the requesting host.

      If you do a kinit on spot, then use SSH to log into rover, you are now stuck as far as Kerberos is concerned.

      Kerberos: The Definitive Guide book - Mon premier blog

      If you want to use some Kerberos service on rover, you must run another kinit, because your credentials cache is stored on spot. And it won't help to copy the credentials cache file from spot to rover because the TGT won't be valid there; you need one issued for rover. If you do another kinit , your password is safe traveling over the network through SSH, but this is still not a single sign-on, and it's annoying.

      SSH has an analogous problem with public-key authentication and solves it with agent forwarding. When it receives the new TGT, the client passes it to sshd , which stores it in the remote account's Kerberos credentials cache. Kerberos V5 TGT passing was successful. This variable controls where the ticket cache is stored.

      The ticket cache contains sensitive information; anyone who steals your ticket cache can impersonate you for the lifetime of its tickets. This directory can easily be an NFS filesystem, allowing the tickets to be stolen by network snooping. Or it can be an inappropriate spot in the filesystem, perhaps one where inherited ACLs give someone else the right to read the file, regardless of the ownership and permissions set by SSH.

      Note that this problem occurs if SSH1 is compiled with Kerberos support, even if Kerberos authentication isn't in use for the session at hand. Kerberos-5 setup notes Here we present an abbreviated "quick-start" menu of steps to set up a working, one-host Kerberos system from scratch, using the MIT Kerberos-5 distribution Version 1. This is far from complete and might be wrong or misleading for some environments or builds. It's just meant to get you started, if you want to give Kerberos a try. Suppose the local host's name is shag. FOO , which allows the KDC software to start without human intervention but which is obviously not wise unless the KDC machine is extremely well protected.

      Issue the following kadmin commands: kadmin. If all goes well, you should be able to use kinit to get a TGT using the password you gave to kadmin. Try out kerberized SSH. The mechanics from a user perspective are mostly the same: in a functioning Kerberos realm, you use kinit to obtain a TGT, and then run the SSH client with KerberosAuthentication turned on which it is by default.

      The host principal is rcmd. Table Direct cross-realm authentication also automatically works using existing cross-realm keys; however, Kerberos-4 doesn't support hierarchical realms.