UNIX Application Servers

An application server is a host that provides one or more services over the network. Application servers can be “secure” or “insecure.” A “secure” host is set up to require authentication from every client connecting to it. An “insecure” host will still provide Kerberos authentication, but will also allow unauthenticated clients to connect.

If you have Kerberos V5 installed on all of your client machines, MIT recommends that you make your hosts secure, to take advantage of the security that Kerberos authentication affords. However, if you have some clients that do not have Kerberos V5 installed, you can run an insecure server, and still take advantage of Kerberos V5’s single sign-on capability.

The keytab file

All Kerberos server machines need a keytab file to authenticate to the KDC. By default on UNIX-like systems this file is named /etc/krb5.keytab. The keytab file is an local copy of the host’s key. The keytab file is a potential point of entry for a break-in, and if compromised, would allow unrestricted access to its host. The keytab file should be readable only by root, and should exist only on the machine’s local disk. The file should not be part of any backup of the machine, unless access to the backup data is secured as tightly as access to the machine’s root password.

In order to generate a keytab for a host, the host must have a principal in the Kerberos database. The procedure for adding hosts to the database is described fully in Adding, modifying and deleting principals. (See Create host keytabs for slave KDCs for a brief description.) The keytab is generated by running kadmin and issuing the ktadd command.

For example, to generate a keytab file to allow the host trillium.mit.edu to authenticate for the services host, ftp, and pop, the administrator joeadmin would issue the command (on trillium.mit.edu):

trillium% kadmin
kadmin5: ktadd host/trillium.mit.edu ftp/trillium.mit.edu
    pop/trillium.mit.edu
kadmin: Entry for principal host/trillium.mit.edu@ATHENA.MIT.EDU with
    kvno 3, encryption type DES-CBC-CRC added to keytab
    FILE:/etc/krb5.keytab.
kadmin: Entry for principal ftp/trillium.mit.edu@ATHENA.MIT.EDU with
    kvno 3, encryption type DES-CBC-CRC added to keytab
    FILE:/etc/krb5.keytab.
kadmin: Entry for principal pop/trillium.mit.edu@ATHENA.MIT.EDU with
    kvno 3, encryption type DES-CBC-CRC added to keytab
    FILE:/etc/krb5.keytab.
kadmin5: quit
trillium%

If you generate the keytab file on another host, you need to get a copy of the keytab file onto the destination host (trillium, in the above example) without sending it unencrypted over the network.

Some advice about secure hosts

Kerberos V5 can protect your host from certain types of break-ins, but it is possible to install Kerberos V5 and still leave your host vulnerable to attack. Obviously an installation guide is not the place to try to include an exhaustive list of countermeasures for every possible attack, but it is worth noting some of the larger holes and how to close them.

We recommend that backups of secure machines exclude the keytab file (/etc/krb5.keytab). If this is not possible, the backups should at least be done locally, rather than over a network, and the backup tapes should be physically secured.

The keytab file and any programs run by root, including the Kerberos V5 binaries, should be kept on local disk. The keytab file should be readable only by root.