MIT Kerberos Documentation

Password management

Your password is the only way Kerberos has of verifying your identity. If someone finds out your password, that person can masquerade as you—send email that comes from you, read, edit, or delete your files, or log into other hosts as you—and no one will be able to tell the difference. For this reason, it is important that you choose a good password, and keep it secret. If you need to give access to your account to someone else, you can do so through Kerberos (see Granting access to your account). You should never tell your password to anyone, including your system administrator, for any reason. You should change your password frequently, particularly any time you think someone may have found out what it is.

Changing your password

To change your Kerberos password, use the kpasswd command. It will ask you for your old password (to prevent someone else from walking up to your computer when you’re not there and changing your password), and then prompt you for the new one twice. (The reason you have to type it twice is to make sure you have typed it correctly.) For example, user david would do the following:

shell% kpasswd
Password for david:    <- Type your old password.
Enter new password:    <- Type your new password.
Enter it again:  <- Type the new password again.
Password changed.

If david typed the incorrect old password, he would get the following message:

shell% kpasswd
Password for david:  <- Type the incorrect old password.
kpasswd: Password incorrect while getting initial ticket

If you make a mistake and don’t type the new password the same way twice, kpasswd will ask you to try again:

shell% kpasswd
Password for david:  <- Type the old password.
Enter new password:  <- Type the new password.
Enter it again: <- Type a different new password.
kpasswd: Password mismatch while reading password

Once you change your password, it takes some time for the change to propagate through the system. Depending on how your system is set up, this might be anywhere from a few minutes to an hour or more. If you need to get new Kerberos tickets shortly after changing your password, try the new password. If the new password doesn’t work, try again using the old one.

Granting access to your account

If you need to give someone access to log into your account, you can do so through Kerberos, without telling the person your password. Simply create a file called .k5login in your home directory. This file should contain the Kerberos principal of each person to whom you wish to give access. Each principal must be on a separate line. Here is a sample .k5login file:


This file would allow the users jennifer and david to use your user ID, provided that they had Kerberos tickets in their respective realms. If you will be logging into other hosts across a network, you will want to include your own Kerberos principal in your .k5login file on each of these hosts.

Using a .k5login file is much safer than giving out your password, because:

  • You can take access away any time simply by removing the principal from your .k5login file.
  • Although the user has full access to your account on one particular host (or set of hosts if your .k5login file is shared, e.g., over NFS), that user does not inherit your network privileges.
  • Kerberos keeps a log of who obtains tickets, so a system administrator could find out, if necessary, who was capable of using your user ID at a particular time.

One common application is to have a .k5login file in root’s home directory, giving root access to that machine to the Kerberos principals listed. This allows system administrators to allow users to become root locally, or to log in remotely as root, without their having to give out the root password, and without anyone having to type the root password over the network.

Password quality verification