If you want kerberized logins on a server you run, you'll need a
*keytab* from accounts. Fill out the
[keytab request form](http://web.mit.edu/accounts/www/srvtabform.html),
-which sends them an e-mail. Ask for a "keytab"; by default they'll
-give you a srvtab, the Kerberos 4 analogue.
+which sends them an e-mail.
Your new keytab will be in
`/mit/accounts/srvtabs/FOR_YOURUSERNAME`, which is AFS and vaguely
insecure. You probably want to install it in `/etc/krb5.keytab`,
-and then randomize the key.
+and then **set a new (random) key**.
# mv -f /etc/krb5.keytab /etc/krb5.keytab.old # back up any keytab you already have
# mv /mit/accounts/srvtabs/FOR_JOEUSER/joeserver-new-keytab /etc/krb5.keytab
# k5srvutil change
+ # k5srvutil delold
-Then make sure your `/etc/ssh/sshd_config` file includes the lines
+Note that the `k5srvutil` command will also generate keys for the 3DES and RC4 ciphers, which are considered weak.
+You are strongly advised to read the "Upgrading cryptographic strength" section below on how to generate only AES keys instead.
+
+If you're using Debathena, you can install the `debathena-ssh-server-config` package to configure Kerberos authentication on the server side. If not, make sure your `/etc/ssh/sshd_config` file includes the lines
GSSAPIAuthentication yes
GSSAPIKeyExchange yes
This will let you SSH in with Kerberos.
Then create a file called `.k5login` in the home directory of
-whichever users you want to be able log into with Kerberos. List the
+whichever users you want to be able to log into with Kerberos. List the
full Kerberos principal of each user, one per line (e.g.,
`joeuser@ATHENA.MIT.EDU`)
-## Dealing with srvtabs
+If you don't want it to be possible to log in to a user account via
+Kerberos, create an empty `.k5login` file in their home directory.
+Otherwise, by default, you can log in to a user account with a Kerberos
+principal from the default realm (ATHENA, presumably) whose username
+matches; that is to say, an Athena user whose username matches a local
+username can log in to that local account. (One option to avoid this is
+to create a `.k5login` file in `/etc/skel` so that new accounts you
+later add get this file by default.)
+
+## Upgrading cryptographic strength
+
+You may wish to change the encryption algorithms (*enctypes*) included in your keytab. With server principals (like `daemon/servername.mit.edu` or `host/servername.mit.edu`) it is particularly important to support *only* strong algorithms. If you support a weak algorithm, an attacker can request a service ticket encrypted with that key, allowing them to do an offline attack and potentially extract your secret key.
+
+To change the supported enctypes, run `kadmin`:
+
+ kadmin -p daemon/kronborg.mit.edu -k -t daemon.kronborg.keytab
+
+From within `kadmin`, to create new keys:
+
+ ktadd -k daemon.kronborg.keytab -e aes256-cts:normal,aes128-cts:normal daemon/kronborg.mit.edu
+
+After all tickets currently issued against your service expire (which will happen after at most one day), you should remove the old keys from your keytab. If there are no outstanding tickets, you can do this from within `kadmin`:
-If you don't specifically mention a "keytab" in your request to
-Accounts, they may give you the Kerberos 4 equivalent, a srvtab.
+ ktremove -k daemon.kronborg.keytab daemon/kronborg.mit.edu old
-In this case you'll want to convert the srvtab to a keytab, like so.
+Before exiting, you may wish to verify in a separate terminal that the correct updated key has been written to the keytab:
- $ ktutil
- ktutil: rst /mit/accounts/srvtabs/FOR_JOEUSER/joeserver-new-srvtab
- ktutil: wkt /etc/krb5.keytab
- ktutil: q
+ kinit -k -t daemon.kronborg.keytab daemon/kronborg.mit.edu
+ kvno daemon/kronborg.mit.edu@ATHENA.MIT.EDU
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