SSH + Kerberos + Keytabs on MacOS
Kerberos is a convenient way to authenticate and obtain access to remote machines via SSH. Instead of typing your password every time you want to access a remote computer, you can type your password only once and obtain a Kerberos ticket, which serves as a ‘passport’ and saves typing effort during subsequent connections.
However, Kerberos tickets usually expire in 24 hours, so you still need to type (and therefore remember) your password at least once a day. Kerberos keytab files make the authentication process even more convenient. With keytabs, you type your password once, it gets encrypted according to the chosen encryption method, and then the encrypted password in the keytab file can be used to obtain a Kerberos ticket, which will subsequently grant access to the remote machine. Never type your password again!
But be careful to make the keytab file readable only by you, the owner, as anyone with a read access to a keytab file will automatically have access to the remote machine as well (though they still won’t know your password, since it’s encrypted).
To get SSH with Kerberos + keytabs working, it’s a long and harrowing road… But let’s start with the first step.
Get SSH + Kerberos + Keytabs working:
(Updated January 2021) Obtain correct SSH binary. If you have MacOS Sierra or later, it comes with an updated version of OpenSSH that drops some options that support Kerberos authentication (the most important being
GSSAPITrustDNS). The best way to obtain an SSH binary with GSSAPITrustDNS support is using Homebrew. After installed, run:
$ brew install rdp/homebrew-openssh-gssapi/openssh-patched --with-gssapi-support
What’s necessary here is to find an SSH binary from El Capitan and then copy it to your
/usr/local/binfolder, and make sure that the
$PATHvariable points first to that folder, before pointing to
/usr/bin(where the native SSH binary lives), so that it will be picked up first by the system.
Adjust Kerberos configuration. This is a complicated process but you can just copy the right
krb5.confto your system, placing it in
/etc/krb5.conf. Here is mine.
Enable Kerberos authentication in SSH config. For each remote network you want to connect to (e.g. Fermilab, CLASSE, CERN), edit the file
~/.ssh/configto be similar to this one: It’s important to have the
GSSAPI\*options and the
PreferredAuthenticationsoption like in the file, to allow Kerberos authentication.
Make a keytab file with your encrypted password. This step is to create a keytab file containing your password that will be fed to the kinit command in order to obtain a Kerberos ticket. On MacOS X (which comes with the Heimdal flavor of Kerberos, and not MIT’s) the command to add a password for CERN’s account is:
$ ktutil -k ~/keytab add -p user@CERN.CH -e arcfour-hmac-md5 -V 3
MacOS will complain about the chosen encryption but it’s the only one that kinit supports for automation of CERN’s network connection (as far as I know). Other encryptions also contain a “salt” that
ktutildoesn’t handle well (see here and here). The equivalent command for a CLASSE entry would be:
$ ktutil -kt ~/keytab add -p user@CLASSE.CORNELL.EDU -e aes256-cts-hmac-sha1-96 -V 1
You can also make your keytab file when logged in to one of the remote machines. For example, if you are on a lxplus machine, you can create a keytab file with a different version of the Kerberos tools (MIT’s one):
$ ktutil addent -password -p user@CERN.CH -k 3 -e arcfour-hmac-md5 (type your password) wkt keytab quit
(for more info see here). Now the keytab file in your home directory contains your encrypted passwords for access to CERN’s and CLASSE’s networks.
Use the keytab file to authenticate to
kinit. The command
$ kinit —afslog -kt ~/keytab user@CERN.CHor
$ kinit -kt ~/keytab user@CLASSE.CORNELL.EDUshould now grant you Kerberos tickets without having to type your password. Note that you could include this command in (start-up) scripts, so that you always have a connection on. Neat!
SSH to remote machine as usual. For example,
ssh email@example.com. Again your password shouldn’t be asked for anymore.
Remember to set the permissions of the keytab file to read-only by you! Anyone with read access to this file could log in to your accounts, so make sure it can only be readable by you.
More (partial) info: http://linux.web.cern.ch/linux/docs/kerberos-access.shtml
PS: Regarding my post about Keychain support with SSH, note that the patched SSH linked above didn’t correctly enable Keychain support. Only the GSSAPI support option worked for me. But the native SSH on macOS does have Keychain support already, so you don’t need a patched version for that.