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SSH without a password with Kerberos

At CMU, we use AFS (Andrew File System) on university-owned servers. Students frequently need to ssh into these servers to do schoolwork. It can get annoying to have to type your password every time to do this.

There is a way to set up SSH for password-less authentication. The key (pun slightly intended) is to use Kerberos and store your authentication information once in a special keytab file.

I learned how to do this by reading another post, as well as some trial and error.

Note that this method was tested on macOS only, on the day of publication of this post. It may not work on other OSes, and it may break in the future.

This guide is also somewhat CMU-specific, but it should be able to be adapted more generally for other use cases.

Check your ktutil

We’re going to be creating a Kerberos keytab file. This keytab file will reside on your local computer, and will allow you to ssh into stuff without typing your password.

To manage keytab files, you’ll need a command-line program called ktutil on your local computer. On my macOS machine, it’s pre-installed:

$ ktutil --version
ktutil (Heimdal 1.5.1apple1)
Copyright 1995-2011 Kungliga Tekniska Högskolan
Send bug-reports to heimdal-bugs@h5l.org

Decide where the keytab file will go

We’ll be using ktutil to create the keytab file. You’ll need to decide where this file should go on your local computer.

My suggestion would be to create a directory called ~/.local and put your keytab named keytab in it:

$ mkdir -p ~/.local
$ ls ~/.local/keytab
ls: /Users/<USERNAME>/.local/keytab: No such file or directory

The usage of ls verifies that there is no pre-existing ~/.local/keytab which would be overwritten with the following steps. Its output will show your local username in place of <USERNAME>.

We’re now going to create the keytab file.

Populate the keytab file

Utter the following magical incantation to set up the keytab file.

$ ktutil \
  -k ~/.local/keytab \
  add \
  -p <ANDREW_USERNAME>@ANDREW.CMU.EDU \
  -e aes256-cts-hmac-sha1-96 \
  -V 1

A few caveats:

Afterwards, you can verify it worked with a usage of the file command:

$ file ~/.local/keytab
/Users/<USERNAME>/.local/keytab: Kerberos Keytab file, realm=ANDREW.CMU.EDU, principal=<ANDREW_USERNAME>/, type=12345, date=Thu Dec 10 01:23:45 2050, kvno=10

The specifics might be slightly different, but the bottom line is that it is a valid “Kerberos Keytab” file.

Set up ssh to use the keytab

We’ll need to configure SSH to use the keytab file we just created.

ssh is configured with the ~/.ssh/config file. Create that file if it doesn’t exist, then add these lines to it, replacing <ANDREW_USERNAME> with your username:

Host cmu
  User <ANDREW_USERNAME>
  Hostname unix.andrew.cmu.edu
  GSSAPIAuthentication yes
  GSSAPIDelegateCredentials yes

This’ll allow you to type

$ ssh cmu

instead of the usual

$ ssh <ANDREW_USERNAME>@unix.andrew.cmu.edu

You may want to add more Hosts with other Hostnames if you e.g. want to use the “shark machines” (not sure if they still use those, but they did from 2016-2020).

The bits with GSSAPI tell ssh to use Kerberos for authentication for ssh cmu.

Check your shell

The last step will involve adding configuration not for ssh, but your shell itself. To know where to add the configuration, you need to know what shell you’re using.

Type:

$ echo $0

at the command line to see what shell program you’re currently running.

I’m not sure on the specifics for bash because I don’t use bash. The default on macOS is zsh nowadays.

Create the appropriate shell configuration file if it doesn’t exist already, so that we can add to it in the next step.

Add shell configuration

Every time you ssh, you’ll need to first generate a “ticket” from the keytab file.

You don’t technically need to make a new ticket every time, but since tickets expires after some time, it’s nice to set up your ssh to make a new one every time.

First create a shell function that’ll generate a ticket for you. Add this function to the shell configuration file you determined from the previous step:

get_kerberos_ticket() {
  kinit -t ~/.local/keytab <ANDREW_USERNAME>@ANDREW.CMU.EDU
}

Again, you’ll need to replace <ANDREW_USERNAME>, as well as the path to the keytab if you changed that.

Now, every time you ssh, you can manually say get_kerberos_ticket to generate a ticket. Like this:

$ get_kerberos_ticket
$ ssh cmu

But you could also set up your shell to do that for you. To do that, add this function to your shell configuration file:

ssh() {
  get_kerberos_ticket
  command ssh -q "$@"
}

This function overrides the ssh command. When called, it:

  1. Gets a ticket
  2. Calls the real ssh command with all the arguments passed to the wrapper ssh function

Note that the -q passed to ssh is optional, but I find it nice to quiet down some informational messages from ssh that I find overly chatty.

If you use scp to transfer files to and from the remote machines, you can set it up with a similar overriding function:

scp() {
  get_kerberos_ticket
  command scp -q "$@"
}

Again, the -q is optional.