Sharing Admin Privileges for Many Hosts Securely

The problem: you have a large team of admins, with a substantial turnover rate. Maybe contractors come and go. Maybe you have tiers of access, due to restrictions based on geography, admin level or even citizenship (as with some US government contracts). You need to give these people administrative access to dozens (perhaps hundreds) of hosts, and you can't manage all their accounts on all the hosts.

This problem arose in the large-scale enterprise in which I work, and our team worked out a solution that:

  • Does not require updating accounts on more than one host whenever a team member arrives or leaves.

  • Does not require deletion or replacement of Secure Shell (SSH) keys.

  • Does not require management of individual SSH keys.

  • Does not require distributed sudoers or other privileged-access management tools (which may not be supported by some Linux-based appliances anyway).

  • And most important, does not require sharing of passwords or key passphrases.

It works between any UNIX or Linux platforms that understand SSH key trust relationships. I personally have made use of it on a half-dozen different Linux distros, as well as Solaris, HP-UX, Mac OS X and some BSD variants.

In our case, the hosts to be managed were several dozen Linux-based special-purpose appliances that did not support central account management tools or sudo. They are intended to be used (when using the shell at all) as the root account.

Our environment also (due to a government contract) requires a two-tier access scheme. US citizens on the team may access any host as root. Non-US citizens may access only a subset of the hosts. The techniques described in this article may be extended for N tiers without any real trouble, but I describe the case N == 2 in this article.

The Scenario

I am going to assume you, the reader, know how to set up an SSH trust relationship so that an account on one host can log in directly, with no password prompting, to an account on another. (Basically, you simply create a key pair and copy the public half to the remote host's ~/.ssh/authorized_keys file.) If you don't know how to do this, stop reading now and go learn. A Web search for "ssh trust setup" will yield thousands of links—or, if you're old-school, the AUTHENTICATION section of the ssh(1) man page will do. Also see ssh-copy-id(1), which can greatly simplify the distribution of key files.

Steve Friedl's Web site has an excellent Tech Tip on these basics, plus some material on SSH agent-forwarding, which is a neat trick to centralize SSH authentication for an individual user. The Tech Tip is available at http://www.unixwiz.net/techtips/ssh-agent-forwarding.html.

I describe key-caching below, as it is not very commonly used and is the heart of the technique described herein.

For illustration, I'm assigning names to players (individuals assigned to roles), the tiers of access and "dummy" accounts.

Hosts:

  • darter — the hostname of the central management host on which all the end-user and utility accounts are active, all keys are stored and caching takes place; also, the sudoers file controlling access to utility accounts is here.

  • n1, n2, ... — hostnames of target hosts for which access is to be granted for all team members ("n" for "non-special").

  • s1, s2, ... — hostnames of target hosts for which access is to be granted only to some team members ("s" for "special").

Accounts (on darter only):

  • univ — the name of the utility account holding the SSH keys that all target hosts (u1, u2, ...) will trust.

  • spec — the name of the utility account holding the SSH keys that only special, restricted-access, hosts (s1, s2, ...) will trust.

  • joe — let's say the name of the guy administering the whole scheme is "Joe" and his account is "joe". Joe is a trusted admin with "the keys to the kingdom"—he cannot be a restricted user.

  • andy, amy — these are users who are allowed to log in to all hosts.

  • alice

  • ned, nora — these are users who are allowed to log in only to "n" (non-special) hosts; they never should be allowed to log in to special hosts s1, s2, ...

  • nancy

You will want to create shared, unprivileged utility accounts on darter for use by unrestricted and restricted admins. These (per our convention) will be called "univ" and "rstr", respectively. No one should actually directly log in to univ and rstr, and in fact, these accounts should not have passwords or trusted keys of their own. All logins to the shared utility accounts should be performed with su(1) from an existing individual account on darter.

The Setup

Joe's first act is to log in to darter and "become" the univ account:


$ sudo su - univ

Then, under that shared utility account, Joe creates a .ssh directory and an SSH keypair. This key will be trusted by the root account on every target host (because it's the "univ"-ersal key):


$ mkdir .ssh    # if not already present
$ ssh-keygen -t rsa -b 2048 -C "universal access 
 ↪key gen YYYYMMDD" -f
.ssh/univ_key
   Enter passphrase (empty for no passphrase):

Very important: Joe assigns a strong passphrase to this key. The passphrase to this key will not be generally shared.

(The field after -C is merely a comment; this format reflects my personal preference, but you are of course free to develop your own.)

This will generate two files in .ssh: univ_key (the private key file) and univ_key.pub (the public key file). The private key file is encrypted, protected by the very strong passphrase Joe assigned to it, above.

Joe logs out of the univ account and into rstr. He executes the same steps, but creates a keypair named rstr_key instead of univ_key. He assigns a strong passphrase to the private key file—it can be the same passphrase as assigned to univ, and in fact, that is probably preferable from the standpoint of simplicity.

Joe copies univ_key.pub and rstr_key.pub to a common location for convenience.

For every host to which access is granted for everyone (n1, n2, ...), Joe uses the target hosts' root credentials to copy both univ_key.pub and rstr_key.pub (on separate lines) to the file .ssh/authorized_keys under the root account directory.

For every host to which access is granted for only a few (s1, s2, ...), Joe uses the target hosts' root credentials to copy only rstr_key.pub (on a single line) to the file .ssh/authorized_keys under the root account directory.

So to review, now, when a user uses su to "become" the univ account, he or she can log in to any host, because univ_key.pub exists in the authorized_keys file of n1, n2, ... and s1, s2, ....

However, when a user uses su to "become" the rstr account, he or she can log in only to n1, n2, ..., because those hosts' authorized_keys files contain rstr_key.pub, but not univ_key.pub.

Of course, in order to unlock the access in both cases, the user will need the strong passphrase with which Joe created the keys. That seems to defeat the whole purpose of the scheme, but there's a trick to get around it.

The Trick

First, let's talk about key-caching. Any user who uses SSH keys whose key files are protected by a passphrase may cache those keys using a program called ssh-agent. ssh-agent does not take a key directly upon invocation. It is invoked as a standalone program without any parameters (at least, none useful to us here).

The output of ssh-agent is a couple environment variable/value pairs, plus an echo command, suitable for input to the shell. If you invoke it "straight", these variables will not become part of the environment. For this reason, ssh-agent always is invoked as a parameter of the shell built-in eval:


$ eval $(ssh-agent)
Agent pid 29013

(The output of eval also includes an echo statement to show you the PID of the agent instance you just created.)

Once you have an agent running, and your shell knows how to communicate with it (thanks to the environment variables), you may cache keys with it using the command ssh-add. If you give ssh-add a key file, it will prompt you for the passphrase. Once you provide the correct passphrase, ssh-agent will hold the unencrypted key in memory. Any invocation of SSH will check with ssh-agent before attempting authentication. If the key in memory matches the public key on the remote host, trust is established, and the login simply happens with no entry of passwords or passphrases.

(As an aside: for those of you who use the Windows terminal program PuTTY, that tool provides a key-caching program called Pageant, which performs much the same function. PuTTY's equivalent to ssh-keygen is a utility called PuTTYgen.)

All you need to do now is set it up so the univ and rstr accounts set themselves up on every login to make use of persistent instances of ssh-agent. Normally, a user manually invokes ssh-agent upon login, makes use of it during that session, then kills it, with eval $(ssh-agent -k), before exiting. Instead of manually managing it, let's write into each utility account's .bash_profile some code that does the following:

  1. First, check whether there is a current instance of ssh-agent for the current account.

  2. If not, invoke ssh-agent and capture the environment variables in a special file in /tmp. (It should be in /tmp because the contents of /tmp are cleared between system reboots, which is important for managing cached keys.)

  3. If so, find the file in /tmp that holds the environment variables and source it into the shell's environment. (Also, handle the error case where the agent is running and the /tmp file is not found by killing ssh-agent and starting from scratch.)

All of the above assumes the key already has been unlocked and cached. (I will come back to that.)

Here is what the code in .bash_profile looks like for the univ account:


/usr/bin/pgrep -u univ 'ssh-agent' >/dev/null

RESULT=$?

if [[ $RESULT -eq 0 ]]  # ssh-agent is running
then
    if [[ -f /tmp/.env_ssh.univ ]]   # bring env in to session
    then
        source /tmp/.env_ssh.univ
    else    # error condition
        echo 'WARNING:  univ ssh agent running, no environment 
         ↪file found'
        echo '          ssh-agent being killed and restarted ... '
        /usr/bin/pkill -u univ 'ssh-agent' >/dev/null
        RESULT=1     # due to kill, execute startup code below
    fi

if [[ $RESULT -ne 0 ]] # ssh-agent not running, start 
 ↪it from scratch
then
    echo "WARNING:  ssh-agent being started now; 
     ↪ask Joe to cache key"
    /usr/bin/ssh-agent > /tmp/.env_ssh.univ
    /bin/chmod 600 /tmp/.env_ssh.univ
    source /tmp/.env_ssh.univ
fi

And of course, the code is identical for the rstr account, except s/univ/rstr/ everywhere.

Joe will have to intervene once whenever darter (the central management host on which all the user accounts and the keys reside) is restarted. Joe will have to log on and become univ and execute the command:


$ ssh-add ~/.ssh/univ_key

and then enter the passphrase. Joe then logs in to the rstr account and executes the same command against ~/.ssh/rstr_key. The command ssh-add -l lists cached keys by their fingerprints and filenames, so if there is doubt about whether a key is cached, that's how to find out. A single agent can cache multiple keys, if you have a use for that, but it doesn't come up much in my environment.

Once the keys are cached, they will stay cached. (ssh-add -t <N> may be used to specify a timeout of N seconds, but you won't want to use that option for this shared-access scheme.) The cache must be rebuilt for each account whenever darter is rebooted, but since darter is a Linux host, that will be a rare event. Between reboots, the single instance (one per utility account) of ssh-agent simply runs and holds the key in memory. The last time I entered the passphrases of our utility account keys was more than 500 days ago—and I may go several hundred more before having to do so again.

The last step is setting up sudoers to manage access to the utility accounts. You don't really have to do this. If you like, you can set (different) passwords for univ and rstr and simply let the users hold them. Of course, shared passwords aren't a great idea to begin with. (That's one of the major points of this whole scheme!) Every time one of the users of the univ account leaves the team, you'll have to change that password and distribute the new one (hopefully securely and out-of-band) to all the remaining users.

No, managing access with sudoers is a better idea. This article isn't here to teach you all of—or any of—the ins and outs of sudoers' Extremely Bizarre Nonsensical Frustration (EBNF) syntax. I'll just give you the cheat code.

Recall that Andy, Amy, Alice and so on were all allowed to access all hosts. These users are permitted to use sudo to execute the su - univ command. Ned, Nora, Nancy and so on are permitted to access only the restricted list of hosts. They may log in only to the rstr account using the su - rstr command. The sudoers entries for these might look like:


User_Alias  UNIV_USERS=andy,amy,alice,arthur        # trusted
User_Alias  RSTR_USERS=ned,nora,nancy,nyarlathotep  # not so much

# Note that there is no harm in putting andy, amy, etc. into
# RSTR_USERS as well. But it also accomplishes nothing.

Cmnd_Alias  BECOME_UNIV = /bin/su - univ
Cmnd_Alias  BECOME_RSTR = /bin/su - rstr

UNIV_USERS   ALL= BECOME_UNIV
RSTR_USERS   ALL= BECOME_RSTR

Let's recap. Every host n1, n2, n3 and so on has both univ and rstr key files in authorized_keys.

Every host s1, s2, s3 and so on has only the univ key file in authorized_keys.

When darter is rebooted, Joe logs in to both the univ and rstr accounts and executes the ssh-add command with the private key file as a parameter. He enters the passphrase for these keys when prompted.

Now Andy (for example) can log in to darter, execute:


$ sudo su - univ

and authenticate with his password. He now can log in as root to any of n1, n2, ..., s1, s2, ... without further authentication. If Andy needs to check the functioning of ntp (for example) on each of 20 hosts, he can execute a loop:


$ for H in n1 n2 n3 [...] n10 s1 s2 s3 [...] s10
> do
>    ssh -q root@$H 'ntpdate -q timeserver.domain.tld'
> done

and it will run without further intervention.

Similarly, nancy can log in to darter, execute:


$ sudo su - rstr

and log in to any of n1, n2 and so on, execute similar loops, and so forth.

Benefits and Risks

Suppose Nora leaves the team. You simply would edit sudoers to delete her from RSTR_USERS, then lock or delete her system account.

"But Nora was fired for misconduct! What if she kept a copy of the keypair?"

The beauty of this scheme is that access to the two key files does not matter. Having the public key file isn't important—put the public key file on the Internet if you want. It's public!

Having the encrypted copy of the private key file doesn't matter. Without the passphrase (which only Joe knows), that file may as well be the output of /dev/urandom. Nora never had access to the raw key file—only the caching agent did.

Even if Nora kept a copy of the key files, she cannot use them for access. Removing her access to darter removes her access to every target host.

And the same goes, of course, for the users in UNIV_USERS as well.

There are two caveats to this, and make sure you understand them well.

Caveat the first: it (almost) goes without saying that anyone with root access to darter obviously can just become root, then su - univ at any time. If you give someone root access to darter, you are giving that person full access to all the target hosts as well. That, after all, is the meaning of saying the target hosts "trust" darter. Furthermore, a user with root access who does not know the passphrase to the keys still can recover the raw keys from memory with a little moderately sophisticated black magic. (Linux memory architecture and clever design of the agent prevent non-privileged users from recovering their own agents' memory contents in order to extract keys.)

Caveat the second: obviously, anyone holding the passphrase can make (and keep) an unencrypted copy of the private keys. In our example, only Joe had that passphrase, but in practice, you will want two or three trusted admins to know the passphrase so they can intervene to re-cache the keys after a reboot of darter.

If anyone with root access to your central management host (darter, in this example) or anyone holding private key passphrases should leave the team, you will have to generate new keypairs and replace the contents of authorized_keys on every target host in your enterprise. (Fortunately, if you are careful, you can use the old trust relationship to create the new one.)

For that reason, you will want to entrust the passphrase only to individuals whose positions on your team are at least reasonably stable. The techniques described in this article are probably not suitable for a high-turnover environment with no stable "core" admins.

One more thing about this: you don't need to be managing tiered or any kind of shared access for this basic trick to be useful. As I noted above, the usual way of using an SSH key-caching agent is by invoking it at session start, caching your key, then killing it before ending your session. However, by including the code above in your own .bash_profile, you can create your own file in /tmp, check for it, load it if present and so on. That way, the host always has just one instance of ssh-agent running, and your key is cached in it permanently (or until the next reboot, anyway).

Even if you don't want to cache your key that persistently, you still can make use of a single ssh-agent and cache your key with the timeout (-t) option mentioned earlier; you still will be saving yourself a step.

Note that if you do this, however, anyone with root on that host will have access to any account of yours that trusts your account on that machine— so caveat actor. (I use this trick only on personal boxes that only I administer.)

The trick for personal use is becoming obsolete, as Mac OS X (via SSHKeyChain) and newer versions of GNOME (via Keyring) automatically know the first time you SSH to a host with which you have a key-based authentication set up, then ask you your passphrase and cache the key for the rest of your GUI login session. Given the lack of default timeouts and warnings about root users' access to unlocked keys, I am not sure this is an unmixed technological advance. (It is possible to configure timeouts in both utilities, but it requires that users find out about the option, and take the effort to configure it.)

Acknowledgements

I gratefully acknowledge the technical review and helpful suggestions of David Scheidt and James Richmond in the preparation of this article.

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