Dynamic SSH Keys

Deprecated: There are several serious drawbacks and security implications inherent in this type. Because of these drawbacks, please use the SSH CA or OTP types whenever possible.

When using this type, the administrator registers a secret key with appropriate sudo privileges on the remote machines; for every authorized credential request, Vault creates a new SSH key pair and appends the newly-generated public key to the authorized_keys file for the configured username on the remote host. Vault uses a configurable install script to achieve this.

The secrets engine does not prompt for sudo passwords; the NOPASSWD option for sudoers should be enabled at all remote hosts for the Vault administrative user.

The private key returned to the user will be leased and can be renewed if desired. Once the key is given to the user, Vault will not know when it gets used or how many time it gets used. Therefore, Vault WILL NOT and cannot audit the SSH session establishments.

When the credential lease expires, Vault removes the secret key from the remote machine.

This page will show a quick start for this secrets engine. For detailed documentation on every path, use vault path-help after mounting the secrets engine.

Drawbacks

The dynamic key type has several serious drawbacks:

Audit logs are unreliable: Vault can only log when users request credentials, not when they use the given keys. If user A and user B both request access to a machine, and are given a lease valid for five minutes, it is impossible to know whether two accesses to that user account on the remote machine were A, A; A, B; B, A; or B, B.
Generating dynamic keys consumes entropy: Unless equipped with a hardware entropy generating device, a machine can quickly run out of entropy when generating SSH keys. This will cause further requests for various Vault operations to stall until more entropy is available, which could take a significant amount of time, after which the next request for a new SSH key will use the generated entropy and cause stalling again.
This type makes connections to client hosts; when this happens the host key is not verified.

sudo

In order to adjust the authorized_keys file for the desired user, Vault connects via SSH to the remote machine as a separate user, and uses sudo to gain the privileges required. An example sudoers file is shown below.

File: /etc/sudoers

# This is a sample sudoers statement; you should modify it
# as appropriate to satisfy your security needs.
vaultadmin   ALL=(ALL)NOPASSWD: ALL

Configuration

Next, infrastructure configuration must be registered with Vault via roles. First, however, the shared secret key must be specified.

Mount the secrets engine

$ vault secrets enable ssh
Successfully mounted 'ssh' at 'ssh'!

Registering the shared secret key

$ vault write ssh/keys/dev_key \
    key=@dev_shared_key.pem

Create a Role

Next, create a role. All of the machines contained within this CIDR block list should be accessible using the registered shared secret key.

$ vault write ssh/roles/dynamic_key_role \
    key_type=dynamic \
    key=dev_key \
    admin_user=username \
    default_user=username \
    cidr_list=x.x.x.x/y
Success! Data written to: ssh/roles/dynamic_key_role

cidr_list is a comma separated list of CIDR blocks for which a role can generate credentials. If this is empty, the role can only generate credentials if it belongs to the set of zero-address roles.

Zero-address roles, configured via /ssh/config/zeroaddress endpoint, takes comma separated list of role names that can generate credentials for any IP address.

Use the install_script option to provide an install script if the remote hosts do not resemble a typical Linux machine. The default script is compiled into the Vault binary, but it is straight forward to specify an alternate. The script takes three arguments which are explained in the comments.

To see the default, see linux_install_script.go

Create a credential

Create a dynamic key for an IP of the remote host that is covered by dynamic_key_role's CIDR list.

$ vault write ssh/creds/dynamic_key_role ip=x.x.x.x
Key             Value
lease_id        ssh/creds/dynamic_key_role/8c4d2042-23bc-d6a8-42c2-6ff01cb83cf8
lease_duration  600
lease_renewable true
ip              x.x.x.x
key             -----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
key_type        dynamic
port            22
username        username

Establish an SSH session

Save the key to a file (e.g. dyn_key.pem) and then use it to establish an SSH session.

$ ssh -i dyn_key.pem username@<IP of remote host>
username@<IP of remote host>:~$

Automate it!

Creation of new key, saving to a file, and using it to establish an SSH session can all be done with a single Vault CLI command.

$ vault ssh -role dynamic_key_role username@<IP of remote host>
username@<IP of remote host>:~$

API

The SSH secret secrets engine has a full HTTP API. Please see the SSH secret secrets engine API for more details.