--- layout: "docs" page_title: "Secret Backend: SSH" sidebar_current: "docs-secrets-ssh" description: |- The SSH secret backend for Vault generates dynamic SSH keys or One-Time-Passwords. --- # SSH Secret Backend Name: `ssh` Vault SSH backend dynamically generates SSH credentials for remote hosts. This increases security by removing the need to share private keys with all users needing access to infrastructure. It also solves the problem of management and distribution of keys belonging to remote hosts. This backend supports two types of credential creation: Dynamic Key and One-Time Password (OTP), which address these problems in different ways. Read and carefully understand both of them before choosing the one which best suits your needs. The Vault team strongly recommends the OTP type whenever possible, and the drawbacks to the dynamic key type should be carefully considered before choosing it. This page will show a quick start for this backend. For detailed documentation on every path, use `vault path-help` after mounting the backend. ### Mounting SSH The `ssh` backend is not mounted by default and needs to be explicitly mounted. This is a common step for both OTP and Dynamic Key types. ```text $ vault mount ssh Successfully mounted 'ssh' at 'ssh'! ``` ---------------------------------------------------- ## I. One-Time-Password (OTP) Type This backend type allows a Vault server to issue an OTP every time a client wants to SSH into a remote host, using a helper command on the remote host to perform verification. An authenticated client requests credentials from the Vault server and, if authorized, is issued an OTP. When the client establishes an SSH connection to the desired remote host, the OTP used during SSH authentication is received by the Vault helper, which then validates the OTP with the Vault server. The Vault server then deletes this OTP, ensuring that it is only used once. Since the Vault server is contacted during SSH connection establishment, every login attempt and the correlating Vault lease information is logged to the audit backend. See [Vault-SSH-Helper](https://github.com/hashicorp/vault-ssh-helper) for details on the helper. ### Drawbacks The main concern with the OTP backend type is the remote host's connection to Vault; if compromised, an attacker could spoof the Vault server returning a successful request. This risk can be mitigated by using TLS for the connection to Vault and checking certificate validity; future enhancements to this backend may allow for extra security on top of what TLS provides. ### Creating a Role Create a role with the `key_type` parameter set to `otp`. All of the machines represented by the role's CIDR list should have helper properly installed and configured. ```text $ vault write ssh/roles/otp_key_role \ key_type=otp \ default_user=username \ cidr_list=x.x.x.x/y,m.m.m.m/n Success! Data written to: ssh/roles/otp_key_role ``` ### Create a Credential Create an OTP credential for an IP that belongs to `otp_key_role`. ```text $ vault write ssh/creds/otp_key_role ip=x.x.x.x Key Value lease_id ssh/creds/otp_key_role/73bbf513-9606-4bec-816c-5a2f009765a5 lease_duration 600 lease_renewable false port 22 username username ip x.x.x.x key 2f7e25a2-24c9-4b7b-0d35-27d5e5203a5c key_type otp ``` ### Establish an SSH session ```text $ ssh username@localhost Password: username@ip:~$ ``` ### Automate it! A single CLI command can be used to create a new OTP and invoke SSH with the correct parameters to connect to the host. ```text $ vault ssh -role otp_key_role username@x.x.x.x OTP for the session is `b4d47e1b-4879-5f4e-ce5c-7988d7986f37` [Note: Install `sshpass` to automate typing in OTP] Password: ``` The OTP will be entered automatically using `sshpass` if it is installed. ```text $ vault ssh -role otp_key_role username@x.x.x.x username@ip:~$ ``` ---------------------------------------------------- ## II. Dynamic Key Type 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 backend 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. ### Drawbacks The dynamic key type has several serious drawbacks: 1. _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. 2. _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. Because of these drawbacks, the Vault team recommends use of the OTP type whenever possible. Care should be taken with respect to the above issues with any deployments using the dynamic key type. ### 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` ```hcl # 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. #### Registering the shared secret key Register a key with a name; this key must have administrative capabilities on the remote hosts. ```text $ 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. ```text $ 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](https://github.com/hashicorp/vault/blob/master/builtin/logical/ssh/linux_install_script.go) ### Create a credential Create a dynamic key for an IP that is covered by `dynamic_key_role`'s CIDR list. ```text $ 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----- MIIEpAIBAAKCAQEA5V/Y95qfGaUXRPkKNK9jgDHXPD2n5Ein+QTNnLSGrHtJUH7+ pgs/5Hc4//124P9qHNmjIYQVyvcLreFgSrQCq4K8193hmypBYtsvCgvpc+jEwaGA zK0QV7uc1z8KL7FuRAxpHJwB6+nubOzzqM03xsViHRhaWhYVHw2Vl4oputSHE7R9 ugaTRg67wge4Nyi5RRL0RQcmW15/Vop8B6HpBSmZQy3enjg+32KbOWCMMTAPuF9/ DgxSgZQaFMjGN4RjDreZI8Vv5zIiFJzZ3KVOWy8piI0PblLnDpU4Q0QSQ9A+Vr7b JS22Lbet1Zbapl/n947/r1wGObLCc5Lilu//1QIDAQABAoIBAHWLfdO9sETjHp6h BULkkpgScpuTeSN6vGHXvUrOFKn1cCfJPNR4tWBuXI6LJM2+9nEccwXs+4IMwjZ0 ZfVCdI/SKtZxBXmP2PxBGMUMP7G/mn0kN64sDlD3ezOvQZgZVEmZFpCrvixYsG+v qlpZ+HhrlJEWds7tvBsyyfNjwWjVIpm08zBmteFj4zu7OEcmGXEHDoxDXxyVP2BG eLU/fM5JA2UEjfCQ1MIZ3rBtPePdz4LRpb+ajklqrUj1OHoiDrXa8EAf0/wDP9re c1iH4bn7ZjYK0+IhZ+Pmw6gUftzZNWSC2kOLnZLdN/K7hgh0l0r0K/1eeXt43upB WALNuiECgYEA8PM2Ob3XXKALF86PUewne4fCz9iixr/cIpvrEGrh9lyQRO8X5Jxb ug38jEql4a574C6TSXfzxURza4P6lnfa0LvymmW0bhxZ5nev9kcAVnLKvpOUArTR 32k9bKXd6zp8Q9ZyVNwHRxcVs4YgwfJlcx8geC4o6YRiIjvcBQ9RVHkCgYEA87OK lZDFBeEY/HVOxAQNXS5fgTd4U4DbwEJLv7SPk02v9oDkGHkpgMs4PcsIpCzsTpJ0 oXMfLSxZ1lmZiuUvAupKj/7RjJ0XyjSMfm1Zs81epWj+boVfM4amZNHVLIWgddmM XzXEZKByvi1gs7qFcjQz2DEbZltWO6dX14O4Fz0CgYEAlWSWyHJWZ02r0xT1c7vS NxtTxH7zXftzR9oYgtNiStfVc4gy7kGr9c3aOjnGZAlFMRhvpevDrxnj3lO0OTsS 5rzBjM1mc6cMboLjDPW01eTSpBroeE0Ym0arGQQ2djSK+5yowsixknhTsj2FbfsW v6wa+6jTIQY9ujAXGOQIbzECgYAYuXlw7SwgCZNYYappFqQodQD5giAyEJu66L74 px/96N7WWoNJvFkqmPOOyV+KEIi0/ATbMGvUUHCY36RFRDU9zXldHJQz+Ogl+qja VsvIAyj8DSfrHJrpBlsxVVyUVMZPzo+ARVs0flbF1qK9+Ul6qbMs1uaZvuCD0tmF ovZ1XQKBgQDB0s7SDmAMgVjG8UBZgUru9vsDrxERT2BloptnnAjSiarLF5M+qeZO 7L4NLyVP39Z83eerEonzDAHHbvhPyi6n2YmnYhGjeP+lPZIVqGF9cpZD3q48YHZc 3ePn2/oLZrXKWOMyMwp2Uj+0SArCW+xMnoNp50sYNVR/JK3BPIdkag== -----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. ```text $ ssh -i dyn_key.pem username@ip username@ip:~$ ``` ### 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. ```text $ vault ssh -role dynamic_key_role username@ip username@ip:~$ ``` ---------------------------------------------------- ## API ### /ssh/keys/ #### POST

Description

Creates or updates a named key. This is a root protected endpoint.

Method

POST

URL

`/ssh/keys/`

Parameters

key required (String) SSH private key with appropriate privileges on remote hosts.

Returns

A `204` response code.

Description: Deletes a named key. This is a root protected endpoint.
Method: DELETE
URL: `/ssh/keys/`
Parameters: None
Returns: A `204` response code.