Signed SSH certificates

The signed SSH certificates is the simplest and most powerful in terms of setup complexity and in terms of being platform agnostic. By leveraging Vault's powerful CA capabilities and functionality built into OpenSSH, clients can SSH into target hosts using their own local SSH keys.

When using the signed SSH certificates, an SSH CA signing key is generated or configured at the secrets engine's mount. This key will be used to sign other SSH keys.

In this section, the term "client" refers to the person or machine performing the SSH operation. The "host" refers to the target machine. If this is confusing, substitute "client" with "user".

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.

Client key signing

Before a client can request their SSH key be signed, the Vault SSH secrets engine must be configured. Usually a Vault administrator or security team performs these steps. It is also possible to automate these actions using a configuration management tool like Chef, Puppet, Ansible, or Salt.

Signing key & role configuration

The following steps are performed in advance by a Vault administrator, security team, or configuration management tooling.

Mount the secrets engine. Like all secrets engines in Vault, the SSH secrets engine must be mounted before use.
```
$ vault secrets enable -path=ssh-client-signer ssh
Successfully mounted 'ssh' at 'ssh-client-signer'!
```
This enables the SSH secrets engine at the path "ssh-client-signer". It is possible to mount the same secrets engine multiple times using different -path arguments. The name "ssh-client-signer" is not special - it can be any name, but this documentation will assume "ssh-client-signer".
Configure Vault with a CA for signing client keys using the /config/ca endpoint. If you do not have an internal CA, Vault can generate a keypair for you.
```
$ vault write ssh-client-signer/config/ca generate_signing_key=true
Key             Value
---             -----
public_key      ssh-rsa AAAAB3NzaC1yc2EA...
```
If you already have a keypair, specify the public and private key parts as part of the payload:
```
$ vault write ssh-client-signer/config/ca \
    private_key="..." \
    public_key="..."
```
Regardless of whether it is generated or uploaded, the client signer public key is accessible via the API at the /public_key endpoint or the CLI (see next step).
Add the public key to all target host's SSH configuration. This process can be manual or automated using a configuration management tool. The public key is accessible via the API and does not require authentication.
```
$ curl -o /etc/ssh/trusted-user-ca-keys.pem http://127.0.0.1:8200/v1/ssh-client-signer/public_key
```
```
$ vault read -field=public_key ssh-client-signer/config/ca > /etc/ssh/trusted-user-ca-keys.pem
```
Add the path where the public key contents are stored to the SSH configuration file as the TrustedUserCAKeys option.
```
# /etc/ssh/sshd_config
# ...
TrustedUserCAKeys /etc/ssh/trusted-user-ca-keys.pem
```
Restart the SSH service to pick up the changes.
Create a named Vault role for signing client keys.
IMPORTANT NOTE: Prior to Vault-1.9, if "allowed_extensions" is either empty or not specified in the role, Vault will assume permissive defaults: any user assigned to the role may specify any arbitrary extension values as part of the certificate request to the Vault server. This may have significant impact on third-party systems that rely on an extensions field for security-critical information. In those cases, consider using a template to specify default extensions, and explicitly setting "allowed_extensions" to an arbitrary, non-empty string if the field is empty or not set.
Because of the way some SSH certificate features are implemented, options are passed as a map. The following example adds the permit-pty extension to the certificate, and allows the user to specify their own values for permit-pty and permit-port-forwarding when requesting the certificate.
```
$ vault write ssh-client-signer/roles/my-role -<<"EOH"
{
  "algorithm_signer": "rsa-sha2-256",
  "allow_user_certificates": true,
  "allowed_users": "*",
  "allowed_extensions": "permit-pty,permit-port-forwarding",
  "default_extensions": {
    "permit-pty": ""
  },
  "key_type": "ca",
  "default_user": "ubuntu",
  "ttl": "30m0s"
}
EOH
```

Client SSH authentication

The following steps are performed by the client (user) that wants to authenticate to machines managed by Vault. These commands are usually run from the client's local workstation.

Locate or generate the SSH public key. Usually this is ~/.ssh/id_rsa.pub. If you do not have an SSH keypair, generate one:
```
$ ssh-keygen -t rsa -C "user@example.com"
```

Ask Vault to sign your public key. This file usually ends in .pub and the contents begin with ssh-rsa ....

$ vault write ssh-client-signer/sign/my-role \
    public_key=@$HOME/.ssh/id_rsa.pub

Key             Value
---             -----
serial_number   c73f26d2340276aa
signed_key      ssh-rsa-cert-v01@openssh.com AAAAHHNzaC1...

The result will include the serial and the signed key. This signed key is another public key.

To customize the signing options, use a JSON payload:

$ vault write ssh-client-signer/sign/my-role -<<"EOH"
{
  "public_key": "ssh-rsa AAA...",
  "valid_principals": "my-user",
  "key_id": "custom-prefix",
  "extensions": {
    "permit-pty": "",
    "permit-port-forwarding": ""
  }
}
EOH

Save the resulting signed, public key to disk. Limit permissions as needed.
```
$ vault write -field=signed_key ssh-client-signer/sign/my-role \
    public_key=@$HOME/.ssh/id_rsa.pub > signed-cert.pub
```
If you are saving the certificate directly beside your SSH keypair, suffix the name with -cert.pub (~/.ssh/id_rsa-cert.pub). With this naming scheme, OpenSSH will automatically use it during authentication.
(Optional) View enabled extensions, principals, and metadata of the signed key.
```
$ ssh-keygen -Lf ~/.ssh/signed-cert.pub
```
SSH into the host machine using the signed key. You must supply both the signed public key from Vault and the corresponding private key as authentication to the SSH call.
```
$ ssh -i signed-cert.pub -i ~/.ssh/id_rsa username@10.0.23.5
```

Host key signing

For an added layer of security, we recommend enabling host key signing. This is used in conjunction with client key signing to provide an additional integrity layer. When enabled, the SSH agent will verify the target host is valid and trusted before attempting to SSH. This will reduce the probability of a user accidentally SSHing into an unmanaged or malicious machine.

Signing key configuration

Mount the secrets engine. For the most security, mount at a different path from the client signer.

$ vault secrets enable -path=ssh-host-signer ssh
Successfully mounted 'ssh' at 'ssh-host-signer'!

Configure Vault with a CA for signing host keys using the /config/ca endpoint. If you do not have an internal CA, Vault can generate a keypair for you.
```
$ vault write ssh-host-signer/config/ca generate_signing_key=true
Key             Value
---             -----
public_key      ssh-rsa AAAAB3NzaC1yc2EA...
```
If you already have a keypair, specify the public and private key parts as part of the payload:
```
$ vault write ssh-host-signer/config/ca \
    private_key="..." \
    public_key="..."
```
Regardless of whether it is generated or uploaded, the host signer public key is accessible via the API at the /public_key endpoint.

Extend host key certificate TTLs.

$ vault secrets tune -max-lease-ttl=87600h ssh-host-signer

Create a role for signing host keys. Be sure to fill in the list of allowed domains, set allow_bare_domains, or both.

$ vault write ssh-host-signer/roles/hostrole \
    key_type=ca \
    algorithm_signer=rsa-sha2-256 \
    ttl=87600h \
    allow_host_certificates=true \
    allowed_domains="localdomain,example.com" \
    allow_subdomains=true

Sign the host's SSH public key.

$ vault write ssh-host-signer/sign/hostrole \
    cert_type=host \
    public_key=@/etc/ssh/ssh_host_rsa_key.pub
Key             Value
---             -----
serial_number   3746eb17371540d9
signed_key      ssh-rsa-cert-v01@openssh.com AAAAHHNzaC1y...

Set the resulting signed certificate as HostCertificate in the SSH configuration on the host machine.

$ vault write -field=signed_key ssh-host-signer/sign/hostrole \
    cert_type=host \
    public_key=@/etc/ssh/ssh_host_rsa_key.pub > /etc/ssh/ssh_host_rsa_key-cert.pub

Set permissions on the certificate to be 0640:

$ chmod 0640 /etc/ssh/ssh_host_rsa_key-cert.pub

Add host key and host certificate to the SSH configuration file.

# /etc/ssh/sshd_config
# ...

# For client keys
TrustedUserCAKeys /etc/ssh/trusted-user-ca-keys.pem

# For host keys
HostKey /etc/ssh/ssh_host_rsa_key
HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pub

Restart the SSH service to pick up the changes.

Client-Side host verification

Retrieve the host signing CA public key to validate the host signature of target machines.

$ curl http://127.0.0.1:8200/v1/ssh-host-signer/public_key

$ vault read -field=public_key ssh-host-signer/config/ca

Add the resulting public key to the known_hosts file with authority.

# ~/.ssh/known_hosts
@cert-authority *.example.com ssh-rsa AAAAB3NzaC1yc2EAAA...

SSH into target machines as usual.

Troubleshooting

When initially configuring this type of key signing, enable VERBOSE SSH logging to help annotate any errors in the log.

# /etc/ssh/sshd_config
# ...
LogLevel VERBOSE

Restart SSH after making these changes.

By default, SSH logs to /var/log/auth.log, but so do many other things. To extract just the SSH logs, use the following:

$ tail -f /var/log/auth.log | grep --line-buffered "sshd"

If you are unable to make a connection to the host, the SSH server logs may provide guidance and insights.

Name is not a listed principal

If the auth.log displays the following messages:

# /var/log/auth.log
key_cert_check_authority: invalid certificate
Certificate invalid: name is not a listed principal

The certificate does not permit the username as a listed principal for authenticating to the system. This is most likely due to an OpenSSH bug (see known issues for more information). This bug does not respect the allowed_users option value of "*". Here are ways to work around this issue:

Set default_user in the role. If you are always authenticating as the same user, set the default_user in the role to the username you are SSHing into the target machine:
```
$ vault write ssh/roles/my-role -<<"EOH"
{
  "default_user": "YOUR_USER",
  // ...
}
EOH
```
Set valid_principals during signing. In situations where multiple users may be authenticating to SSH via Vault, set the list of valid principles during key signing to include the current username:
```
$ vault write ssh-client-signer/sign/my-role -<<"EOH"
{
  "valid_principals": "my-user"
  // ...
}
EOH
```

If you do not see a prompt after authenticating to the host machine, the signed certificate may not have the permit-pty extension. There are two ways to add this extension to the signed certificate.

As part of the role creation

$ vault write ssh-client-signer/roles/my-role -<<"EOH"
{
  "default_extensions": {
    "permit-pty": ""
  }
  // ...
}
EOH

As part of the signing operation itself:

$ vault write ssh-client-signer/sign/my-role -<<"EOH"
{
  "extensions": {
    "permit-pty": ""
  }
  // ...
}
EOH

No port forwarding

If port forwarding from the guest to the host is not working, the signed certificate may not have the permit-port-forwarding extension. Add the extension as part of the role creation or signing process to enable port forwarding. See no prompt after login for examples.

{
  "default_extensions": {
    "permit-port-forwarding": ""
  }
}

No x11 forwarding

If X11 forwarding from the guest to the host is not working, the signed certificate may not have the permit-X11-forwarding extension. Add the extension as part of the role creation or signing process to enable X11 forwarding. See no prompt after login for examples.

{
  "default_extensions": {
    "permit-X11-forwarding": ""
  }
}

No agent forwarding

If agent forwarding from the guest to the host is not working, the signed certificate may not have the permit-agent-forwarding extension. Add the extension as part of the role creation or signing process to enable agent forwarding. See no prompt after login for examples.

{
  "default_extensions": {
    "permit-agent-forwarding": ""
  }
}

Key comments

There are additional steps needed to preserve comment attributes in keys which ought to be considered if they are required. Private and public key may have comments applied to them and for example where ssh-keygen is used with its -C parameter - similar to:

ssh-keygen -C "...Comments" -N "" -t rsa -b 4096 -f host-ca

Adapted key values containing comments must be provided with the key related parameters as per the Vault CLI and API steps demonstrated below.

# Using CLI:
vault secrets enable -path=hosts-ca ssh
KEY_PRI=$(cat ~/.ssh/id_rsa | sed -z 's/\n/\\n/g')
KEY_PUB=$(cat ~/.ssh/id_rsa.pub | sed -z 's/\n/\\n/g')
# Create / update keypair in Vault
vault write ssh-client-signer/config/ca \
  generate_signing_key=false \
  private_key="${KEY_PRI}" \
  public_key="${KEY_PUB}"

# Using API:
curl -X POST -H "X-Vault-Token: ..." -d '{"type":"ssh"}' http://127.0.0.1:8200/v1/sys/mounts/hosts-ca
KEY_PRI=$(cat ~/.ssh/id_rsa | sed -z 's/\n/\\n/g')
KEY_PUB=$(cat ~/.ssh/id_rsa.pub | sed -z 's/\n/\\n/g')
tee payload.json <<EOF
{
  "generate_signing_key" : false,
  "private_key"          : "${KEY_PRI}",
  "public_key"           : "${KEY_PUB}"
}
EOF
# Create / update keypair in Vault
curl -X POST -H "X-Vault-Token: ..." -d @payload.json http://127.0.0.1:8200/v1/hosts-ca/config/ca

IMPORTANT: Do NOT add a private key password since Vault can't decrypt it. Destroy the keypair and payload.json from your hosts immediately after they have been confirmed as successfully uploaded.

Known issues

On SELinux-enforcing systems, you may need to adjust related types so that the SSH daemon is able to read it. For example, adjust the signed host certificate to be an sshd_key_t type.
On some versions of SSH, you may get the following error:
```
no separate private key for certificate
```
This is a bug introduced in OpenSSH version 7.2 and fixed in 7.5. See OpenSSH bug 2617 for details.
On some versions of SSH, you may get the following error on target host:
```
userauth_pubkey: certificate signature algorithm ssh-rsa: signature algorithm not supported [preauth]
```
Fix is to add below line to /etc/ssh/sshd_config
```
CASignatureAlgorithms ^ssh-rsa
```
The ssh-rsa algorithm is no longer supported in OpenSSH 8.2

API

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