---
title: Vault SSH Certificate Authority
description: Steps followed to enable using Vault as a CA for ssh login
---

# Setup

[Reference Vault Documentation](https://developer.hashicorp.com/vault/docs/secrets/ssh/signed-ssh-certificates#host-key-signing)

I have set the documenation to use my own defaults and configuration. This also assumes a running and active Vault instance.

## Enable the SSH CA

I followed the defaults mostly in the docs, reference the above link for details. Use either root or a role with permissions for the endpoints.

Start with enabling the mount.
```bash
vault secrets enable -path=ssh-client-signer ssh
```

Generate a key. This will be used only for signing and not for client authentication. Keep it in a secure location, rename the path the key will be written to.
```bash
ssh-keygen -t rsa -C "alexanderlebens@gmail.com"
```

Add the above signing key.
```bash
vault write ssh-client-signer/config/ca private_key="..." public_key="..."
```

## Create Client Role and Key

Once the above is complete, create a role to use to sign your own client cert. I used my common username and configurations. This can also be done in the Vault UI.
```bash
vault write ssh-client-signer/roles/alexlebens -<<"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": "alexlebens",
  "ttl": "30m0s"
}
EOH
```

## Create Client Key

Generate the ssh key to use to authenticate to your hosts. This is the one to keep in ~/.ssh.
```bash
ssh-keygen -t rsa -C "alexanderlebens@gmail.com"
```

## Configure SSH to use the Key and Cert

SSH will defailt to using the cert when using the matching name "id_rsa_host-cert.pub" as shown in the renewal certificate section. Use the principal as signed by Vault as the User and set the IdentityFile to the Key as generated above.
```
Host ps08rp
    Hostname 10.232.1.51
    User alexlebens
    IdentityFile ~/.ssh/id_rsa_host
```

# Operations

## Prepare Target Host

Download the public cert from the endpoint.
```bash
curl -o /etc/ssh/trusted-user-ca-keys.pem https://vault.alexlebens.net/v1/ssh-client-signer/public_key
```

Then add that file to the sshd config.
```
TrustedUserCAKeys /etc/ssh/trusted-user-ca-keys.pem
```

### Automation

This step is currently manual as I have few hosts that I need ssh for. The most common tool for automation would be Ansible. But this would only be useful for my RaspberyPis and I plan to migrate those to Talos and Kubernetes in the future.

## Renew Client Certificate

Sign the client cert, on your machine, with the Vault CA.
```bash
vault write -field=signed_key ssh-client-signer/sign/alexlebens public_key=@$HOME/.ssh/id_rsa_host.pub > ~/.ssh/id_rsa_host-cert.pub
```

I added the following to my .zshrc to make this easier. So now I just run "vault-renew" before I need to ssh.
```
# Vault
export VAULT_ADDR="https://vault.alexlebens.net"
alias vault-renew='vault write -field=signed_key ssh-client-signer/sign/alexlebens public_key=@$HOME/.ssh/id_rsa_host.pub > ~/.ssh/id_rsa_host-cert.pub'
```

### View Cert Details

For troubleshooting purposes or clafification use the follow to inspect the cert.
```bash
ssh-keygen -Lf ~/.ssh/id_rsa_host-cert.pub
```