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SSH Keys

Read when:

  • changing local key storage or key generation;
  • debugging SSH authentication or host-key trust;
  • changing how provider key pairs are imported or cleaned up.

Crabbox generates a fresh SSH key per lease by default. This keeps a long-lived personal key out of every runner and gives the provider layer a predictable, per-lease resource name it can import and later delete.

Per-lease key generation

When a lease is created, the CLI runs ssh-keygen to produce a key it stores locally. The key type is ed25519 for most leases, and rsa (4096-bit) only for AWS and Azure Windows targets, where the platform requires RSA. Generation is idempotent: if a key already exists for the lease ID, it is reused as-is.

Local key storage lives under the Crabbox user config directory, outside the repository:

macOS:   ~/Library/Application Support/crabbox/testboxes/<lease>/id_ed25519
Linux:   ~/.config/crabbox/testboxes/<lease>/id_ed25519

The matching <lease>/id_ed25519.pub sits beside it. The key directory is created with 0700 permissions.

Host-key trust and connection reuse

A per-lease known_hosts file lives next to the key (<lease>/known_hosts). All SSH connections use:

  • StrictHostKeyChecking=accept-new — trust a host's key on first contact, then pin it;
  • UserKnownHostsFile pointed at the per-lease known_hosts;
  • IdentitiesOnly=yes with -i <key> so only the lease key is offered.

Because host keys are scoped to the lease's own file, a reused provider IP from a previous lease never poisons the user's global ~/.ssh/known_hosts, and two leases sharing an address do not cross host-key state.

On macOS and Linux, connection multiplexing is enabled (ControlMaster=auto, ControlPersist=10m) with a ControlPath scoped by the key path, so reused IPs do not share a control socket between leases. Windows OpenSSH and secret-authenticated targets disable multiplexing (ControlMaster=no).

What the broker sees

In brokered mode the CLI sends only the public key to the coordinator; the private key never leaves the local machine. The Worker imports or reuses that public key in the target provider under a stable per-lease name derived from the lease ID (crabbox-<lease>, with _ rewritten to -):

  • Hetzner uploads it as an SSH key, reusing an existing key with matching contents instead of creating a duplicate;
  • AWS imports it as an EC2 key pair;
  • Azure and GCP inject it through their respective instance metadata / key paths.

When the coordinator assigns a different final lease ID than the provisional one the CLI started with, the CLI renames the local key directory to the final ID so later status, ssh, run --id, and stop commands keep finding the key.

Cleanup

Provider delete paths remove the per-lease cloud key or key pair when the machine is deleted (for example AWS DeleteKeyPair, Hetzner SSH-key delete, and the equivalent on other adapters). Several provider backends also remove the local key directory when they release or clean up a lease (for example the Parallels, local-container, Semaphore, Blacksmith, and Sprites adapters).

Bringing your own key

Setting CRABBOX_SSH_KEY (or the ssh.key config value) points the CLI at an existing private key instead of a generated per-lease one. doctor validates that key — checking the private path and its .pub sibling — only when CRABBOX_SSH_KEY is set; otherwise it reports the default per-lease mode as healthy.

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