SSH uses native SFTP. It can be used for sources, destinations, or both:
```yaml
backend: ssh
host: example.com
user: distributor
port: 2222
path: /remote/root
ssh_key_file: /home/distributor/.ssh/id_ed25519
known_hosts: /home/distributor/.ssh/known_hosts
host_key_policy: accept-new
```
Authentication uses SSH agent identities first when `SSH_AUTH_SOCK` is set, then `ssh_key_file` if configured. Password authentication in YAML is not supported.
Host key policies:
-`strict`, `true`, and `"true"` require a matching known host key.
-`accept-new` accepts and persists a new host key, but fails if an existing key changed. During `run --dry-run`, new host keys are accepted only for the current connection and are not persisted.
-`off`, `false`, and `"false"` disable host key checking and are insecure.
`accept-new` and `strict` use `known_hosts` when configured. If omitted, distributor uses the current service user's default OpenSSH `known_hosts` path where practical. `accept-new` fails when it needs to persist a new host key and no writable `known_hosts` path is available. It does not create a missing parent `.ssh` directory.
`endpoint` and `bucket` are required. `prefix` is an optional backend root; it is treated as an object-key prefix, not a real directory. Prefixes must be clean slash-separated paths after trimming leading and trailing slashes. `http://` endpoints are allowed for explicitly configured local development or local S3-compatible test services.
If either credential environment variable name is configured, both must be configured and both referenced variables must resolve to non-empty values through the real process environment or `secrets.directory`. Explicit credentials take precedence over the AWS SDK default credential chain. If credential environment variable names are omitted, the SDK default credential chain is used and `secrets.directory` values are not injected into the process environment.
At least one output type must be enabled. When `publish.html` is true, `transform.markdown_to_html.enabled` must be `true` and `transform.markdown_to_html.mode` must be `sidecar`.
Transfer policy:
-`transfer.on_destination_same`: `skip` or `fail`; defaults to `skip`.
-`transfer.on_destination_older`: `replace` or `fail`; defaults to `replace`.
-`transfer.on_destination_newer`: `skip`, `replace`, or `fail`; defaults to `skip`.
-`transfer.on_conflict`: `fail` or `replace`; defaults to `fail`.
`replace` for `on_destination_newer` and `on_conflict` is honored only when `run --force` is used for that invocation. Force is CLI-only; there is no persistent config field that enables forced replacement by default.
Do not put literal secrets in config files. `secrets.directory` lets deployments provide credential values as files:
```yaml
secrets:
directory: /run/secrets/distributor
```
Each regular file in the directory becomes an internal credential environment value named by the filename. Valid filenames must match `[A-Za-z_][A-Za-z0-9_]*`. Directories are ignored, and symlinks to regular files are followed. Exactly one trailing LF or CRLF is trimmed from each file; other whitespace is preserved.
The resolver checks the real process environment first, then the secrets directory. If both define the same variable with different values, `run` prints a warning with the variable name and uses the real environment value. Secret values are not printed. The process environment is not modified, so SDK default credential chains see only real environment variables.
