Config parsing rejects unknown YAML fields. The executable backends are `local` and `ssh`. S3 config fields are accepted by config validation, but runtime execution for S3 is unavailable.
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.
-`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.
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` or `fail`; defaults to `skip`.
-`transfer.on_conflict`: only `fail`; defaults to `fail`.
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.
