SSH hook

Sarus also includes the source code for a hook capable of enabling SSH connections inside containers. The SSH hook is an executable binary that performs different ssh-related operations depending on the argument it receives from the runtime. For the full details about the implementation and inner workings, please refer to the related developer documentation.

Hook installation

The hook is written in C++ and it will be compiled along with Sarus if the ENABLE_SSH=TRUE CMake option has been used when configuring the build (the option is enabled by default). The Sarus installation scripts will also automatically install the hook in the <CMAKE_INSTALL_PREFIX>/bin directory.

A custom SSH software (statically linked Dropbear) will also be built and installed in the <CMAKE_INSTALL_PREFIX>/dropbear directory. This directory must satisfy the security requirements for critical files and directories.

Hook configuration

The SSH hook must be configured to run as a prestart hook. It expects to receive its own name/location as the first argument, and the string start-ssh-daemon as positional argument. In addition, the following environment variables must be defined:

  • HOOK_BASE_DIR: Absolute base path to the directory where the hook will create and access the SSH keys. The keys directory will be located in <HOOK_BASE_DIR>/<username>/.oci-hooks/ssh/keys.

  • PASSWD_FILE: Absolute path to a password file (PASSWD(5)). The file is used by the hook to retrieve the username of the user.

  • DROPBEAR_DIR: Absolute path to the location of the custom SSH software.

  • "SERVER_PORT: TCP port on which the SSH daemon will listen. This must be an unused port and is tipically set to a value different than 22 in order to avoid clashes with an SSH daemon that could be running on the host.

The following is an example of OCI hook JSON configuration file enabling the SSH hook:

    "version": "1.0.0",
    "hook": {
        "path": "/opt/sarus/bin/ssh_hook",
        "env": [
        "args": [
    "when": {
        "annotations": {
            "^com.hooks.ssh.enabled$": "^true$"
    "stages": ["prestart"]

Sarus support at runtime

The command sarus ssh-keygen will call the hook without creating a container, passing the appropriate arguments to generate dedicated keys to be used by containers.

The com.hooks.ssh.enabled=true annotation that enables the hook is automatically generated by Sarus if the --ssh command line option is passed to sarus run.


The SSH hook currently does not implement a poststop functionality and requires the use of a private PID namespace to cleanup the Dropbear daemon. Thus, the hook currently requires the use of a private PID namespace for the container. Thus, the --ssh option of sarus run implies --pid=private, and is incompatible with the use of --pid=host.