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 OpenSSH software will also be built and installed in the <CMAKE_INSTALL_PREFIX>/openssh directory. This directory must satisfy the security requirements for critical files and directories.

Sarus 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-sshd as positional argument. In addition, the following environment variables must be defined:

• SARUS_PREFIX_DIR: Absolute path to the installation directory of Sarus.
• SARUS_OPENSSH_DIR: Absolute path to the location of Sarus’s custom OpenSSH software.

The following is an example OCIHooks object enabling the SSH hook:

{
    "prestart": [
        {
            "path": "/opt/sarus/bin/ssh_hook",
            "env": [
                "SARUS_PREFIX_DIR=/opt/sarus",
                "SARUS_OPENSSH_DIR=/opt/sarus/openssh"
            ],
            "args": [
                "ssh_hook",
                "start-sshd"
            ]
        }
    ]
}

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 hook bind-mounts the custom OpenSSH software and starts the sshd daemon only if the environment variable SARUS_SSH_HOOK=1 is present in the container. This environment variable is automatically set by Sarus if the --ssh command line option is passed to sarus run.