Snap Package Format and Linux Sandboxing Explained

The Snap package format is a universal Linux packaging system created by Canonical that bundles an application along with all its required dependencies, runtimes, and libraries. Designed to run consistently across any Linux distribution, Snaps eliminate dependency conflicts and streamline software distribution. Crucially, Snap packages provide built-in security through strict sandboxing mechanisms—utilizing Linux kernel features such as AppArmor, cgroups, seccomp filters, and mount namespaces to isolate applications completely from the host system and other software.

Understanding the Snap Package Format

At its core, a Snap is a single compressed file using the SquashFS filesystem format. When an application packaged as a Snap is installed, it is not unpacked into system directories like traditional .deb or .rpm packages. Instead, the Snap daemon (snapd) mounts the SquashFS image as a read-only loop device.

Because each Snap includes its own set of dependencies, developers no longer have to worry about the specific library versions installed on the host operating system. This self-contained architecture enables transactional updates, automated rollbacks if an update fails, and the ability to install multiple versions of the same application concurrently without conflicts.

How Snap Sandboxing Works

Snap relies on three distinct confinement levels to manage application security:

1. Strict: The default and most secure mode. The application runs fully sandboxed and has no access to system resources, user files, or hardware unless explicitly granted.
2. Classic: Used for developer tools and utilities that require broad system access (such as compilers or shell utilities). Applications in this mode run with the same access levels as traditional Linux packages and are not sandboxed.
3. Devmode: A diagnostic mode where confinement rules are monitored rather than enforced, primarily used by developers to debug sandboxing issues.

Under Strict confinement, snapd orchestrates several foundational Linux kernel security features to establish an isolated environment:

• AppArmor: Enforces Mandatory Access Control (MAC). AppArmor profiles specify precisely which files, directories, network sockets, and D-Bus endpoints an application can read, write, or execute.
• Seccomp (Secure Computing Mode): Restricts the system calls (syscalls) an application can make to the Linux kernel. If an application attempts an unauthorized syscall that is not explicitly whitelisted, the kernel blocks the action or terminates the process.
• Mount Namespaces: Creates a customized, isolated view of the filesystem for each application. A strictly confined Snap cannot see the host root filesystem directly; instead, it sees a controlled environment where paths like /tmp are private to that application.
• Control Groups (cgroups): Manages and restricts system resources, such as memory and CPU usage, ensuring that a misbehaving or compromised Snap cannot starve the host system of resources.

Secure Communication via Interfaces: Plugs and Slots

Because strictly confined applications are isolated by default, they require a secure mechanism to access external hardware or host resources, such as the network, webcams, audio devices, or personal files. Snap achieves this through an interface system based on plugs and slots:

• Slot: A provider of a system resource or service (often provided by the core system or another Snap).
• Plug: A consumer requesting access to that resource.

When a plug is connected to a slot, snapd automatically updates the application's AppArmor and seccomp profiles to allow the requested access. While safe interfaces (such as network access) are often connected automatically during installation, sensitive interfaces (such as raw disk access or camera control) typically require manual approval from the user via the command line or a graphical software center. This permission-based model ensures users maintain granular control over application capabilities.