landstrip

landstrip runs a tool in an OS-level sandbox using Landlock LSM on Linux, Seatbelt on macOS, and LPAC AppContainer on Windows. It accepts the Anthropic Sandbox Runtime JSON subset as the policy, in JSON or YAML syntax.

Installation

npm

npm install --save-dev @landstrip/landstrip

npx landstrip -p policy.json cargo test

The npm package installs a small Node.js wrapper and a platform-specific native binary package.

Platforms

Windows AppContainer

Win32 API provides AppContainer for application level sandboxing. The platform creates a per-run LPAC AppContainer profile, grants its SID access to the lowered read and write roots, and removes those grants after the sandboxed process tree exits. Windows policies must use explicit read allowlists.

Landstrip assigns the sandboxed process to a Job Object with KILL_ON_JOB_CLOSE, so child processes are kept in the sandbox process tree and are terminated when the launcher exits.

allowNetwork grants the internet and private-network AppContainer capabilities, while the default container holds none and denies all network access.

AppContainer capabilities are coarse: fine-grained TCP policies by host or port require Windows Filtering Platform rules keyed by the AppContainer SID. I.e., this would require elevated privileges, which is not sustainable for a agent sandbox runtime, which should rely on unprivileged tools and techniques.

Policy Format

JSON is the default policy format. Use --format yaml for YAML policy files or YAML read from standard input.

landstrip --format yaml -p policy.yaml cargo test

YAML path fields can use normal lists or one statement per line:

filesystem:
  allowWrite: |
    .
    ~/.cargo
  denyRead: |
    ~/.ssh
  allowRead: |
    ~/.ssh/config
network:
  allowNetwork: true

Windows-only hardening options live under windows. They are optional because some tools, shells, JITs, and GUI helpers may rely on the blocked behaviors:

{
  "windows": {
    "disableWin32k": true,
    "disableExtensionPoints": true,
    "strictHandleChecks": true,
    "imageLoadNoRemote": true,
    "imageLoadNoLowLabel": true,
    "imageLoadPreferSystem32": true
  }
}

Network Policy

Sandbox mode denies direct network access by default. Proxy ports, local binding, and Unix sockets can be allowed with the Anthropic Sandbox Runtime network fields.

For a filesystem-only sandbox with unrestricted direct network access, set:

{
  "network": {
    "allowNetwork": true
  }
}

allowNetwork disables landstrip network enforcement while leaving filesystem policy enforcement in place. On Windows this grants the AppContainer its network capabilities; without it the container denies all network access.

Error Output

Failures reported by landstrip are printed as JSON objects on standard error, one object per line. Each object is tagged by the trap kind, with the kind name as the single top-level key.

{"Internal":{"file":"policy.json","source":"expected value at line 1 column 1"}}

{"Launch":["cargo","No such file or directory"]}

The trap kinds are:

• Filesystem: a filesystem access denial, as [operation, path, mechanism] where the operation is read or write and the mechanism is the kernel enforcement layer that detected the denial.
• Network: a denied TCP connect or bind, as [operation, target, mechanism] where the operation is connect or bind and the target is address:port.
• Launch: the tool could not be started, as [program, message].
• Usage: a command-line usage error, as a message string. Usage errors exit with status 2.
• Internal: any other policy, platform, or system error, as an object of diagnostic key/value pairs (for example source, file, or platform API details).

Logs and sandboxed tool output are not part of the response. Normal successful tool execution does not print a landstrip response unless a write denial was observed ({"Filesystem":["write","/repo/out","seccomp"]}), because standard error belongs to landstrip; standard output belongs to the sandboxed tool.

Trap FD

Use --trap-fd FD to write landstrip trap denial blocks to an already-open file descriptor as JSON objects, one per line followed by a newline.

landstrip --trap-fd 3 -p policy.json cargo test 3>landstrip-traps.txt

Linux filesystem and network denials observed by the seccomp broker are emitted with the same shapes as standard error:

{"Filesystem":["write","/repo/out","seccomp"]}
{"Network":["connect","127.0.0.1:9999","seccomp"]}

The mechanism element records the kernel enforcement layer that detected the denial (e.g. seccomp or landlock).

This stream is separate from the sandboxed tool's output. If the option is omitted, landstrip is quiet unless it has to report a policy, launch, or platform error. These long-lived error messages remain on standard error and are not duplicated in the trap stream.

Trap responses are informational. The configured sandbox policy always applies. However, writing trap responses requires an already-open file descriptor and a readable file path. If the sandbox blocks writing to the descriptor, or if writing fails, the denial is quietly dropped and the policy remains in effect. On backends without per-denial callbacks the option is best-effort.

The descriptor must be 3 or greater (standard I/O descriptors 0-2 are reserved).

Development

Commit messages

• <subsystem>: <message>
• Long description for non-trivial changes.
• Kernel style commit messages.
• Signed-off-by

Documenting errors

The following snippet demonstrates the recommended pattern for documenting the return values on error:

/// # Errors
///
/// Returns [`<variant's unqualified name>`](<variant's unqualified name>)
/// Returns ...

Licensing

The JavaScript npm wrapper is licensed under Apache-2.0. The Rust source and native binaries are licensed under LGPL-2.1-or-later. Corresponding source for each published native binary is available from the GitHub repository tag that matches the package version.