nsjail

nsjail is Boundary's default jail type that uses Linux namespaces to provide process isolation. It creates unprivileged network namespaces to control and monitor network access for processes running under Boundary.

Overview

nsjail leverages Linux namespace technology to isolate processes at the network level. When Boundary runs with nsjail, it creates a separate network namespace for the isolated process, allowing Boundary to intercept and filter all network traffic according to the configured policy.

This jail type requires Linux capabilities to create and manage network namespaces, which means it has specific runtime requirements when running in containerized environments like Docker.

Architecture

Runtime & Permission Requirements for Running the Boundary in Docker

This section describes the Linux capabilities and runtime configurations required to run the Agent Boundary with nsjail inside a Docker container. Requirements vary depending on the OCI runtime and the seccomp profile in use.

1. Default runc runtime with CAP_NET_ADMIN

When using Docker's default runc runtime, the Boundary requires the container to have CAP_NET_ADMIN. This is the minimal capability needed for configuring virtual networking inside the container.

Docker's default seccomp profile may also block certain syscalls (such as clone) required for creating unprivileged network namespaces. If you encounter these restrictions, you may need to update or override the seccomp profile to allow these syscalls.

see Docker Seccomp Profile Considerations

2. Default runc runtime with CAP_SYS_ADMIN (testing only)

For development or testing environments, you may grant the container CAP_SYS_ADMIN, which implicitly bypasses many of the restrictions in Docker's default seccomp profile.

• The Boundary does not require CAP_SYS_ADMIN itself.
• However, Docker's default seccomp policy commonly blocks namespace-related syscalls unless CAP_SYS_ADMIN is present.
• Granting CAP_SYS_ADMIN enables the Boundary to run without modifying the seccomp profile.

⚠️ Warning: CAP_SYS_ADMIN is extremely powerful and should not be used in production unless absolutely necessary.

3. sysbox-runc runtime with CAP_NET_ADMIN

When using the sysbox-runc runtime (from Nestybox), the Boundary can run with only:

• CAP_NET_ADMIN

The sysbox-runc runtime provides more complete support for unprivileged user namespaces and nested containerization, which typically eliminates the need for seccomp profile modifications.

Docker Seccomp Profile Considerations

Docker's default seccomp profile frequently blocks the clone syscall, which is required by the Boundary when creating unprivileged network namespaces. If the clone syscall is denied, the Boundary will fail to start.

To address this, you may need to modify or override the seccomp profile used by your container to explicitly allow the required clone variants.

You can find the default Docker seccomp profile for your Docker version here (specify your docker version):

https://github.com/moby/moby/blob/v25.0.13/profiles/seccomp/default.json#L628-L635

If the profile blocks the necessary clone syscall arguments, you can provide a custom seccomp profile that adds an allow rule like the following:

This example unblocks the clone syscall entirely.

Example: Overriding the Docker Seccomp Profile

To use a custom seccomp profile, start by downloading the default profile for your Docker version:

https://github.com/moby/moby/blob/v25.0.13/profiles/seccomp/default.json#L628-L635

Save it locally as seccomp-v25.0.13.json, then insert the clone allow rule shown above (or add "clone" to the list of allowed syscalls).

Once updated, you can run the container with the custom seccomp profile:

This instructs Docker to load your modified seccomp profile while granting only the minimal required capability (CAP_NET_ADMIN).