NS Jail on Docker

This page describes the runtime and permission requirements for running Agent Boundaries with the nsjail jail type on Docker.

For an overview of nsjail, see nsjail.

Runtime & Permission Requirements for Running Boundary in Docker

This section describes the Linux capabilities and runtime configurations required to run Agent Boundaries 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, Agent Boundaries 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.

• Agent Boundaries 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 Agent Boundaries 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), Agent Boundaries 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 Agent Boundaries when creating unprivileged network namespaces. If the clone syscall is denied, Agent Boundaries 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).