Nucleus

Extremely lightweight Docker alternative for agents and production services

Nucleus is a minimalist container runtime for Linux. It provides isolated execution environments using Linux kernel primitives without the overhead of traditional container runtimes. Nucleus supports two operating modes:

• Agent mode (default) — ephemeral, fast-startup sandboxes for AI agent workloads
• Production mode — strict isolation for long-running, network-bound NixOS services with declarative configuration, egress policy enforcement, health checks, and systemd integration

Why Nucleus?

• Zero-overhead isolation – Direct use of cgroups, namespaces, pivot_root, capabilities, seccomp, and Landlock
• Memory-backed filesystems – Container disk mapped to tmpfs, pre-populated with agent context
• gVisor integration – Optional application kernel for enhanced security, including networked service mode
• Production service support – Declarative NixOS module, egress policies, health checks, secrets mounting, sd_notify, and journald integration
• Minimal rootfs – Replace host bind mounts with a purpose-built Nix store closure for production services
• External security policies – Per-service seccomp profiles (JSON), capability policies (TOML), and Landlock rules (TOML) with SHA-256 pinning
• Seccomp profile generation – Trace mode records syscalls, then nucleus seccomp generate creates a minimal allowlist profile
• Multi-container topologies – Compose-equivalent TOML format with dependency DAG, reconciliation, and NixOS systemd integration
• Integrity & audit controls – Structured audit log, context hashing, rootfs attestation, seccomp deny logging, mount flag verification, and kernel lockdown assertions
• Structured telemetry – Optional OpenTelemetry export for container lifecycle tracing
• Linux-native – Runs on standard Linux and NixOS

Architecture

Nucleus leverages Linux kernel isolation primitives:

• Namespaces – PID, mount, network, UTS, IPC, user, cgroup, and optional time isolation
• cgroups v2 – Resource limits (CPU, memory, PIDs, I/O)
• pivot_root – Filesystem isolation (chroot fallback available in agent mode only)
• Capabilities – All capabilities dropped by default, or configured via TOML policy file (irreversible)
• seccomp – Syscall whitelist filtering with per-service JSON profiles and trace-based generation (irreversible)
• Landlock – Path-based filesystem access control via hardcoded defaults or TOML policy file (Linux 5.13+)
• gVisor – Optional application kernel (runsc) with None/Sandbox/Host network modes
• PID 1 init – Mini-init supervisor in production mode for zombie reaping and signal forwarding
• In-memory secrets – Dedicated tmpfs at /run/secrets with volatile zeroing of source buffers
• Mount audit – Post-setup verification of mount flags in production mode

Container filesystem is backed by tmpfs and either populated with context files (agent mode) or mounted from a pre-built Nix rootfs closure (production mode).

Platform Support

• Linux (kernel 6.x+) on x86_64
• NixOS (first-class NixOS module support)
• Not supported: macOS, Windows, BSDs, 32-bit Linux

Installation

cargo install nucleus

Or via Nix:

nix run github:0kenx/nucleus

Usage

Agent Mode (default)

# Run agent in isolated container with pre-populated context
nucleus run --context ./agent-context/ -- /usr/bin/agent

# Specify resource limits
nucleus run --memory 512M --cpus 2 --context ./ctx/ -- ./agent

# Name your container
nucleus run --name my-agent --context ./ctx/ -- ./agent

# Use gVisor for enhanced isolation
nucleus run --runtime gvisor --context ./ctx/ -- ./agent

# Rootless mode
nucleus run --rootless -- /bin/sh

# Optional networking
nucleus run --network host --allow-host-network -- curl https://example.com
nucleus run --network bridge -p 8080:80 -- ./server

# Context streaming (bind mount for instant access)
nucleus run --context ./large-dir/ --context-mode bind -- ./agent

# Integrity and audit hardening
nucleus run --context ./ctx/ --verify-context-integrity --seccomp-log-denied -- ./agent

# Environment variables
nucleus run -e DEBUG=1 -- ./agent

# Pass sensitive values via --secret (mounted in-memory at /run/secrets)
nucleus run --secret /path/to/api-key:/run/secrets/api_key -- ./agent

Production Mode

Production mode enforces strict security invariants:

• Forbids --allow-degraded-security, --allow-chroot-fallback, and --allow-host-network
• Requires explicit --memory limit
• Requires successful cgroup creation (no fallback to running without limits)
• Egress policy failures are fatal (no silent degradation)
• Bridge DNS must be configured explicitly (no public resolver defaults)

# Run a long-running service with production hardening
nucleus run \
  --service-mode production \
  --trust-level trusted \
  --memory 1G --cpus 2 --pids 256 \
  --rootfs /nix/store/...-my-service-rootfs \
  --verify-rootfs-attestation \
  --require-kernel-lockdown integrity \
  --network bridge --dns 10.0.0.1 \
  --egress-allow 10.0.0.0/8 --egress-tcp-port 443 --egress-tcp-port 8443 \
  --health-cmd "curl -sf http://localhost:8080/health" \
  --health-interval 30 --health-retries 3 \
  --secret /run/secrets/tls-cert:/etc/tls/cert.pem \
  -e CONFIG_PATH=/etc/myservice/config.toml \
  --sd-notify \
  -p 8080:8080 \
  -- /bin/my-service --config /etc/myservice/config.toml

# gVisor with network access (sandbox network stack)
nucleus run \
  --service-mode production \
  --runtime gvisor \
  --gvisor-platform kvm \
  --memory 512M \
  --network bridge --dns 10.0.0.1 \
  --rootfs /nix/store/...-proxy-rootfs \
  -- /bin/proxy

Security Policy Files

Nix defines the service (what runs). Separate files define security policy (what the process is allowed to do at the kernel level). This separation keeps security config auditable, tool-compatible, and on its own change cadence.

# Run with external security policies
nucleus run \
  --service-mode production \
  --rootfs /nix/store/...-my-service-rootfs \
  --memory 512M --cpus 1 \
  --seccomp-profile ./config/my-service.seccomp.json \
  --seccomp-profile-sha256 abc123... \
  --caps-policy ./config/my-service.caps.toml \
  --landlock-policy ./config/my-service.landlock.toml \
  -- /bin/my-service

Seccomp profile (JSON — OCI-native format, tooling emits it directly):

{
  "defaultAction": "SCMP_ACT_KILL_PROCESS",
  "architectures": ["SCMP_ARCH_X86_64"],
  "syscalls": [
    {
      "names": ["read", "write", "close", "openat", "fstat",
                "mmap", "munmap", "brk", "futex", "clock_gettime"],
      "action": "SCMP_ACT_ALLOW"
    }
  ]
}

Capability policy (TOML):

# config/my-service.caps.toml
[bounding]
keep = []          # empty = drop all

[ambient]
keep = []

Landlock policy (TOML):

# config/my-service.landlock.toml
min_abi = 3

[[rules]]
path = "/bin"
access = ["read", "execute"]

[[rules]]
path = "/etc/myservice"
access = ["read"]

[[rules]]
path = "/run/secrets"
access = ["read"]

[[rules]]
path = "/tmp"
access = ["read", "write", "create", "remove"]

Seccomp Profile Generation

Profiles shouldn't be hand-written from scratch. Use trace mode to record actual syscall usage, then generate a minimal profile:

# 1. Run in trace mode — all syscalls allowed but logged
nucleus run \
  --seccomp-mode trace \
  --seccomp-log ./trace.ndjson \
  --rootfs /nix/store/...-my-service-rootfs \
  --memory 512M \
  -- /bin/my-service

# 2. Generate minimal profile from trace
nucleus seccomp generate ./trace.ndjson -o config/my-service.seccomp.json

# 3. Review and tighten (remove anything surprising)
# 4. Commit — Nix pins the SHA-256 hash
# 5. Run in enforce mode
nucleus run \
  --seccomp-profile ./config/my-service.seccomp.json \
  --seccomp-profile-sha256 "$(sha256sum config/my-service.seccomp.json | cut -d' ' -f1)" \
  -- /bin/my-service

Trace mode requires root or CAP_SYSLOG (reads /dev/kmsg). It is rejected in production mode — it is a development tool only.

Multi-Container Topologies

Nucleus includes a Compose-equivalent for managing multi-container stacks using TOML configuration with dependency ordering.

# topology.toml
name = "myapp"

[networks.internal]
subnet = "10.42.0.0/24"

[services.postgres]
rootfs = "/nix/store/...-postgres"
command = ["postgres", "-D", "/var/lib/postgresql/data"]
memory = "2G"
cpus = 2.0
networks = ["internal"]
health_check = "pg_isready -U myapp"

[services.web]
rootfs = "/nix/store/...-web"
command = ["/bin/web-server"]
memory = "512M"
networks = ["internal"]
port_forwards = ["8443:8443"]
egress_allow = ["10.42.0.0/24"]

[[services.web.depends_on]]
service = "postgres"
condition = "healthy"

# Validate topology and show dependency order
nucleus compose validate -f topology.toml

# Bring up all services in dependency order
nucleus compose up -f topology.toml

# Show service status
nucleus compose ps -f topology.toml

# Tear down in reverse dependency order
nucleus compose down -f topology.toml

Container Management

# List running containers
nucleus ps

# List all containers (including stopped)
nucleus ps --all

# Show resource usage statistics
nucleus stats

# Stop a container (SIGTERM, then SIGKILL after timeout)
nucleus stop <container>
nucleus stop --timeout 30 <container>

# Kill a container with a specific signal
nucleus kill <container>
nucleus kill --signal TERM <container>

# Remove a stopped container
nucleus rm <container>
nucleus rm --force <container>

# Attach to a running container
nucleus attach <container>
nucleus attach <container> -- /bin/bash

# Checkpoint a running container (requires root, CRIU)
nucleus checkpoint <container> --output /path/to/checkpoint

# Restore from checkpoint
nucleus restore --input /path/to/checkpoint

NixOS Module

Nucleus provides a declarative NixOS module for running containers as systemd services. Each container is managed as a nucleus-<name>.service unit with journald logging, sd_notify readiness, and automatic restart.

Flake Setup

{
  inputs.nucleus.url = "github:0kenx/nucleus";

  outputs = { self, nixpkgs, nucleus, ... }: {
    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        nucleus.nixosModules.default
        ./configuration.nix
      ];
    };
  };
}

Service Configuration

{ pkgs, nucleus, ... }:

let
  # Build a minimal rootfs containing only the packages your service needs.
  # This replaces host bind mounts with a locked-down Nix closure.
  proxyRootfs = nucleus.lib.mkRootfs {
    inherit pkgs;
    packages = [ my-proxy-pkg pkgs.cacert pkgs.curl ];
  };
in
{
  services.nucleus = {
    enable = true;
    package = nucleus.packages.x86_64-linux.default;

    containers.sigid-proxy = {
      enable = true;
      command = [ "/bin/sigid-proxy" "--config" "/etc/sigid/proxy.toml" ];
      rootfs = proxyRootfs;

      # Resource limits (required in production mode)
      memory = "1G";
      cpus = 2.0;
      pids = 256;

      # Security policy files (separate from Nix, auditable by security engineers)
      seccompProfile = {
        path = ./config/sigid-proxy.seccomp.json;
        sha256 = "abc123...";  # Nix verifies at build time
      };
      capsPolicy = ./config/sigid-proxy.caps.toml;
      landlockPolicy = ./config/sigid-proxy.landlock.toml;

      # Optional hardening toggles
      verifyRootfsAttestation = true;
      seccompLogDenied = true;
      requireKernelLockdown = "integrity";

      # Networking
      network = "bridge";
      dns = [ "10.0.0.1" ];  # internal resolver — no public DNS default
      portForwards = [ "8080:8080" "8443:8443" ];

      # Egress policy — audited outbound access
      egressAllow = [ "10.0.0.0/8" ];
      egressTcpPorts = [ 443 8443 ];

      # Health checking
      healthCheck = "curl -sf http://localhost:8080/health";
      healthInterval = 30;
      healthRetries = 3;
      healthStartPeriod = 10;

      # Secrets (mounted read-only)
      secrets = [
        { source = config.age.secrets.proxy-tls.path; dest = "/etc/tls/cert.pem"; }
      ];

      # Environment
      environment = {
        RUST_LOG = "info";
        CONFIG_PATH = "/etc/sigid/proxy.toml";
      };

      # systemd integration
      sdNotify = true;  # Type=notify, passes NOTIFY_SOCKET into container
    };
  };
}

Topology Services

Topologies can also be managed as systemd services:

{
  services.nucleus = {
    enable = true;
    package = nucleus.packages.x86_64-linux.default;

    topologies.myapp = {
      enable = true;
      configFile = ./topology.toml;
    };
  };
}

This creates a nucleus-topology-myapp.service (Type=oneshot, RemainAfterExit) that runs nucleus compose up on start and nucleus compose down on stop.

What the Module Generates

For each enabled container, the module creates a systemd service:

• Unit: nucleus-<name>.service, ordered after network-online.target
• Type: notify (when sdNotify = true) or simple
• Restart: on-failure with 5s backoff
• Logging: stdout/stderr captured to journald with SyslogIdentifier=nucleus-<name>
• Command: nucleus run --service-mode production ... with all configured options
• Hardening: ProtectSystem=strict, ProtectHome=true at the systemd level (defense-in-depth)

Building a Rootfs

Use nucleus.lib.mkRootfs to build a minimal, reproducible root filesystem:

nucleus.lib.mkRootfs {
  inherit pkgs;
  name = "my-service-rootfs";  # optional, defaults to "nucleus-rootfs"
  packages = [
    my-service-package
    pkgs.cacert       # TLS certificates
    pkgs.curl         # for health checks
    pkgs.busybox      # minimal coreutils
  ];
}

This produces a Nix store path containing /bin, /lib, /etc, etc. from the specified packages. It is mounted read-only inside the container, replacing the host bind mounts used in agent mode.

mkRootfs also emits a .nucleus-rootfs-sha256 manifest at the root of the closure. Use --verify-rootfs-attestation or verifyRootfsAttestation = true; to require that manifest to match the mounted rootfs at startup.

Security Notes

Do not pass secrets via -e / --env. Environment variables are visible in /proc/<pid>/environ to any process that can read it (mitigated by hidepid=2 in production mode, but not in agent mode). Use --secret instead — secrets are mounted on an in-memory tmpfs at /run/secrets with volatile source buffer zeroing.

Agent mode is not hardened. By design, agent mode applies several security mechanisms on a best-effort basis: seccomp and Landlock failures are warn-and-continue (with --allow-degraded-security), chroot fallback is available (with --allow-chroot-fallback), bridge DNS defaults to public resolvers (8.8.8.8), and cgroup creation failures are non-fatal. Operators requiring strict isolation should use production mode, which makes all of these fatal.

Production Mode vs Agent Mode

Egress Policy

When --egress-allow is specified, Nucleus applies iptables OUTPUT chain rules inside the container's network namespace:

1. Allow loopback traffic
2. Allow established/related connections
3. Allow DNS to configured resolvers
4. Allow traffic to permitted CIDRs (optionally restricted to specific ports)
5. Log denied packets (rate-limited, nucleus-egress-denied: prefix)
6. Drop everything else

# Allow outbound to internal network on HTTPS only
nucleus run --network bridge --dns 10.0.0.1 \
  --egress-allow 10.0.0.0/8 --egress-tcp-port 443 \
  -- ./my-service

# Deny-all egress (only DNS to configured resolvers is allowed)
nucleus run --network bridge --dns 10.0.0.1 \
  --egress-allow "" \
  -- ./isolated-service

gVisor Network Modes

When using gVisor (--runtime gvisor), the network mode is automatically selected:

The sandbox mode gives gVisor-isolated services full network access through gVisor's user-space TCP/IP stack, without exposing the host kernel's network code.

Additional Hardening Flags

• --seccomp-profile <path> loads a custom per-service seccomp profile (OCI JSON format).
• --seccomp-profile-sha256 <hex> verifies the profile's SHA-256 hash before loading.
• --seccomp-mode trace|enforce switches between trace (record all syscalls) and enforce (default).
• --seccomp-log <path> writes NDJSON syscall trace when in trace mode.
• --caps-policy <path> loads a TOML capability policy (replaces default drop-all).
• --caps-policy-sha256 <hex> verifies the capability policy hash.
• --landlock-policy <path> loads a TOML Landlock filesystem policy (replaces default rules).
• --landlock-policy-sha256 <hex> verifies the Landlock policy hash.
• --verify-context-integrity hashes the source context tree before launch and verifies the populated /context tree matches.
• --verify-rootfs-attestation requires a .nucleus-rootfs-sha256 manifest and verifies the mounted rootfs against it.
• --seccomp-log-denied requests kernel logging for denied seccomp decisions when the host supports SECCOMP_FILTER_FLAG_LOG.
• --require-kernel-lockdown integrity|confidentiality refuses startup unless /sys/kernel/security/lockdown satisfies the requested mode.
• --gvisor-platform systrap|kvm|ptrace selects the runsc backend explicitly.
• --time-namespace enables Linux time namespaces for native containers.
• --disable-cgroup-namespace turns off cgroup namespace isolation when a workload needs the host cgroup view.

If NUCLEUS_OTLP_ENDPOINT or OTEL_EXPORTER_OTLP_ENDPOINT is set, Nucleus exports lifecycle spans over OTLP in addition to normal local logging.

Development

This project uses Nix flakes for reproducible builds:

# Enter development shell
nix develop

# Build
cargo build

# Run tests
cargo test

# Run with Apalache installed (for TLA+ trace replay)
cargo test -- --include-ignored

# Build release binary
cargo build --release

# Clippy
cargo clippy --all-targets -- --deny warnings

Project Structure

nucleus/
├── src/
│   ├── container/      # Container orchestration, lifecycle, state, config
│   ├── isolation/      # Namespace management, user mapping, attach
│   ├── resources/      # cgroup v2 resource control, stats
│   ├── filesystem/     # tmpfs, rootfs mounting, context population, secrets, attestation
│   ├── security/       # Capabilities, seccomp, Landlock, gVisor, OCI, policy files
│   │   ├── caps_policy.rs       # TOML capability policy loader
│   │   ├── landlock_policy.rs   # TOML Landlock policy loader
│   │   ├── seccomp_trace.rs     # Seccomp trace mode (syscall recording)
│   │   ├── seccomp_generate.rs  # Profile generator from traces
│   │   └── policy.rs            # Shared policy infrastructure (SHA-256, TOML/JSON loaders)
│   ├── network/        # Networking (none/host/bridge), egress policy
│   ├── topology/       # Multi-container topology (Compose equivalent)
│   │   ├── config.rs   # TOML topology config (services, networks, volumes)
│   │   ├── dag.rs      # Dependency DAG with topological sort
│   │   ├── reconcile.rs # Diff running vs desired state, apply changes
│   │   └── dns.rs      # Per-topology /etc/hosts DNS
│   ├── checkpoint/     # CRIU checkpoint/restore
│   ├── audit.rs        # Structured audit log (JSON events)
│   └── error.rs        # Error types
├── nix/
│   └── module.nix      # NixOS module (containers + topologies)
├── config/             # Security policy files (per-service)
│   ├── *.seccomp.json  # Seccomp syscall allowlists (OCI format)
│   ├── *.caps.toml     # Capability bounding set policies
│   └── *.landlock.toml # Landlock filesystem access rules
├── tests/
│   ├── model_based_*   # Property-based tests from TLA+ specs
│   └── tla_*           # tla-connect driver tests
├── formal/tla/         # TLA+ formal specifications
├── intent/             # Intent high-level specs
└── flake.nix           # Nix flake (packages, modules, lib.mkRootfs)

Testing

Nucleus uses spec-driven development with comprehensive testing:

• Unit tests: Individual component functionality
• Model-based tests: Property-based tests verifying TLA+ specifications
• tla-connect tests: TLA+ to Rust state machine mapping
• Integration tests: Complete container lifecycle

All state machines are formally verified using TLA+ and the Apalache model checker.

System-Level TLA+ Model

A composed system model verifies cross-subsystem ordering, authorization, and end-to-end progress:

apalache-mc check --config=formal/tla/Nucleus_System.cfg formal/tla/Nucleus_System.tla

License

Licensed under either of:

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.