vlan-probe 🛡️

VLAN Isolation & Network Permission Probe Tool — verify your firewall rules by probing target VLAN subnets, IPs, and ports from the host, and get alerted to any unauthorized inter-VLAN access.

✨ Features

• Multi-protocol probing — tcp, udp (DNS-aware on port 53, NTP-aware on port 123), icmp (ping), and sctp.
• Policy-first configuration — declare what should be blocked vs. reachable; every deviation is reported as a violation 🔴.
• Concurrent execution — multi-threaded worker pool (-j / --concurrency) to probe dozens of targets simultaneously.
• Flexible output — human-friendly table, structured json, or streaming ndjson.
• Alerting built-in — --strict exit codes for automation, plus scheduled MQTT reporting via systemd timers 📡.
• Boring & reliable — typed Python 3.10+, zero long-running daemons, 100% test coverage.

📑 Table of contents

• Requirements
• Installation
• Quick start
• Configuration
• Usage
• Scheduled MQTT reporting
• Development

Requirements

• Python 3.10 or newer
• Linux (uses the ip and ping utilities)

Installation

Install with pipx (recommended)

vlan-probe is a command-line tool — pipx runs it in an isolated environment while keeping the vlan-probe command on your PATH.

pipx install vlan-probe

Upgrade to a newer release:

pipx upgrade vlan-probe

Don't have pipx yet? Install it first:

brew install pipx && pipx ensurepath      # macOS/Homebrew
apt install pipx && pipx ensurepath       # Debian/Ubuntu
pacman -S python-pipx && pipx ensurepath  # Arch

Install from source

pipx install .          # via pipx
uv tool install .       # via uv
pip install .           # into the active environment

Quick start

# 1. copy the example config and edit it to match your network
cp vlan_probe.toml.example /etc/vlan_probe.toml

# 2. probe everything and show a human-friendly table
vlan-probe -f table

# 3. or go headless: JSON to stdout, exit 1 on any violation
vlan-probe -f json -s

Configuration

Configuration lives in a single TOML file (or JSON file) — readable, diff-able, and easy to keep in git. By default it is read from /etc/vlan_probe.toml (or /etc/vlan_probe.json); use -c <path> for another location.

[[targets]]
name = "Internal - Device A SSH"
vlan = "Internal"
ip = "10.10.1.10"
port = 22
protocol = "tcp"
expected_blocked = true

[[targets]]
name = "External - Public DNS"
vlan = "External"
ip = "8.8.8.8"
port = 53
protocol = "udp"
expected_blocked = false

[[targets]]
name = "Internal - Gateway Ping"
vlan = "Internal"
ip = "10.10.1.1"
port = 0
protocol = "icmp"
expected_blocked = true

[[targets]]
name = "Internal - Diameter Signalling"
vlan = "Internal"
ip = "10.10.1.20"
port = 3868
protocol = "sctp"
expected_blocked = false

Each target declares the firewall policy it expects:

Supported protocols: tcp · udp (probed with DNS query on port 53, NTP request on port 123, or null byte; ICMP port-unreachable is detected) · icmp (via ping) · sctp.

Note on UDP probing: UDP is connectionless. For DNS (port 53) and NTP (port 123), vlan-probe sends valid protocol request packets and awaits responses. For other UDP ports, it sends a probe byte and waits for a response or an ICMP Port Unreachable message. Silent UDP endpoints that drop unsolicited packets will time out.

Environment variables

Every setting that isn't a probe target can come from an environment variable instead of a flag or the config file. CLI flags win over env vars; env vars win over the [mqtt] config section. This keeps secrets out of the config file and makes containerized/systemd deployments trivial.

CLI defaults:

MQTT ([mqtt] section, overridable per-key):

Booleans accept 1/true/yes/on. A VLAN_PROBE_MQTT_HOST alone is enough to enable MQTT — no [mqtt] section needed:

export VLAN_PROBE_MQTT_HOST=mqtt.example.com
export VLAN_PROBE_MQTT_USERNAME=vlan-probe
export VLAN_PROBE_MQTT_PASSWORD='s3cret'
export VLAN_PROBE_MQTT_TLS=true
vlan-probe --mqtt

Usage

-v, --version             Show program's version number and exit
-c, --config PATH         Path to config TOML/JSON file (default: /etc/vlan_probe.toml)
-f, --format FORMAT       Output format: ndjson, json, or table (default: ndjson)
-t, --timeout SECONDS     Socket connection timeout (default: 2.0)
-j, --concurrency INT     Number of concurrent worker threads (default: 10)
-s, --strict              Exit with code 1 if any violations occur
--mqtt                    Publish results to MQTT (requires [mqtt] section or VLAN_PROBE_MQTT_HOST)
--color [auto|always|never]  Colorize output (default: auto)

# table output
vlan-probe -c ./vlan_probe.toml -f table

# json output
vlan-probe -f json

# strict mode with a custom timeout and concurrency
vlan-probe -s -t 5.0 -j 20

# no color
vlan-probe -f table --color never

Exit codes

Output formats

Table (IPs are fictional):

VLAN         TARGET                         ENDPOINT               STATUS   DETAILS
------------------------------------------------------------------------------------------
Internal     Internal - Device A SSH        10.10.1.10:22 (tcp)    PASS     OK
Internal     Internal - DNS Server          10.10.1.1:53 (udp)     PASS     OK
IoT          IoT - Device B                 10.10.2.50:80 (tcp)    PASS     OK
DMZ          DMZ - Web Server               10.10.4.50:80 (tcp)    FAIL     EXPECTED_CONNECTIVITY_FAILED: Failed to connect to DMZ - Web Server (10.10.4.50:80)
External     External - Public DNS          8.8.8.8:53 (udp)       PASS     OK

JSON — a run summary with totals and violations:

{
  "timestamp": "2026-08-15T07:56:17.397044+00:00",
  "total_probed": 12,
  "passed": 10,
  "failed": 2,
  "violations": [
    {
      "vlan": "DMZ",
      "target": "DMZ - Web Server",
      "ip": "10.10.4.50",
      "port": 80,
      "error": "EXPECTED_CONNECTIVITY_FAILED: Failed to connect to DMZ - Web Server (10.10.4.50:80)"
    }
  ]
}

NDJSON — one object per probed target, ideal for streaming/ingestion:

{"timestamp": "2026-08-15T07:55:15.797713+00:00", "target_name": "Internal - Device A SSH", "target_vlan": "Internal", "target_ip": "10.10.1.10", "port": 22, "protocol": "tcp", "reachable": false, "expected_blocked": true, "status": "PASS", "latency_ms": 1001.0, "error": null}

Strict mode (-s) writes failures to stderr and exits with code 1:

🚨 2 unauthorized connection(s) detected!
  - EXPECTED_CONNECTIVITY_FAILED: Failed to connect to DMZ - Web Server (10.10.4.50:80)
  - EXPECTED_CONNECTIVITY_FAILED: Failed to connect to DMZ - Web Server HTTPS (10.10.4.50:443)

Scheduled MQTT reporting ⏱️

Run the probe on a schedule and publish results to an MQTT broker so dashboards and automations can track VLAN isolation state over time. A systemd timer wakes a short-lived process that probes, publishes, and exits — no daemon, no state to babysit.

Add an [mqtt] section to the config:

[mqtt]
host = "mqtt.example.com"
port = 8883
username = "vlan-probe"               # optional
password = "s3cret"                   # optional
tls = true                            # optional, default false
ca_certs = "/etc/ssl/certs/ca-certificates.crt"  # optional
insecure = false                      # optional
topic_prefix = "vlan-probe"           # optional, default "vlan-probe"
retain = true                         # optional, default true
qos = 1                               # optional, default 1
connect_timeout = 5.0                 # optional, default 5.0

…and run with --mqtt:

vlan-probe --mqtt

Topics published (all retained at QoS 1):

• vlan-probe/<hostname>/summary — run summary (totals + violations)
• vlan-probe/<hostname>/targets/<vlan>/<target> — one message per target

A failed MQTT delivery is fatal (exit code 2); probe violations in strict mode still exit 1. --mqtt without an [mqtt] section or VLAN_PROBE_MQTT_HOST is a config error.

systemd timer

Ship the example units and enable the timer (adjust the interval in vlan-probe.timer, default every 5 minutes):

# system-wide (root)
sudo cp deploy/systemd/vlan-probe.service deploy/systemd/vlan-probe.timer /etc/systemd/system/
sudo systemctl daemon-reload
sudo systemctl enable --now vlan-probe.timer

# user (no root)
mkdir -p ~/.config/systemd/user
cp deploy/systemd/user/vlan-probe.service deploy/systemd/user/vlan-probe.timer ~/.config/systemd/user/
systemctl --user daemon-reload
systemctl --user enable --now vlan-probe.timer

Ops tips:

• journalctl -u vlan-probe — probe + delivery logs per run
• systemctl list-timers vlan-probe — next scheduled run
• Broker credentials: the [mqtt] config section (chmod 600) or the VLAN_PROBE_MQTT_* environment variables in a systemd EnvironmentFile

Development 🧑‍💻

The repository ships a Makefile wrapping uv — that's the supported way to work in this project.

make venv    # one-time setup (also configures git hooks)
make lint    # before pushing
make test    # before pushing

Git hooks 🪝

A pre-commit hook is provided in .githooks/pre-commit to automatically run ruff, mypy, and pytest before every commit.

To install / enable the hook:

make hooks
# or directly with git:
git config core.hooksPath .githooks

Test coverage 🛡️

Every code path is covered by a meaningful test — real loopback sockets and real config files, mocking only system boundaries. pytest fails if coverage drops below 100% (--cov-fail-under=100). Run the suite with a coverage report:

make test                      # mypy + pytest with coverage
make run ARGS="-f table"       # try it out

License

MIT