🚀 automtu — Automatic Path MTU Detection for Linux & WireGuard

automtu is a small but powerful CLI tool that automatically detects your egress interface, probes the Path MTU (PMTU), computes a safe MTU for WireGuard, and optionally applies it — or simply prints it for automation tools like Ansible.

Perfect for:

• Docker-in-Docker (DiD)
• WireGuard tunnels
• Cloud VPS setups
• CI runners
• NAT / overlay networks

No more TLS handshake timeouts. No more guessing MTU values. Just reliable networking. ✅

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✨ Features

• 🔍 Auto-detects your egress interface (eth0, ens3, etc.)
• 📡 Probes Path MTU using DF-ping (ping -M do)
• 🧮 Computes a safe WireGuard MTU (effective_mtu - overhead)
• 🔐 Supports WireGuard peer auto-discovery
• ⚙️ Optional automatic MTU application
• 🤖 Machine-readable output for Ansible / CI
• 📦 JSON output for automation pipelines
• 🧪 Fully unit-tested

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📦 Installation

Option 1 — Install via pip (recommended)

pip install automtu

or editable mode for development:

pip install -e .

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Option 2 — Local installation from source

git clone https://github.com/kevinveenbirkenbach/automtu.git
cd automtu
pip install -e .

Now the automtu command is available system-wide.

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🚦 Usage

🔍 Show detected MTU (no changes)

automtu --dry-run

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📡 Probe PMTU and apply to egress interface

sudo automtu --pmtu-target 1.1.1.1 --apply-egress-mtu

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🔐 Auto-detect WireGuard peers and apply WG MTU

sudo automtu --auto-pmtu-from-wg --apply-wg-mtu

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⚡ Prefer WireGuard as egress if default route uses wg0

sudo automtu --prefer-wg-egress --auto-pmtu-from-wg --apply-wg-mtu

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🧮 Print only the MTU number (for Ansible & scripts)

automtu --print-mtu effective

Output:

1452

Perfect for:

- name: Detect MTU
  command: automtu --print-mtu effective
  register: mtu

- name: Apply MTU
  command: ip link set mtu {{ mtu.stdout }} dev eth0

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📤 JSON output (CI / automation)

automtu --print-json

Output:

{
  "egress": { "iface": "eth0", "base_mtu": 1500, "effective_mtu": 1452 },
  "pmtu": { "policy": "min", "chosen": 1452 },
  "wg": { "iface": "wg0", "mtu": 1372 },
  "dry_run": true
}

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🛡 Notes

• Applying MTU requires root (sudo) unless --dry-run is used
• PMTU probing may fail if ICMP is blocked — fallback is automatic
• MTU changes are runtime-only (not persistent across reboot)

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🧠 Why automtu?

Because networking stacks are layered, tunneled, NATed and virtualized. Path MTU matters — especially for:

• Docker-in-Docker
• VPN tunnels
• Cloud providers
• Kubernetes nodes
• CI runners

automtu removes guesswork and prevents hard-to-debug TLS timeouts.

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👤 Author

Kevin Veen-Birkenbach 🌍 https://www.iveen.world

Open Source, Automation, Infrastructure & Digital Sovereignty.

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❤️ Support

If automtu helped you, consider supporting the project:

• ⭐ Star on GitHub
• ☕ Buy Me a Coffee
• 💙 GitHub Sponsors

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Happy networking! 🌐✨