automtu/Guide.md

automtu — Practical Guide

The tool can:

• auto-detect your egress interface (e.g., eth0)
• probe Path MTU (PMTU) using ping -M do
• compute a safe WireGuard MTU: effective_mtu - overhead (default overhead=80)
• apply MTU to egress and/or WireGuard (explicit flags)

Recipes

1. Only compute/show (safe default): automtu --dry-run

2. Probe PMTU and apply to egress: sudo automtu --pmtu-target 1.1.1.1 --apply-egress-mtu

3. Auto-add WireGuard peer endpoints as PMTU targets and apply WG MTU: sudo automtu --auto-pmtu-from-wg --apply-wg-mtu

4. Prefer WireGuard as egress basis if default route uses wg0: sudo automtu --prefer-wg-egress --auto-pmtu-from-wg --apply-wg-mtu

5. Force WG MTU: sudo automtu --set-wg-mtu 1372 --apply-wg-mtu

Notes

• Applying MTU requires root (unless --dry-run).
• PMTU probing can fail if ICMP is blocked; the tool then falls back to egress MTU.
• Runtime MTU changes are not persistent across reboots.