Kevin's Blog

On May 12, 2026, two of my Arch Linux + LUKS servers at Hetzner became unreachable at the same moment. Both had been running for 4+ months without issue. Both had received the same pacman -Syyu the day before, but had stayed on the old kernel until the morning the websites stopped responding. I rebooted — SSH never came back. nmap -Pn -p 22 showed filtered from anywhere. No ping. No banner. The Hetzner Robot panel insisted the hardware was fine.

Several hours went into hypotheses that turned out to be wrong:

• The encryptssh initcpio hook referencing a /usr/lib/initcpio/udev/11-dm-initramfs.rules file that no longer exists. Real bug, no boot impact — the initramfs rebuilds anyway.
• PermitRootLogin no in sshd_config. Real misconfiguration, fixed it, didn’t help. A refusing sshd shows closed, not filtered.
• Predictable interface-naming drift after the systemd 260 upgrade. Patched the .network config to match by MAC. Useful hardening; not the cause.
• Stale GRUB stage1 + core.img in the MBR. Arch never re-runs grub-install after a grub package upgrade. Refreshed it. Still filtered.
• Kernel 7.0.5 regression. Downgraded to 6.18.3, the kernel that had run for 4 months. Still filtered. So the kernel itself wasn’t it either.

The clue was in the persistent journal: a single recorded boot from December 31 to May 12 10:13 UTC, and absolutely nothing after. Every reboot since the upgrade was failing before systemd-journald could flush to disk — so the failure had to be in the initramfs, before the root filesystem was even mounted.

What it almost certainly was

Hetzner Dedicated servers configure the initramfs network with ip=dhcp on the kernel command line. That depends on Hetzner’s DHCP server replying to whatever request format the current kernel sends. Somewhere between kernel 6.18 / iproute2 6.18 and kernel 7.0 / iproute2 7.0, the request format changed enough that Hetzner’s DHCP stopped responding. Effects:

• Old kernel at runtime kept the interface already configured (Phase A — 32 hours of healthy operation after the package upgrade).
• New kernel cold-boots, hits DHCP, never gets an IP, dropbear cannot listen, port 22 stays filtered.

Hetzner’s own documentation has been quietly moving away from ip=dhcp toward static IPv4 in the kernel command line. The fix is exactly that:

GRUB_CMDLINE_LINUX="cryptdevice=/dev/md1:cryptroot ip=A.B.C.D::GATEWAY:255.255.255.255:hostname:eth0:none"

One line in /etc/default/grub, grub-mkconfig, reboot. No more dependency on Hetzner’s DHCP responding to whatever your current kernel sends.

Why it matters for anyone running this stack

If you run Arch on Hetzner Dedicated with full-disk encryption and remote unlock via dropbear, the ip=dhcp shipped by installimage is a latent bug. It can keep working for years and then break overnight, on every machine you have, after a routine pacman -Syyu. The static-IP version is what Hetzner now recommends and removes the entire dependency.

Tooling

While debugging, I turned the whole rescue / chroot / diagnose / fix workflow into a Python CLI (hal) — including hal fix static-ip, which derives the static cmdline directly from your existing systemd-networkd .network file:

→ github.com/kevinveenbirkenbach/hetzner-arch-luks

Single command, idempotent, reversible (the original /etc/default/grub is backed up to .hal-backup). If you’re on this stack, switch to static IP before the next kernel upgrade catches you.