Linux Security

The “Host key verification failed” error after renaming a jump host

When you rename a jump host (or any SSH‑accessible machine) the client still keeps the old hostname in its ~/.ssh/known_hosts. On the next connection the client pulls the key presented by the renamed host and compares it with the stored key for the old name. The mismatch triggers the classic

Host key verification failed.

It’s a safety net against man‑in‑the‑middle attacks, but it can be a nuisance when you legitimately change the hostname. Below is a practical, step‑by‑step fix, a few automation tricks, and some security notes I’ve learned after 20 years on the grind.

Why the $USER Variable Matters for Systemd Timers

Systemd timers are the go‑to replacement for cron on almost every distro. They’re declarative, mesh cleanly with the unit graph, and bring a bunch of goodies—persistent timers, calendar syntax, and dependency handling. The catch? If your timer’s service unit runs a script that relies on $USER, you’ll see a silent failure after a reboot. Systemd doesn’t set $USER for system‑wide units; it only populates it for user‑specific units that run in a logged‑in session.

When a service spits out a JSON blob that contains a list of users, the data is usually nested and has more fields than you actually need. Turning that into a flat CSV is a common thing to do for automation scripts, reporting, or feeding into spreadsheets. The two most reliable command‑line tools for this job are jq (a JSON processor) and awk (a text filter). Together they give you a fast, secure, and scriptable pipeline.

The sticky bit in a nutshell

The sticky bit (+t) on a directory means that only the file’s owner, the directory’s owner, or root can delete or rename files inside it. /tmp is traditionally set to 1777 (octal) – owner root, group root, permissions rwxrwxrwt. That lets any user create files, but stops them from messing with each other’s data.

What happened after the update

Some kernel releases changed the default mount options for tmpfs or how tmpfs‑based /tmp is handled. If /tmp is mounted as tmpfs without the sticky bit, the kernel quietly clears the +t flag. The directory stays world‑writable, but the protection that stops users from deleting each other’s files disappears. Applications that expect the sticky bit now refuse to write, because they see the directory as insecure.

Why Move /var/log to a tmpfs?

On most production boxes and home‑lab rigs the root file system is a single ext4 or btrfs partition that also holds /var/log. After a few months the logs can eat up gigabytes of space, especially on servers that churn out audit, kernel, or app logs. A full root partition can block package upgrades, kernel updates, or even prevent a boot if the filesystem gets remounted read‑only. Moving /var/log to a tmpfs frees the disk right away, keeps the logs in RAM, and cuts out the need to compress or rotate them manually.

Why a kernel update can drop you into emergency mode because /boot won’t mount

A fresh kernel can change the initramfs or the boot‑loader’s config, and if the loader can’t find the /boot partition the system drops into that classic emergency shell:

Emergency shell (root=rw) – press Ctrl‑D to continue

Below is a no‑fluff, hands‑on walk‑through that keeps the commands you need, the trade‑offs, and a few security reminders.

1. Inspect the emergency environment

First, make sure the culprit is really the /boot mount. In the emergency shell run:

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When a VPS loses its /etc directory, the system can become unbootable or at least unusable until the configuration files are restored. If you have been running Borg backups (the recommended tool for incremental, deduplicated snapshots) the recovery path is clear: locate the snapshot that contains the last good /etc, extract it, and replace the missing directory. This walkthrough shows the exact commands, trade‑offs, and security checks you should perform on a current 2026 VPS environment.

Why Bluetooth headphones keep dropping on GNOME 48

Bluetooth audio on GNOME 48 is now handled by PipeWire instead of the old PulseAudio stack. The new stack is cleaner, but it also turns on a “low‑power” mode that can make a headset drop after a few minutes of idle time or when you close the laptop lid. The real culprit is usually the kernel’s power‑management for the Bluetooth controller and PipeWire’s default settings.