Linux Security

When you power on a server and hit ssh user@host, the connection dies in a few seconds even though the key works on a fresh machine. The usual suspect is the SSH agent – it’s either not running or not forwarding the key after a reboot. The fix is straightforward: make the agent start automatically, add your key once, and let ssh_config do the rest.

1. Verify the agent is running

ssh-add -l

If you get The agent has no identities, the agent isn’t started or $SSH_AUTH_SOCK isn’t set. On most modern distros the agent is started by the display manager or a systemd‑user unit. Check it with:

Why /tmp on a tmpfs is a lifesaver for log‑heavy workloads

If a service starts spamming logs, a full /tmp can bring the whole system to a halt. On low‑RAM servers or in containers where /tmp lives on a regular disk partition, the problem is even worse. Mounting /tmp as a tmpfs keeps it in memory, guarantees instant cleanup, and stops disk‑space exhaustion. Below is a practical, step‑by‑step guide that covers the trade‑offs and security notes you’ll need to keep in mind.

Why the “Broken Packages” Hang is a Common Pain Point

When I run a fresh upgrade from Ubuntu 22.04 LTS to 24.04 LTS on a home server, the first thing that can bite me is the infamous “Broken Packages” dialog that stalls the upgrade process. It isn’t a bug in the installer; it’s the package manager trying to juggle a huge shift in dependencies between releases. The result is a long pause—sometimes hours—before the system finally moves on. For a server that should stay online, that pause is unacceptable.

When systemd Unit Files Don’t Pick Up Changes

You tweak a unit file, hit systemctl, and nothing happens.
That’s the classic “I changed the file, but systemd still runs the old version” problem.
The daemon is still holding the old config in memory. systemd keeps a cache of unit files; it only rereads them after a daemon‑reload. If you skip that step, the changes stay invisible until you reboot or reload manually.

Sticky‑Bit Directories and ACLs: Why “Permission denied” Still Happens

When you try to move a file into a directory that has the sticky bit set, the kernel first checks the directory’s owner and the user’s write permission. If the directory is owned by someone else, the sticky bit forces it to also check the file’s owner. ACLs can grant write access to a group or user, but the sticky‑bit logic still requires the file’s owner to match the directory’s owner unless the moving user has the write permission on the file itself. That subtle rule is the source of a lot of “Permission denied” errors that only surface when ACLs are involved.

Web Application and API Protection on Linux

Why WAAP and NGWAF matter for self‑hosters

When you expose a service over HTTP(S), the attack surface expands beyond the kernel. Modern web apps and APIs get hit with injection, XSS, CSRF, and the newer API‑specific threats like replay or rate‑limit abuse. A WAAP (Web Application and API Protection) or NGWAF (Next‑Generation Web Application Firewall) sits in front of your HTTP stack and can stop most of those before they even reach your code.

Why a Local Search Domain Matters

In a home network you often want to reach devices by simple names—printer, raspberrypi, nas—instead of remembering IP addresses. A local search domain lets the resolver append a suffix automatically, turning printer into printer.home.local. systemd‑resolved can supply that suffix for the host itself, and, when coupled with a lightweight DNS forwarder, can expose the same domain to all clients.

Prerequisites

All examples assume a Debian‑based system; adjust package names for your distro.

Pulling a single file from a Borg backup without unpacking the whole archive is a common task for sysadmins who need quick recovery of a configuration file, a database dump, or a user document.
The following walkthrough shows how to do it safely, efficiently, and with a minimal security footprint.

Why pull a single file?

• Speed – Extracting an entire archive can take minutes or hours on large repositories.
• Disk space – You may not have room to unpack thousands of files.
• Security – Extracting only what you need reduces the attack surface; you avoid exposing the rest of the backup to the current filesystem.
• Automation – Scripts that restore a single file can run in CI/CD pipelines or as part of a systemd timer.

Borg basics recap

Borg is a deduplicating backup program that stores data in a repository.
Each backup is an archive that can contain one or more directories.
The repository can be local or remote (SSH, HTTP, or a network filesystem).
Borg supports strong encryption (repokey, passphrase, or keyfile) and can be configured to use a keyfile stored on the backup host.

Why journald can eat your SD card

Running a Raspberry Pi on an 8 GB SD card is a rite of passage for home labs, IoT experiments, and low‑cost servers.
On Pi OS the default is to keep systemd‑journald persistent, so every log line ends up on that tiny flash drive.
In practice the Pi runs a handful of services—networking, SSH, Docker, custom scripts—that churn out logs faster than the card can keep up.
The result? The SD card fills up, you can’t install updates, and the OS itself starts to feel cramped.

Monitoring /home Usage with a Systemd Timer

A home server rarely needs a full‑blown monitoring stack.
If you just want a quick alert when /home fills up, a tiny systemd service plus curl will do the trick. The example below keeps the footprint minimal, respects security best practices, and uses the official Telegram Bot API.

1. Create a Bot and Grab Credentials

1. Talk to BotFather on Telegram and create a bot.
2. Note the bot token (e.g. 123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11).
3. Find your chat ID by sending a message to the bot and querying https://api.telegram.org/bot<token>/getUpdates.
4. Store both values in a read‑only file:

sudo mkdir -p /etc/telegram-alert
sudo tee /etc/telegram-alert/credentials <<EOF
TOKEN=123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11
CHAT_ID=987654321
EOF
sudo chmod 600 /etc/telegram-alert/credentials

2. Service Unit – home-usage-alert.service

# /etc/systemd/system/home-usage-alert.service
[Unit]
Description=Send Telegram alert when /home exceeds 90%% usage
After=network.target

[Service]
Type=oneshot
EnvironmentFile=/etc/telegram-alert/credentials
ExecStart=/usr/bin/bash -c '
  USAGE=$(df --output=pcent /home | tail -1 | tr -d "%")
  if [ "$USAGE" -ge 90 ]; then
    curl -s -X POST https://api.telegram.org/bot$TOKEN/sendMessage \
      -d chat_id=$CHAT_ID \
      -d text="⚠️ /home is at ${USAGE}%% capacity. Free space needed." \
      -H "Content-Type: application/x-www-form-urlencoded"
  fi
'
PrivateTmp=yes
ProtectSystem=full
RestrictAddressFamilies=AF_INET AF_INET6
Restart=no

[Install]
WantedBy=multi-user.target

Security notes