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CompTIA Linux+ — Security and Troubleshooting flashcards

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Hardening and fixing Linux for Linux+: SSH keys and sshd settings, sudoers and PAM, SELinux (and AppArmor, no longer in XK0-006), firewalld, ufw and nftables, file integrity and auditd, the journal and /var/log, and diagnosing storage, CPU, memory, network and boot faults with the right commands.

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Learn deck · part of CompTIA Linux+ · written against: Current Linux+ exam objectives (topics common to XK0-005 and its successor). Updated 2026-10-10.

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  1. Which sshd_config settings stop direct root logins and password authentication?
    PermitRootLogin no and PasswordAuthentication no in /etc/ssh/sshd_config, followed by a reload of sshd.

    Watch out: a file in /etc/ssh/sshd_config.d/ that sets the same option can override the main file, so check the result with sshd -T; confirm key-based login works in a second session first so you cannot lock yourself out.
    Security › SSH and Remote Access
  2. Which commands create an Ed25519 key pair and install the public key on a server?
    ssh-keygen -t ed25519 creates the pair; ssh-copy-id admin@server01 appends the public key to ~/.ssh/authorized_keys on the server.
    Security › SSH and Remote Access
  3. Key login fails, and the server log complains about bad ownership or modes. What does sshd expect?
    With StrictModes on, the user's home directory, ~/.ssh and authorized_keys must be owned by the user and not writable by others — typically 700 on ~/.ssh and 600 on authorized_keys.
    Security › SSH and Remote Access
  4. What does a passphrase on an SSH private key protect against, and how does ssh-agent keep it convenient?
    If the key file is stolen, the passphrase stops it being used. ssh-agent holds the decrypted key in memory for the session (ssh-add loads it), so the passphrase is typed once.
    Security › SSH and Remote Access
  5. An SSH client warns that the remote host identification has changed. What does it mean, and what should the admin do?
    The server's host key no longer matches the entry in known_hosts — either the server was rebuilt or someone is intercepting the connection. Verify the new fingerprint out of band before removing the old entry with ssh-keygen -R server01.
    Security › SSH and Remote Access
  6. Which journalctl command shows only messages of priority error and worse from the current boot?
    journalctl -b -p err — -b limits it to this boot and -p err to error and more severe priorities.
    Troubleshooting › Logs
  7. How do you follow one service's log live with journalctl?
    journalctl -u nginx -f — -u selects the unit and -f follows new entries as they arrive.
    Troubleshooting › Logs
  8. After a reboot, journalctl -b -1 reports that no previous boot exists. Why, and how is it fixed?
    The journal is volatile, kept only in /run/log/journal. Create /var/log/journal or set Storage=persistent in /etc/systemd/journald.conf, then restart systemd-journald.
    Troubleshooting › Logs
  9. How can journalctl show only the logs from a particular time window?
    With --since and --until, e.g. journalctl --since "09:00" --until "10:30" or journalctl --since "2 hours ago".
    Troubleshooting › Logs
  10. On a Red Hat-family host, which file collects authentication messages such as failed SSH logins, and what is the Debian and Ubuntu equivalent?
    /var/log/secure on Red Hat-family systems and /var/log/auth.log on Debian and Ubuntu; on systemd hosts the same events are also in the journal.

    Watch out: Debian 12 and later no longer install rsyslog by default, so a fresh Debian system has no auth.log; read the events with journalctl.
    Troubleshooting › Logs

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About these cards

Review status: Second-reader reviewed, with corrections applied.

Checked against:

  • CompTIA Linux+ exam objectives — topic outline only; no text copied. Tags follow the four XK0-005 domains; as of October 2026 the current exam is XK0-006 (launched 2025-07-15), and XK0-005 retired on 2026-01-13.

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