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Your server's authentication logs record every login attempt, successful or failed. Reviewing them regularly — or better, automating the review — is how you catch brute-force attempts, unusual login times, or unauthorized access before they become a bigger problem.

Prerequisites

• Ubuntu or Debian VPS
• Root or sudo access

Step 1 — Locate the Auth Log

On systemd-based Ubuntu/Debian, authentication events are in the journal, and also typically in:

/var/log/auth.log

Step 2 — View Recent Login Attempts

sudo grep "sshd" /var/log/auth.log | tail -50

Or via journalctl:

sudo journalctl -u ssh --since "24 hours ago"

Step 3 — List Successful Logins

last -a | head -20

Step 4 — List Failed Login Attempts

sudo grep "Failed password" /var/log/auth.log | tail -50

Step 5 — Count Attempts by Source IP

sudo grep "Failed password" /var/log/auth.log | awk '{print $(NF-3)}' | sort | uniq -c | sort -nr | head -10

This quickly reveals which IPs are hammering your server — useful for manually banning persistent offenders or confirming Fail2Ban is catching them.

Step 6 — Install logwatch for Daily Summaries

sudo apt install logwatch -y

Run an on-demand report:

sudo logwatch --detail high --range today --service sshd --print

Step 7 — Enable Daily Email Reports (Optional)

sudo nano /etc/cron.daily/00logwatch

/usr/sbin/logwatch --output mail --mailto [email protected] --detail high

This requires a working local mail setup (e.g. postfix configured to relay outbound mail).

What to Look For

• A sudden spike in failed logins from a single IP or range
• Successful logins at unusual hours or from unfamiliar countries
• Logins to accounts that shouldn't normally be used interactively (e.g. service accounts)
• New entries in authorized_keys you don't recognize

Common Errors

auth.log is empty or missing — some minimal images only log to the systemd journal; use journalctl instead, or install rsyslog.

Best Practices

• Pair log monitoring with Fail2Ban so repeat offenders are blocked automatically, not just logged
• Ship logs off-server for critical production systems, so an attacker can't erase evidence
• Review logs on a fixed schedule, not just reactively

FAQ

Is a few failed SSH attempts per day normal?
Yes — automated internet-wide scanning constantly probes port 22/2222 on every public IP. Fail2Ban handles this automatically; the concern is sustained targeted activity or successful unauthorized logins.

Continue Reading

• How to Install and Configure Fail2Ban
• VPS Security Checklist for Beginners
• How to Detect and Remove Rootkits on a Linux VPS

Browse more articles in Server Security & Hardening.