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Fail2Ban Reference & Useful Commands

Category: All About Ubuntu
Last Updated: May 11, 2025
Applies To: Ubuntu Server 22.04+

Fail2Ban Jail Configuration

Fail2Ban jails control how long an IP remains banned after matching filters. To increase ban duration (e.g., to 48 hours):

Configuration File

sudo nano /etc/fail2ban/jail.local

Example Jail Settings for SSH and UFW Block:

[sshd]
enabled = true
port    = ssh
logpath = %(sshd_log)s
bantime = 172800
findtime = 600
maxretry = 3

[ufw-block]
enabled = true
filter  = ufw-block
logpath = /var/log/ufw.log
bantime = 172800
findtime = 600
maxretry = 3

✅ bantime is in seconds → 172800 equals 48 hours
✅ findtime is the window (in seconds) to detect repeated offenses
✅ maxretry is the number of failed attempts before banning

After changes:

sudo systemctl restart fail2ban

Useful Commands

Task Command
Check fail2ban service status sudo systemctl status fail2ban
Start fail2ban sudo systemctl start fail2ban
Restart fail2ban sudo systemctl restart fail2ban
View all jail statuses sudo fail2ban-client status
View a specific jail (e.g., sshd) sudo fail2ban-client status sshd
See currently banned IPs in a jail sudo fail2ban-client get sshd banned
Unban an IP from a jail sudo fail2ban-client set sshd unbanip <IP>
Get ignore list for a jail sudo fail2ban-client get sshd ignoreip
Manually test a filter (dry run) fail2ban-regex /var/log/auth.log /etc/fail2ban/filter.d/sshd.conf

Filter & Jail File Paths

File Purpose Path
Jail configuration /etc/fail2ban/jail.local
Custom filters /etc/fail2ban/filter.d/
Fail2Ban main log /var/log/fail2ban.log
UFW log (for ufw-block) /var/log/ufw.log

Dynamically Updating ignoreip in Fail2Ban with DDNS

To prevent your own dynamic IP from being blocked by Fail2Ban (especially on services like sshd or custom UFW blocks), you can automate the injection of a DDNS-resolved IP into the ignoreip configuration.

Script Overview

Location:
/usr/local/bin/update-fail2ban-ignoreip.sh

Purpose:
Resolves a DDNS hostname to an IPv4 address and updates the ignoreip line in /etc/fail2ban/jail.local. This helps Fail2Ban ignore your dynamic IP address automatically.

Key Script Breakdown

#!/bin/bash
DDNS_HOST="your-ddns.example.com"
RESOLVED_IP=$(dig +short "$DDNS_HOST" | grep -Eo '([0-9]{1,3}\.){3}[0-9]{1,3}' | head -n 1)

  • Resolves your DDNS hostname to a valid IPv4 address.

JAIL_FILE="/etc/fail2ban/jail.local"

  • Points to the jail config you want to modify.

sed -i -E "s|^(ignoreip\s*=).*|\1 127.0.0.1/8 ::1 $RESOLVED_IP fe80::/10|" "$JAIL_FILE"

  • Uses sed to replace the entire ignoreip line with:

    • localhost + loopback (127.0.0.1/8 ::1)

    • your resolved DDNS IP

    • and optional link-local IPv6 scope (fe80::/10)

systemctl restart fail2ban

  • Restarts Fail2Ban so the updated IP takes effect immediately.

Example Output

Resolved IP: <your ip>
ignoreip updated in jail.local
Fail2Ban restarted successfully

Cron Job (Optional)

To schedule it daily or multiple times a day, add to root’s crontab:

*/15 * * * * /usr/local/bin/update-fail2ban-ignoreip.sh >> /var/log/update-fail2ban-ignoreip.log 2>&1

Notes

  • Use ignoreip to exempt safe IPs (including local/DDNS).

  • Consider rotating logs weekly to avoid bloated logs.

  • Fail2Ban can be extended to cover other services (Apache, Postfix, etc.).

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