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• fail2ban
• Jonathan Haack
• Haack's Networking
• support@haacksnetworking.org

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fail2ban

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This tutorial is designed to help you install fail2ban and get a basic set of configurations in place.

sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
sudo nano /etc/fail2ban/jail.local

Once inside the configuration file jail.local edit the destination email and the action parameter. Read the conf file and decide which combination of m, w, l is right for your situation.

<destemail = email>
<action = %(action_mwl)s>

Default policy targets the middle in this example. Later, jails like sshd or recidive are stricter and get maxretry = 1 overrides, where apache or public servers are overridden to maxretry = 5 for more tolerance:

[DEFAULT]
bantime  = 1h
findtime  = 30d 
maxretry = 3

Increase db purge age so as to retain enough for the jail parameters:

sudo nano /etc/fail2ban/fail2ban.conf
<dbpurgeage = 30d>  

Add enabled = true to desired jails. Here's an example of me setting ssh to something very strict. Only do this in tightly monitored and low access scenarios, or you will have a lot of false positives from user error:

[sshd]
enabled = true
port    = ssh
logpath = %(sshd_log)s
backend = %(sshd_backend)s
maxretry = 1

The repeat offender, or recidivist jail, is listed under [recidive] and I give it particular attention. In a tightly controlled environment, if someone has banged once on ssh invalidly and does it again, they have no reason to bang again indefinitely. Again, in larger environments, it might not be possible to enforce maxretry = 1.

[recidive]
enabled            = true
logpath            = /var/log/fail2ban.log
banaction          = iptables-multiport[blocktype=DROP]
banaction_allports = iptables-allports[blocktype=DROP]
bantime            = 100y
maxretry           = 1

Here's an example of keeping postfix more tolerant, so that you don't get false positives on more common services while users are setting up stuff or accessing public facing resources:

[apache-auth]
enabled  = true
port     = http,https
logpath  = %(apache_error_log)s
maxretry = 5 #increased to 5

Once you activate desired jails, restart service or reload config:

sudo systemctl restart fail2ban.service 
sudo fail2ban-client reload 

Hope this helps! Oh yeah … here is how to remove a false positive!

fail2ban-client set ssh unbanip 10.xx.15x.12x
fail2ban-client unban --all

Another method that does more than individual services, and instead zaps all records:

sudo systemctl stop fail2ban
sudo truncate -s 0 /var/log/fail2ban.log
sudo rm /var/lib/fail2ban/fail2ban.sqlite3
sudo systemctl restart fail2ban

Systemd log issues. Change the sshd jail as follows

sudo nano /etc/fail2ban/jail.local
backend = systemd
#backend = %(sshd_backend)s

Some recommend adding backend = systemd into jail.conf, but I've found that does nothing. The error over ipv6 not being set and using auto can be removed as follows:

sudo nano /etc/fail2ban/fail2ban.conf 
'allowipv6 = auto'

To check a particular jail's statistics:

sudo fail2ban-client status recidive

Install rpl and use it to change default banaction to DROP:

sudo apt install rpl
sudo rpl -q 'banaction = iptables-multiport' 'banaction = iptables-multiport[blocktype=DROP]' /etc/fail2ban/jail.local && 
sudo rpl -q 'banaction_allports = iptables-allports' 'banaction_allports = iptables-allports[blocktype=DROP]' /etc/fail2ban/jail.local && 
sudo fail2ban-client reload

Small script / one-liner to avoid remembering iptables flags for jails I monitor a lot:

cat << 'EOF' > /usr/local/bin/list-recidive-ips.sh
#!/bin/bash
iptables -L f2b-recidive -v -n
EOF
chmod 750 /usr/local/bin/list-recidive-ips.sh

Change all reject rules to drop for a given iptables fail2ban managed jail/entry:

sudo iptables -L f2b-recidive -n --line-numbers | grep REJECT | awk '{print $1}' | sort -r | xargs -I {} sudo iptables -R f2b-recidive {} -j DROP

Script, fail2ban-stats.sh, which queries all jails for historical and current bans:

#!/bin/bash
# /usr/local/bin/fail2ban-stats.sh
 
#header
echo "Jail                          | Banned now | Total failed | Total banned | Actions taken"
echo "------------------------------|------------|--------------|--------------|--------------"
 
# Get list of jails
jails=$(sudo fail2ban-client status | grep "Jail list" | sed 's/.*Jail list://' | tr -d ' ' | tr ',' ' ')
 
for jail in $jails; do
  stats=$(sudo fail2ban-client status "$jail" 2>/dev/null)
  if [ -z "$stats" ]; then
    printf "%-30s | inactive or error\n" "$jail"
    continue
  fi
 
  # Safer extraction without variable-length lookbehind
  banned=$(echo "$stats" | awk '/Currently banned:/ {print $NF}' || echo 0)
  failed=$(echo "$stats" | awk '/Total failed:/ {print $NF}' || echo 0)
  tbanned=$(echo "$stats" | awk '/Total banned:/ {print $NF}' || echo 0)
  actions=$(echo "$stats" | awk '/Actions executed:/ {print $NF}' || echo 0)  # or "Actions taken" if your version says that
 
  printf "%-30s | %10s | %12s | %12s | %12s\n" "$jail" "$banned" "$failed" "$tbanned" "$actions"
done

— oemb1905 2026/03/21 21:58