Block suspicious IP with Linux firewall daemon

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Block Brute-Force requests with Firewall Daemon from Bash Script

Firewall Daemon can help to protect against ongoing brute force attacks by detecting attempted attacks on the Linux host. To permanently protect the host from suspicious sources they can be blocked. The following bash script prevent from suspicious requests by append the IP address as argument and set it to reject using firewall-cmd.

#!/bin/bash
# permanently reject or remove suspicious sources by kernel firewall
# dependency: firewalld (fail2ban optional)
if [[ $1 == "add" && $2 =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
   echo "$2 add to reject"
   firewall-cmd --permanent --zone=public --add-rich-rule='rule family="ipv4" source address="'$2'" reject'

elif [[ $1 == "remove" && $2 =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
   echo "$2 being to remove"
   firewall-cmd --permanent --zone=public --remove-rich-rule='rule family="ipv4" source address="'$2'" reject'

 elif [[ $1 == "add6" && $2 =~ ^[A-Za-z0-9]{1,4}\:[A-Za-z0-9]{1,4}\: ]]; then
    echo "$2 add to reject"
    firewall-cmd --permanent --zone=public --add-rich-rule='rule family="ipv6" source address="'$2'" reject'

  elif [[ $1 == "remove6" && $2 =~ ^[A-Za-z0-9]{1,4}\:[A-Za-z0-9]{1,4}\: ]]; then
    echo "$2 being to remove"
    firewall-cmd --permanent --zone=public --remove-rich-rule='rule family="ipv6" source address="'$2'" reject'

  else
    echo "Usage: fw add|remove [source IPv4]"
    echo "       fw add6|remove6 [source IPv6]"
    exit 0
fi
firewall-cmd --reload
sleep 1
if [[ -f /run/fail2ban/fail2ban.pid ]]; then
  systemctl restart fail2ban
else
  echo "fail2ban not running"
fi
sleep 2
firewall-cmd --list-all

The script was written on Debian 10 (buster), Debian has ufw installed by default, so you have to install firewalld and disable ufw, as well as on Ubuntu 20. On RHEL and CentOS 7+ family and Fedora firewalld is default and the script can be used.

Save the script lines into a file, for instance as fw and make them executable, then run the script to block an ip address with add.

chmod u+x fw
./fw add 192.168.89.56

Use remove to swipe the ip address from the chain if desired.

How to enable Firewalld on Debian

The firewalld package is available on the official Debian 10 repositories. Installation is quick as shown below commands.

sudo apt update
sudo apt -y install firewalld

Install firewalld in the terminal as root or user with sudo privileges.

sudo ufw disable

If ufw is activated, the uncomplicated firewall (ufw) for managing the netfilter must be deactivated in order to make firewalld to the standard firewall.

$ sudo systemctl enable firewalld
$ sudo systemctl start firewalld

Run the firewall daemon and activate it for the system start.

$ sudo firewall-cmd --state
running

Check if firewall daemon is running and the service is available.

$ sudo firewall-cmd --reload

Load the new firewall rules and keep the status information.

Using Debian after run firewall-cmd --reload the error appears:

Error: COMMAND_FAILED: ‘/usr/sbin/ip6tables-restore -w -n’ failed: ip6tables-restore v1.8.2 (nf_tables:
line 4: RULE_REPLACE failed (no such file ordirectory): rule in chain OUTPUT

The solution is to run update-alternatives to force Debian to use iptables instead of nftables.

$ sudo update-alternatives --set iptables /usr/sbin/iptables-legacy
$ sudo update-alternatives --set ip6tables /usr/sbin/ip6tables-legacy

After switching from nftables to iptables, restart the Linux machine with reboot.

Firewalld configuring

Firewalld is a firewall-management solution that acts as a front-end for the iptables packet filter system provided by the Linux kernel. firewall-cmd is the utility used to manage the firewall configuration. The firewalld daemon manages groups of rules using entities called “zones”. Zones are like sets of rules that determine what traffic to allow based on the known trust of the networks to which the computer is connected. A zone is assigned to the network interfaces in order to determine the behavior that the firewall should allow.

Assign an interface to the zone public using the firewall-cmd tool, check zones and interfaces with the command.

$ sudo firewall-cmd --zone=public --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces:
  sources:
  services: dhcpv6-client https ssh
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:
        rule family="ipv4" source address="10.10.10.1" reject

If no network interface appears at interfaces, this must still be assigned to the zone, to querying the interface name use ip or ifconfig (net-tools).

$ ip link
$ ip addr
$ ifconfig

Here at the virtual Debian (buster) it is Link 2 ens33.

The interface ens33 we assign to the default zone public.

$ sudo firewall-cmd --zone=public --change-interface=ens33

Check the interface assigned to the zone use get-active-zones.

$ firewall-cmd --get-active-zones
public
  interfaces: ens33
$ sudo firewall-cmd --zone=public --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: ens33
  sources:
  services: dhcpv6-client https ssh
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:
        rule family="ipv4" source address="10.10.10.1" reject

The interface ens33 is assigned to zone public.

Interacting Fail2ban and firewalld

Fail2ban (failure leads to ban) is an IPS framework developed in Python to prevent attacks. It runs on all Unixoid OS that is based on a managable packet filter system or a firewall such as iptables or firewalld on Linux.

ln the script (above), if available and executed, the addresses banned by fail2ban are restored to their previously active state after firewalld has been processed.

firewall-cmd add | remove source address

Using firewall-cmd to reject suspicious requests from sources.

$ sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.10.10.1" reject'

The locked IP address can be removed with the following command line.

$ sudo firewall-cmd --permanent --remove-rich-rule='rule family="ipv4" source address="10.10.10.1" reject'

Execute the rule entered through the firewall daemon.

$ sudo firewall-cmd --reload

Help to use firewall-cmd

Check the entered and enabled rules of the Public zone.

$ sudo firewall-cmd --zone=public --list-all

Get current firewall rules with the following commands.

$ sudo firewall-cmd --list-all

Use the iptables command to list current rules.

$ sudo iptables -vxnL

Show standard zone for connections and interfaces.

$ firewall-cmd --get-default-zone

Set a zone as the default zone.

$ sudo firewall-cmd --set-default-zone=zone_name

Output currently active zones.

$ firewall-cmd --get-active-zones

Get output of predefined zones.

$ firewall-cmd --get-zones

Get help and man page of firewall-cmd.

$ firewall-cmd --help
$ man firewall-cmd

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