Wissensdatenbank

Introduction

iptables is a powerful tool used to configure the Linux-kernel's integrated firewall. It comes preinstalled on most Ubuntu distributions, however if you are using a customized Ubuntu version or running inside a container you will most likely have to install it manually.

sudo apt-get install iptables iptables-persistent

After installation, if you are asked whether to save your current rules, it does not matter at the moment because you are going to remove or create new rules later.

Tips

You can use the netcat command (on a different computer than your server) to test which of your ports are open or closed.

nc -z -w5 -v SERVER_IP PORT

• nc is the netcat command.
• -z just send a packet without payload.
• -w5 wait up to 5 seconds for a response.
• -v verbose mode.
• Replace SERVER_IP with your server address.
• Replace PORT with the port you want to test if it's open (e.g. 22).

On your server you can use the netstat command to see which ports are currently listening for connections.

sudo netstat -tulpn

Note: Although netstat is a handy to find the ports you want to work with, you should be aware of the applications you currently have installed on your server and which ports are listening, you do not have to allow every port you find in the netstat output.

Syntax

sudo iptables -A INPUT -p tcp -m tcp --dport 22 --m geoip --src-cc PE -j ACCEPT

• -A INPUT add a rule to the INPUT chain, a chain is a group of rules, the ones we use most on this guide will be INPUT, OUTPUT and PREROUTING.
• -p tcp set tcp as the protocol this rule will apply to, you can also use other protocols such as udp, icmp or all.
• -m tcp use the tcp module. iptables supports additional features via modules, some of which come already preinstalled with iptables and others, such as the geoip module.
• --dport 22 the commands starting with -- indicate additional options for the previously used module, in this case we will tell the tcp module to only apply to port 22.
• -m geoip use the geoip module. It will limit packets on a country basis (more information at step 5).
• --src-cc PE tell the geoip module to limit the incoming packets to the ones that come from Peru. For more country codes search for ISO 3166 country codes on the internet.
• -j ACCEPT the -j argument tells iptables what to do if a packet matches the constraints specified in the previous arguments. In this case it will ACCEPT those packets, other options are REJECT, DROP and more. You can find more options by searching iptables jump targets on the internet.

1. Basics

List all rules.

sudo iptables -L

List all commands that were used to create the currently used rules, useful to edit or delete rules.

sudo iptables -S

To delete a specific rule choose a rule from sudo iptables -S and replace -A with -D.

# -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT

sudo iptables -D INPUT -p tcp -m tcp --dport 22 -j ACCEPT

List all numbered rules in the INPUT chain.

sudo iptables -L INPUT --line-numbers

Delete a numbered rule.

sudo iptables -D INPUT 2

To clear all rules.

sudo iptables -F

Warning: you might lose connection if connected by SSH.

Clear only rules in the OUTPUT chain.

sudo iptables -F OUTPUT

2. Create initial rules

Allow SSH on eth0 interface

sudo iptables -A INPUT -i eth0 -p tcp -m tcp --dport 22 -j ACCEPT

• -i eth0 apply rule to a specific interface, to allow from any interface remove this command.

To limit incoming packets to a specific IP (i.e. 10.0.3.1/32).

sudo iptables -A INPUT -i eth0 -s 10.0.3.1/32 -p tcp -m tcp --dport 22 -j ACCEPT

• -s 10.0.3.1/32 specifies an IP/subnet to allow connections from.

Set default chain rules.

Warning: before proceeding make sure you have applied the correct SSH rules if working on a remote server.

sudo iptables -P INPUT DROP
sudo iptables -P FORWARD DROP 
sudo iptables -P OUTPUT ACCEPT 

• -P INPUT DROP denies all incoming packets (i.e. no one will be able to connect to your running servers such as Apache, SQL, etc).
• -P FORWARD DROP denies all forwarded packets (i.e. when you are using your system as router).
• -P OUTPUT ACCEPT allows all outgoing packets (i.e. when you perform an HTTP request).

Allow all traffic on loopback interface (recommended).

sudo iptables -A INPUT -i lo -j ACCEPT
sudo iptables -A OUTPUT -o lo -j ACCEPT

3. Make rules persistent

Save the current iptables rules.

sudo netfilter-persistent save
sudo netfilter-persistent reload

If you are running inside a container the netfilter-persistent command most likely will not work, so you need to reconfigure the iptables-persistent package.

sudo dpkg-reconfigure iptables-persistent

4. Allow outgoing connections

Allow DNS queries.

sudo iptables -A OUTPUT -p tcp --dport 53 -m state --state NEW -j ACCEPT
sudo iptables -A OUTPUT -p udp --dport 53 -m state --state NEW -j ACCEPT

Use the state module to allow RELATED and ESTABLISHED outgoing packets.

sudo iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Allow the desired ports; in this case, HTTP ports.

sudo iptables -A OUTPUT -p tcp --dport 80 -m state --state NEW -j ACCEPT

More ports you might want to use.

• FTP: tcp at port 21
• HTTPS: tcp at port 443
• DHCP: udp at port 67
• NTP: udp at port 123

Note: If you want to allow apt-get it might be necessary to allow FTP and HTTPS.

Allow the returned traffic only for RELATED and already ESTABLISHED connections (recommended because sometimes bidirectional communication is required).

sudo iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Other useful rules

Allow ping requests from outside.

sudo iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
sudo iptables -A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT

Forward traffic on eth0 port 2200 to 10.0.3.21:22 (useful if you want to expose an SSH server that is running inside a container).

sudo iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 2200 -j DNAT --to-destination 10.0.3.21:22

If you successfully login to your server by using SSH, a persistent connection will be created (i.e. no new connections even if you are connected for more than 1 hour). If you fail and try to login again, a new connection will be created. This will block continuous SSH login attempts by limiting new connections per hour.

sudo iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
sudo iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 3600 --hitcount 4 -j DROP

Redirect all requests on port 443 to port 4430 (useful if you want to bind to port 443 without root).

sudo iptables -t nat -A PREROUTING -i ens3 -p tcp --dport 443 -j REDIRECT --to-port 4430
sudo iptables -A INPUT -p tcp -m tcp --dport 4430 -m geoip --src-cc PE -j ACCEPT

• ens3 the network interface.
• -m geoip country block module (see step 5).

Warning: Do not use lo, the OS will discard all packets redirected to the loopback interface.

5. Allow or block whole countries

5.1 Install xtables-addons

You can install the xtables-addons module using various methods, feel free to use the installation method that works best for you.

• Install usingapt-get.

  sudo apt-get install xtables-addons-common
  

• Install using module-assistant.

  sudo apt-get install module-assistant xtables-addons-source
  sudo module-assistant --verbose --text-mode auto-install xtables-addons
  

• Install from source.

  sudo apt-get install git bc libncurses5-dev libtext-csv-xs-perl autoconf automake libtool xutils-dev iptables-dev
  git clone git://git.code.sf.net/p/xtables-addons/xtables-addons
  cd xtables-addons
  ./autogen.sh
  ./configure
  make
  sudo make install
  

Build a "countries" database.

sudo apt-get install libtext-csv-xs-perl unzip
sudo mkdir /usr/share/xt_geoip
sudo /usr/lib/xtables-addons/xt_geoip_dl
sudo /usr/lib/xtables-addons/xt_geoip_build -D /usr/share/xt_geoip *.csv
sudo rm GeoIPCountryCSV.zip GeoIPCountryWhois.csv GeoIPv6.csv

Reboot your system.

sudo reboot

After xtables-addons has been successfully installed, after the first reboot, run depmod otherwise country blocking will not work properly (this is only required for the first time).

sudo depmod 

Create a script at /etc/cron.monthly/geoip-updater to update the geoip database monthly.

#!/usr/bin/env bash
# this script is intended to run with sudo privileges

echo 'Removing old database---------------------------------------------------'
rm -rf /usr/share/xt_geoip/*
mkdir -p /usr/share/xt_geoip

echo 'Downloading country databases-------------------------------------------'
mkdir /tmp/geoip-updater
cd /tmp/geoip-updater
/usr/lib/xtables-addons/xt_geoip_dl

echo 'Building geoip database-------------------------------------------------'
/usr/lib/xtables-addons/xt_geoip_build -D /usr/share/xt_geoip *.csv

echo 'Removing temporary files------------------------------------------------'
cd /tmp
rm -rf /tmp/geoip-updater

Make /etc/cron.monthly/geoip-updater executable.

sudo chmod +x /etc/cron.monthly/geoip-updater

5.2 Example rules

_Note: If you are receiving an iptables: No chain/target/match by that name error when trying to apply a geoip rule, it's possible that xtables-addons has not been installed correctly. Try another installation method.

Block all incoming packets from China, Hong Kong, Russia and Korea.

sudo iptables -A INPUT -m geoip --src-cc CN,HK,RU,KR -j DROP

Allow incoming packets on port 80 from everywhere except the countries above.

sudo iptables -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT

Allow incoming packets on the ens3 interface on port 22 only from Peru (feel free to choose the country code you want to accept packets from, for example, US for United States).

sudo iptables -A INPUT -i ens3 -p tcp -m tcp --dport 22 -m geoip --src-cc PE -j ACCEPT

Allow incoming packets on port 443 only from Peru.

sudo iptables -A INPUT -p tcp -m tcp --dport 443 -m geoip --src-cc PE -j ACCEPT