Unexpected DDOS: Blocking China with ipset and iptables

Set up iptables

If you already have iptables set up and in use, skip this section and go straight to the ipset section.

Create a file where we can declare some rules to use:

sudo nano /etc/iptables.firewall.rules

Inside there you'll want to paste the following:

*filter

#  Allow all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT -d 127.0.0.0/8 -j REJECT

#  Accept all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

#  Allow all outbound traffic - you can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT

#  Allow HTTP and HTTPS connections from anywhere (the normal ports for websites and SSL).
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT

#  Allow SSH connections
#
#  The -dport number should be the same port number you set in sshd_config
#
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT

#  Allow ping
-A INPUT -p icmp --icmp-type echo-request -j ACCEPT

#  Log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

#  Drop all other inbound - default deny unless explicitly allowed policy
-A INPUT -j DROP
-A FORWARD -j DROP

COMMIT

Save that. Next, we need to apply those rules – this is just a text file, and we need to instruct iptables to actually use it.

sudo iptables-restore < /etc/iptables.firewall.rules

That should have loaded the rules and applied them; you can check by

iptables -L

The output of that command ought to look like

Chain INPUT (policy ACCEPT)
   target     prot opt source               destination
   ACCEPT     all  --  anywhere             anywhere
   REJECT     all  --  anywhere             127.0.0.0/8          reject-with icmp-port-unreachable
   ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
   ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
   ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:https
   ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
   ACCEPT     icmp --  anywhere             anywhere
   LOG        all  --  anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables denied: "
   DROP       all  --  anywhere             anywhere

   Chain FORWARD (policy ACCEPT)
   target     prot opt source               destination
   DROP       all  --  anywhere             anywhere

   Chain OUTPUT (policy ACCEPT)
   target     prot opt source               destination
   ACCEPT     all  --  anywhere             anywhere

Great, it's working! But if you reboot the server it won't be. So lets fix that by creating a file which will run at boot.

sudo nano /etc/network/if-pre-up.d/firewall

Inside that file paste:

#!/bin/sh
/sbin/iptables-restore < /etc/iptables.firewall.rules

Save it. Now we must make sure it's allowed to execute:

sudo chmod +x /etc/network/if-pre-up.d/firewall

Done. The firewall is now running with those rules applied and those rules will be re-applied every time the server reboots. But it's not blocking China yet; it's only blocking anything not on port 80 or 443 (http and https).

Using ipset to block China

You can't manually add a few thousand IP addresses to your iptables, and even doing it automatically is a bad idea because it can cause a lot of CPU load (or so I've read). Instead we can use ipset which is designed for this sort of thing. ipset handles big lists of ip addresses; you just create a list and then tell iptables to use that list in a rule.

Note; I assume that the entirety of the following is done as root. Adjust accordingly if your system is based on sudo.

apt-get install ipset

Next, I wrote a small Bash script to do all the work, which you should be able to understand from the comments in it. Create a file:

nano /etc/block-china.sh

Here's what you want to paste into it:

# Create the ipset list
ipset -N china hash:net

# remove any old list that might exist from previous runs of this script
rm cn.zone

# Pull the latest IP set for China
wget -P . http://www.ipdeny.com/ipblocks/data/countries/cn.zone

# Add each IP address from the downloaded list into the ipset 'china'
for i in $(cat /etc/cn.zone ); do ipset -A china $i; done

# Restore iptables
/sbin/iptables-restore < /etc/iptables.firewall.rules

Save the file. Make it executable:

chmod +x /etc/block-china.sh

This hasn't done anything yet, but it will in a minute when we run the script. First, we need to add a rule into iptables that refers to this new ipset list the script above defines:

nano /etc/iptables.firewall.rules

Add the following line:

-A INPUT -p tcp -m set --match-set china src -j DROP

Save the file. To be clear, my full iptables.firewall.rules now looks like this:

*filter

#  Allow all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT -d 127.0.0.0/8 -j REJECT

#  Accept all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Block anything from China
# These rules are pulled from ipset's china list
# The source file is at /etc/cn.zone (which in turn is generated by a shell script at /etc/block-china.sh )
-A INPUT -p tcp -m set --match-set china src -j DROP

#  Allow all outbound traffic - you can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT

#  Allow HTTP and HTTPS connections from anywhere (the normal ports for websites and SSL).
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT

#  Allow SSH connections
#
#  The -dport number should be the same port number you set in sshd_config
#
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT

#  Allow ping
-A INPUT -p icmp -j ACCEPT

#  Log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

#  Drop all other inbound - default deny unless explicitly allowed policy
-A INPUT -j DROP
-A FORWARD -j DROP

COMMIT

Right now, nothing has changed with the server because no new rules have been applied; to do so, run the block-china.sh script:

/etc/block-china.sh

This should show some output as it pulls a fresh list of Chinese based IPs and then, after a few seconds or so, it will complete and drop you back to a command prompt.

To test if it worked, run:

iptables -L

You should now see a new rule blocking China – the output ought to look like this:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
REJECT     all  --  anywhere             loopback/8           reject-with icmp-port-unreachable
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
DROP       tcp  --  anywhere             anywhere             match-set china src
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:https
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
ACCEPT     icmp --  anywhere             anywhere
LOG        all  --  anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables denied: "
DROP       all  --  anywhere             anywhere

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere

Almost done! This works, and will continue to work on re-boots. But, IP addresses change and that list will grow stale over time. If you want to pull and apply an updated list of IPs you can just run the block-china.sh script again.

We can also set the machine to do that automatically via a cron job:

crontab -e

Add a line such as this:

* 5 * * * /etc/block-china.sh

This will run /etc/block-china.sh at 5am every day. The user running the script will need to be root or have root privileges