Drop IP traffic using iptables in Ubuntu

Once in a while my server was getting a lot of traffic. As I discovered that was unwanted traffic, different IP's were doing POST calls to my WordPress login. Not only I didn't feel secure (I was being attacked) but they put my CPU at I had the CPU at 80% all the time.

iptables to the rescue

I already had a bunch of iptables rules set on my Ubuntu. Thing was, I didn't know how to block a specific IP.

Some people, including myself, have a set of firewall rules in a separate file so every time Ubuntu restarts uses that set of rules.

nano /etc/iptables.firewall.rules

On that file I was supposed to add the blocking (DROP) IP rules. They look like this:

-A INPUT -s xx.xx.xxx.xx -j DROP

Use the iptables Ban Generator

It's critical to put the DROP rules at the beggining, otherwise it doesn't work. It looks like if a packet meets a rule, none of the following rules will apply. Meaning, if you put the DROP rules at the end, the packets will probably have met a previous ACCEPT rule.

Range Ban

To block 116.10.191.* addresses:

-A INPUT -s 116.10.191.0/24 -j DROP

To block 116.10.. addresses:

-A INPUT -s 116.10.0.0/16 -j DROP

To block 116...* addresses:

-A INPUT -s 116.0.0.0/8 -j DROP

Tips

Restore the iptables based on the firewall rules file:

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

Print the iptables with the IPs:

iptables -L -n

Starter file with some firewall rules:

*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 -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

Further information in Linode's Documentation.