How to block an IP address with ufw on Ubuntu Linux server

ufw block specific IP address

The syntax is:
sudo ufw deny from {ip-address-here} to any
To block or deny all packets from 192.168.1.5, enter:
sudo ufw deny from 192.168.1.5 to any

Block an IP address ufw

Instead of deny rule we can reject connection from any IP as follows:
sudo ufw reject from 202.54.5.7 to any
You use reject when you want the other end (attacker) to know the port or IP is unreachable. However, we use deny for connections to attackers (hosts) you don’t want people to see at all. In other words the reject sends a reject response to the source, while the deny (DROP) target sends nothing at all.

Show firewall status including your rules

Verify newly added rules, enter:
$ sudo ufw status numbered
OR
$ sudo ufw status

Fig.01: ufw firewall status

ufw block specific IP and port number

The syntax is:
ufw deny from {ip-address-here} to any port {port-number-here}
To block or deny spammers IP address 202.54.1.5 to port 80, enter:
sudo ufw deny from 202.54.1.5 to any port 80
Again verify with the following command:
$ sudo ufw status numbered
Sample outputs:

Status: active
 
	 To                         Action      From
	 --                         ------      ----
[ 1] 192.168.1.10 80/tcp        ALLOW       Anywhere
[ 2] 192.168.1.10 22/tcp        ALLOW       Anywhere
[ 3] Anywhere                   DENY        192.168.1.5
[ 4] 80                         DENY IN     202.54.1.5

ufw deny specific IP, port number, and protocol

The syntax is:
sudo ufw deny proto {tcp|udp} from {ip-address-here} to any port {port-number-here}
For example block hacker IP address 202.54.1.1 to tcp port 22, enter:
$ sudo ufw deny proto tcp from 202.54.1.1 to any port 22
$ sudo ufw status numbered

ufw block subnet

The syntax is same:
$ sudo ufw deny proto tcp from sub/net to any port 22
$ sudo ufw deny proto tcp from 202.54.1.0/24 to any port 22

How do I delete blocked IP address or unblock an IP address again?

The syntax is:
$ sudo ufw status numbered
$ sudo ufw delete NUM
To delete rule number # 4, enter:
$ sudo ufw delete 4
Sample outputs:

Deleting:
 deny from 202.54.1.5 to any port 80
Proceed with operation (y|n)? y
Rule deleted

Tip: UFW NOT blocking an IP address

UFW (iptables) rules are applied in order of appearance, and the inspection ends immediately when there is a match. Therefore, for example, if a rule is allowing access to tcp port 22 (say using sudo ufw allow 22), and afterward another Rule is specified blocking an IP address (say using ufw deny proto tcp from 202.54.1.1 to any port 22), the rule to access port 22 is applied and the later rule to block the hacker IP address 202.54.1.1 is not. It is all about the order. To avoid such problem you need to edit the /etc/ufw/before.rules file and add a section to “Block an IP Address” after “# End required lines” section.
$ sudo vi /etc/ufw/before.rules
Find line that read as follows:

# End required lines

Append your rule to block spammers or hackers:

# Block spammers 
-A ufw-before-input -s 178.137.80.191 -j DROP
# Block ip/net (subnet) 
-A ufw-before-input -s 202.54.1.0/24 -j DROP

Save and close the file. Finally, reload the firewall:
$ sudo ufw reload
As noted below in the comment section, we can skip the whole process and use the following simple syntax:
$ sudo ufw insert 1 deny from {BADIPAddress-HERE}
$ sudo ufw insert 1 deny from 178.137.80.191 comment 'block spammer'
$ sudo ufw insert 1 deny from 202.54.1.0/24 comment 'Block DoS attack subnet'

Conclusion

We learned how to block an IP address or network subnet/CIDR (Classless Inter-Domain Routing) using the ufw based firewall to protect our server from bad guys.