UFW - Uncomplicated Firewall The default firewall configuration tool for Ubuntu is ufw. Developed to ease iptables firewall configuration, ufw provides a user friendly way to create an IPv4 or IPv6 host-based firewall. By default UFW is disabled. Gufw is a GUI that is available as a frontend.
Basic Syntax and Examples
Default rules are fine for the average home user When you turn UFW on, it uses a default set of rules (profile) that should be fine for the average home user. That's at least the goal of the Ubuntu developers. In short, all 'incoming' is being denied, with some exceptions to make things easier for home users.
Enable and Disable
Enable UFW To turn UFW on with the default set of rules: sudo ufw enable To check the status of UFW: sudo ufw status verbose The output should be like this: youruser@yourcomputer:~$ sudo ufw status verbose [sudo] password for youruser: Status: active Logging: on (low) Default: deny (incoming), allow (outgoing) New profiles: skip youruser@yourcomputer:~$ Note that by default, deny is being applied to incoming. There are exceptions, which can be found in the output of this command: sudo ufw show raw You can also read the rules files in /etc/ufw (the files whose names end with .rules).
Disable UFW To disable ufw use: sudo ufw disable
Allow and Deny (specific rules)
Allow sudo ufw allow
Deny sudo ufw deny
Delete Existing Rule To delete a rule, simply prefix the original rule with delete. For example, if the original rule was: ufw deny 80/tcp Use this to delete it: sudo ufw delete deny 80/tcp
Services You can also allow or deny by service name since ufw reads from /etc/services To see get a list of services: less /etc/services
Allow by Service Name sudo ufw allow
Deny by Service Name sudo ufw deny
Status
Checking the status of ufw will tell you if ufw is
enabled or disabled and also list the current ufw rules that are applied to
your iptables.
To check the status of ufw:
sudo ufw status
Firewall loaded
To Action From
-- ------ ----
22:tcp DENY 192.168.0.1
22:udp DENY 192.168.0.1
22:tcp DENY 192.168.0.7
22:udp DENY 192.168.0.7
22:tcp ALLOW 192.168.0.0/24
22:udp ALLOW 192.168.0.0/24
if ufw was not enabled the output would be:
sudo ufw status
Status: inactive
Logging To enable logging use: sudo ufw logging on To disable logging use: sudo ufw logging off
Advanced Syntax You can also use a fuller syntax, specifying the source and destination addresses, ports and protocols.
Allow Access This section shows how to allow specific access. Allow by Specific IP sudo ufw allow from
Allow by Subnet You may use a net mask : sudo ufw allow from 192.168.1.0/24 Allow by specific port and IP address sudo ufw allow from
Allow by specific port, IP address and protocol sudo ufw allow from
Enable PING Note: Security by obscurity may be of very little actual benefit with modern cracker scripts. By default, UFW allows ping requests. You may find you wish to leave (icmp) ping requests enabled to diagnose networking problems. In order to disable ping (icmp) requests, you need to edit /etc/ufw/before.rules and remove the following lines: # ok icmp codes -A ufw-before-input -p icmp --icmp-type destination-unreachable -j ACCEPT -A ufw-before-input -p icmp --icmp-type source-quench -j ACCEPT -A ufw-before-input -p icmp --icmp-type time-exceeded -j ACCEPT -A ufw-before-input -p icmp --icmp-type parameter-problem -j ACCEPT -A ufw-before-input -p icmp --icmp-type echo-request -j ACCEPT or change the "ACCEPT" to "DROP" # ok icmp codes -A ufw-before-input -p icmp --icmp-type destination-unreachable -j DROP -A ufw-before-input -p icmp --icmp-type source-quench -j DROP -A ufw-before-input -p icmp --icmp-type time-exceeded -j DROP -A ufw-before-input -p icmp --icmp-type parameter-problem -j DROP -A ufw-before-input -p icmp --icmp-type echo-request -j DROP
Deny Access
Deny by specific IP sudo ufw deny from
Deny by specific port and IP address sudo ufw deny from
Working with numbered rules Listing rules with a reference number You may use status numbered to show the order and id number of rules: sudo ufw status numbered
Editing numbered rules
Delete numbered rule You may then delete rules using the number. This will delete the first rule and rules will shift up to fill in the list. sudo ufw delete 1
Insert numbered rule sudo ufw insert 1 allow from
Advanced Example Scenario: You want to block access to port 22 from 192.168.0.1 and 192.168.0.7 but allow all other 192.168.0.x IPs to have access to port 22 using tcp sudo ufw deny from 192.168.0.1 to any port 22 sudo ufw deny from 192.168.0.7 to any port 22 sudo ufw allow from 192.168.0.0/24 to any port 22 proto tcp
This puts the specific rules first and the generic second. Once a rule is
matched the others will not be evaluated (see manual below) so you must put
the specific rules first. As rules change you may need to delete old rules
to ensure that new rules are put in the proper order.
To check your rules orders you can check the status; for the scenario the
output below is the desired output for the rules to work properly
sudo ufw status
Firewall loaded
To Action From
-- ------ ----
22:tcp DENY 192.168.0.1
22:udp DENY 192.168.0.1
22:tcp DENY 192.168.0.7
22:udp DENY 192.168.0.7
22:tcp ALLOW 192.168.0.0/24
Scenario change: You want to block access to port 22 to 192.168.0.3 as well
as 192.168.0.1 and 192.168.0.7.
sudo ufw delete allow from 192.168.0.0/24 to any port 22
sudo ufw status
Firewall loaded
To Action From
-- ------ ----
22:tcp DENY 192.168.0.1
22:udp DENY 192.168.0.1
22:tcp DENY 192.168.0.7
22:udp DENY 192.168.0.7
sudo ufw deny 192.168.0.3 to any port 22
sudo ufw allow 192.168.0.0/24 to any port 22 proto tcp
sudo ufw status
Firewall loaded
To Action From
-- ------ ----
22:tcp DENY 192.168.0.1
22:udp DENY 192.168.0.1
22:tcp DENY 192.168.0.7
22:udp DENY 192.168.0.7
22:tcp DENY 192.168.0.3
22:udp DENY 192.168.0.3
22:tcp ALLOW 192.168.0.0/24
If you simply add the deny rule the allow would have been above it and
been
applied instead of the deny
Interpreting Log Entries Based on the response to the post UFW log guide/tutorial ?. The SPT and DPT values, along with SRC and DST values, will typically be the values you’ll focus on when analysing the firewall logs.
Pseudo Log Entry Feb 4 23:33:37 hostname kernel: [ 3529.289825] [UFW BLOCK] IN=eth0 OUT= MAC=00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd SRC=444.333.222.111 DST=111.222.333.444 LEN=103 TOS=0x00 PREC=0x00 TTL=52 ID=0 DF PROTO=UDP SPT=53 DPT=36427 LEN=83
Date It's good practice to watch the dates and times. If things are out of order or blocks of time are missing then an attacker probably messed with your logs.
Hostname The server’s hostname
Uptime The time in seconds since boot.
Logged Event Short description of the logged event; e.g. [UFW BLOCK]
IN If set, then the event was an incoming event.
OUT If set, then the event was an outgoing event.
MAC This provides a 14-byte combination of the Destination MAC, Source MAC, and EtherType fields, following the order found in the Ethernet II header. See Ethernet frame and EtherType for more information.
SRC This indicates the source IP, who sent the packet initially. Some IPs are routable over the internet, some will only communicate over a LAN, and some will only route back to the source computer. See IP address for more information.
DST This indicates the destination IP, who is meant to receive the packet. You can use whois.net or the cli whois to determine the owner of the IP address.
LEN This indicates the length of the packet.
TOS I believe this refers to the TOS field of the IPv4 header. See TCP Processing of the IPv4 Precedence Field for more information.
PREC I believe this refers to the Precedence field of the IPv4 header.
TTL This indicates the “Time to live” for the packet. Basically each packet will only bounce through the given number of routers before it dies and disappears. If it hasn’t found its destination before the TTL expires, then the packet will evaporate. This field keeps lost packets from clogging the internet forever. See Time to live for more information.
ID Not sure what this one is, but it's not really important for reading logs. It might be ufw’s internal ID system, it might be the operating system’s ID.
PROTO This indicates the protocol of the packet - TCP or UDP. See TCP and UDP Ports Explained for more information.
SPT This indicates the source. I believe this is the port, which the SRC IP sent the IP packet over. See List of TCP and UDP port numbers for more information.
DPT This indicates the destination port. I believe this is the port, which the SRC IP sent its IP packet to, expecting a service to be running on this port.
WINDOW This indicates the size of packet the sender is willing to receive.
RES This bit is reserved for future use & is always set to 0. Basically it’s irrelevant for log reading purposes.
SYN URGP SYN indicates that this connection requires a three-way handshake, which is typical of TCP connections. URGP indicates whether the urgent pointer field is relevant. 0 means it's not. Doesn’t really matter for firewall log reading.
IP Blocklist for UFW If your Ubuntu system is directly exposed to the internet, either via a public IP address or port forwarding, an IP blocklist will add another layer of protection. I created a couple of scripts that integrate with ufw and update the list daily. Full details are available on the GitHub project: https://github.com/poddmo/ufw-blocklist.
Other Resources
- For instructions on using ufw first see the official server guide.
- The most recent syntax and manual can be retrieved by getting the man page. Otherwise open a terminal window and type: man ufw
- Firewall - wiki homepage for firewall related documentation.
- Iptables - interface to the netfilter subsystem in the Linux kernel.
- UncomplicatedFirewall - UFW Project wiki page.
- Gufw - Graphic User Interface for UFW.