Firewall Ubuntu

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Introduction

For an introduction to firewalls, please see Firewall.

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

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

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 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 <port>/<optional: protocol>

example: To allow incoming tcp and udp packet on port 53

sudo ufw allow 53

example: To allow incoming tcp packets on port 53

sudo ufw allow 53/tcp

example: To allow incoming udp packets on port 53

sudo ufw allow 53/udp

Deny

sudo ufw deny <port>/<optional: protocol>

example: To deny tcp and udp packets on port 53

sudo ufw deny 53

example: To deny incoming tcp packets on port 53

sudo ufw deny 53/tcp

example: To deny incoming udp packets on port 53

sudo ufw deny 53/udp

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

example: to allow ssh by name

sudo ufw allow ssh Deny by Service Name

sudo ufw deny

example: to deny ssh by name

sudo ufw deny ssh Status IconsPage/important.png 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

example:To allow packets from 207.46.232.182:

sudo ufw allow from 207.46.232.182 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 to port

example: allow IP address 192.168.0.4 access to port 22 for all protocols

sudo ufw allow from 192.168.0.4 to any port 22 Allow by specific port, IP address and protocol

sudo ufw allow from to port proto

example: allow IP address 192.168.0.4 access to port 22 using TCP

sudo ufw allow from 192.168.0.4 to any port 22 proto tcp

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

example:To block packets from 207.46.232.182:

sudo ufw deny from 207.46.232.182 Deny by specific port and IP address

sudo ufw deny from to port

example: deny ip address 192.168.0.1 access to port 22 for all protocols

sudo ufw deny from 192.168.0.1 to any port 22 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 IconsPage/important.png 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 IconsPage/important.png 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.

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