Provided by: ufw_0.36-0ubuntu0.18.04.2_all 
NAME
ufw-framework - using the ufw framework
DESCRIPTION
ufw provides both a command line interface and a framework for managing a netfilter firewall. While the
ufw command provides an easy to use interface for managing a firewall, the ufw framework provides the
administrator methods to customize default behavior and add rules not supported by the command line tool.
In this way, ufw can take full advantage of Linux netfilter's power and flexibility.
OVERVIEW
The framework provides boot time initialization, rules files for adding custom rules, a method for
loading netfilter modules, configuration of kernel parameters and configuration of IPv6. The framework
consists of the following files:
/lib/ufw/ufw-init
initialization script
/etc/ufw/before.init
initialization customization script run before ufw is initialized
/etc/ufw/after.init
initialization customization script run after ufw is initialized
/etc/ufw/before[6].rules
rules file containing rules evaluated before UI added rules
/etc/ufw/user[6].rules
rules file containing UI added rules (managed with the ufw command)
/etc/ufw/after[6].rules
rules file containing rules evaluated after UI added rules
/etc/default/ufw
high level configuration
/etc/ufw/sysctl.conf
kernel network tunables
/etc/ufw/ufw.conf
additional high level configuration
BOOT INITIALIZATION
ufw is started on boot with /lib/ufw/ufw-init. This script is a standard SysV style initscript used by
the ufw command and should not be modified. The /etc/before.init and /etc/after.init scripts may be used
to perform any additional firewall configuration that is not yet supported in ufw itself and if they
exist and are executable, ufw-init will execute these scripts. ufw-init will exit with error if either of
these scripts exit with error. ufw-init supports the following arguments:
start: loads the firewall
stop: unloads the firewall
restart:
reloads the firewall
force-reload:
same as restart
status:
basic status of the firewall
force-stop:
same as stop, except does not check if the firewall is already loaded
flush-all:
flushes the built-in chains, deletes all non-built-in chains and resets the policy to ACCEPT
ufw-init will call before.init and after.init with start, stop, status and flush-all, but typically, if
used, these scripts need only implement start and stop.
ufw uses many user-defined chains in addition to the built-in iptables chains. If MANAGE_BUILTINS in
/etc/default/ufw is set to 'yes', on stop and reload the built-in chains are flushed. If it is set to
'no', on stop and reload the ufw secondary chains are removed and the ufw primary chains are flushed. In
addition to flushing the ufw specific chains, it keeps the primary chains in the same order with respect
to any other user-defined chains that may have been added. This allows for ufw to interoperate with other
software that may manage their own firewall rules.
To ensure your firewall is loading on boot, you must integrate this script into the boot process. Consult
your distribution's documentation for the proper way to modify your boot process if ufw is not already
integrated.
RULES FILES
ufw is in part a front-end for iptables-restore, with its rules saved in /etc/ufw/before.rules,
/etc/ufw/after.rules and /etc/ufw/user.rules. Administrators can customize before.rules and after.rules
as desired using the standard iptables-restore syntax. Rules are evaluated as follows: before.rules
first, user.rules next, and after.rules last. IPv6 rules are evaluated in the same way, with the rules
files named before6.rules, user6.rules and after6.rules. Please note that ufw status only shows rules
added with ufw and not the rules found in the /etc/ufw rules files.
Important: ufw only uses the *filter table by default. You may add any other tables such as *nat, *raw
and *mangle as desired. For each table a corresponding COMMIT statement is required.
After modifying any of these files, you must reload ufw for the rules to take effect. See the EXAMPLES
section for common uses of these rules files.
MODULES
Netfilter has many different connection tracking modules. These modules are aware of the underlying
protocol and allow the administrator to simplify his or her rule sets. You can adjust which netfilter
modules to load by adjusting IPT_MODULES in /etc/default/ufw. Some popular modules to load are:
nf_conntrack_ftp
nf_nat_ftp
nf_conntrack_irc
nf_nat_irc
nf_conntrack_netbios_ns
nf_conntrack_pptp
nf_conntrack_tftp
nf_nat_tftp
nf_conntrack_sane
KERNEL PARAMETERS
ufw will read in /etc/ufw/sysctl.conf on boot when enabled. Please note that /etc/ufw/sysctl.conf
overrides values in the system systcl.conf (usually /etc/sysctl.conf). Administrators can change the file
used by modifying /etc/default/ufw.
IPV6
IPv6 is enabled by default. When disabled, all incoming, outgoing and forwarded packets are dropped, with
the exception of traffic on the loopback interface. To adjust this behavior, set IPV6 to 'yes' in
/etc/default/ufw. See the ufw manual page for details.
EXAMPLES
As mentioned, ufw loads its rules files into the kernel by using the iptables-restore and
ip6tables-restore commands. Users wanting to add rules to the ufw rules files manually must be familiar
with these as well as the iptables and ip6tables commands. Below are some common examples of using the
ufw rules files. All examples assume IPv4 only and that DEFAULT_FORWARD_POLICY in /etc/default/ufw is
set to DROP.
IP Masquerading
To allow IP masquerading for computers from the 10.0.0.0/8 network on eth1 to share the single IP address
on eth0:
Edit /etc/ufw/sysctl.conf to have:
net.ipv4.ip_forward=1
Add to the end of /etc/ufw/before.rules, after the *filter section:
*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 10.0.0.0/8 -o eth0 -j MASQUERADE
COMMIT
If your firewall is using IPv6 tunnels or 6to4 and is also doing NAT, then you should not usually
masquerade protocol '41' (ipv6) packets. For example, instead of the above, /etc/ufw/before.rules can be
adjusted to have:
*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 10.0.0.0/8 ! --protocol 41 -o eth0 -j MASQUERADE
COMMIT
Add the ufw route to allow the traffic:
ufw route allow in on eth1 out on eth0 from 10.0.0.0/8
Port Redirections
To forward tcp port 80 on eth0 to go to the webserver at 10.0.0.2:
Edit /etc/ufw/sysctl.conf to have:
net.ipv4.ip_forward=1
Add to the end of /etc/ufw/before.rules, after the *filter section:
*nat
:PREROUTING ACCEPT [0:0]
-A PREROUTING -p tcp -i eth0 --dport 80 -j DNAT \
--to-destination 10.0.0.2:80
COMMIT
Add the ufw route rule to allow the traffic:
ufw route allow in on eth0 to 10.0.0.2 port 80 proto tcp
Egress filtering
To block RFC1918 addresses going out of eth0:
Add the ufw route rules to reject the traffic:
ufw route reject out on eth0 to 10.0.0.0/8
ufw route reject out on eth0 to 172.16.0.0/12
ufw route reject out on eth0 to 192.168.0.0/16
Full example
This example combines the other examples and demonstrates a simple routing firewall. Warning: this setup
is only an example to demonstrate the functionality of the ufw framework in a concise and simple manner
and should not be used in production without understanding what each part does and does not do. Your
firewall will undoubtedly want to be less open.
This router/firewall has two interfaces: eth0 (Internet facing) and eth1 (internal LAN). Internal clients
have addresses on the 10.0.0.0/8 network and should be able to connect to anywhere on the Internet.
Connections to port 80 from the Internet should be forwarded to 10.0.0.2. Access to ssh port 22 from the
administrative workstation (10.0.0.100) to this machine should be allowed. Also make sure no internal
traffic goes to the Internet.
Edit /etc/ufw/sysctl.conf to have:
net.ipv4.ip_forward=1
Add to the end of /etc/ufw/before.rules, after the *filter section:
*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A PREROUTING -p tcp -i eth0 --dport 80 -j DNAT \
--to-destination 10.0.0.2:80
-A POSTROUTING -s 10.0.0.0/8 -o eth0 -j MASQUERADE
COMMIT
Add the necessary ufw rules:
ufw route reject out on eth0 to 10.0.0.0/8
ufw route reject out on eth0 to 172.16.0.0/12
ufw route reject out on eth0 to 192.168.0.0/16
ufw route allow in on eth1 out on eth0 from 10.0.0.0/8
ufw route allow in on eth0 to 10.0.0.2 port 80 proto tcp
ufw allow in on eth1 from 10.0.0.100 to any port 22 proto tcp
NOTES
When using ufw with libvirt and bridging, packets may be blocked. The libvirt team recommends that the
following sysctl's be set to disable netfilter on the bridge:
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
Note that the bridge module must be loaded in to the kernel before these values are set. One way to
ensure this works properly with ufw is to add 'bridge' to IPT_MODULES in /etc/default/ufw, and then add
the above rules to /etc/ufw/sysctl.conf.
Alternatively to disabling netfilter on the bridge, you can configure iptables to allow all traffic to be
forwarded across the bridge. Eg, add to /etc/ufw/before.rules within the *filter section:
-I FORWARD -m physdev --physdev-is-bridged -j ACCEPT
SEE ALSO
ufw(8), iptables(8), ip6tables(8), iptables-restore(8), ip6tables-restore(8), sysctl(8), sysctl.conf(5)
AUTHOR
ufw is Copyright 2008-2014, Canonical Ltd.
ufw and this manual page was originally written by Jamie Strandboge <jamie@canonical.com>