UFW FRAMEWORK(8) May 2023 UFW FRAMEWORK(8)
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 in-
terface for managing a firewall, the ufw framework provides the admin-
istrator methods to customize default behavior and add rules not sup-
ported by the command line tool. In this way, ufw can take full advan-
tage 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 initial-
ized
/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 com-
mand)
/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 stan-
dard 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 sup-
ported 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 argu-
ments:
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 imple-
ment 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 pri-
mary 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 fire-
wall 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 fol-
lows: before.rules first, user.rules next, and after.rules last. IPv6
rules are evaluated in the same way, with the rules files named be-
fore6.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 mod-
ules 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
Unconditional loading of connection tracking modules (nf_conntrack_*)
in this manner is deprecated. ufw continues to support the functional-
ity but new configuration should only contain the specific modules re-
quired for the site. For more information, see CONNECTION HELPERS.
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 ex-
amples 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 ad-
justed to have:
*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 10.0.0.0/8 ! --protocol 41 -o eth0 -j MAS-
QUERADE
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. Con-
nections 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
CONNECTION HELPERS
Various protocols require the use of netfilter connection tracking
helpers to group related packets into RELATED flows to make rulesets
clearer and more precise. For example, with a couple of kernel modules
and a couple of rules, a ruleset could simply allow a connection to FTP
port 21, then the kernel would examine the traffic and mark the other
FTP data packets as RELATED to the initial connection.
When the helpers were first introduced, one could only configure the
modules as part of module load (eg, if your FTP server listened on a
different port than 21, you'd have to load the nf_conntrack_ftp module
specifying the correct port). Over time it was understood that uncondi-
tionally using connection helpers could lead to abuse, in part because
some protocols allow user specified data that would allow traversing
the firewall in undesired ways. As of kernel 4.7, automatic conntrack
helper assignment (ie, handling packets for a given port and all IP ad-
dresses) is disabled (the old behavior can be restored by setting
net/netfilter/nf_conntrack_helper=1 in /etc/ufw/sysctl.conf). Firewalls
should now instead use the CT target to associate traffic with a par-
ticular helper and then set RELATED rules to use the helper. This al-
lows sites to tailor the use of helpers and help avoid abuse.
In general, to use helpers securely, the following needs to happen:
1. net/netfilter/nf_conntrack_helper should be set to 0 (default)
2. create a rule for the start of a connection (eg for FTP, port
21)
3. create a helper rule to associate the helper with this connec-
tion
4. create a helper rule to associate a RELATED flow with this con-
nection
5. if needed, add the corresponding nf_conntrack_* module to
IPT_MODULES
6. optionally add the corresponding nf_nat_* module to IPT_MODULES
In general it is desirable to make connection helper rules as specific
as possible and ensure anti-spoofing is correctly setup for your site
to avoid security issues in your ruleset. For more information, see
ANTI-SPOOFING, above, and <https://home.regit.org/netfilter-en/secure-
use-of-helpers/>.
Currently helper rules must be managed in via the RULES FILES. A future
version of ufw will introduce syntax for working with helper rules.
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-re-
store(8), sysctl(8), sysctl.conf(5)
AUTHOR
ufw is Copyright 2008-2023, Canonical Ltd.
May 2023 UFW FRAMEWORK(8)
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