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

Unconditional loading of connection tracking modules (nf_conntrack_*) in this manner is deprecated. ufw continues to support the functionality but new configuration should only contain the specific modules required 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 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.

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 unconditionally 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 addresses) 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 particular helper and then set RELATED rules to use the helper. This allows 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 connection

4.

create a helper rule to associate a RELATED flow with this connection

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

When using ufw with LXD or Incus, additional rules for the bridge (by default, lxdbr0) are needed for the guests for DHCP, DNS and outbound connections. Eg:

ufw allow in on lxdbr0 to any port 67 proto udp
ufw allow in on lxdbr0 to any port 53
ufw route allow in on lxdbr0 from XXX.YYY.ZZZ.0/24

The first two rules limit access from the guests to the host to only DHCP and DNS while the 3rd rule allows access from the guests on the XXX.YYY.ZZZ.0/24 subnet (as configured by LXD or Incus) to the world. Adjust as necessary for your environment (site requirements, name of the bridge, bridge network, etc).

To allow traffic from the outside to your guests, then additional rules are needed. Eg, to allow HTTPS to the 10.210.254.123 guest:

ufw route allow out on lxdbr0 to 10.210.254.123 port 443 proto tcp

Note: when thinking of 'in on lxdbr0' vs 'out on lxdbr0', think of the traffic in the context of the host and the guest: if the traffic is from the guest into or through the host, then an 'in on lxdbr0' should be used; if the traffic is from anywhere out to the guest, then an 'out on lxdbr0' should be used. As always, additional rules may be needed depending on your default policy.

SEE ALSO¶

ufw(8), iptables(8), ip6tables(8), iptables-restore(8), ip6tables-restore(8), sysctl(8), sysctl.conf(5)

AUTHOR¶

ufw is Copyright 2008-2024, Canonical Ltd.