SuSEfirewall2 is a script that reads the variables set in
/etc/sysconfig/SuSEfirewall2
to generate a set of
iptables rules. It defines three security zones, although only the first
and the second one are considered in the following sample configuration:
Given that there is no way to control what is happening on the external network, the host needs to be protected from it. In most cases, the external network is the Internet, but it could be another insecure network, such as a WLAN.
This refers to the private network, in most cases the LAN. If the hosts on this network use IP addresses from the private range (see Abschnitt „Netzmasken und Routing“ (Kapitel 21, Grundlegendes zu Netzwerken, ↑Referenz)), enable network address translation (NAT), so hosts on the internal network can access the external one.
While hosts located in this zone can be reached both from the external and the internal network, they cannot access the internal network themselves. This setup can be used to put an additional line of defense in front of the internal network, because the DMZ systems are isolated from the internal network.
Any kind of network traffic not explicitly allowed by the filtering rule set is suppressed by iptables. Therefore, each of the interfaces with incoming traffic must be placed into one of the three zones. For each of the zones, define the services or protocols allowed. The rule set is only applied to packets originating from remote hosts. Locally generated packets are not captured by the firewall.
The configuration can be performed with YaST (see
Section 14.4.1, “Configuring the Firewall with YaST”). It can also be
made manually in the file
/etc/sysconfig/SuSEfirewall2
, which is well
commented. Additionally, a number of example scenarios are available in
/usr/share/doc/packages/SuSEfirewall2/EXAMPLES
.
![]() | Automatic Firewall Configuration |
---|---|
After the installation, YaST automatically starts a firewall on all configured interfaces. If a server is configured and activated on the system, YaST can modify the automatically-generated firewall configuration with the options or in the server configuration modules. Some server module dialogs include a button for activating additional services and ports. The YaST firewall configuration module can be used to activate, deactivate, or reconfigure the firewall. |
The YaST dialogs for the graphical configuration can be accessed from the YaST Control Center. Select
+ . The configuration is divided into seven sections that can be accessed directly from the tree structure on the left side.Set the start-up behavior in this dialog. In a default installation, SuSEfirewall2 is started automatically. You can also start and stop the firewall here. To implement your new settings in a running firewall, use
.All known network interfaces are listed here. To remove an interface from a zone, select the interface, press
, and choose . To add an interface to a zone, select the interface, press and choose any of the available zones. You may also create a special interface with your own settings by using .You need this option to offer services from your system to a zone from which it is protected. By default, the system is only protected from external zones. Explicitly allow the services that should be available to external hosts. After selecting the desired zone in
, activate the services from the list.Masquerading hides your internal network from external networks (such as the Internet) while enabling hosts in the internal network to access the external network transparently. Requests from the external network to the internal one are blocked and requests from the internal network seem to be issued by the masquerading server when seen externally. If special services of an internal machine need to be available to the external network, add special redirect rules for the service.
In this dialog, configure the UDP ports that allow broadcasts. Add
the required port numbers or services to the appropriate zone,
separated by spaces. See also the file
/etc/services
.
The logging of broadcasts that are not accepted can be enabled here. This may be problematic, because Windows hosts use broadcasts to know about each other and so generate many packets that are not accepted.
Configure whether the IPsec service should be available to the external network in this dialog. Configure which packets are trusted under
.There are two rules for the logging: accepted and not accepted packets. Packets that are not accepted are DROPPED or REJECTED. Select from
, , or for both of them.Here, set special firewall rules that allow connections, matching specified citeria such as source network, protocol, destination port, and source port. Configure such rules for external, internal, and demilitarized zones.
When finished with the firewall configuration, exit this dialog with
. A zone-oriented summary of your firewall configuration then opens. In it, check all settings. All services, ports, and protocols that have been allowed and all custom rules are listed in this summary. To modify the configuration, use . Press to save your configuration.
The following paragraphs provide step-by-step instructions for a
successful configuration. Each configuration item is marked as to
whether it is relevant to firewalling or masquerading. Use port range
(for example, 500:510
) whenever appropriate. Aspects
related to the DMZ (demilitarized zone) as mentioned in the
configuration file are not covered here. They are applicable only to a
more complex network infrastructure found in larger organizations
(corporate networks), which require extensive configuration and in-depth
knowledge about the subject.
First, use the YaST module System Services (Runlevel) to enable SuSEfirewall2 in
your runlevel (3 or 5 most likely). It sets the symlinks for the
SuSEfirewall2_* scripts in the /etc/init.d/rc?.d/
directories.
FW_DEV_EXT
(firewall, masquerading)
The device linked to the Internet. For a modem connection, enter
ppp0
. For an ISDN link, use
ippp0
. DSL connections use
dsl0
. Specify auto
to use the
interface that corresponds to the default route.
FW_DEV_INT
(firewall, masquerading)
The device linked to the internal, private network (such as
eth0
). Leave this blank if there is no internal
network and the firewall protects only the host on which it runs.
FW_ROUTE
(firewall, masquerading)
If you need the masquerading function, set this to
yes
. Your internal hosts will not be visible to
the outside, because their private network addresses (e.g.,
192.168.x.x
) are ignored by Internet routers.
For a firewall without masquerading, set this to
yes
if you want to allow access to the internal
network. Your internal hosts need to use officially registered IP
addresses in this case. Normally, however, you should
not allow access to your internal network from
the outside.
FW_MASQUERADE
(masquerading)
Set this to yes
if you need the masquerading
function. This provides a virtually direct connection to the Internet
for the internal hosts. It is more secure to have a proxy server
between the hosts of the internal network and the Internet.
Masquerading is not needed for services that a proxy server provides.
FW_MASQ_NETS
(masquerading) Specify the hosts or networks to masquerade, leaving a space between the individual entries. For example:
FW_MASQ_NETS="192.168.0.0/24 192.168.10.1"
FW_PROTECT_FROM_INT
(firewall)
Set this to yes
to protect your firewall host from
attacks originating in your internal network. Services are only
available to the internal network if explicitly enabled. Also see
FW_SERVICES_INT_TCP
and
FW_SERVICES_INT_UDP
.
FW_SERVICES_EXT_TCP
(firewall) Enter the TCP ports that should be made available. Leave this blank for a normal workstation at home that should not offer any services.
FW_SERVICES_EXT_UDP
(firewall) Leave this blank unless you run a UDP service and want to make it available to the outside. The services that use UDP include include DNS servers, IPsec, TFTP, DHCP and others. In that case, enter the UDP ports to use.
FW_SERVICES_ACCEPT_EXT
(firewall)
List services to allow from the Internet. This is a more generic form
of the FW_SERVICES_EXT_TCP
and
FW_SERVICES_EXT_UDP
settings, and more
specific than FW_TRUSTED_NETS
. The notation
is a space-separated list of
,
for example net
,protocol
[,dport
][,sport
]0/0,tcp,22
or
0/0,tcp,22,,hitcount=3,blockseconds=60,recentname=ssh
,
which means: allow at most three ssh connects per minute from the
same IP address.
FW_SERVICES_INT_TCP
(firewall)
With this variable, define the services available for the internal
network. The notation is the same as for
FW_SERVICES_EXT_TCP
, but the settings are
applied to the internal network. The variable
only needs to be set if FW_PROTECT_FROM_INT
is set to yes
.
FW_SERVICES_INT_UDP
(firewall)
See FW_SERVICES_INT_TCP
.
FW_SERVICES_ACCEPT_INT
(firewall)
List services to allow from internal hosts. See
FW_SERVICES_ACCEPT_EXT.
FW_SERVICES_ACCEPT_RELATED_*
(firewall)
This is how the SuSEfirewall2 implementation considers packets
RELATED
by netfilter.
For example, to allow finer grained filtering of Samba broadcast
packets, RELATED
packets are not accepted
unconditionally. Variables starting with
FW_SERVICES_ACCEPT_RELATED_
allow
restricting RELATED
packets handling to certain
networks, protocols and ports.
This means that adding connection tracking modules (conntrack
modules) to FW_LOAD_MODULES
does not
automatically result in accepting the packets tagged by those
modules. Additionally, you must set variables starting with
FW_SERVICES_ACCEPT_RELATED_
to a suitable
value.
FW_CUSTOMRULES
(firewall)
Uncomment this variable to install custom rules. Find examples in
/etc/sysconfig/scripts/SuSEfirewall2-custom
.
After configuring the firewall, test your setup. The
firewall rule sets are created by entering SuSEfirewall2
start as root
.
Then use telnet, for example, from an external host
to see whether the connection is actually denied. After that, review
/var/log/messages
, where you should see something
like this:
Mar 15 13:21:38 linux kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=00:80:c8:94:c3:e7:00:a0:c9:4d:27:56:08:00 SRC=192.168.10.0 DST=192.168.10.1 LEN=60 TOS=0x10 PREC=0x00 TTL=64 ID=15330 DF PROTO=TCP SPT=48091 DPT=23 WINDOW=5840 RES=0x00 SYN URGP=0 OPT (020405B40402080A061AFEBC0000000001030300)
Other packages to test your firewall setup are nmap or nessus. The
documentation of nmap
is found at
/usr/share/doc/packages/nmap
and the documentation
of nessus
resides in the directory
/usr/share/doc/packages/nessus-core
after
installing the respective package.