Masquerading and a firewall ensure a controlled data flow and data exchange. SSH (secure shell) enables you to log in to remote hosts over an encrypted connection. The encryption of files or entire partitions protects your data in the event that third parties gain access to your system. Along with technical instructions, find information about security aspects of Linux networks.
Whenever Linux is used in a networked environment, you can use the kernel functions that allow the manipulation of network packets to maintain a separation between internal and external network areas. The Linux netfilter framework provides the means to establish an effective firewall that keeps different networks apart. With the help of iptables—a generic table structure for the definition of rule sets—precisely control the packets allowed to pass a network interface. Such a packet filter can be set up quite easily with the help of SuSEfirewall2 and the corresponding YaST module.
The components netfilter and iptables are responsible for the filtering and manipulation of network packets as well as for network address translation (NAT). The filtering criteria and any actions associated with them are stored in chains, which must be matched one after another by individual network packets as they arrive. The chains to match are stored in tables. The iptables command allows you to alter these tables and rule sets.
The Linux kernel maintains three tables, each for a particular category of functions of the packet filter:
This table holds the bulk of the filter rules, because it implements the
packet filtering mechanism in the stricter sense,
which determines whether packets are let through
ACCEPT) or discarded (
This table defines any changes to the source and target addresses of packets. Using these functions also allows you to implement masquerading, which is a special case of NAT used to link a private network with the Internet.
The rules held in this table make it possible to manipulate values stored in IP headers (such as the type of service).
These tables contain several predefined chains to match packets:
This chain is applied to incoming packets.
This chain is applied to packets destined for the system's internal processes.
This chain is applied to packets that are only routed through the system.
This chain is applied to packets originating from the system itself.
This chain is applied to all outgoing packets.
Figure 4.1, “iptables: A Packet's Possible Paths” illustrates the paths along which a network packet may travel on a given system. For the sake of simplicity, the figure lists tables as parts of chains, but in reality these chains are held within the tables themselves.
In the simplest of all possible cases, an incoming packet destined for the
system itself arrives at the
eth0 interface. The packet
is first referred to the
PREROUTING chain of the
mangle table then to the
PREROUTING chain of the
nat table. The
following step, concerning the routing of the packet, determines that
the actual target of the packet is a process of the system itself. After
INPUT chains of the
filter table, the packet finally reaches its
target, provided that the rules of the
filter table are
Masquerading is the Linux-specific form of NAT (network address
translation). It can be used to connect a small LAN (where hosts use IP
addresses from the private range—see Section 18.1.2, “Netmasks and Routing”) with the
Internet (where official IP addresses are used). For the LAN hosts to be
able to connect to the Internet, their private addresses are translated to
an official one. This is done on the router, which acts as the gateway
between the LAN and the Internet. The underlying principle is a simple one:
The router has more than one network interface, typically a network card and
a separate interface connecting with the Internet. While the latter links
the router with the outside world, one or several others link it with the
LAN hosts. With these hosts in the local network connected to the network
card (such as
eth0) of the router, they can send any
packets not destined for the local network to their default gateway or
|Using the Correct Network Mask|
When configuring your network, make sure both the broadcast address and the netmask are the same for all local hosts. Failing to do so prevents packets from being routed properly.
As mentioned, whenever one of the LAN hosts sends a packet destined for an
Internet address, it goes to the default router. However, the
router must be configured before it can forward such packets. For
reasons, SUSE Linux does not enable this in a default installation.
To enable it, set the variable
IP_FORWARD in the
The target host of the connection can see your router, but knows nothing about the host in your internal network where the packets originated. This is why the technique is called masquerading. Because of the address translation, the router is the first destination of any reply packets. The router must identify these incoming packets and translate their target addresses, so packets can be forwarded to the correct host in the local network.
With the routing of inbound traffic depending on the masquerading table, there is no way to open a connection to an internal host from the outside. For such a connection, there would be no entry in the table. In addition, any connection already established has a status entry assigned to it in the table, so the entry cannot be used by another connection.
As a consequence of all this, you might experience some problems with a number of application protocols, such as ICQ, cucme, IRC (DCC, CTCP), and FTP (in PORT mode). Netscape, the standard FTP program, and many others use the PASV mode. This passive mode is much less problematic as far as packet filtering and masquerading are concerned.
Firewall is probably the term most widely used to describe a mechanism that provides and manages a link between networks while also controlling the data flow between them. Strictly speaking, the mechanism described in this section is called a packet filter. A packet filter regulates the data flow according to certain criteria, such as protocols, ports, and IP addresses. This allows you to block packets that, according to their addresses, are not supposed to reach your network. To allow public access to your Web server, for example, explicitly open the corresponding port. However, a packet filter does not scan the contents of packets with legitimate addresses, such as those directed to your Web server. For example, if incoming packets were intended to compromise a CGI program on your Web server, the packet filter would still let them through.
A more effective but more complex mechanism is the combination of several
types of systems, such as a packet filter interacting with an application
gateway or proxy. In this case, the packet filter rejects any packets
destined for disabled ports. Only packets directed to the application gateway
are accepted. This gateway or proxy pretends to be the actual client of the
server. In a sense, such a proxy could be considered a masquerading host on
the protocol level used by the application. One example for such a proxy is
Squid, an HTTP proxy server. To use
Squid, the browser must be configured to
communicate via the proxy. Any HTTP pages requested are served from the
proxy cache and pages not found in the cache are fetched from the Internet by
the proxy. As another example, the SUSE
proxy-suite) provides a proxy for the FTP
The following section focuses on the packet filter that comes with
SUSE Linux. For further information about packet filtering and firewalling,
read the Firewall HOWTO included in the
package. If this package is installed, read the HOWTO with
SuSEfirewall2 is a script that reads the
variables set in
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
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 Section 18.1.2, “Netmasks and Routing”), 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 126.96.36.199, “Configuring 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
|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 optionsor 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. Activate the services after selecting the desired zone in.
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 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.
When completed 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 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. 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
The device linked
to the Internet. For a modem connection, enter
ppp0. For an ISDN link, use
DSL connections use
auto to use the interface that
corresponds to the default route.
The device linked
to the internal, private network (such as
Leave this blank if there is no internal network and the firewall
protects only the host on which it runs.
If you need the
masquerading function, set this to
Your internal hosts will not be visible to the outside, because their
private network addresses (e.g.,
are ignored by Internet routers.
For a firewall without masquerading, only set this to
yes if you want to allow access to the internal
network. Your internal hosts need to use officially registered IPs in
this case. Normally, however, you should not allow
access to your internal network from the outside.
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 a proxy server provides.
Specify the hosts or networks to masquerade, leaving a space between the individual entries. For example:
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
Enter the TCP ports that should be made available. Leave this blank for a normal workstation at home that should not offer any services.
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.
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
FW_PROTECT_FROM_INT is set to
After configuring the firewall, test your setup.
The firewall rule sets are created by entering SuSEfirewall2
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
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
installing the respective package.
The most up-to-date information and other documentation about the
SuSEfirewall2 package is found in
/usr/share/doc/packages/SuSEfirewall2. The home page of
the netfilter and
iptables project, http://www.netfilter.org, provides a large collection of documents
in many languages.