D-Link NetDefend DFL-260E User Manual Download

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Action

Src Iface

Src Net

Dest Iface

Dest Net

Parameters

NAT

lan

lannet

wan

all-nets

All

Now, what is wrong with this rule set?

If we assume that we want to implement address translation for reasons of security as well as functionality, we
discover that this rule set makes our internal addresses visible to machines in the DMZ. When internal machines
connect to wan_ip port 80, they will be allowed to proceed by rule 2 as it matches that communication. From an
internal perspective, all machines in the DMZ should be regarded as any other Internet-connected servers; we do
not trust them, which is the reason for locating them in a DMZ in the first place.

There are two possible solutions:

Rule 2 can be changed so that it only applies to external traffic.

Rules 2 and 3 can be swapped so that the NAT rule is carried out for internal traffic before the Allow rule
matches.

Which of these two options is the best? For this configuration, it makes no difference. Both solutions work just as
well.

However, suppose that we use another interface, ext2, in the NetDefend Firewall and connect it to another
network, perhaps to that of a neighboring company so that they can communicate much faster with our servers.

If option 1 was selected, the rule set must be adjusted like this:

Action

Src Iface

Src Net

Dest Iface

Dest Net

Parameters

SAT

any

all-nets

core

wan_ip

http SETDEST 10.10.10.5 80

Allow

wan

all-nets

core

wan_ip

http

Allow

ext2

ext2net

core

wan_ip

http

NAT

lan

lannet

any

all-nets

All

This increases the number of rules for each interface allowed to communicate with the web server. However, the
rule ordering is unimportant, which may help avoid errors.

If option 2 was selected, the rule set must be adjusted like this:

Action

Src Iface

Src Net

Dest Iface

Dest Net

Parameters

SAT

any

all-nets

core

wan_ip

http SETDEST 10.10.10.5 80

NAT

lan

lannet

core

wan_ip

All

Allow

any

all-nets

core

wan_ip

http

This means that the number of rules does not need to be increased. This is good as long as all interfaces can be
trusted to communicate with the web server. If, however, at a later point we add an interface that cannot be
trusted to communicate with the web server, separate Drop rules would have to be placed before the rule granting
all machines access to the web server.

Determining the best course of action must be done on a case-by-case basis, taking all circumstances into
account.

Example 7.4. Enabling Traffic to a Web Server on an Internal Network

The example we have decided to use is that of a web server with a private address located on an internal
network. From a security standpoint, this approach is wrong, as web servers are very vulnerable to attack and
should therefore be located in a DMZ. However, due to its simplicity, we have chosen to use this model in our
example.

In order for external users to access the web server, they must be able to contact it using a public address. In this
example, we have chosen to translate port 80 on the NetDefend Firewall's external address to port 80 on the web
server:

Action

Src Iface

Src Net

Dest Iface

Dest Net

Parameters

SAT

any

all-nets

core

wan_ip

http SETDEST wwwsrv 80