A firewall is a device designed to prevent outside individuals from accessing a network or a group of networks.

What is a firewall?

What is a firewall ?

A firewall is a device designed to prevent outside individuals from accessing a network or a group of networks.

It is usually a stand-alone computer, router or proprietary hardware device.

A firewall acts as an entry point to your system and evaluates connection requests as they are received.

It only processes those from authorized hosts, and can delete the others.

Firewall the tasks performed :

Firewalls can scan incoming packets from various protocols and take different actions depending on the scan result.

They are therefore able to carry out conditional evaluations, of the type “if this type of package is encountered, do this”.

These conditional interpretations are called rules.

Generally, when you install a firewall, you implement rules that reflect your company’s access policy.

For example, if you do not want or no longer want the network hosts 206.246.131.xxx,

connect to your system, you can forbid them,

access by rejecting all the packets they can send to you.

For their part, they will be offered a nice message indicating that the connection is refused, or simply their blocked connection.

In this respect, firewalls are for a network, what user permissions are for an operating system.

For example, under Windows NT it is possible to specify which users are allowed to access a given file or directory.

Discretionary access control is implemented at the operating system level.

Similarly, firewalls allow this type of access control to be applied to interconnected workstations and the company’s website.

However, this access filtering is only one of the many possibilities that firewalls offer today.

For some years now, their manufacturer has been following the “kitchen sink” approach in developing functionalities

, i.e. they have included all functionalities in their offers with the exception of the kitchen sink.

Well, if you don’t understand, it’s okay, it’s not the most important thing to remember… You can find it:

Content filtering :

Some companies may prevent their users from visiting certain sites:

Web-based e-mail sites, underground sites, stock exchange gateways, pornographic sites, etc….

This feature allows you to reject certain types of ActiveX code or Java applets.

The Virtual Private Network (VPN):

VPNs allow traffic to be safely channelled from one point to another,

on generally hostile networks (such as the Internet).

Even if there is nowadays a wide variety of VPN devices, manufacturers such as Checkpoint, Cisco integrate VPNs into their firewalls.

Many products offer both business-to-business and LAN VPN capabilities, to LAN ( Local Network Area, i.e., a kind of network.).

VPN and Firewall.

Network Adress Translation (NAT) :

This service is often used to match illegal address blocks, or reserved (see RFC 1918) with valid blocks (for example, with

Although NAT is not necessarily a security feature, the first NAT products that appear in companies are often firewall products.

Load balancing :

The most generic of all, the expression load balancing, is the art of segmenting traffic in a distributed way.

Some firewalls now allow WEB and FTP traffic to be directed in this way.

Fault tolerance :

Some of the most successful firewalls, such as CISC PIX and the Nokia/Checkpoint union, support quite complex functionalities.

Often referred to as HA (High-Availability).

Sophisticated fault tolerance features often make it possible to execute, firewalls in pairs, one of the devices operating as a “hot reserve”,

in case of failure of the other.

Firewall and intrusion detection :

The term “intrusion detection” can have several meanings.

Here, this is a completely new type of product that some manufacturers are starting to develop, integrate into their firewall offer.

Even if this is not a problem in itself, people should get tired of the workload this imposes on their firewall.

Even if it may seem attractive to be able to manage all these features, you have to be sceptical about the approach to the kitchen sink.

Firewalls have always been considered as playing a central role in, enterprise security models.

To borrow from the KISS principle (Keep It Simple, Stupid) so dear to the world, of network administration.

I would say that the overcrowding of features, may not be the best that can be done when it comes to safety products.

But there’s no need to theorize about that… the last rounds of firewall vulnerabilities confirmed some doubts.

This is explained below.

Firewall, they’re not infallible !

Even if firewall manufacturers like to think that their products are safe, problems that plague operating system and application developers,

the fact is that they are in every respect just as vulnerable.

Let’s look at some examples:

May 1998: It was discovered that Firewall-1 contained some keywords that,

when used to represent a network object, opened a gaping security hole.


July 1999: Problems were discovered in “ipchains”, the native Linux firewall code.

Remote attackers could use the feed to,

transmit data to allegedly blocked ports.


June 2000: A denial of service attack using fragmented packets was discovered,

capable of disabling all Firewall-1 firewalls.

And the patch was available too long after that.

This list is only an overview of the problems discovered and still found in the, current firewalls, some of which are directly related to functionality,

outside the product, and later added: content filtering and encapsulation ( for the use of VPN).

It remains to be seen whether firewall manufacturers will treat security with the same, interest only the addition of features.

They claim that most of their customers do not want more security, but more features.

I’m asking you the question:

“What do you think is the most important thing in firewall?

Tell your supplier what you think! ».

If you wish to have more details on: What is a firewall?

We redirect you to a nice reseller from Cisco explaining everything in detail :

Cisco firewall.

Our personal opinion is that a firewall, if he can’t manage security,

is really not useful, and rather cumbersome !

Now you know: What is a firewall !

Check us other articles : Hackcess.