Design And Implementation Three Multi Tier Firewall

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It is really important to understand the security needs of any financial organization. Firewall plays a very important role in it. Firewalls are deployed to defend the network. They are usually placed on first and second line of defense. By deploying a firewall in a network we can restrict the traffic that is entering in the network and also traversing through different zones. But all these things depend upon the proper design and the placement of firewall in a network.

In Three-tier deployment architecture is the deployment of multiple subnet between the private network and the internet separated by firewall. Each subsequent has more specific filtering rules to restrict the traffic only from the trusted sources.

In this IS I will discuss and propose a design for deploying Firewall on Network Perimeter. Generally in old trends firewalls were deployed in two Tier firewall architecture in which the private network is secure from the public network by defining the two separate interfaces but here I am proposing Firewall architecture in a Multiple Tier architecture manner.

Now a day's Applications are created in form of modules that generally resides on different machines or servers and are structured or you can say housed in different groups so as to secure and maintain segregations.

Like if security is breached on one module it won't harm the other one. In other words if a Server is compromised other may possible be safe.

The outer most subnets are proposed as DMZs. Middle subnets should be served as transaction subnets where the system needs to support complex web applications placed in the DMZs, now the third or back-end subnet would be the private network that is trusted network.

This architecture is most secure but however it is also the most complex to design and implement. Like the Database Server that contains client's account details is more sensitive and require more protection and security than the Web servers that is used for the Front-end.

The Concept of my Independent Study is to protect and secure our private traffic from the public Network. This can be done by creating different subnets and restrict them according to the needs. For creating different subnets we require different interfaces physically or virtually on the firewall device. If you use physical interfaces for the devices it limits to the number of ports available on the devices. As general we usually don't have that much physical interface available on the device as we require so I would propose to create virtual interfaces on the firewall. Now these interfaces are assigned in different ZONES termed as DMZs. This limitation can be overcome by creating different Virtual interfaces on that device and assigned them in appropriate zones.

So that as more number of VLANs are created more security can be achieved by assigning different Servers in different VLANs. I am proposing this design to a Financial Institute and it contains Zones, VLANs, and Access Control Policies.

defining firewall:

The purpose of firewall is to monitor, examine and control the network traffic to protect the Network devices and system that are critical for any financial organization. Firewall first lookup the policies for the traffic passing through it and drops the packets that don't meet the policy statements.

Firewall provides filtering of unwanted/ non legitimate traffic from the outside world as well as from the inside network also.

Firewalls are designed to block illegal unauthorized access and it only allows the traffic that is permitted in the policy defined.

Transmission of each packet is checked first, firewall contains some rules/ policies in it and each rule has some action against it either permit or deny.

Firewalls are available in both hardware and software form. The basic purpose of firewall is to protect our private network from internet and unauthorized access and to protect our private network.

Two Tier Three Tier Or multiple tier

The idea of providing this tier base architecture is to secure multi-tier application environment. There is no specific definition of two-tier or three-tier firewall. They came from different ideas like the term tier refers to the number of interfaces available on the firewall.

A two-tier firewall contains two interfaces each assigned to a different zone like:

Inside/ Private network/ Trusted

Outside/ Untrusted network

A three-tier firewall generally having three zones like:

Inside/ Private network/ Trusted

Outside/ Untrusted network

A DMZ (Demilitarized zone)

Use the DMZ zone to host the servers that needs to be accessed from the outside world. It plays a vital role for any organization in which a lot of business services depends on the internet. Like e-commerce based services and also a lot of Banks are giving Internet banking facilities to their customer these days and by implementing such kind of architecture and adopting such recommendations in our network we can improve the availability and security.

Email servers, web servers and DNS servers are some of the servers that needs to be accessed publically from the outside network so they needs some extra security and protection.

Now let's see the other usage of tier based architecture. Here tier does not mean the interfaces a firewall have but the layers of firewall you provide. In such kind of deployment a firewall is needed at each tier. Like one firewall for outside public network, one for the DMZ and one for you private network.


An ISP has to protect a host (web server) using a firewall to restrict all the connection except those required ones. So as it is a web server it only requires TCP port 80 for http session and TCP port 443 for https sessions.

So after that inner communication starts with based on inner firewall rules.

Multi tier applications over view

Now a days applications are designed in multiple logical tiers, software engineers has segregated the major functional areas into logical groupings that can be design, implemented and run independently of each other. Like if we take an example of a web-based application following tiers may possibly present there.




4.1 Presentation

This tier directly interacts with the users that are coming from the internet. This tier is closest to internet. Such kind of publically accessed services are generally implemented using web, DNS and email servers.

The purpose of these servers is to present the application in front of user. This tier handles the interaction between users coming from public network and back-end components.

4.2 Middleware

In this tier such components are placed that performs business logic of the application in response to the queries requested by the servers hosted in presentation layer on behalf of internet users.

4.3 Data

In Data Tier core servers such as database servers, directory servers that contain confidential database are placed.

This tier contains most confidential data of bank like account information of users and customer record.

Following diagram illustrate of workflow of a web-based multi-tier application.

Users from the internet generate a request to web server via web browser.

The request is then processed by web server and being sent to middleware tire.

Then the middleware component interacts with the database servers for the requested query. After processing the query the request is being responded to the web server then the web server relays the result to the internet user directly.

By using this methodology there is no such direct communication between the public user and the core database servers.

By deploying firewalls in multi-tier architecture we can segregate our network in segments based upon their criticality, confidentiality, sensitivity, exposure, and the possibility of that they can be compromised.

Explaing firewall deployment using single subnet

After segregating the segments into groups it help us to analyze the risk and exposure of the devices over public network that how we restrict the direct interaction of critical servers from the internet users. The acceptable amount of risk on each of the server vary from case to case so there are reasons behind to create different kind of zones and VLANs and put these servers in the relevant zones and VLANs and which security level is needed by each server.

An example of Internet banking application that works on different servers. Different types of servers are playing different roles in the overall workflow of this application. The server that is playing the role of FRONT-end server doesn't require such strict level of security policy as compare to the server on which customer account information exist(Core Database server).

But in single subnet methodology all the servers are place behind the firewall and same security level is provided to each server either web server or bank's database server. They all will be equally protected from the threats both from internet users and from the locally compromised server.

Explaing single firewall deployment with multiple subnet:

Deploying firewall in such manner that using physical and virtual interfaces of the firewall to create different subnets. Segregate the network into particular logical tiers create different subnet and inside each subnet each tier will provide more strict level of security than using single subnet. In this type of deployment the outer most tier (presentation tier) only interacts with middle one (middleware Tier) and middleware tier only interacts with inner most tier (data tier) only.

Proposing Solution to a Financial Organization


7.2 Explanation of the design

This is the proposed logical design containing internet facing routers that are serving as perimeter routers and acting as first line of defense. Both routers are working in High availability mode.

After that we have two firewalls performing second line of defense to the Servers that contains all the Zones and VLANs on it. Rules will be created here. Application flow control will be handling at this level.

Both of the Firewalls are working in a high availability mode providing backup to each other. In case of physical interface or logical interface failure or the whole device failure network will be run smoothly.

The inter connectivity between the firewall shown in the diagram is used for control signal and data switchover in case of failure.

These Firewalls are then connected to Layer two switches using gigabit interfaces. Servers will be terminating on the same switches.

Layer two trunks will be created between the switches as well in order to cater the case of device or interface failure.

Spanning tree would be configured on the switches in order to avoid loop between the switches and provide contingency.

7.3 Inventory Chart

Device Recommended


Brief description

Firewall Recommended

SRX 650

Secure Router provides Firewalling upto 7.0 Gbps

Router Recommended

Cisco 2821

This router will be used for the link termination.

Switch Recommended

Cisco 2960

This switch will be used for establishing trunk between SRX firewall and SWITCH and also it will be served as Server farm switch.

7.4Detail diagram:

This would be the proposed solution for the financial organization that is going to be created on same firewall.

The basic theme is to create different zones according to the relevant security levels.

Here in the above diagram you can see four different zones named as

Internet Access Zone

Public Access Zone

Trusted Sever Zone

Business Access Zone

Internet Access Zone

The router on which internet link is terminating should be assigned in this zone. Strict rulebase /policies would be implemented. Only one way traffic from inside zone to outside zones would be permitted.

Public Access Zone

The VLANs that need to be accessed from the internet by any mean would be assigned in this Zone. Different VLANs are created in this zone to block the traffic within the same zone.

Like Internet Backing Front end server, and Email servers.

Trusted Sever Zone

VLANs created for core Business Application and other critical financial Application are assigned in this zone. These servers are critical servers and very strict policies would be implemented for these servers. Only legitimate traffic would be permitted. Between the zones and within the zones between the VLANs. Following are some example of VLANs that would be created in this zone.

Core Business Application VLAN, Internet banking DB VLAN, ATM PHEONIX VLAN, CTL VLAN

Business Access Zone

These are the extranets or you can say external connectivity between the Bank and the other corporate entities. Like NADRA, UFONE

This zone is used to host the servers for the following VLANs like i.e. NADRA, SWIFT VLAN, UFONE VLAN, SMS VLAN, 1-Link VLAN, CENTRAL Banking servers.

Explaining Traffic Flow between different zones / within the zones between the VLANs

In the above diagram a case is being discussed. As shown in the diagram there are four zones mentioned.

Named as:

Internet Access Zone

Public Access Zone

Transition Server zone

Trusted Zone

Internet banking application is design to work in multi tier architecture. Bank clients that are coming from the internet will first hit the front-end servers which are publically available, that's why these servers are placed in Public Access Zone.

Then restricted policies are implemented between Public Access Zone and Transition Server Zone. Only these servers can send request for communication to Transition Server zone's VLANs.

Then only these servers will communicate with the Trusted Zone's VLANs.

Only these transition application servers will communicate with Bank's Core Database Servers.

This model is beneficial for the bank so as to secure Bank's critical servers. There is no direct communication between outside network like internet users and core business servers.

Implementation of the Design:

To implement the design a lot of configuration needs to be done on the different devices. Following are some of the configuration templates that are required to implement the design.

Configuration Template for creating ZONES, VLANs and Access control Policies:

10.1 Creating Zones:

Following commands will create zones on the SRX firewall.

set security zones security-zone <Name-Of-The-Zone>

set security zones security-zone Internet Access Zone

set security zones security-zone Public Access Zone

set security zones security-zone Transition Server zone

set security zones security-zone Trusted Zone

set security zones security-zone Business Access Zone

10.2 Creating virtual lans:

Following commands will create VLANs on the SRX firewall.

set interfaces <interface> vlan-tagging

set interfaces <interface> unit <tag-number> description "<description-String>"

set interfaces <interface> unit <tag-number> vlan-id <tag-number>

set interfaces ge-0/0/0 vlan-tagging

set interfaces ge-0/0/0 unit 10 description "ATM-VLAN "

set interfaces ge-0/0/0 unit 10 vlan-id 10

10.3 Creating access control policies:

Following commands will create ACL on the SRX firewall.

set security policies from-zone <ZONE-NAME> to-zone <ZONE-NAME> policy <Policy-Name> match source-address <Address>

set security policies from-zone <ZONE-NAME> to-zone <ZONE-NAME> policy <Policy-Name> match destination-address <Address>

set security policies from-zone <ZONE-NAME> to-zone <ZONE-NAME> policy <Policy-Name> match application any

set security policies from-zone <ZONE-NAME> to-zone <ZONE-NAME> policy <Policy-Name> then permit

set security policies from-zone INTERNET-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 match source-address x.x.x.x/x

set security policies from-zone INTERNET-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 match destination-address x.x.x.x/x

set security policies from-zone INTERNET-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 match application any

set security policies from-zone INTERNET-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 then permit

set security policies from-zone PUBLIC-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 match source-address x.x.x.x/x

set security policies from-zone PUBLIC-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 match destination-address x.x.x.x/x

set security policies from-zone PUBLIC-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 match application any

set security policies from-zone PUBLIC-ACCESS-ZONE to-zone PUBLIC-ACCESS-ZONE policy Policy-1 then permit


For any financial organization Security is an indispensable concern. Core Business servers needs to protected not just from the Outside public world but also from the Inside entities. For this a proper Network design should be implemented in which the placement and role of the firewall is very important. The Solution proposed in this independent study is how the applications that are working in multiple tiers can be secured properly and by segregating each type of application in separate zone you can restrict the non legitimate traffic from the other zone and also within the zone by creating different types of VLANs that restrict the intra zone unwanted traffic. By using this methodology traffic flow can be control much more tightly without the need of creating as number of zones as equal to number of VLANs. This tightly controlled traffic flow will restrict the interaction between each tier. In short this methodology will restrict inter-zone traffic and inter-zone traffic as well. Any traffic like intra-zone or inter-zone should be first lookup in the access control policy if it exists then communication will occur else the packets would be just dropped. The Caveat of using this methodology can be bottleneck occurrence due to traffic load between the zones and within the zones, every traffic should be passed first through firewall but to overcome this issue deploy the firewall and switch in a manner using gigabit interface trunks between them and also calculate the inter-zone and intra-zone traffic by traffic analyzers and if needed built bundles between Firewall and Switches. And moving in such manner will help us to protect our network and not to compromise on security. Lastly I would say that this Independent Study provides recommendations and secure model for Multi-Teir environments.

Key words/ acronyms:

ACL- Access Control List

VLAN- Virtual Local Area Network

WAN -Wide Area Network

DMZ - Demilitarized Zone