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Chapter: Cryptography and Network Security

Firewalls: design principles, characteristics, Limitations, Types

1. Firewall design principles 2. Firewall characteristics: 3. Limitations of firewall 4 Types of firewalls 5. Advantages of packet filter router 6. Application level gateway 7 Circuit level gateway



1. Firewall design principles


Internet connectivity is no longer an option for most organizations. However, while internet access provides benefits to the organization, it enables the outside world to reach and interact with local network assets. This creates the threat to the organization. While it is possible to equip each workstation and server on the premises network with strong security features, such as intrusion protection, this is not a practical approach. The alternative, increasingly accepted, is the firewall.


The firewall is inserted between the premise network and internet to establish a controlled link and to erect an outer security wall or perimeter. The aim of this perimeter is to protect the premises network from internet based attacks and to provide a single choke point where security and audit can be imposed. The firewall can be a single computer system or a set of two or more systems that cooperate to perform the firewall function.


2. Firewall characteristics:


·        All traffic from inside to outside, and vice versa, must pass through the firewall. This is achieved by physically blocking all access to the local network except via the firewall.

·        Various configurations are possible.  


·        Only authorized traffic, as defined by the local security policy, will be allowed to pass.


·        Various types of firewalls are used, which implement various types of security policies.  



·        The firewall itself is immune to penetration. This implies that use of a trusted system with a secure operating system. This implies that use of a trusted system with a secure operating system.  


Four techniques that firewall use to control access and enforce the site‟s security policy is as follows:


1.     Service control – determines the type of internet services that can be accessed, inbound or outbound. The firewall may filter traffic on this basis of IP address and TCP port number; may provide proxy software that receives and interprets each service request before passing it on; or may host the server software itself, such as web or mail service.  


2.     Direction control – determines the direction in which particular service request may be initiated and allowed to flow through the firewall.



3.     User control – controls access to a service according to which user is attempting to access it.  


4.     Behavior control – controls how particular services are used.

Capabilities of firewall


A firewall defines a single choke point that keeps unauthorized users out of the protected network, prohibits potentially vulnerable services from entering or leaving the network, and provides protection from various kinds of IP spoofing and routing attacks.  


A firewall provides a location for monitoring security related events. Audits and alarms can be implemented on the firewall system.  


A firewall is a convenient platform for several internet functions that are not security related.  


A firewall can serve as the platform for IPsec.  


3. Limitations of firewall


·        The firewall cannot protect against attacks that bypass the firewall. Internal systems may have dial-out capability to connect to an ISP. An internal LAN may support a modem pool that provides dial-in capability for traveling employees and telecommuters.  


·        The firewall does not protect against internal threats. The firewall does not protect against internal threats, such as a disgruntled employee or an employee who unwittingly cooperates with an external attacker.  


·        The firewall cannot protect against the transfer of virus-infected programs or files. Because of the variety of operating systems and applications supported inside the perimeter, it would be impractical and perhaps impossible for the firewall to scan all incoming files, e-mail, and messages for viruses.  


4 Types of firewalls

There are 3 common types of firewalls.


·        Packet filters  


·        Application-level gateways  


·        Circuit-level gateways  


Packet filtering router


A packet filtering router applies a set of rules to each incoming IP packet and then forwards or discards the packet. The router is typically configured to filter packets going in both directions. Filtering rules are based on the information contained in a network packet:


·        Source IP address – IP address of the system that originated the IP packet.   Destination IP address – IP address of the system, the IP is trying to reach.   Source and destination transport level address – transport level port number.   IP protocol field – defines the transport protocol.  


·        Interface – for a router with three or more ports, which interface of the router the packet come from or which interface of the router the packet is destined for.



The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header. If there is a match to one of the rules, that rule is invoked to determine whether to forward or discard the packet. If there is no match to any rule, then a default action is taken.


Two default policies are possible:


· Default = discard: That which is not expressly permitted is prohibited.   

· Default = forward: That which is not expressly prohibited is permitted.  


The default discard policy is the more conservative. Initially everything is blocked, and services must be added on a case-by-case basis. This policy is more visible to users, who are most likely to see the firewall as a hindrance. The default forward policy increases ease of use for end users but provides reduced security.


5. Advantages of packet filter router

·  Simple  


·  Transparent to users  


Very fast  


Weakness of packet filter firewalls


·       Because packet filter firewalls do not examine upper-layer data, they cannot prevent attacks that employ application specific vulnerabilities or functions.  


·         Because of the limited information available to the firewall, the logging functionality present in packet filter firewall is limited.  


·         It does not support advanced user authentication schemes.  


·        They are generally vulnerable to attacks such as layer address spoofing.


Some of the attacks that can be made on packet filtering routers and the appropriate counter measures are the following:


·       IP address spoofing – the intruders transmit packets from the outside with a source IP address field containing an address of an internal host.  

Countermeasure: to discard packet with an inside source address if the packet arrives on an external interface.  


·        Source routing attacks – the source station specifies the route that a packet should take as it crosses the internet; i.e., it will bypass the firewall.

·     Tiny fragment attacks – the intruder create extremely small fragments and force the TCP header information into a separate packet fragment. The attacker hopes that only the first fragment is examined and the remaining fragments are passed through.  

Countermeasure: to discard all packets where the protocol type is TCP and the IP fragment offset is equal to 1.  


6. Application level gateway


An Application level gateway, also called a proxy server, acts as a relay of application level traffic. The user contacts the gateway using a TCP/IP application, such as Telnet or FTP, and the gateway asks the user for the name of the remote host to be accessed. When the user responds and provides a valid user ID and authentication information, the gateway contacts the application on the remote host and relays TCP segments containing the application data between the two endpoints.


Application level gateways tend to be more secure than packet filters. It is easy to log and audit all incoming traffic at the application level. A prime disadvantage is the additional processing overhead on each connection.


7 Circuit level gateway


Circuit level gateway can be a stand-alone system or it can be a specified function performed by an application level gateway for certain applications. A Circuit level gateway does not permit an end-to-end TCP connection; rather, the gateway sets up two TCP connections, one between itself and a TCP user on an inner host and one between itself and a TCP user on an outer host. Once the two connections are established, the gateway typically relays TCP segments from one connection to the other without examining the contents. The security function consists of determining which connections will be allowed.


A typical use of Circuit level gateways is a situation in which the system administrator trusts the internal users. The gateway can be configured to support application level or proxy service on inbound connections and circuit level functions for outbound connections.


Basiton host


It is a system identified by the firewall administrator as a critical strong point in the network‟s security. The Bastion host serves as a platform for an application level and circuit level gateway.


Common characteristics of a Basiton host are as follows:


·        The Bastion host hardware platform executes a secure version of its operating system, making it a trusted system.

·        Only the services that the network administrator considers essential are installed on the Bastion host.


·        It may require additional authentication before a user is allowed access to the proxy services.


·        Each proxy is configured to support only a subset of standard application‟s command set.

·        Each proxy is configured to allow access only to specific host systems.  


·        Each proxy maintains detailed audit information by logging all traffic, each connection

·         and the duration of each connection.  


·        Each proxy is independent of other proxies on the Bastion host.  


·        A proxy generally performs no disk access other than to read its initial configuration file.   


·        Each proxy runs on a non privileged user in a private and secured directory on the Bastion host.



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