• Animesh Gupta

What is a Firewall?

Updated: Aug 1, 2018


A firewall is a hardware or software system that prevents unauthorized access to or from a network. It can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet. All data entering or leaving the intranet pass through the firewall, which examines each packet and blocks those that do not meet the specified security criteria.


Generally, firewalls are configured to protect against unauthenticated interactive logins from the outside world. This helps prevent hackers from logging into machines on your network. More sophisticated firewalls block traffic from the outside to the inside, but permit users on the inside to communicate a little more freely with the outside.


Firewalls are essential since they provide a single block point, where security and auditing can be imposed. Firewalls provide an important logging and auditing function; often, they provide summaries to the administrator about what type/volume of traffic has been processed through it. This is an important benefit: Providing this block point can serve the same purpose on your network as an armed guard does for your physical premises.


Different Types of Firewalls


There are several types of firewalls that work on different layers of the OSI model. Depending on the kind of service and security you need for your network, you need to choose the right type of firewall. The following are the list of seven different types  firewalls that are widely used for network security.

  • Screened host firewalls

  • Screened subnet firewalls

  • Packet filter firewalls

  • Stateful inspection firewalls

  • Hybrid firewalls

  • Proxy server firewalls

  • Application level Gateway Firewalls

  • Circuit Level Gateway Firewalls

Screened host firewalls

There are two types of screened host-one is single homed bastion host and the other one is dual homed bastion host. In case of single homed bastion host the firewall system consists of a packet filtering router and a bastion host. A bastion host is basically a single computer with high security configuration, which has the following characteristics:

  • Traffic from the Internet can only reach the bastion host; they cannot reach the internal network.

  • Traffic having the IP address of the bastion host can only go to the Internet. No traffic from the internal network can go to the Internet.


This type of configuration can have a web server placed in between the router and the bastion host in order to allow the public to access the server from the Internet. The main problem with the single homed bastion host is that if the packet filter route gets compromised then the entire network will be compromised. To eliminate this drawback we can use the dual homed bastion host firewall system, where a bastion host has two network cards- one is used for internal connection and the second one is  used for connection with the router. In this case, even if, the router got compromised, the internal network will remain unaffected since it is in the separate network zone.


Screened Subnet Firewalls


This is one of the most secured firewall configurations. In this configuration, two packet filtering routers are used and the bastion host is positioned in between the two routers. In a typical case, both the Internet and the internal users have access to the screened subnet, but the traffic flow between the two subnets (one is from bastion host to the internal network and the other is the sub-network between the two routers) is blocked.



Packet filtering firewalls


This type of firewall is the most common and easy to deploy in a small-sized network. A router functions as a firewall by examining every packet passing through the network. Based on access control list, the router either forward or drop packets. Normally, the IP address of the source and destination, port number and type of traffic are taken into account when the router processes each data packet. Since a router cannot check packet in the application layer, this type of firewall cannot defend attacks that use application layers vulnerabilities. They also fail to fight against spoofing attacks. You can use this configuration if you need higher network speed and do need limited login and authentication capacity.

Stateful inspection


Stateful inspection firewall works at the network layer in the OSI model. It monitors both the header and contents of the traffic.  The main difference between the packet filtering and the stateful inspection is that it the later one analyzes not only the packet headers but also inspects the state of the packets along with providing proxy services. Stateful inspection firewalls maintain a state table and a set of instructions to inspect each packet and store the information based on the type of traffic. It also monitors each TCP connection and remembers which ports are being used by that connection. If there is any port not required by the connection, then that port get closed.

Proxy server firewalls


Proxy allows users to run specific service (FTP, TELNET, HTTP etc.) or  type of connection by enforcing authentication, filtering and logging.  For specific service there will be a specific proxy. For example, if you want to allow only HTTP connection to the Internet for your internal network users, then you must allow only HTTP proxy, nothing else. Users who need to go to Internet create a virtual circuit with the proxy server and the proxy server sends the request to connect to a specific site on behalf of that particular user. Proxy server changes the IP of the request so as the Internet or the outside world can see only the IP of the proxy server. Thus proxy server hides the internal network behind it. When a proxy receives the data from the Internet it sends the data back to its intended internal user via the virtual circuit. The main advantage of using proxy is that it is fully aware of the type of data it handles and can give protection to it. One disadvantage of proxy is that if there is an update  of protocol that is used by the Internet, then the proxy software also needs to be updated to allow a specific service related to that protocol.


Circuit Level Gateways


A circuit-level gateway is a firewall that provides User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) connection security, and works between an Open Systems Interconnection (OSI) network model’s transport and application layers such as the session layer. Unlike application gateways, circuit-level gateways monitor TCP data packet handshaking and session fulfillment of firewall rules and policies.


A proxy server is a security barrier between internal and external computers, while a circuit-level gateway is a virtual circuit between the proxy server and internal client.

For example, when a user Web page access request passes through the circuit gateway, basic internal user information, such as IP address, is exchanged for proper feedback. Then, the proxy server forwards the request to the Web server. Upon receiving the request, the external server sees the proxy server’s IP address but does not receive any internal user information. The Web or real server sends the proxy server a proper response, which is forwarded to the client or end user via the circuit-level gateway.


Application Level Gateways


An application gateway or application level gateway (ALG) is a firewall proxy which provides network security. It filters incoming node traffic to certain specifications which mean that only transmitted network application data is filtered. Such network applications include File Transfer Protocol (FTP), Telnet, Real Time Streaming Protocol (RTSP) and BitTorrent.

Application gateways provide high-level secure network system communication. For example, when a client requests access to server resources such as files, Web pages and databases, the client first connects with the proxy server, which then establishes a connection with the main server.

The application gateway resides on the client and server firewall. The proxy server hides Internet Protocol (IP) addresses and other secure information on the client’s behalf. A computer’s internal system may communicate with an external computer using firewall protection. The application gateway and external computer function without client information or knowledge of the proxy server IP address.


Hybrid firewalls


They function almost the same way the  stateful inspection type firewalls work, which means they can work both in network and in application level. Normally, in a hybrid system some hosts reside inside the firewall while the others reside outside of the firewall. To communicate with the machine outside the central network IPsec tunnels are used. An example where this type of configuration is suitable is a major site connected with its branch sites via VPN. One distinct feature of this configuration is the firewall administration at the major site distribute the security policy to its branch site so as a uniform security is maintained throughout the organization.

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