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Firewall

A firewall is a device designed to control the flow of traffic into and out-of a network. In general, firewalls are installed to prevent attacks. It disrupts free communication between a trusted and un-trusted network in order to mange the information flow and restrict unwarranted dangerous access. 

Hardware firewall is a physical device between internet and your computer while Software firewall is installed as a program on computer.

Types of Firewalls

Packet Filtering Firewall
It applies a set of rules to each incoming packet and then forwards/discards (two default policies) the packet. It filters packets going in both directions. The packet filter is typically set up as a list of rules based on matches to fields in IP or TCP header. Some of the attacks that can be made on packet filtering routers are: IP address spoofing, Source Routing attacks, Tiny Fragment attacks.

Advantages: Simplicity, transparency to users and high speed.

Disadvantages: Difficult to set up, lack of authentication.

Application Level Gateway Firewall
In this, when a client establishes connection with the destination service, it connects to an application gateway. It is also called a proxy server. The proxy then establishes the connection with destination behind the firewall and acts on behalf of client, making all packet forwarding decisions. It acts as a relay of application level traffic. Also, it needs separate proxies for each service- SMTP, DNS and other custom services.

Advantages: Higher security than packet filters, Easy to log and audit all incoming traffic.

Disadvantages: Requires great memory and processor resources.

Circuit Level Gateway Firewall
This firewall works at session layer of OSI model. Traffic is allowed only if a session request is legitimate. It sets up two TCP connections. The gateway typically relays TCP segments from one connection to other without examining the contents and security function determines whether to allow it further or not. A proxy server as explained above is a security barrier between internal and external computers while a circuit level gateway is a virtual circuit between proxy server and internal hosts.

Advantages: Hides private network data, doesn't need a separate proxy server for each application, simple to implement.

Disadvantages: Doesn't filter individual packets, attacker may take advantage after establishing a connection.

Bastion Host

It is a system identified by firewall administrator as a crucial point in network's security. It executes a secure version of OS and is trusted. It requires additional authentication before access is allowed. It is designed specifically to withstand attacks as it is on public side of the network. Firewalls and Routers can be considered as Bastion Hosts.