Firewall and Types of firewall

FIREWALL
• Almost every medium and large-scale organization has a presence on the Internet and has an organizational network
connected to it. Network partitioning at the boundary between the outside Internet and the internal network is
essential for network security. Sometimes the inside network (intranet) is referred to as the “trusted” side and the
external Internet as the “un-trusted” side
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TYPES OF FIREWALL
• Firewall is a network device that isolates organization’s internal network from larger
outside network/Internet. It can be a hardware, software, or combined system/both that
prevents unauthorized access to or from internal network. All data packets entering or leaving
the internal network pass through the firewall, which examines each packet and blocks those
that do not meet the specified security criteria.

Cont.…
Firewall is categorized into three basic types:
• Packet filter (Stateless & Stateful)
• Application-level gateway
• Circuit-level gateway

Network Layer or Packet Filtering Firewall
The first type of network firewall was the packet filter which would look at network addresses and ports of
the packet to determine if that packet should be allowed or blocked.
In this type of firewall deployment, the internal network is connected to the external network/Internet via a
router firewall. The firewall inspects and filters data packet-by packet.
Packet-filtering firewalls allow or block the packets mostly based on criteria such as source and/or
destination IP addresses, protocol, source and/or destination port numbers, and various other parameters
within the IP header.

Cont.…
• Filtering rules are based on information contained in a network packet:
• Source IP address: The IP address of the system that originated the IP packet (e.g.,192.178.1.1)
• Destination IP address: The IP address of the system the IP packet is trying to reach (e.g.,192.168.1.2)
• Source and destination transport-level address: The transport level {(e.g., Transmission Control
Protocol (TCP) or User Datagram Protocol (UDP)} port number, which defines applications such as
SNMP or TELNET
• IP protocol field: It’s work on a transport protocol which is transfer of data between end system using
the services of the network layer.
• Interface: For a router with three or more ports, which interface of the router the packet came from or
which interface of the router the packet is destined.

Network Layer firewall generally fall into two
sub-categories:
Stateful:
• Stateful firewall can watch traffic streams from
end-to-end. They are aware of communication
paths and can implement various IP security
functions such as tunnels and encryption. If a
packet does not match an exiting connection, it
will be evaluated according to the ruleset for new
connections. If a packet matches an existing
connection based on comparison with firewall’s
state table, it will be allowed to pass without
further processing.
Stateless:
• Stateless firewalls require less memory , and
can be faster for simple filters that require less
time to filter than to look up a session. They
may also be necessary for filtering stateless
network protocols that have no concept of a
session. However, they cannot make more
complex decisions based on what stage
communications between hosts have reached.

Application-level Gateways
• Application-level gateways tend to be more secure than packet
filters. Rather than trying to deal with the numerous possible
combinations that are to be allowed and forbidden at the TCP and
IP level, the application-level gateway need only scrutinize a few
allowable applications. In addition, it is easy to log and audit all
incoming traffic at the application level.

Application-level Gateways
TErminal NETworkFileTransfer Protocol
Simple MailTransfer ProtocolHypertextTransfer Protocol

Circuit-Level Gateway
• Circuit-level gateway work at the session layer of the OSI model, or as a “shim-layer”
between the application layer and the transport layer of the TCP/IP stack.
• 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 outside host. Once the two connections are established,
the gateway typically relays TCP segments from one connection to the other without
examining the contents.
• 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.

Firewall and Types of firewall

Proxies
• A proxy server may act as a firewall by responding to input packets in the manner of an
application, while blocking other packets. A proxy server is a gateway from one network to
another for a specific network application, in the sense that it functions as a proxy on behalf
of the network user.
• Proxies make tempering with an internal system from external network more difficult, so
that misuse of one internal system would not necessarily cause a security reach exploitable
from outside the firewall.Conversely, intruders may hijack a publicly reachable system and
use it as proxy for their own purpose; the proxy then masquerades as that system to other
internal machines.While use of internal address spaces enhances security, crackers may
still employ method such as IP spoofing to attempt to pass packets to a target network.

NAT (Network AddressTranslation)
• Firewall often have network address translation(NAT) functionally, and the host
protected behind a firewall commonly have address in the “private address range”, as
defined in RFC 1918. firewall often have such functionality to hide the true address
of protected hosts. Originally, the NAT function was developed to address the limited
number of IPv4 routable addresses that could be used or assigned to companies or
individuals as well as reduce both the amount and therefore cost of obtaining enough
public addresses for every computer in an organization. Although NAT on its own is
not considered a security feature, hiding the addresses devices has become an often
used defence against network. (ex: VPN)

Firewall and Types of firewall

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