Page 137 - DCAP309_INFORMATION_SECURITY_AND_PRIVACY

Unit 9: Firewalls

A firewall proxy server fundamentally turns a two-party session into a four-party session, with the Notes
middle procedure emulating the two real hosts. A proxy service must be run for every type of
Internet application the firewall will support — a Simple Mail Transport Protocol (SMTP) proxy
for e-mail, an HTTP proxy for Web services and so on.

Did u know? Since firewall proxy server function at the application layer, proxy servers are
also known as application layer firewalls.

Notes Proxy servers are approximately always one-way preparations running from the
internal network to the external network.

Self Assessment

Fill in the blanks:
6. Firewall proxy servers function at the ........................ layer of the firewall, where both ends
of an association are forced to carry out the session through the proxy.
7. A firewall proxy server fundamentally turns a two-party session into a ........................ session,
with the middle procedure emulating the two real hosts.

9.4 Packet Filtering

In a packet filtering firewall, the firewall inspects five packet traits:
1. Source IP address
2. Source port
3. Destination IP address
4. Destination port
5. IP protocol (TCP or UDP)
Depending upon rules configured into the firewall, the packet will either be permitted through,
rejected, or dropped. If the firewall discards the packet, it sends a message back to the sender
allowing him/her recognize that the packet was discarded. If the packet was dropped, the
firewall just does not respond to the packet. The sender must linger for the communication to
time out. Dropping packets rather than rejecting them greatly enlarges the time needed to scan
the network. Packet filtering firewalls function on Layer 3 of the OSI model, the Network Layer.
Routers are a very common form of packet filtering firewall.
An enhanced structure of the packet filtering firewall is a packet filtering firewall with a state
oriented examination engine. With this augmentation, the firewall “remembers” conversations
among systems and networks. It is then essential to fully scrutinize only the conversation’s first
packet.

Self Assessment

Fill in the blanks:
8. In packet filtering, depending upon rules configured into the firewall, the ........................
will either be permitted through, rejected, or dropped.

9. ........................ firewalls function on Layer 3 of the OSI model, the Network Layer.

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