B2B Integration : A Practical Guide to Collaborative E-commerce

Internet Security 305
the IP address of its source and destination. Packet filters check this
address information of each and every packet, to determine whether it
is acceptable or not. The administrators set up the firewall rules on the
basis of whether it filters the incoming traffic and allows/disallows
connections.
Routers, which direct information packets, can provide some firewall
support because their function is to forward and filter data packets.
Some sophisticated routers provide additional facilities such as encryption
features.
These types of firewalls are usually the least expensive. However,
they do not offer much flexibility and also slow down the performance
of the network considerably.
Application level firewalls
Application level firewalls work by handling the traffic destined for a
specific application such as e-mail or FTP, rather than all network
traffic (see Figure 10.14).
Application level firewalls are like a relay system between the
Internet and the internal network; they do not allow direct traffic to
pass between the two networks. In these, a highly isolated machine,
called an application gateway, proxy gateway, or proxy server is used.
This machine runs programs called proxies; a separate proxy is run for
each Internet service, such as HTTP and FTP.
Since all interaction of the external users is with the proxies, this
allows all passwords and internal IP addresses to remain internal,
making them more difficult to detect and misuse.
External
Network
Network
Laval
Figure 10.14. — Application level firewall schematic