1. PROS & CONS of Proxy Firewall

2. Advantages of Proxy Firewall
Proxy firewalls provide comprehensive, protocol-aware security analysis for the protocols they support. By working at the application layer, they are able to make better security decisions than products that focus purely on packet header information.
The topology of the internal protected network is hidden by proxy firewalls. Internal IP addresses are shielded from the external world because proxy services do not allow direct communications between external servers and internal computers. Although this can also be accomplished using Network Address Translation techniques, it occurs by default with proxy firewalls.

3. Advantages of Proxy Firewall
Network discovery is made substantially more difficult because attackers do not receive packets created directly by their target systems.
Attackers can often develop detailed information about the types of hosts and services located on a network by observing packet header information from the hosts.
How different systems set fields such as the Time to Live (TTL) field, window size, and TCP options can help an attacker determine which operating system is running on a server.

4. Advantages of Proxy Firewall
This technique, known as fingerprinting, is used by an attacker to determine what kinds of exploits to use against the client system.
Proxies can prevent much of this activity because the attacking system does not receive any packets directly created by the server.
Robust, protocol-aware logging is possible in proxy firewalls. This can make it significantly easier to identify the methods of an attack.
It also provides a valuable backup of the logs that exist on the servers being protected by the proxy.

5. Disadvantages Of Proxy Firewall
Proxy firewalls are not compatible with all network protocols. A new proxy agent must be developed for each new application or protocol to pass through the firewall.
A reduction of performance occurs due to the additional processing requests required for application services. The extra overhead implied by setting up two connections for every conversation, combined with the time needed to validate requests at the application layer, adds up to slower performance. In some cases, this can be balanced by choosing higher-end servers to run your proxy. However, for some extremely high-bandwidth networks, a proxy firewall may become a performance bottleneck.

6. Disadvantages Of Proxy Firewall
Virtual Private Networks (VPNs) may not function through a proxy firewall. VPN packet authentication will fail if the IP address of the sender is modified during the transmission. Although this is normally thought of as an issue with Network Address Translation, the same issue occurs with proxy firewalls. Of course, if the VPN endpoint is the firewall, this will not be a problem.
The configuration of proxy firewalls can be more difficult than other firewall technologies. Especially when using older proxies, it can be difficult to properly install and configure the set of proxies necessary for your network.

7. Note
It is also worth noting that the number of proxy firewall products on the market is decreasing.
The commercial firewall industry is moving away from proxy firewalls, due mainly to performance and compatibility concerns. Many of these vendors are dropping their proxy product lines in exchange for stateful products that make use of Deep Packet Inspection techniques.
Deep Packet Inspection allows security tests at the application layer. However, unlike proxies, it allows direct connections to occur between computer systems.
Deep Packet Inspection firewalls tend to be more flexible than proxies and they can be designed to handle very high-speed networks.