Proxy firewall filters operate at the Application layer.

The Application Layer:
The Application layer is the topmost layer in the OSI model and is responsible for providing network services to user applications. It interacts directly with the application software and enables communication between different applications. This layer includes protocols such as HTTP, FTP, SMTP, etc.

Proxy Firewall:
A proxy firewall is a network security device that acts as an intermediary between an internal network and the external network (usually the internet). It receives requests from clients inside the network and forwards them to the destination on behalf of the client. It also receives responses from the destination and sends them back to the client.

Filters at the Application Layer:
Proxy firewalls can implement various filtering techniques to control and manage network traffic. These filters are applied at different layers of the OSI model. In the case of proxy firewalls, the filters are typically implemented at the Application layer.

Reasons for Application Layer Filtering:
1. Granular Control: Filtering at the Application layer allows for more granular control over network traffic. It enables the firewall to inspect the content of application-layer protocols and make decisions based on specific application-level data.

2. Protocol-specific Filtering: Since different application-layer protocols have their own unique characteristics and vulnerabilities, filtering at the Application layer allows for protocol-specific filtering. For example, a proxy firewall can inspect HTTP requests and responses, filtering out malicious or unauthorized content.

3. Content Filtering: Application layer filtering enables content filtering, which involves analyzing the content of the data packets. This allows the firewall to block or allow specific types of content based on predefined rules or policies.

4. User Authentication: Application layer filtering can also be used for user authentication purposes. The firewall can require users to authenticate themselves before granting access to certain applications or services.

Conclusion:
Proxy firewall filters operate at the Application layer of the OSI model. This allows for more granular control, protocol-specific filtering, content filtering, and user authentication. By filtering traffic at this layer, proxy firewalls provide an additional layer of security for the network.