Firewall Design for File Uploads to Internal Network

To allow Internet-based users to upload files to a protected internal network server while maintaining security, I would design a firewall setup involving a Demilitarized Zone (DMZ). 1. Network Segmentation with DMZ: A DMZ creates a buffer zone between the external (Internet) and internal networks. The file upload server would reside within this DMZ, not directly on the internal network. This isolates the server from the more sensitive internal systems, limiting the impact of a potential compromise. Resources such as Kizza's Guide to Computer Network Security and Adams' paper on FTP server security in the DMZ discuss this architecture. 2. Two-Firewall Architecture: Two firewalls or distinct firewall rule sets would be used:

• External Firewall (Internet to DMZ): This firewall sits between the Internet and the DMZ. Its rules would allow only specific inbound connections for the file transfer protocol (e.g., SFTP over TCP port 22, FTPS over TCP ports 990/21 and dynamic data ports, or a secure web-based upload interface over TCP port 443) destined for the designated file upload server in the DMZ. All other inbound traffic would be denied.
• Internal Firewall (DMZ to Internal Network): This firewall sits between the DMZ and the internal network. Its rules would be highly restrictive. It would primarily block all unsolicited connections initiated from the DMZ to the internal network. If files need to move from the DMZ upload server to an internal network server, the transfer should ideally be initiated from the internal network (a 'pull' mechanism) or through a secure, application-level gateway that performs deep packet inspection and content scanning before allowing data to proceed.

3. Protocol Selection: For file uploads, I would prioritize secure protocols such as SFTP (SSH File Transfer Protocol) or FTPS (FTP Secure) over plain FTP. These protocols encrypt both control and data channels, mitigating risks like credential sniffing and data tampering during transit. Plain FTP is problematic due to its use of multiple dynamic ports for data channels, which complicates stateful firewall configurations. Semeria's technology overview on Internet firewalls highlights aspects of protocol handling. 4. Content Scanning and Security Services: Beyond basic packet filtering, the design would incorporate security services:

• Application-Level Gateway/Proxy: For deeper inspection of file contents, an application-level gateway could be used. This acts as a proxy, terminating the external connection and creating a new one to the internal server (or the DMZ server), allowing for thorough content inspection.
• Malware Scanning and Intrusion Prevention Systems (IPS): Systems for anti-malware scanning and intrusion prevention would be deployed, preferably within the DMZ or immediately behind the internal firewall. These systems would scan all uploaded files for malicious code before they are processed or moved further into the internal network. SANS Institute's papers on Web Application Firewalls can offer further context on content-aware security.

This layered approach, using network segmentation and strict firewall rules with integrated security services, balances the need for external access with robust internal network protection.