New SMTP Tunnel Tool Bypasses Deep Packet Inspection

A new networking utility called SMTP Tunnel has been developed to help users bypass Deep Packet Inspection (DPI) firewalls by disguising traffic as normal email communications.

The tool functions as a SOCKS5 proxy that tunnels data through what appears to be standard SMTP email sessions, making it difficult for network monitoring systems to detect and block the connection.

Key capabilities include:

• Local proxy running on 127.0.0.1:1080
• Disguised SMTP traffic using EHLO, STARTTLS, and AUTH commands
• One-line installation on Linux VPS systems
• Multi-user support with per-user secrets and IP whitelists
• Auto-generated client packages for easy deployment

The system uses Python/asyncio with TLS 1.2+ encryption and HMAC-SHA256 authentication to secure communications while maintaining the appearance of legitimate email traffic.

The SMTP Tunnel operates by creating a local SOCKS5 proxy that runs on 127.0.0.1:1080. When applications connect to this proxy, their traffic is captured and sent to a remote server.

On the server side, the incoming connection is disguised as a standard SMTP email session. The system uses legitimate email protocol commands including:

• EHLO - Extended Hello command to initiate SMTP sessions
• STARTTLS - Command to upgrade connection to encrypted TLS
• AUTH - Authentication mechanism for email servers

Deep Packet Inspection systems examining the traffic see what appears to be a normal email conversation rather than a VPN or proxy connection. This allows the tunnel to pass through firewalls that would normally block standard proxy protocols.

Installation is designed to be straightforward with a one-liner command that works on any Linux VPS. This simplified deployment process makes the tool accessible to users without extensive technical configuration.

The tool supports multi-user environments with several security features:

• Per-user secrets - Each user has unique authentication credentials
• IP whitelists - Access can be restricted to specific IP addresses
• Auto-generated client packages - Clients can be created that only require double-clicking to run

Additional functionality includes auto-reconnection capabilities that automatically restore connections if they are lost. The proxy works with any application that supports SOCKS5 protocol, providing broad compatibility across different software and use cases.

The SMTP Tunnel is built using Python/asyncio, which provides asynchronous I/O operations suitable for handling multiple concurrent connections efficiently.

Security is implemented through:

• TLS 1.2+ encryption for all tunnel communications
• HMAC-SHA256 authentication to verify message integrity and authenticity

The combination of these technologies ensures that while the traffic appears as SMTP email to network inspection tools, the actual data remains encrypted and secure between the client and server endpoints.

The project source code is available on GitHub, allowing users to review the implementation and deploy their own instances of the tunnel system.

The SMTP Tunnel project has been published on GitHub, making the source code publicly available for review and deployment. The tool was shared through a Hacker News post, where it received community attention and engagement.

According to available information, the project post generated 11 points and 1 comment on the platform, indicating initial interest from the technical community. The GitHub repository provides access to the codebase for users who want to implement the tunneling solution.