Printing Across VLANs: A Secure and Scalable Solution

In my previous configuration, I used a CUPS server with multiple network interfaces to enable printing from computers on restricted networks onto the printer located in the admin network. However, after some research and exploration, I discovered a more efficient way to achieve this goal by utilizing VLANs for network segmentation.

Creating a Dedicated Printer VLAN

To start, I created a dedicated "printer" VLAN that can only communicate with the internet but is isolated from other VLANs. Networks from which I want to enable printing, are configured to allow creating connections to the printer VLAN. This ensures that printing traffic within the restricted networks cannot accidentally traverse to other parts of the network, while computers can print to the printer. The printer VLAN is now the primary focus for setting up the UDP proxy container.

Configuring the UDP Proxy Container

To enable printing across multiple networks, I created a Linux container, for a UDP proxy, with network interfaces on both the printer VLAN and any additional VLANs that need to support printing. In this setup, the UDP proxy container acts enable notifications of the printers on the printer VLAN to computers.

Here's an example of how to configure the UDP proxy relay using the udpbroadcastrelay command:

# Configure the first relay for NetBIOS
./udpbroadcastrelay --id 1 --port 137 --dev printer-vlan-interface --dev computer-vlan-interface -f
# Configure the second relay for NetBIOS
./udpbroadcastrelay --id 2 --port 138 --dev printer-vlan-interface --dev computer-vlan-interface -f
# Configure the third relay for SSDP
./udpbroadcastrelay --id 3 --port 1900 --dev printer-vlan-interface --dev computer-vlan-interface --multicast 239.255.255.250 -f

In this example:

• --id specifies the ID of the relay instance.
• --port sets the port number for the UDP broadcast.
• --dev 
• The first instance represents the network interface to listen for messages from
• The remaining instance specifies the network interface(s) to relay the broadcasts to.
• --multicast enables multicast support if needed (in this case, we're using a fixed IP address).
• The -f option tells udpbroadcastrelay to run in background mode.

Why This Approach?

This approach offers several benefits over my initial CUPS server configuration:

1. Network Segregation: By isolating printing traffic within the printer VLAN, we minimize the risk of unauthorized access or eavesdropping on network communications.
2. Scalability and Flexibility: Using UDP proxy relays allows us to easily add or remove printers and networks as needed without requiring significant changes to the underlying infrastructure.
3. Improved Security: The use of dedicated VLANs reduces the attack surface and minimizes potential vulnerabilities.

Conclusion

By implementing a printer VLAN with UDP proxy relays, we've successfully created an efficient printing system that offers improved network security, scalability, and flexibility compared to my initial setup. This approach allows us to easily manage printers across multiple networks while maintaining isolation between them.