Masscan: A Deep Dive

Masscan leverages a user-space TCP/IP stack for its network scanning, setting it apart from traditional tools by avoiding kernel-based networking stack overhead. This technique underpins its capability for rapid, large-scale scans.

Architecture and Design

Operating entirely in user-space allows Masscan to directly manipulate packet data, bypassing the kernel's network stack. This is achieved through raw sockets for packet crafting and sending, alongside custom packet capture logic for receiving responses.

Operation

• Asynchronous I/O: Masscan uses non-blocking operations to send and receive packets concurrently, utilizing an event-driven model for efficient packet handling.
• Direct Interface Access: By interacting directly with network interfaces, Masscan can operate in promiscuous mode, capturing all traffic and filtering for relevant responses.
• Efficient Packet Filtering: Custom logic is employed to sift through incoming packets, identifying those pertinent to the scan, ensuring performance efficiency.

Benefits

• Reduced Latency: The user-space stack significantly cuts down latency by eliminating kernel stack processing, crucial for high-speed scanning.
• Enhanced Control and Flexibility: Direct packet crafting and sending provide unparalleled control over the scanning process, allowing for detailed customization and optimization.
• Scalability: The architecture's inherent scalability ensures performance scales linearly with network and hardware capabilities, supporting high packet rates.

Challenges and Considerations

Despite its advantages, Masscan's user-space operation requires careful resource management and presents compatibility challenges across different platforms and network interfaces. Additionally, security implications and the potential for network disruption necessitate responsible use.

The following pseudocode segments offer insight into how Masscan efficiently crafts, sends, and listens for packets, leveraging a user-space stack for unparalleled speed and flexibility in network scanning:

Setting Up the Scanner

    function setupScanner(targetIPs, portRange, rate):
        for each IP in targetIPs:
            for each port in portRange:
                schedulePacketSend(IP, port, rate)
        

Crafting and Sending Packets

    function schedulePacketSend(IP, port, rate):
        while true:
            wait 1/rate seconds
            sendPacket(IP, port)
    
    function sendPacket(IP, port):
        packet = createPacket(IP, port)
        rawSocket.send(packet)
        log("Packet sent to " + IP + ":" + port)
    
    function createPacket(IP, port):
        packet = new Packet()
        packet.destinationIP = IP
        packet.destinationPort = port
        packet.flags = SYN
        return packet
        

Listening for Responses

    function listenForResponses():
        while true:
            response = rawSocket.receive()
            if response is not None:
                if response.flags == SYN-ACK:
                    log("Port " + response.sourcePort + " on " + response.sourceIP + " is open")
                else if response.flags == RST:
                    log("Port " + response.sourcePort + " on " + response.sourceIP + " is closed")