Port scanning is a network reconnaissance technique used to discover open network ports on a target computer or device. Network ports are communication endpoints that allow data to be sent and received over a network. Port scanning is commonly employed for various purposes, including network management, security assessment, and troubleshooting. Here's how it works:
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Port: A port is identified by a numeric value and is associated with a specific protocol or service. For example, Port 80 is typically associated with HTTP (web) traffic, Port 22 is used for SSH (secure shell), and Port 25 is commonly used for SMTP (email) traffic.
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Port Status: A port can be in one of three states: open, closed, or filtered.
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Open: An open port means that the target device is actively listening for incoming connections on that port. This could indicate that a service or application is running and ready to accept data.
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Closed: A closed port means that there is no active service listening on that port. The device may respond to a port scan, indicating that the port is closed.
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Filtered: A filtered port typically means that a firewall, intrusion detection system, or some other security mechanism is actively blocking access to the port. The port may not respond to a port scan, or it may respond with a special message indicating that access is denied.
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Port Scanning Techniques: Port scanning tools and methods vary, but they all involve sending network packets to a target device's IP address and different ports to determine the status of those ports. Common port scanning techniques include:
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TCP Connect Scanning: This method attempts to establish a full connection to each port. If a connection is successfully established, the port is considered open.
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SYN Scanning (Half-Open Scanning): In this approach, the scanner sends a TCP SYN packet to the target port. If the target responds with a SYN/ACK (synchronization acknowledgment) packet, the port is considered open. If the target responds with a RST (reset) packet, the port is closed.
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UDP Scanning: Similar to TCP scanning, but for UDP ports. UDP scanning is often more challenging, as many UDP services do not respond to scans.
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Banner Grabbing: This technique involves making a connection to an open port and capturing the "banner" or response from the service running on that port. The banner can reveal information about the service, which is useful for further analysis.
Port scanning can be used for legitimate network management tasks, such as identifying open ports for services and diagnosing network issues. However, it can also be used by malicious actors to identify potential vulnerabilities and attack points in a network. Therefore, ethical and responsible use of port scanning is essential, and organizations often employ port scanning as part of their cybersecurity and network monitoring efforts to safeguard their systems and networks.
