Network Scanning Basics
Carefully review scan results to map out your target’s attack surface. Remember that frequent scanning may trigger alerts on well monitored systems. Network scanning is one of the most fundamental skills in penetration testing and network administration. The goal is simple: enumerate active hosts, identify open ports, and determine what services are listening. By understanding which services are exposed, you can focus your efforts on areas that present the highest risk. The most popular tool for the job is Nmap, but a seasoned security professional should be comfortable with several scanning methods.
Begin with a basic ping sweep to see which IP addresses respond. Tools like nmap -sn or the fping utility can quickly check entire subnets for active hosts. Not all devices respond to ICMP requests, so be prepared for stealthier techniques like ARP pings on local networks or TCP SYN scans when ICMP is blocked. Knowing how to vary your approach ensures you collect accurate information regardless of firewall rules.
Once you identify live systems, move on to port scanning. A TCP SYN scan (nmap -sS) is efficient and widely used, sending a SYN packet to start the three-way handshake without completing it. Closed ports respond with RST, while open ports reply with SYN/ACK. This approach is fast and relatively stealthy because it does not actually establish a connection. For a more thorough analysis, consider a TCP connect scan (nmap -sT), which completes the handshake and is useful when SYN scanning is not an option, for example when running without raw socket privileges.
Service detection adds another layer. With nmap -sV, you can determine which service versions are running on open ports. Nmap sends a series of probes and matches the responses against its vast signature database. This is critical for identifying outdated software or services with known vulnerabilities. Combined with the results of vulnerability scanners like Nessus or OpenVAS, service detection data helps you prioritize patching or additional investigation.
Timing and performance are also crucial considerations. Nmap allows you to adjust the -T option, ranging from -T0 (paranoid) to -T5 (insane). Faster scans send packets more aggressively but risk missing responses or triggering intrusion detection systems. Slower scans are more reliable but take significantly longer to complete. Striking a balance is key: -T3 (normal) is safe in most situations, while -T4 can speed things up without being too aggressive. When scanning large networks, consider using the --min-hostgroup and --max-hostgroup options to control how many hosts are scanned in parallel.
Sometimes you want to go beyond standard TCP scans. UDP scanning (nmap -sU) is notoriously slow because UDP is connectionless and lacks handshake mechanisms. When you send a probe to a UDP port, you might not get a response even if the service is running. To confirm, you often need to send application-specific payloads or rely on service discovery protocols. You can also use masscan for extremely fast scanning if you just need a high-level overview, though accuracy may suffer with that speed.
After collecting scan data, be meticulous in your analysis. Look for unusual ports or services that might indicate misconfigurations. Document your findings carefully, including any timestamps and command-line options used for each scan. This not only helps you track your own work but also provides a clear audit trail. Many organizations require explicit authorization before scanning, especially for more intrusive techniques. Keeping detailed notes ensures you can respond to questions or concerns from administrators.
Remember that repeated or high-intensity scans can set off alarms. Modern intrusion detection systems look for patterns such as sweeps across large port ranges or single hosts targeted by multiple probes in quick succession. A good rule of thumb is to start with conservative settings, review the results, and escalate if you need deeper information. Whenever possible, coordinate your scanning activities with network administrators so they are aware of your testing schedule and can whitelist your scanner’s IP address if necessary.
Finally, network scanning should never be a one-time event. Environments change rapidly, and new vulnerabilities appear daily. Integrating regular scans into your security program helps identify new assets and catch configuration drift before attackers exploit it. Automate routine scans with cron jobs or continuous integration tools and review the output regularly. Effective scanning is a cornerstone of proactive defense and helps you stay one step ahead of potential intruders.