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Nmap Network Scanning

Chapter 6. Optimizing Nmap Performance

Introduction

One of my highest Nmap development priorities has always been performance. A default scan (nmap <hostname>) of a host on my local network takes a fifth of a second. That is barely enough time to blink, but adds up when you are scanning hundreds or thousands of hosts. Moreover, certain scan options such as UDP scanning and version detection can increase scan times substantially. So can certain firewall configurations, particularly response rate limiting. While Nmap utilizes parallelism and many advanced algorithms to accelerate these scans, the user has ultimate control over how Nmap runs. Expert users carefully craft Nmap commands to obtain only the information they care about while meeting their time constraints.

While Nmap performance is a high priority, accuracy is even more important. Authors of competing scanners have given high-profile conference presentations about how their scanner only takes four seconds to scan an entire class B address space. These scanners are actually trivial to write, since they omit all the congestion control and packet loss detection algorithms, leaving just a tight loop spewing probe packets as fast as the system can generate or the wire can bear. Such scanners are often promoted as stateless—meaning they have also omitted the code to track and retransmit probes. You can achieve similar behavior with Nmap by adding flags such as --min-rate 1000 to request that Nmap send at least 1,000 packets per second, and --max-retries 0 to disable retransmission of timed-out probes. Yet I rarely recommend this. Ninety-nine percent of the packets may be dropped by the next router upstream, and the scanner will never know the difference.

Unmetered packet blasting scanners such as Scanrand are useful in some situations, but Nmap takes a much more conservative and accurate route. Nmap assumes the worst (high latency and packet loss) of the target networks at first, then speeds up as it gathers statistics showing that it can safely do so. While this happens automatically, an administrator can quicken the learning process by passing hints about the network to Nmap. An example of such a hint would be --max-rtt-timeout 200ms, which allows Nmap to assume that any responses to a target host probe will come within 200 milliseconds.

This chapter first discusses high-level methodologies for improving scan times. Then it covers how timing templates and low-level controls are used to speed up Nmap without impacting accuracy. It finishes with a tutorial by Jack Mogren of the Mayo Clinic, detailing how he improved scan time against his 676,352-IP network from nearly a week to 46 hours. Considering the huge importance of scanner performance, this chapter may seem short. This is because the chapter focuses on high-level general scanning performance tips, while tips for optimizing specific scan techniques are spread throughout this book where those techniques are covered.

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