Free Nmap security tool: Improving scan times

The free Nmap security tool offers flexibility in terms of how you run scans. Learn tips for customizing scans based on what you're trying to accomplish on your customer's network.

The free, open source Nmap security scanner has a variety of configuration options that can be adjusted by resellers to suit individual client needs. In this tip, learn how you can configure Nmap scan times to achieve a proper balance between speed, stability and functionality.

How to improve Nmap scan times
Your objectives for running an Nmap scan will determine how you want it to run: slow and quietly, fast and furious, or somewhere in between. Therefore, Nmap includes a variety of timing options that allow you to affect almost every aspect of a scan.

By default, Nmap is set to not abort a scan due to time -- no matter how long it may take to complete. This can be overridden with the Host Timeout option (--host_timeout), which sets the amount of time a scan will wait before giving up on an IP address. This can be useful when scanning network devices over a slow connection or when the scan comes across a device that is slow in responding.

Nmap's other timing options can basically be split into four categories: round trip time, delay, parallel host scanning and parallel port scanning. Round trip time is the number of milliseconds required to receive a response to an Nmap request. Nmap automatically adjusts its response time timeout during a scan. However, you can force it to use a larger timeout value using the Minimum Round Trip Time Timeout option (--min_rtt_timeout) if, for example, your network is experiencing dropped packets. The Maximum Round Trip Time Timeout (--max_rtt_timeout) is useful for ensuring an accurate scan across slow or problematic networks.

Read more on custom scans using Nmap security scanner.

About the author
Michael Cobb, CISSP-ISSAP is the founder and managing director of Cobweb Applications Ltd., a consultancy that offers IT training and support in data security and analysis. He co-authored the book
IIS Security and has written numerous technical articles for leading IT publications. Mike is the guest instructor for SearchSecurity's Web Security School and, as a SearchSecurity.com site expert, answers user questions on application and platform security.

This tip originally appeared on SearchSecurity.com.


This was first published in December 2006
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