Linux Server Security, Second Edition

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OS Hardening Principles

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If you state a port range using the -p flag, you can combine commas and dashes to

create a very specific group of ports to be scanned. For example, typing

-p 20-

23,80,53,600-1024

tells nmap to scan ports 20 through 23, 80, 53, and 600 through

1024. Don’t use any spaces in your port range, however. Alternatively, you can use

the

-F flag (short for “fast scan”), which tells nmap to scan only those ports listed in

the file /usr/share/nmap/nmap-services; these are ports Fyodor has found to fre-

quently yield interesting results.

The “target’’ expression can be a hostname, a host IP address, a network IP address,

or a range of IP addresses. Wildcards may be used. For example,

192.168.17.*

expands to all 255 IP addresses in the network 192.168.17.0/24 (in fact, you could

use

192.168.17.0/24 instead); 10.13.[1,2,4].* expands to 10.13.1.0/24, 10.13.2.0/

24, and 10.13.4.0/24. As you can see, nmap is very flexible in the types of target

expressions it understands.

Figure 3-7. Sample nmapfe session

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Chapter 3: Hardening Linux and Using iptables

Some simple por