Ossec kan lese nmap grepable filer og bruke de som en verktøy for å sjekke og varsle om host endringer.
Følgende endringer er gjort for å få det til å fungere.

Nmap correlation
Ossec can read nmap grepable output files to use as a correlation tool and also to alert based on host information changes. Follow the step by step below on how to configure ossec:

1.	Add the nmap output file on ossec.conf (generally at /var/ossec/etc/ossec.conf):

<ossec_config>
 <localfile>
    <log_format>nmapg</log_format>
    <location>/var/log/nmap-out.log</location>
  </localfile>
</ossec_config>

2.	If the file does not exist, touch it:

ossec-test# touch /var/log/nmap-out.log

3.	Restart ossec:

ossec-test# /var/ossec/bin/ossec-control restart

4.	Run your nmap scans (example scanning 192.168.2.0/24 network):

ossec-test# nmap --append_output -sU -sT -oG /var/log/nmap-out.log 192.168.2.0-255

NOTE:  I've found that this is handiest when paired with a routine nmap scan.

5.	So add the above line to your crontab.

ossec-test#crontab -e
15 * * * * nmap --append_output -sU -sT -oG /var/log/nmap-out.log 192.168.2.0-255

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[edit]

Example of alert when a new host is found:

** Alert 1152058913.238: mail
2006 Jul 04 20:21:53 /var/log/nmap-out.log
Rule: 15 (level 8) -> 'New host information added.'
Src IP: (none)
User: (none)
Host: 192.168.2.10, open ports: 21(tcp) 22(tcp) 80(tcp) 113(tcp) 514(udp) 1514(udp) 4500(udp)

[edit]

Example of alert when a new a host information is changed:

** Alert 1152058983.487: mail
2006 Jul 04 20:23:03 /var/log/nmap-out.log
Rule: 15 (level 8) -> 'Host information changed.'
Src IP: (none)
User: (none)
Host: 192.168.2.1, open ports: 54(udp) 8080(tcp) 161(udp) 520(udp) 1025(udp) 1900(udp)
Previously open ports: 53(udp) 80(tcp) 161(udp) 520(udp) 1025(udp) 1900(udp)