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onerr keyword (PAM, listfile module)
only_from and no_access keywords, xinetd.conf
open relay mail servers
open servers, testing FTP server for possible exploitation as a proxy
open-source integrity checkers [See Tripwire]
openlog function
    using in C program
OpenSSH [See SSH]
OpenSSL
    CA.pl, Perl script creating Certifying Authority
    PEM encoding, converting DER certificate to
    testing SSL connection to POP/IMAP server
    web site
Openwall Project, John the Ripper
operating system fingerprints
    nmap -O command
    nmap command, using for
outgoing network connections [See networks, protecting outgoing traffic]
ownership, file
    inability to track with manual integrity check
    verifying for RPM-installed files