Real-World experience with configuration, maintenance and monitoring of Linux/Unix systems and network services. Especially with following technologies and services:
Firewalls (iptables, shorewall, pfsense, ModSecurity)
VPNs (OpenVPN, openswan, strongswan, ipsec)
Web services (DNS, bind, apache, nginx, wordpress, php )
Mail (smtp, pop3, postfix, dovecot, spamassasin, webmails, MX, SPF, DKIM)
Database (MySQL, PostgreSQL)
Configuration Management (Puppet)
Programming (Python, Bash, Perl, php, SQL)