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A bastion host is a specialized computer that is deliberately exposed on a public network. From a secured network perspective, it is the only node exposed to the outside world and is therefore very prone to attack. It is placed outside the firewall in single firewall systems or, if a system has two firewalls, it is often placed between the two firewalls or on the public side of a demilitarized zone (DMZ).
The bastion host processes and filters all incoming traffic and prevents malicious traffic from entering the network, acting much like a gateway. The most common examples of bastion hosts are mail, domain name system, Web and File Transfer Protocol (FTP) servers. Firewalls and routers can also become bastion hosts.
The bastion host node is usually a very powerful server with improved security measures and custom software. It often hosts only a single application because it needs to be very good at what it does. The software is usually customized, proprietary and not available to the public. This host is designed to be the strong point in the network to protect the system behind it. Therefore, it often undergoes regular maintenance and audit. Sometimes bastion hosts are used to draw attacks so that the source of the attacks may be traced.
To maintain the security of bastion hosts, all unnecessary software, daemons and users are removed. The operating system is continually updated with the latest security updates and an intrusion detection system is installed.