A bastion host is a dedicated system that we intentionally expose on a public network. From a secured network point-of-view, it is the only node that we expose to the outside world, and thus, it is very vulnerable to attack. We place it outside the firewall in one firewall system, or if the system has two firewalls, we place it between two firewalls.
Bastion Host filters and processes the incoming traffic and averts the vicious traffic from entering the network, serving as a gateway. General examples for bastion host are domain name system, mail.
