A bastion host is a special-purpose computer on a network specifically designed and configured to withstand attacks. The computer generally hosts a single application, for example a proxy server, and all other services are removed or limited to reduce the threat to the computer. It is hardened in this manner primarily due to its location and purpose, which is either on the outside of a firewall or in a demilitarized zone and usually involves access from untrusted networks or computers.
Definitions
The term is generally attributed to a 1990 article discussing firewalls by Marcus J. Ranum. Ranum defined a Bastion host as Krutz and Vines have described a bastion host as "any computer that is fully exposed to attack by being on the public side of the DMZ, unprotected by a firewall or filtering router. Firewalls and routers, anything that provides perimeter access control security can be considered bastion hosts. Other types of bastion hosts can include web, mail, DNS, and FTP servers...Due to their exposure, a great deal of effort must be put into designing and configuring bastion hosts to minimize the chances of penetration." In an Amazon Web Services context, a bastion host is defined as "a server whose purpose is to provide access to a private network from an external network, such as the Internet. Because of its exposure to potential attack, a bastion host must minimize the chances of penetration.". A further AWS related definition is that bastion hosts are "instances that sit within your public subnet and are typically accessed using SSH or RDP. Once remote connectivity has been established with the bastion host, it then acts as a ‘jump’ server, allowing you to use SSH or RDP to log in to other instances deeper within your VPC. When properly configured through the use of security groups and Network ACLs, the bastion essentially acts as a bridge to your private instances via the internet."
Placement
There are two common network configurations that include bastion hosts and their placement. The first requires two firewalls, with bastion hosts sitting between the first "outside world" firewall, and an inside firewall, in a DMZ. Often smaller networks do not have multiple firewalls, so if only one firewall exists in a network, bastion hosts are commonly placed outside the firewall. Bastion hosts are related to multi-homed hosts and screened hosts. While a dual-homed host often contains a firewall it is also used to host other services as well. A screened host is a dual-homed host that is dedicated to running the firewall. A bastion server can also be set up using ProxyCommand with OpenSSH.
Examples
These are several examples of bastion host systems/services:
