A bastion host is a publicly accessible device for the network's security, which has a direct connection to a public network such as the Internet. The bastion host serves as a platform for any one of the three types of firewalls: packet filter, circuit-level gateway, or application-level gateway.
Bastion hosts must check all incoming and outgoing traffic and enforce the rules specified in the security policy. They must be prepared for attacks from external and possibly internal sources. They should be built with the least amount of hardware and software in order for a potential hacker to have less opportunity to overcome the firewall. Bastion hosts are armed with logging and alarm features to prevent attacks.
The bastion host's role falls into the following three common types:
Single-homed bastion host.
This is a device with only one network interface, normally used for an application-level gateway. The external router is configured to send all incoming data to the bastion host, and all internal clients are configured to send all outgoing data to the host. Accordingly, the host will test the data according to security guidelines.
Dual-homed bastion host.
This is a firewall device with at least two network interfaces. Dual-homed bastion hosts serve as application-level gateways, and as packet filters and circuit-level gateways as well. The advantage of using such hosts is that they create a complete break between the external network and the internal network. This break forces all incoming and outgoing traffic to pass through the host. The dual-homed bastion host will prevent a security break-in when a hacker tries to access internal devices.
Multihomed bastion host
Single-purpose or internal bastion hosts can be classified as either single-homed or multihomed bastion hosts. The latter are used to allow the user to enforce strict security mechanisms. When the security policy requires all inbound and outbound traffic to be sent through a proxy server, a new proxy server should be created for the new streaming application. On the new proxy server, it is necessary to implement strict security mechanisms such as authentication. When multihomed bastion hosts are used as internal bastion hosts, they must reside inside the organization's internal network, normally as application gateways that receive all incoming traffic from external bastion hosts. They provide an additional level of security in case the external firewall devices are compromised. All the internal network devices are configured to communicate only with the internal bastion host.
A tri-homed firewall connects three network segments with different network addresses. This firewall may offer some security advantages over firewalls with two interfaces. An attacker on the unprotected Internet may compromise hosts on the DMZ but still not reach any hosts on the protected internal network.