What is Bastion host?

Bharat Kalluri / 2020-11-14

The function of a bastion host is to provide a safe and secure entry point into servers/instances in the private cluster/subnet.

It is not advised for servers to have a public IP. Usually in set ups involving AWS, the server runs on an auto scaling group. And there will be a load balancer sitting in front of the auto scaling group serving the website/API's. The servers are placed in a private subnet, none of them will get a public IP, which also implies direct SSH is also not possible. This is a good thing. But there is sometimes a need to login into the server to check on processes. In that case, there will be a dummy instance in the public subnet which also has a public IP. Since they are in the same VPC, SSH can be done from the this dummy instance. This dummy instance is called as a bastion host.

Bastion host infra illustration

Bastion host infra illustration

So now to SSH into the server, we first SSH into the bastion host and then SSH into the server. Now since this is a single entry point into your private subnets, it is very important to make sure that this system is hardened security wise. Have a SSH key with a strong password, allow SSH only through particular IP's and make sure the bastion host is shut down when no one is accessing the private systems.

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