In practice, it's often a hassle for users to go through this TOFU confirmation step on each new machine or user account they use, and this amount of explicit coordination between SSH administrators and SSH users imposes significant operational costs for large organizations.
However, rotating cryptographic keys is generally considered good security practice, so it's also possible that a host key verification failure simply means that your infrastructure has been updated. This is called a host key verification failure and is intended to prevent Machine-in-the-Middle (MitM) and various other forms of traffic interception attacks. If the server's SSH host key changes in the future, your SSH client can now issue a warning because the host key fingerprints will no longer match. ssh/known_hosts || echo "You do not yet have an SSH known hosts database." This is called Trust On First Use (TOFU) and, if you accepted the connection, you will have added a line to your "known hosts database," typically a file called known_hosts in your ~/.ssh directory containing some information about this connection. Try this now: ssh host01Īccept the connection by answering yes: yes The first time you connect to an SSH server using a plain host key for authentication, you'll see a prompt asking you to confirm the connection. Without SSH certificates, authenticating an SSH server is usually done by noting its host key's fingerprint in your user's. Let’s get started learning about SSH certificates. Refer to the Tech Learning Collective Events calendar for the latest information about upcoming workshops and other educational events. Once again, consider enrolling in Tech Learning Collective’s SYS101 course or attending some networking- and system administration-related workshops if these topics are unfamiliar to you. a working knowledge of basic OpenSSH server administration when run on a GNU/Linux Operating System.a basic understanding of asymmetric (public-key) cryptography, and.To complete this exercise, you should have: certificate authentication with SSH products such as Tectia® Server.the converse, i.e., user authentication with SSH certificates.Topics not (currently) included in this workshop are: This means manually managing entries in your known_hosts file for each host you connect to becomes a thing of the past, as does the initial “Are you sure you want to continue connecting (yes/no)?” question when you connect to a new SSH server for the first time. Instead, on each connection attempt, your SSH client refers to a single public key contained in a certificate issued by your organization’s own Certificate Authority (CA) and checks to see whether the host key of the server you are connecting to was signed (approved) by that authority. Learn more about system administration in Tech Learning Collective’s SYS101 course.Īs an SSH user (client-side), using SSH certificates for host authentication means you no longer have to remember the individual fingerprints of the SSH host keys that servers present to you when you connect to them. Certificate-based authentication is also at the heart of many other authentication systems, such as VPNs and the ubiquitous SSL/TLS (HTTPS) technology that produces a padlock icon in Web browsers, so what you learn here will help you in understanding and setting up those systems in your networks. Benefits of SSH CertificatesĪs a system administrator (server-side), using SSH certificates for host authentication enables you to more easily rotate SSH host keys on some periodic schedule without triggering SSH host key verification failures in clients. It is part of Tech Learning Collective’s “ Securing Our Servers: Basic Network Operations for Autonomous Communities” workshop as well as our SEC101 (security fundamentals) course. This module walks you through the process of creating a simple, OpenSSH-based Public Key Infrastructure (PKI) for the purpose of authenticating SSH servers to connecting clients.