Jan 12, 2019 How to know/find out/see my ssh host key I am using putty on a windows and managed to log in to my linux although it screamed for the unknow host key as usual for 1st time log-in. I know nothing will go wrong so I just accepted it. Host keys are normally generated automatically when OpenSSH is first installed or when the computer is first booted. The ssh-keygen program can be used for generating additional host keys or for replacing existing keys. Known Host Keys. SSH clients store host keys for hosts they have ever connected to.

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For a Linux VPS, all you have to do to generate new hostkeys on the server is use ssh-keygen (included with the standard OpenSSH package) to perform the following commands for both SSH protocols (ultimately you must perform all three commands). Make sure that the -N is followed by two single quotation marks and that all three file names are different.

Nov 30, 2018 Sample set up for our RHEL 8 server. Where, You generate a key pair on your Linux/Unix/macOS desktop. Place the public key on RHEL 8 server. One can unlock public key using a private key stored on your desktop with the help of ssh command. Apr 12, 2018 SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with a CentOS server, chances are, you will spend most of your time in a terminal session connected to your server through SSH. In this guide, we’ll focus on setting up SSH keys for a vanilla CentOS 7 installation.

SSH1 protocol

For the SSH1 protocol you only require an RSA1 key:

ssh-keygen -f /etc/ssh/ssh_host_key -N ' -t rsa1
SSH2 protocol

For the SSH2 protocol you need two keys, for rsa and dsa:

ssh-keygen -f /etc/ssh/ssh_host_rsa_key -N ' -t rsa
ssh-keygen -f /etc/ssh/ssh_host_dsa_key -N ' -t dsa
The authenticity of host xxx can't be established

If you have logged into the VPS via SSH in the past, after you regenerate the hostkeys for a new SSH connection you should get a message stating that the authenticity of the server cannot be established. This is true, because the server's fingerprint is stored in a local cache on your system:

On Linux this is stored in ~/.ssh/known_hosts
On Windows it is stored in the registry under HKEY_CURRENT_USERSoftwareSimonTathamPuTTYSshHostKeys
Every time a connection is established, a comparison of the fingerprints (which look something like xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx (for rsa)) is made between the local database and the server you are contacting.

Removing the offending key

You can remove the locally saved cache for the server and re-establish the connection, in which case you will be prompted to save the fingerprint again. On a Linux terminal, you will see the following warning:

Offending key in /home/USER/.ssh/known_hosts:15
where /home/USER/.ssh/known_hosts is the file location and 15 is the line number. This enables you to use vi to jump directly to the right line so you can delete it:

vi /home/USER/.ssh/known_hosts +15
For Windows, you use regedit to find the correct registry entry and delete the offending key:

HKEY_CURRENT_USERSoftwareSimonTathamPuTTYSshHostKeys

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With a secure shell (SSH) key pair, you can create virtual machines (VMs) in Azure that use SSH keys for authentication, eliminating the need for passwords to sign in. This article shows you how to quickly generate and use an SSH public-private key file pair for Linux VMs. You can complete these steps with the Azure Cloud Shell, a macOS or Linux host, the Windows Subsystem for Linux, and other tools that support OpenSSH.

Note

VMs created using SSH keys are by default configured with passwords disabled, which greatly increases the difficulty of brute-force guessing attacks.

For more background and examples, see Detailed steps to create SSH key pairs.

For additional ways to generate and use SSH keys on a Windows computer, see How to use SSH keys with Windows on Azure.

Supported SSH key formats

Azure currently supports SSH protocol 2 (SSH-2) RSA public-private key pairs with a minimum length of 2048 bits. Other key formats such as ED25519 and ECDSA are not supported.

Create an SSH key pair

Use the ssh-keygen command to generate SSH public and private key files. By default, these files are created in the ~/.ssh directory. You can specify a different location, and an optional password (passphrase) to access the private key file. If an SSH key pair with the same name exists in the given location, those files are overwritten.

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The following command creates an SSH key pair using RSA encryption and a bit length of 4096:

If you use the Azure CLI to create your VM with the az vm create command, you can optionally generate SSH public and private key files using the --generate-ssh-keys option. The key files are stored in the ~/.ssh directory unless specified otherwise with the --ssh-dest-key-path option. The --generate-ssh-keys option will not overwrite existing key files, instead returning an error. In the following command, replace VMname and RGname with your own values:

Provide an SSH public key when deploying a VM

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To create a Linux VM that uses SSH keys for authentication, specify your SSH public key when creating the VM using the Azure portal, Azure CLI, Azure Resource Manager templates, or other methods:

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If you're not familiar with the format of an SSH public key, you can display your public key with the following cat command, replacing ~/.ssh/id_rsa.pub with the path and filename of your own public key file if needed:

A typical public key value looks like this example:

If you copy and paste the contents of the public key file to use in the Azure portal or a Resource Manager template, make sure you don't copy any trailing whitespace. To copy a public key in macOS, you can pipe the public key file to pbcopy. Similarly in Linux, you can pipe the public key file to programs such as xclip.

The public key that you place on your Linux VM in Azure is by default stored in ~/.ssh/id_rsa.pub, unless you specified a different location when you created the key pair. To use the Azure CLI 2.0 to create your VM with an existing public key, specify the value and optionally the location of this public key using the az vm create command with the --ssh-key-values option. In the following command, replace VMname, RGname, and keyFile with your own values:

If you want to use multiple SSH keys with your VM, you can enter them in a space-separated list, like this --ssh-key-values sshkey-desktop.pub sshkey-laptop.pub.

SSH into your VM

With the public key deployed on your Azure VM, and the private key on your local system, SSH into your VM using the IP address or DNS name of your VM. In the following command, replace azureuser and myvm.westus.cloudapp.azure.com with the administrator user name and the fully qualified domain name (or IP address):

If you specified a passphrase when you created your key pair, enter that passphrase when prompted during the login process. The VM is added to your ~/.ssh/known_hosts file, and you won't be asked to connect again until either the public key on your Azure VM changes or the server name is removed from ~/.ssh/known_hosts.

If the VM is using the just-in-time access policy, you need to request access before you can connect to the VM. For more information about the just-in-time policy, see Manage virtual machine access using the just in time policy.

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Next steps

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For more information on working with SSH key pairs, see Detailed steps to create and manage SSH key pairs.
If you have difficulties with SSH conne