SSHKNOWNHOSTS FILE FORMAT. The /etc/ssh/sshknownhosts and /.ssh/knownhosts files contain host public keys for all known hosts. The global file should be prepared by the administrator (optional), and the per-user file is maintained automatically: whenever the user connects from an unknown host, its key is added to the per-user file.
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.
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
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:
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
A CentOS Linux user account created to provide security barriers between various apps and other users of the systems. For example, Apache or Nginx server runs as www-data user. Webmasters can upload files using another user called ftp. By separating them, you can improve the security of your system. This page explains how to create, modify, and delete user accounts on a CentOS Linux 7/8 server.
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Create a new user account in CentOS Linux 7/8
The procedure is as follows for creating a new user account on CentOS Linux:
Important files
Above commands modify the following files to create or delete user accounts:
Do not modify the following files using a text editor such as vim or nano to avoid misconfiguration.
Let us see all commands and examples in details.
Display a list of all Linux user account
The /etc/passwd file contains one line for each user account, with seven fields delimited by colons. Type cat command to list users:
cat /etc/passwd Want to see information about the currently logged-in user? Type id command: id Sample outputs:
Finally, use the ls command to see the owner and group of a file named /etc/hosts:
ls -l /path/to/file Run the ps command/top command to see owner of process: ps
See how to “Show All Running Processes in Linux using ps/htop commands” for more information.
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Creating a new CentOS user account from the command line
The syntax is:
sudo useradd userName
Examples
Create the vivek user account, run:
sudo adduser vivek Verify it: id vivek Or use the grep command as follows: grep -w '^vivek' /etc/passwd Finally, set or change the password for vivek user by typing the following passwd command: sudo passwd vivek A new user account was created. Next, verify it by log in using the ssh command: ssh vivek@your-centos-8-server-ip
Deleting user account in CentOS Linux 7/8
The syntax is:
sudo userdel {userName} The -r option remove home directory and mail spool of given user account. So make a backup if you need user data. Let us delete the vivek user along with all personal data: sudo userdel -r vivek Verify that the user vivek is successfully deleted from CentOS server: id vivek Advance examples
Let us create a new accout named www-files as follows:
$ sudo useradd -d /home/www-files -m Where,
How to modify user accounts
You need to usermod as follows:
usermod [options] {username} Please note that the user account must exists in order to use usermod. Let us see some common examples. Set new value of the GECOS field (commentsudo usermod -c 'Vivek Gite' vivek
Update home directory location for the user account
Change /home/vivek to /home/ftpvivek, run:
sudo usermod -d /home/ftpvivek -m vivek
Lock the user accountsudo usermod -L vivek
Unlock the user accountsudo usermod -U vivek
Create a Sudo User on CentOS![]()
All members of the wheel group have sudo access. So all you have to do is append user account to the wheel group as follows:
sudo usermod -aG wheel {username} Next, login as vivek user and test sudo access: {vivek@my-centos8-box:~ }$ sudo ls /root/ See “Linux Add User To Group” for more info. Linux Change or Rename User Name and UID (user-id)sudo usermod -l login-name old-name See “How to Change a USER and GROUP ID on Linux For All Owned Files” for more info. ![]() Getting help
Use man command or pass the --help as follows:
usermod command
Run:
man 8 usermod
passwd commandCentos 7 Generate New Ssh Host Keys List
Execute:
man 8 passwd
useradd command
Type:
man 8 useradd
Conclusion
You learned how to create a new user account and modify or delete user them on CentOS Enterprise Linux 7 or 8. See CentOS/RHEL 8 docs here for more info.
Centos 7 Ssh Keygen
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