![]() There is a limit on attempts before the SSH server will fail the authentication. The client will not try other identities, even if offered by ssh-agent or a PK11 provider. IdentitiesOnly defaults to no, but when set to yes, tells SSH to use only the identity specified on the command line or in the configuration file. ![]() I may set this option to no if I know I need to be prompted for a password, such as to add or replace a key using ssh-copy-id. PubkeyAuthentication defaults to yes so that key authentication is attempted. I only need to specify PasswordAuthentication=yes if I am trying to override a locally customized configuration file. If I see a prompt, I know it is a passphrase or Kerberos prompt. I sometimes disable this setting to ensure that I am authenticating with a method other than SSH password authentication. PasswordAuthentication defaults to yes so that if other methods fail, the user will see a password prompt. If I know I need to be prompted for a password, such as when copying a new public key to a host, I use -o PreferredAuthentications=password. The default generally has five to six options listed with Kerberos first, keys in the middle, and password last. ![]() PreferredAuthentications specifies the order of methods to try. ![]() The following command has the same result as the one above: $ ssh -o IdentityFile=~/.ssh/id_somehubs options I use include: The IdentityFile SSH option can be used instead of -i. These options are described in the ssh_config man page. I also use a handful of other options specified with -o. The -i option specifies the key to use and works the same with all of the SSH client utilities, including the ssh, ssh-copy-id, and scp commands: $ ssh -i ~/.ssh/id_somehubs option can be given muliple times to limit which keys to try, if you know it is one of a handful of keys, but I usually only need to specify the exact key. There are a few options I use on the command line during setup, or for verification and then later in the configuration file for future use. The ssh man page not only describes the -i option, but also has a section titled AUTHENTICATION which further explains the steps used to determine which key or other method is used. When I use a client command such as ssh or scp, the utility selects a file based on command-line options, a per-host basis in the configuration file, or program defaults: I pick a name that hopefully makes as much sense to future me as it does currently. When I generate an SSH key pair, I get prompted for the name of the public key (identity) file with a default of ~/.ssh/id_rsa. How does my system decide which key to use? In addition to the keys used from my workstation, I also have separate keys for any shared applications, plus the keys that need to be uploaded to an automation system such as Ansible Tower. I passphrase protect all (ok, most of) the keys, and am careful about access to the private key files. Of course, I need to keep all of these keys secure. (Again, the public key is often uploaded to a central site and propagated in an automated manner.) Each upstream community that allows SSH access, usually to gain write access for source control commits.Networks I manage where the public key is loaded into an identity management system that propagates it out to the systems I access interactively.Training classrooms and similar environments that use shared keys.In particular, I use different key pairs for: I currently have about a half dozen places where I use SSH keys on a regular basis and several other less frequently accessed locations. Old Linux commands and their modern replacements.Linux system administration skills assessment.A guide to installing applications on Linux.Download RHEL 9 at no charge through the Red Hat Developer program.
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