secsh-agent is a program to hold private keys used for public key authentication (RSA, DSA, ECDSA, Ed25519). The idea is that secsh-agent is usually started in the beginning of an X-session or a login session, and all other windows or programs are started as clients to the secsh-agent program. Through use of environment variables, the agent can be located and automatically used for authentication when logging in to other machines using secsh.
If a command line is given, this is executed as a subprocess of the agent. When the command dies, so does the agent.
The agent initially does not have any private keys. Keys are added using secsh(see AddKeysToAgent for details), or secsh-add. Multiple identities may be stored in secsh-agent concurrently and secshwill automatically use them if present. secsh-add is also used to remove keys from secsh-agentand to query the keys that are held in one.
The idea is that the agent is run in the user's local PC, laptop, or terminal. Authentication data need not be stored on any other machine, and authentication passphrases never go over the network. However, the connection to the agent is forwarded over SSH remote logins, and the user can thus use the privileges given by the identities anywhere in the network in a secure way.
There are two main ways to get an agent setup: Either the agent starts a new subcommand into which some environment variables are exported, or the agent prints the needed shell commands (either sh or csh syntax can be generated) which can be evaluated in the calling shell. Later secsh looks at these variables and uses them to establish a connection to the agent.
A unix-domain socket is created ($TMPDIR/ssh-XXXXXXXX/agent.<pid>), and the name of this socket is stored in the SSH_AUTH_SOCK environment variable. The socket is made accessible only to the current user. This method is easily abused by root or another instance of the same user.
The SSH_AGENT_PID environment variable holds the agent's PID.
The agent exits automatically when the command given on the command line terminates.
You can also call secsh-agent as ssh-agent.
Binds the agent to the unix-domain socket bind address. The default is $TEMP/ssh-XXXXXXXXXX/agent.ppid.
Generates C-shell commands on stdout. This is the default if SHELL looks like it's a csh style of shell.
Foreground mode. When this option is specified, secsh-agent will not fork.
Enables debug mode. When this option is specified, secsh-agent will not fork.
Specifies the hash algorithm used when displaying key fingerprints. Valid options are: md5 and sha256. The default is sha256.
Kills the current agent (given by the SSH_AGENT_PID environment variable).
Generate Bourne shell commands on stdout. This is the default if SHELL does not look like it's a csh style of shell.
sets a default value for the maximum lifetime of identities added to the agent. The lifetime may be specified in seconds or in a time format specified in sshd_config. A lifetime specified for an identity with secsh-add overrides this value. Without this option, the default maximum lifetime is forever.
Contains the protocol version 2 DSA authentication identity of the user.
Contains the protocol version 2 ECDSA authentication identity of the user.
Contains the protocol version 2 ED25519 authentication identity of the user.
Contains the protocol version 2 RSA authentication identity of the user.
Unix-domain sockets used to contain the connection to the authentication agent. These sockets should only be readable by the owner. The sockets should get automatically removed when the agent exits.
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
All UNIX systems. Windows 7. Windows Server 2008 R2. Windows 8. Windows Server 2012. Windows 10. Windows Server 2016. Windows Server 2019.
The secsh-add utility, by default, loads keys from the $USERPROFILE directory on 7/2008R2/8/2012/10/2016/2019 platforms instead of ~/. This was done because the ~/ directory might not be available for the case of a domain machine that cannot contact a domain controller. Note that the secure shell service continues to use the ~/.ssh directory to store configuration files.
The authentication agent only works when connecting to secure shell servers that are based upon OpenSSH. The agent does not work when connecting to ssh.com-based servers.
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