OpenSSH authentication agent
ssh-agent is a program to hold private
keys used for public key authentication. Through use of environment
variables the agent can be located and automatically used for authentication
when logging in to other machines using
The options are as follows:
- Bind the agent to the UNIX-domain socket bind_address. The default is $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>.
- Generate C-shell commands on
stdout. This is the default if
SHELLlooks like it's a csh style of shell.
- Foreground mode. When this option is specified,
ssh-agentwill not fork.
- Debug mode. When this option is specified,
ssh-agentwill not fork and will write debug information to standard error.
- Specifies the hash algorithm used when displaying key fingerprints. Valid options are: “md5” and “sha256”. The default is “sha256”.
- Kill the current agent (given by the
- Specify an option when starting
ssh-agent. Currently only one option is supported:
no-restrict-websafe. This instructs
ssh-agentto permit signatures using FIDO keys that might be web authentication requests. By default,
ssh-agentrefuses signature requests for FIDO keys where the key application string does not start with “ssh:” and when the data to be signed does not appear to be a ssh(1) user authentication request or a ssh-keygen(1) signature. The default behaviour prevents forwarded access to a FIDO key from also implicitly forwarding the ability to authenticate to websites.
- Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO
authenticator middleware shared libraries that may be used with the
-soptions to ssh-add(1). Libraries that do not match the pattern list will be refused. See PATTERNS in ssh_config(5) for a description of pattern-list syntax. The default list is “/usr/lib/*,/usr/local/lib/*”.
- Generate Bourne shell commands on
stdout. This is the default if
SHELLdoes not look like it's a csh style of shell.
- Set 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(5). A lifetime specified for an identity with ssh-add(1) overrides this value. Without this option the default maximum lifetime is forever.
- command [arg ...]
- If a command (and optional arguments) is given, this is executed as a subprocess of the agent. The agent exits automatically when the command given on the command line terminates.
There are two main ways to get an agent set up. The first is at
the start of an X session, where all other windows or programs are started
as children of the
ssh-agent program. The agent
starts a command under which its environment variables are exported, for
ssh-agent xterm &. When the command
terminates, so does the agent.
The second method is used for a login session. When
ssh-agent is started, it prints the shell commands
required to set its environment variables, which in turn can be evaluated in
the calling shell, for example
In both cases, ssh(1) looks at these environment variables and uses them to establish a connection to the agent.
The agent initially does not have any private keys. Keys are added
using ssh-add(1) or by ssh(1) when
AddKeysToAgent is set in
ssh_config(5). Multiple identities may be stored in
ssh-agent concurrently and
automatically use them if present.
ssh-add(1) is also used to remove keys from
ssh-agent and to query the keys that are held in
ssh-agent may be forwarded
from further remote hosts using the
-A option to
ssh(1) (but see
the caveats documented therein), avoiding the need for authentication data
to be stored on other machines. Authentication passphrases and private keys
never go over the network: the connection to the agent is forwarded over SSH
remote connections and the result is returned to the requester, allowing the
user access to their identities anywhere in the network in a secure
ssh-agentstarts, it stores the name of the agent's process ID (PID) in this variable.
ssh-agentstarts, it creates a UNIX-domain socket and stores its pathname in this variable. It is accessible only to the current user, but is easily abused by root or another instance of the same 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.
ssh(1), ssh-add(1), ssh-keygen(1), ssh_config(5), sshd(8)
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.