Linux "ssh-agent" Command Line Options and Examples
ssh-agent is a program to hold private keys used for public key authentication (RSA, DSA, ECDSA, Ed25519). ssh-agent is usually started in the beginning of an X-session or a login session, and all other windows or pro‐ grams are started as clients to the ssh-agent program.
ssh-agent [-c | -s] [-Dd] [-a bind_address] [-E fingerprint_hash] [-P pkcs11_whitelist] [-t life]
[command [arg ...]]
ssh-agent [-c | -s] -k
Command Line Options:
Bind the agent to the UNIX-domain socket bind_address. The default is$TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>.
ssh-agent -a ...
Generate C-shell commands on stdout. This is the default if SHELL looks like it's a csh style of shell.
ssh-agent -c ...
Foreground mode. When this option is specified ssh-agent will not fork.
ssh-agent -D ...
Specifies the hash algorithm used when displaying key fingerprints. Valid options are: “md5” and“sha256”. The default is “sha256”.
ssh-agent -E ...
Kill the current agent (given by the SSH_AGENT_PID environment variable).
ssh-agent -k ...
Specify a pattern-list of acceptable paths for PKCS#11 shared libraries that may be added using the -soption to ssh-add(1). The default is to allow loading PKCS#11 libraries from“/usr/lib/*,/usr/local/lib/*”. PKCS#11 libraries that do not match the whitelist will be refused. SeePATTERNS in ssh_config(5) for a description of pattern-list syntax.
ssh-agent -P ...
Set a default value for the maximum lifetime of identities added to the agent. The lifetime may bespecified in seconds or in a time format specified in sshd_config(5). A lifetime specified for an iden‐tity with ssh-add(1) overrides this value. Without this option the default maximum lifetime is forever.If a command line is given, this is executed as a subprocess of the agent. When the command dies, so does theagent.The idea is that the agent is run in the user's local PC, laptop, or terminal. Authentication data need not bestored on any other machine, and authentication passphrases never go over the network. However, the connectionto the agent is forwarded over SSH remote logins, and the user can thus use the privileges given by the identi‐ties anywhere in the network in a secure way.There are two main ways to get an agent set up: The first is that the agent starts a new subcommand into whichsome environment variables are exported, eg ssh-agent xterm &. The second is that the agent prints the neededshell commands (either sh(1) or csh(1) syntax can be generated) which can be evaluated in the calling shell, egeval `ssh-agent -s` for Bourne-type shells such as sh(1) or ksh(1) and eval `ssh-agent -c` for csh(1) and deriv‐atives.Later ssh(1) looks at these variables and uses them to establish a connection to the agent.The agent will never send a private key over its request channel. Instead, operations that require a privatekey will be performed by the agent, and the result will be returned to the requester. This way, private keysare not exposed to clients using the agent.A UNIX-domain socket is created 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 anotherinstance of the same user.The SSH_AGENT_PID environment variable holds the agent's process ID.The agent exits automatically when the command given on the command line terminates.In Debian, ssh-agent is installed with the set-group-id bit set, to prevent ptrace(2) attacks retrieving privatekey material. This has the side-effect of causing the run-time linker to remove certain environment variableswhich might have security implications for set-id programs, including LD_PRELOAD, LD_LIBRARY_PATH, and TMPDIR.If you need to set any of these environment variables, you will need to do so in the program executed by ssh-agent.FILES$TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>UNIX-domain sockets used to contain the connection to the authentication agent. These sockets shouldonly be readable by the owner. The sockets should get automatically removed when the agent exits.
ssh-agent -t ...