SSH-AGENT (1) | General commands | Unix Manual Pages | :man▋
ssh-agent - authentication agent
ssh-agent [-a bind_address] [-c |-s ] [-t life] [-d] [command [args ...]] ssh-agent [-c |-s ] -k
ssh-agent is a program to hold private keys used for public key authentication (RSA, DSA). The idea is that ssh-agent is started in the beginning of an X-session or a login session, and all other windows or programs are started as clients to the ssh-agent program. Through use of environment variables the agent can be located and automatically used for authentication when logging in to other machines using ssh(1).
The options are as follows:
|-a bind_address |
| || Bind the agent to the unix-domain socket bind_address. The default is /tmp/ssh-XXXXXXXX/agent.<ppid>. |
|-c || Generate C-shell commands on stdout. This is the default if SHELL looks like its a csh style of shell. |
|-s || Generate Bourne shell commands on stdout. This is the default if SHELL does not look like its a csh style of shell. |
|-k || Kill the current agent (given by the SSH_AGENT_PID environment variable). |
|-t life |
| || 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(8). A lifetime specified for an identity with ssh-add(1) overrides this value. Without this option the default maximum lifetime is forever. |
|-d || Debug mode. When this option is specified ssh-agent will not fork. |
If a commandline 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 ssh-add(1). When executed without arguments, ssh-add(1) adds the files ~/.ssh/id_rsa, ~/.ssh/id_dsa and ~/.ssh/identity. If the identity has a passphrase, ssh-add(1) asks for the passphrase (using a small X11 application if running under X11, or from the terminal if running without X). It then sends the identity to the agent. Several identities can be stored in the agent; the agent can automatically use any of these identities. ssh-add -l displays the identities currently held by the agent.
The idea is that the agent is run in the users 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 set up: The first is that the agent starts a new subcommand into which some environment variables are exported, eg ssh-agent xterm &. The second is that the agent prints the needed shell commands (either sh(1) or csh(1) syntax can be generated) which can be evalled in the calling shell, eg eval ssh-agent -s for Bourne-type shells such as sh(1) or ksh(1) and eval ssh-agent -c for csh(1) and derivatives.
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 private key will be performed by the agent, and the result will be returned to the requester. This way, private keys are 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 another instance of the same user.
The SSH_AGENT_PID environment variable holds the agents process ID.
The agent exits automatically when the command given on the command line terminates.
| ~/.ssh/identity |
| || Contains the protocol version 1 RSA authentication identity of the user. |
| ~/.ssh/id_dsa |
| || Contains the protocol version 2 DSA authentication identity of the user. |
| ~/.ssh/id_rsa |
| || Contains the protocol version 2 RSA authentication identity of the user. |
| /tmp/ssh-XXXXXXXX/agent.<ppid> |
| || 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), sshd(8)
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