SSH-AGENT(1)

NAME
ssh-agent - authentication agent

SYNOPSIS

ssh-agent command

DESCRIPTION

Ssh-agent is a program to hold authentication private keys. 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 children of the ssh-agent program (the command normally starts X or is the user shell). Programs started under the agent inherit a connection to the agent, and the agent is automatically used for RSA authentication when logging to other machines using ssh.

The agent initially does not have any private keys. Keys are added using ssh-add. When executed without arguments, ssh-add adds the $HOME/.ssh/identity file. If the identity has a passphrase, ssh-add 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 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.

A connection to the agent is inherited by child programs. There are two alternative methods for inheriting the agent. The preferred method is to have an open file descriptor which is inherited, and have an environment variable (SSH_AUTHENTICATION_FD) contain the number of this descriptor. This restricts access to the authentication agent to only those programs that are siblings of the agent, and it is fairly difficult even for root to get unauthorized access to the agent.

On some machines, an alternative method is used. A unix-domain socket is created (/tmp/ssh_agent.*), and the name of this socket is stored in the SSH_AUTHENTICATION_SOCKET 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 socket is only used if ssh is unable to find a file descriptor that would not be closed by shells.

The agent exits automatically when the command given on the command line terminates.

FILES

$HOME/.ssh/identity
    Contains the RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file. This file is not used by ssh-agent, but is normally added to the agent using ssh-add at login time.

/tmp/ssh_agent.<pid>
    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.

AUTHOR

Tatu Ylonen <ylo@cs.hut.fi>

SEE ALSO
ssh-add(1), ssh-keygen(1), ssh(1), sshd(8)