from small one page howto to huge articles all in one place

search text in:

Which filesystem do you use?

poll results

Last additions:
using iotop to find disk usage hogs

using iotop to find disk usage hogs






average rating: 1.7 (82 votes) (1=very good 6=terrible)

May 25th. 2007:




why adblockers are bad

Workaround and fixes for the current Core Dump Handling vulnerability affected kernels

Workaround and fixes for the current Core Dump Handling vulnerability affected kernels






average rating: 1.3 (27 votes) (1=very good 6=terrible)

April, 26th. 2006:

You are here: manpages


Section: User Commands (1)
Index Return to Main Contents

BSD mandoc


ssh-agent - authentication agent  


ssh-agent [-c | s ] [-d ] [-a bind_address ] [-t life ] [command [arg ... ] ]
ssh-agent [-c | s ] -k  


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 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 agent initially does not have any private keys. Keys are added using ssh-add1. Multiple identities may be stored in ssh-agent concurrently and ssh(1) will automatically use them if present. ssh-add1 is also used to remove keys from ssh-agent and to query the keys that are held in one.

The options are as follows:

-a bind_address
Bind the agent to the UNIX socket bind_address The default is $TMPDIR/ssh-XXXXXXXXXX/agent.ppid
Generate C-shell commands on stdout This is the default if SHELL looks like it's a csh style of shell.
Debug mode. When this option is specified ssh-agent will not fork.
Kill 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.
-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_config5. A lifetime specified for an identity with ssh-add1 overrides this value. Without this option the default maximum lifetime is forever.

If a commandline is given, this is executed as a subprocess of the agent. When the command dies, so does 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.

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 evaluated 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 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 agent's process ID.

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


UNIX 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-add1, ssh-keygen1, 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.




Please read "Why adblockers are bad".

Other free services
Shorten long
URLs to short
links like
Reverse DNS lookup
Find out which hostname(s)
resolve to a
given IP or other hostnames for the server
rdf newsfeed | rss newsfeed | Atom newsfeed
- Powered by LeopardCMS - Running on Gentoo -
Copyright 2004-2013 Sascha Nitsch Unternehmensberatung UG(haftungsbeschränkt)
Valid XHTML1.1 : Valid CSS : buttonmaker
- Level Triple-A Conformance to Web Content Accessibility Guidelines 1.0 -
- Copyright and legal notices -
Time to create this page: 3.5 ms