ssh-agent (1)
Leading comments
$OpenBSD: ssh-agent.1,v 1.64 2016/11/30 06:54:26 jmc Exp $
Author: Tatu Ylonen <ylo@cs.hut.fi>
Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
All rights reserved
As far as I am concerned, the code I have written for this software
can be used freely for any purpose. Any derived versions of this
software must be clearly marked as such, and if the derived work is
incompatible with the protocol description in the RFC file, it must be
called by a name other th...
NAME
ssh-agent - authentication agentSYNOPSIS
ssh-agent [-c | s ] [-[-a bind_address ] ] [-E fingerprint_hash ] [-t life ] [command [arg ... ] ]
ssh-agent [-c | s ] -k
DESCRIPTION
is a program to hold private keys used for public key authentication (RSA, DSA, ECDSA, Ed25519). 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(1) (see AddKeysToAgent in ssh_config5 for details) or ssh-add1. Multiple identities may be stored in concurrently and ssh(1) will automatically use them if present. ssh-add1 is also used to remove keys from 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
- -c
- Generate C-shell commands on stdout This is the default if SHELL looks like it's a csh style of shell.
- -D
- Foreground mode. When this option is specified will not fork.
- -d
- Debug mode. When this option is specified will not fork and will write debug information to standard error.
- -E fingerprint_hash
- Specifies the hash algorithm used when displaying key fingerprints. Valid options are: ``md5'' and ``sha256'' The default is ``sha256''
- -k
- Kill the current agent (given by the SSH_AGENT_PID environment variable).
- -s
- 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 command line 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.
In Debian, is installed with the set-group-id bit set, to prevent ptrace(2) attacks retrieving private key material. This has the side-effect of causing the run-time linker to remove certain environment variables which 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 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.