ssh-agent(1)
NAME
ssh-agent --- authentication agent
SYNOPSIS
ssh-agent [-c | -s] [-Dd] [-a bind_address] [-E fingerprint_hash]
[-P pkcs11_whitelist] [-t life] [command [arg ...]]
ssh-agent [-c | -s] -k
DESCRIPTION
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(1) (see AddKeysToAgent in ssh_config(5) for details) or ssh-add(1).
Multiple identities may be stored in ssh-agent concurrently and ssh(1)
will automatically use them if present. ssh-add(1) 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-domain 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 ssh-agent will
not fork.
-d Debug mode. When this option is specified ssh-agent 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).
-P pkcs11_whitelist
Specify a pattern-list of acceptable paths for PKCS#11 shared
libraries that may be added using the -s option 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. See PATTERNS in
ssh_config(5) for a description of pattern-list syntax.
-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_config(5). A lifetime specified
for an identity 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 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-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 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 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-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.
SEE ALSO
ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)
AUTHORS
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.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
