openssh/ssh-add.1

.\"	$OpenBSD: ssh-add.1,v 1.83 2021/12/22 06:56:41 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 than "ssh" or "Secure Shell".
.\"
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.Dd $Mdocdate: December 22 2021 $
.Dt SSH-ADD 1
.Os
.Sh NAME
.Nm ssh-add
.Nd adds private key identities to the OpenSSH authentication agent
.Sh SYNOPSIS
.Nm ssh-add
.Op Fl cDdKkLlqvXx
.Op Fl E Ar fingerprint_hash
.Op Fl H Ar hostkey_file
.Op Fl h Ar destination_constraint
.Op Fl S Ar provider
.Op Fl t Ar life
.Op Ar
.Nm ssh-add
.Fl s Ar pkcs11
.Nm ssh-add
.Fl e Ar pkcs11
.Nm ssh-add
.Fl T
.Ar pubkey ...
.Sh DESCRIPTION
.Nm
adds private key identities to the authentication agent,
.Xr ssh-agent 1 .
When run without arguments, it adds the files
.Pa ~/.ssh/id_rsa ,
.Pa ~/.ssh/id_dsa ,
.Pa ~/.ssh/id_ecdsa ,
.Pa ~/.ssh/id_ecdsa_sk ,
.Pa ~/.ssh/id_ed25519 ,
and
.Pa ~/.ssh/id_ed25519_sk .
After loading a private key,
.Nm
will try to load corresponding certificate information from the
filename obtained by appending
.Pa -cert.pub
to the name of the private key file.
Alternative file names can be given on the command line.
.Pp
If any file requires a passphrase,
.Nm
asks for the passphrase from the user.
The passphrase is read from the user's tty.
.Nm
retries the last passphrase if multiple identity files are given.
.Pp
The authentication agent must be running and the
.Ev SSH_AUTH_SOCK
environment variable must contain the name of its socket for
.Nm
to work.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl c
Indicates that added identities should be subject to confirmation before
being used for authentication.
Confirmation is performed by
.Xr ssh-askpass 1 .
Successful confirmation is signaled by a zero exit status from
.Xr ssh-askpass 1 ,
rather than text entered into the requester.
.It Fl D
Deletes all identities from the agent.
.It Fl d
Instead of adding identities, removes identities from the agent.
If
.Nm
has been run without arguments, the keys for the default identities and
their corresponding certificates will be removed.
Otherwise, the argument list will be interpreted as a list of paths to
public key files to specify keys and certificates to be removed from the agent.
If no public key is found at a given path,
.Nm
will append
.Pa .pub
and retry.
If the argument list consists of
.Dq -
then
.Nm
will read public keys to be removed from standard input.
.It Fl E Ar fingerprint_hash
Specifies the hash algorithm used when displaying key fingerprints.
Valid options are:
.Dq md5
and
.Dq sha256 .
The default is
.Dq sha256 .
.It Fl e Ar pkcs11
Remove keys provided by the PKCS#11 shared library
.Ar pkcs11 .
.It Fl H Ar hostkey_file
Specifies a known hosts file to look up hostkeys when using
destination-constrained keys via the
.Fl h
flag.
This option may be specified multiple times to allow multiple files to be
searched.
If no files are specified,
.Nm
will use the default
.Xr ssh_config 5
known hosts files:
.Pa ~/.ssh/known_hosts ,
.Pa ~/.ssh/known_hosts2 ,
.Pa /etc/ssh/ssh_known_hosts ,
and
.Pa /etc/ssh/ssh_known_hosts2 .
.It Fl h Ar destination_constraint
When adding keys, constrain them to be usable only through specific hosts or to
specific destinations.
.Pp
Destination constraints of the form
.Sq [user@]dest-hostname
permit use of the key only from the origin host (the one running
.Xr ssh-agent 1 )
to the listed destination host, with optional user name.
.Pp
Constraints of the form
.Sq src-hostname>[user@]dst-hostname
allow a key available on a forwarded
.Xr ssh-agent 1
to be used through a particular host (as specified by
.Sq src-hostname )
to authenticate to a further host,
specified by
.Sq dst-hostname .
.Pp
Multiple destination constraints may be added when loading keys.
When attempting authentication with a key that has destination constraints,
the whole connection path, including
.Xr ssh-agent 1
forwarding, is tested against those constraints and each
hop must be permitted for the attempt to succeed.
For example, if key is forwarded to a remote host,
.Sq host-b ,
and is attempting authentication to another host,
.Sq host-c ,
then the operation will be successful only if
.Sq host-b
was permitted from the origin host and the subsequent
.Sq host-b>host-c
hop is also permitted by destination constraints.
.Pp
Hosts are identified by their host keys, and are looked up from known hosts
files by
.Nm .
Wildcards patterns may be used for hostnames and certificate host
keys are supported.
By default, keys added by
.Nm
are not destination constrained.
.Pp
Destination constraints were added in OpenSSH release 8.9.
Support in both the remote SSH client and server is required when using
destination-constrained keys over a forwarded
.Xr ssh-agent 1
channel.
.Pp
It is also important to note that destination constraints can only be
enforced by
.Xr ssh-agent 1
when a key is used, or when it is forwarded by a
.Sy cooperating
.Xr ssh 1 .
Specifically, it does not prevent an attacker with access to a remote
.Ev SSH_AUTH_SOCK
from forwarding it again and using it on a different host (but only to
a permitted destination).
.It Fl K
Load resident keys from a FIDO authenticator.
.It Fl k
When loading keys into or deleting keys from the agent, process plain private
keys only and skip certificates.
.It Fl L
Lists public key parameters of all identities currently represented
by the agent.
.It Fl l
Lists fingerprints of all identities currently represented by the agent.
.It Fl q
Be quiet after a successful operation.
.It Fl S Ar provider
Specifies a path to a library that will be used when adding
FIDO authenticator-hosted keys, overriding the default of using the
internal USB HID support.
.It Fl s Ar pkcs11
Add keys provided by the PKCS#11 shared library
.Ar pkcs11 .
.It Fl T Ar pubkey ...
Tests whether the private keys that correspond to the specified
.Ar pubkey
files are usable by performing sign and verify operations on each.
.It Fl t Ar life
Set a maximum lifetime when adding identities to an agent.
The lifetime may be specified in seconds or in a time format
specified in
.Xr sshd_config 5 .
.It Fl v
Verbose mode.
Causes
.Nm
to print debugging messages about its progress.
This is helpful in debugging problems.
Multiple
.Fl v
options increase the verbosity.
The maximum is 3.
.It Fl X
Unlock the agent.
.It Fl x
Lock the agent with a password.
.El
.Sh ENVIRONMENT
.Bl -tag -width Ds
.It Ev "DISPLAY", "SSH_ASKPASS" and "SSH_ASKPASS_REQUIRE"
If
.Nm
needs a passphrase, it will read the passphrase from the current
terminal if it was run from a terminal.
If
.Nm
does not have a terminal associated with it but
.Ev DISPLAY
and
.Ev SSH_ASKPASS
are set, it will execute the program specified by
.Ev SSH_ASKPASS
(by default
.Dq ssh-askpass )
and open an X11 window to read the passphrase.
This is particularly useful when calling
.Nm
from a
.Pa .xsession
or related script.
.Pp
.Ev SSH_ASKPASS_REQUIRE
allows further control over the use of an askpass program.
If this variable is set to
.Dq never
then
.Nm
will never attempt to use one.
If it is set to
.Dq prefer ,
then
.Nm
will prefer to use the askpass program instead of the TTY when requesting
passwords.
Finally, if the variable is set to
.Dq force ,
then the askpass program will be used for all passphrase input regardless
of whether
.Ev DISPLAY
is set.
.It Ev SSH_AUTH_SOCK
Identifies the path of a
.Ux Ns -domain
socket used to communicate with the agent.
.It Ev SSH_SK_PROVIDER
Specifies a path to a library that will be used when loading any
FIDO authenticator-hosted keys, overriding the default of using
the built-in USB HID support.
.El
.Sh FILES
.Bl -tag -width Ds -compact
.It Pa ~/.ssh/id_dsa
.It Pa ~/.ssh/id_ecdsa
.It Pa ~/.ssh/id_ecdsa_sk
.It Pa ~/.ssh/id_ed25519
.It Pa ~/.ssh/id_ed25519_sk
.It Pa ~/.ssh/id_rsa
Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
authenticator-hosted Ed25519 or RSA authentication identity of the user.
.El
.Pp
Identity files should not be readable by anyone but the user.
Note that
.Nm
ignores identity files if they are accessible by others.
.Sh EXIT STATUS
Exit status is 0 on success, 1 if the specified command fails,
and 2 if
.Nm
is unable to contact the authentication agent.
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-askpass 1 ,
.Xr ssh-keygen 1 ,
.Xr sshd 8
.Sh 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.