SSH(1)SSH(1)NAME
ssh - OpenSSH SSH client (remote login program)
SYNOPSIS
ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D\fP
[bind_address:] port ] [-e escape_char] [-F configfile] [-i iden‐
tity_file] [-L\fP [bind_address:] port:host:hostport ] [-l login_name]
[-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-R\fP
[bind_address:] port:host:hostport ] [-S ctl_path] [-w tunnel :tunnel]
[user@]hostname [command]
DESCRIPTION
ssh (SSH client) is a program for logging into a remote machine and for
executing commands on a remote machine. It is intended to replace
rlogin and rsh, and provide secure encrypted communications between two
untrusted hosts over an insecure network. X11 connections and arbi‐
trary TCP ports can also be forwarded over the secure channel.
ssh connects and logs into the specified hostname (with optional user
name). The user must prove his/her identity to the remote machine
using one of several methods depending on the protocol version used
(see below).
If command is specified, it is executed on the remote host instead of a
login shell.
The options are as follows:
-1 Forces ssh to try protocol version 1 only.
-2 Forces ssh to try protocol version 2 only.
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
-A Enables forwarding of the authentication agent connection. This
can also be specified on a per-host basis in a configuration
file.
Agent forwarding should be enabled with caution. Users with the
ability to bypass file permissions on the remote host (for the
agent's Unix-domain socket) can access the local agent through
the forwarded connection. An attacker cannot obtain key mate‐
rial from the agent, however they can perform operations on the
keys that enable them to authenticate using the identities
loaded into the agent.
-a Disables forwarding of the authentication agent connection.
-b bind_address
Use bind_address on the local machine as the source address of
the connection. Only useful on systems with more than one
address.
-C Requests compression of all data (including stdin, stdout,
stderr, and data for forwarded X11 and TCP connections). The
compression algorithm is the same used by gzip(1), and the
``level'' can be controlled by the CompressionLevel option for
protocol version 1. Compression is desirable on modem lines and
other slow connections, but will only slow down things on fast
networks. The default value can be set on a host-by-host basis
in the configuration files; see the Compression option.
-c cipher_spec
Selects the cipher specification for encrypting the session.
Protocol version 1 allows specification of a single cipher. The
supported values are ``3des'', ``blowfish'', and ``des''. 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three
different keys. It is believed to be secure. blowfish is a
fast block cipher; it appears very secure and is much faster
than 3des. des is only supported in the ssh client for interop‐
erability with legacy protocol 1 implementations that do not
support the 3des cipher. Its use is strongly discouraged due to
cryptographic weaknesses. The default is ``3des''.
For protocol version 2, cipher_spec is a comma-separated list of
ciphers listed in order of preference. The supported ciphers
are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr,
aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow‐
fish-cbc, and cast128-cbc. The default is:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr
-D [bind_address:] port
Specifies a local ``dynamic'' application-level port forwarding.
This works by allocating a socket to listen to port on the local
side, optionally bound to the specified bind_address. Whenever
a connection is made to this port, the connection is forwarded
over the secure channel, and the application protocol is then
used to determine where to connect to from the remote machine.
Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh
will act as a SOCKS server. Only root can forward privileged
ports. Dynamic port forwardings can also be specified in the
configuration file.
IPv6 addresses can be specified with an alternative syntax:
[bind_address/] port or by enclosing the address in square
brackets. Only the superuser can forward privileged ports. By
default, the local port is bound in accordance with the Gateway‐
Ports setting. However, an explicit bind_address may be used to
bind the connection to a specific address. The bind_address of
``localhost'' indicates that the listening port be bound for
local use only, while an empty address or `*' indicates that the
port should be available from all interfaces.
-e escape_char
Sets the escape character for sessions with a pty (default: `~'
) . The escape character is only recognized at the beginning of
a line. The escape character followed by a dot (`.') closes
the connection; followed by control-Z suspends the connection;
and followed by itself sends the escape character once. Setting
the character to ``none'' disables any escapes and makes the
session fully transparent.
-F configfile
Specifies an alternative per-user configuration file. If a con‐
figuration file is given on the command line, the system-wide
configuration file (/usr/local/etc/ssh_config) will be ignored.
The default for the per-user configuration file is ~/.ssh/con‐
fig.
-f Requests ssh to go to background just before command execution.
This is useful if ssh is going to ask for passwords or
passphrases, but the user wants it in the background. This
implies -n. The recommended way to start X11 programs at a
remote site is with something like ssh -f host xterm.
-g Allows remote hosts to connect to local forwarded ports.
-I smartcard_device
Specify the device ssh should use to communicate with a smart‐
card used for storing the user's private RSA key. This option
is only available if support for smartcard devices is compiled
in (default is no support).
-i identity_file
Selects a file from which the identity (private key) for RSA or
DSA authentication is read. The default is ~/.ssh/identity for
protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro‐
tocol version 2. Identity files may also be specified on a per-
host basis in the configuration file. It is possible to have
multiple -i options (and multiple identities specified in con‐
figuration files).
-k Disables forwarding (delegation) of GSSAPI credentials to the
server.
-L [bind_address:] port:host:hostport
Specifies that the given port on the local (client) host is to
be forwarded to the given host and port on the remote side.
This works by allocating a socket to listen to port on the local
side, optionally bound to the specified bind_address. Whenever
a connection is made to this port, the connection is forwarded
over the secure channel, and a connection is made to host port
hostport from the remote machine. Port forwardings can also be
specified in the configuration file. IPv6 addresses can be
specified with an alternative syntax:
[bind_address/] port/host/ hostport or by enclosing the address
in square brackets. Only the superuser can forward privileged
ports. By default, the local port is bound in accordance with
the GatewayPorts setting. However, an explicit bind_address may
be used to bind the connection to a specific address. The
bind_address of ``localhost'' indicates that the listening port
be bound for local use only, while an empty address or `*' indi‐
cates that the port should be available from all interfaces.
-l login_name
Specifies the user to log in as on the remote machine. This
also may be specified on a per-host basis in the configuration
file.
-M Places the ssh client into ``master'' mode for connection shar‐
ing. Multiple -M options places ssh into ``master'' mode with
confirmation required before slave connections are accepted.
Refer to the description of ControlMaster in ssh_config(5) for
details.
-m mac_spec
Additionally, for protocol version 2 a comma-separated list of
MAC (message authentication code) algorithms can be specified in
order of preference. See the MACs keyword for more information.
-N Do not execute a remote command. This is useful for just for‐
warding ports (protocol version 2 only).
-n Redirects stdin from /dev/null (actually, prevents reading from
stdin). This must be used when ssh is run in the background. A
common trick is to use this to run X11 programs on a remote
machine. For example, ssh -n shadows.cs.hut.fi emacs & will
start an emacs on shadows.cs.hut.fi, and the X11 connection will
be automatically forwarded over an encrypted channel. The ssh
program will be put in the background. (This does not work if
ssh needs to ask for a password or passphrase; see also the -f
option.)
-O ctl_cmd
Control an active connection multiplexing master process. When
the -O option is specified, the ctl_cmd argument is interpreted
and passed to the master process. Valid commands are: ``check''
(check that the master process is running) and ``exit'' (request
the master to exit).
-o option
Can be used to give options in the format used in the configura‐
tion file. This is useful for specifying options for which
there is no separate command-line flag. For full details of the
options listed below, and their possible values, see ssh_con‐
fig(5).
AddressFamily
BatchMode
BindAddress
ChallengeResponseAuthentication
CheckHostIP
Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
DynamicForward
EscapeChar
ForwardAgent
ForwardX11
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
HashKnownHosts
Host
HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
HostName
IdentityFile
IdentitiesOnly
KbdInteractiveDevices
LocalCommand
LocalForward
LogLevel
MACs
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
Port
PreferredAuthentications
Protocol
ProxyCommand
PubkeyAuthentication
RekeyLimit
RemoteForward
RhostsRSAAuthentication
RSAAuthentication
SendEnv
ServerAliveInterval
ServerAliveCountMax
SmartcardDevice
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UsePrivilegedPort
User
UserKnownHostsFile
VerifyHostKeyDNS
XAuthLocation
-p port
Port to connect to on the remote host. This can be specified on
a per-host basis in the configuration file.
-q Quiet mode. Causes all warning and diagnostic messages to be
suppressed.
-R [bind_address:] port:host:hostport
Specifies that the given port on the remote (server) host is to
be forwarded to the given host and port on the local side. This
works by allocating a socket to listen to port on the remote
side, and whenever a connection is made to this port, the con‐
nection is forwarded over the secure channel, and a connection
is made to host port hostport from the local machine.
Port forwardings can also be specified in the configuration
file. Privileged ports can be forwarded only when logging in as
root on the remote machine. IPv6 addresses can be specified by
enclosing the address in square braces or using an alternative
syntax:
[bind_address/] host/port/ hostport
By default, the listening socket on the server will be bound to
the loopback interface only. This may be overriden by specify‐
ing a bind_address. An empty bind_address, or the address `*',
indicates that the remote socket should listen on all inter‐
faces. Specifying a remote bind_address will only succeed if
the server's GatewayPorts option is enabled (see sshd_config(5))
.
-S ctl_path
Specifies the location of a control socket for connection shar‐
ing. Refer to the description of ControlPath and ControlMaster
in ssh_config(5) for details.
-s May be used to request invocation of a subsystem on the remote
system. Subsystems are a feature of the SSH2 protocol which
facilitate the use of SSH as a secure transport for other appli‐
cations (eg. sftp(1)) . The subsystem is specified as the
remote command.
-T Disable pseudo-tty allocation.
-t Force pseudo-tty allocation. This can be used to execute arbi‐
trary screen-based programs on a remote machine, which can be
very useful, e.g., when implementing menu services. Multiple -t
options force tty allocation, even if ssh has no local tty.
-V Display the version number and exit.
-v Verbose mode. Causes ssh to print debugging messages about its
progress. This is helpful in debugging connection, authentica‐
tion, and configuration problems. Multiple -v options increase
the verbosity. The maximum is 3.
-w tunnel :tunnel
Requests a tun(4) device on the client (first tunnel arg) and
server (second tunnel arg). The devices may be specified by
numerical ID or the keyword ``any'', which uses the next avail‐
able tunnel device. See also the Tunnel directive in ssh_con‐
fig(5).
-X Enables X11 forwarding. This can also be specified on a per-
host basis in a configuration file.
X11 forwarding should be enabled with caution. Users with the
ability to bypass file permissions on the remote host (for the
user's X authorization database) can access the local X11 dis‐
play through the forwarded connection. An attacker may then be
able to perform activities such as keystroke monitoring.
For this reason, X11 forwarding is subjected to X11 SECURITY
extension restrictions by default. Please refer to the ssh -Y
option and the ForwardX11Trusted directive in ssh_config(5) for
more information.
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
subjected to the X11 SECURITY extension controls.
ssh may additionally obtain configuration data from a per-user
configuration file and a system-wide configuration file. The
file format and configuration options are described in ssh_con‐
fig(5).
ssh exits with the exit status of the remote command or with 255
if an error occurred.
AUTHENTICATION
The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is
the default, with ssh falling back to protocol 1 if it detects protocol
2 is unsupported. These settings may be altered using the Protocol
option in ssh_config(5), or enforced using the -1 and -2 options (see
above). Both protocols support similar authentication methods, but
protocol 2 is preferred since it provides additional mechanisms for
confidentiality (the traffic is encrypted using AES, 3DES, Blowfish,
CAST128, or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-
ripemd160). Protocol 1 lacks a strong mechanism for ensuring the
integrity of the connection.
The methods available for authentication are: host-based authentica‐
tion, public key authentication, challenge-response authentication, and
password authentication. Authentication methods are tried in the order
specified above, though protocol 2 has a configuration option to change
the default order: PreferredAuthentications.
Host-based authentication works as follows: If the machine the user
logs in from is listed in /etc/hosts.equiv or
/usr/local/etc/shosts.equiv on the remote machine, and the user names
are the same on both sides, or if the files ~/.rhosts or ~/.shosts
exist in the user's home directory on the remote machine and contain a
line containing the name of the client machine and the name of the user
on that machine, the user is considered for login. Additionally, the
server must be able to verify the client's host key (see the descrip‐
tion of /usr/local/etc/ssh_known_hosts and ~/.ssh/known_hosts, below)
for login to be permitted. This authentication method closes security
holes due to IP spoofing, DNS spoofing, and routing spoofing. [Note to
the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh pro‐
tocol in general, are inherently insecure and should be disabled if
security is desired.]
Public key authentication works as follows: The scheme is based on pub‐
lic-key cryptography, using cryptosystems where encryption and decryp‐
tion are done using separate keys, and it is unfeasible to derive the
decryption key from the encryption key. The idea is that each user
creates a public/private key pair for authentication purposes. The
server knows the public key, and only the user knows the private key.
ssh implements public key authentication protocol automatically, using
either the RSA or DSA algorithms. Protocol 1 is restricted to using
only RSA keys, but protocol 2 may use either. The HISTORY section of
ssl(8) contains a brief discussion of the two algorithms.
The file ~/.ssh/authorized_keys lists the public keys that are permit‐
ted for logging in. When the user logs in, the ssh program tells the
server which key pair it would like to use for authentication. The
client proves that it has access to the private key and the server
checks that the corresponding public key is authorized to accept the
account.
The user creates his/her key pair by running ssh-keygen(1). This
stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa
(protocol 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the pub‐
lic key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (proto‐
col 2 DSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
directory. The user should then copy the public key to ~/.ssh/autho‐
rized_keys in his/her home directory on the remote machine. The autho‐
rized_keys file corresponds to the conventional ~/.rhosts file, and has
one key per line, though the lines can be very long. After this, the
user can log in without giving the password.
The most convenient way to use public key authentication may be with an
authentication agent. See ssh-agent(1) for more information.
Challenge-response authentication works as follows: The server sends an
arbitrary Qq challenge text, and prompts for a response. Protocol 2
allows multiple challenges and responses; protocol 1 is restricted to
just one challenge/response. Examples of challenge-response authenti‐
cation include BSD Authentication (see login.conf(5)) and PAM (some
non-OpenBSD systems).
Finally, if other authentication methods fail, ssh prompts the user for
a password. The password is sent to the remote host for checking; how‐
ever, since all communications are encrypted, the password cannot be
seen by someone listening on the network.
ssh automatically maintains and checks a database containing identifi‐
cation for all hosts it has ever been used with. Host keys are stored
in ~/.ssh/known_hosts in the user's home directory. Additionally, the
file /usr/local/etc/ssh_known_hosts is automatically checked for known
hosts. Any new hosts are automatically added to the user's file. If a
host's identification ever changes, ssh warns about this and disables
password authentication to prevent server spoofing or man-in-the-middle
attacks, which could otherwise be used to circumvent the encryption.
The StrictHostKeyChecking option can be used to control logins to
machines whose host key is not known or has changed.
When the user's identity has been accepted by the server, the server
either executes the given command, or logs into the machine and gives
the user a normal shell on the remote machine. All communication with
the remote command or shell will be automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the
user may use the escape characters noted below.
If no pseudo-tty has been allocated, the session is transparent and can
be used to reliably transfer binary data. On most systems, setting the
escape character to ``none'' will also make the session transparent
even if a tty is used.
The session terminates when the command or shell on the remote machine
exits and all X11 and TCP connections have been closed.
ESCAPE CHARACTERS
When a pseudo-terminal has been requested, ssh supports a number of
functions through the use of an escape character.
A single tilde character can be sent as ~~ or by following the tilde by
a character other than those described below. The escape character
must always follow a newline to be interpreted as special. The escape
character can be changed in configuration files using the EscapeChar
configuration directive or on the command line by the -e option.
The supported escapes (assuming the default `~' ) are:
~. Disconnect.
~^Z Background .
~# List forwarded connections.
~& Background ssh at logout when waiting for forwarded connection /
X11 sessions to terminate.
~? Display a list of escape characters.
~B Send a BREAK to the remote system (only useful for SSH protocol
version 2 and if the peer supports it).
~C Open command line. Currently this allows the addition of port
forwardings using the -L and -R options (see above). It also
allows the cancellation of existing remote port-forwardings
using -KR hostport. ! Ns command allows the user to execute a
local command if the PermitLocalCommand option is enabled in
ssh_config(5). Basic help is available, using the -h option.
~R Request rekeying of the connection (only useful for SSH protocol
version 2 and if the peer supports it).
TCP FORWARDING
Forwarding of arbitrary TCP connections over the secure channel can be
specified either on the command line or in a configuration file. One
possible application of TCP forwarding is a secure connection to a mail
server; another is going through firewalls.
In the example below, we look at encrypting communication between an
IRC client and server, even though the IRC server does not directly
support encrypted communications. This works as follows: the user con‐
nects to the remote host using , specifying a port to be used to for‐
ward connections to the remote server. After that it is possible to
start the service which is to be encrypted on the client machine, con‐
necting to the same local port, and ssh will encrypt and forward the
connection.
The following example tunnels an IRC session from client machine
``127.0.0.1'' (localhost) to remote server ``server.example.com :''
$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
$ irc -c '#users' -p 1234 pinky 127.0.0.1
This tunnels a connection to IRC server ``server.example.com'', joining
channel ``#users'', nickname ``pinky'', using port 1234. It doesn't
matter which port is used, as long as it's greater than 1023 (remember,
only root can open sockets on privileged ports) and doesn't conflict
with any ports already in use. The connection is forwarded to port
6667 on the remote server, since that's the standard port for IRC ser‐
vices.
The -f option backgrounds ssh and the remote command ``sleep 10'' is
specified to allow an amount of time (10 seconds, in the example) to
start the service which is to be tunnelled. If no connections are made
within the time specified, ssh will exit.
X11 FORWARDING
If the ForwardX11 variable is set to ``yes'' (or see the description of
the -X, -x, and -Y options above) and the user is using X11 (the DIS‐
PLAY environment variable is set), the connection to the X11 display is
automatically forwarded to the remote side in such a way that any X11
programs started from the shell (or command) will go through the
encrypted channel, and the connection to the real X server will be made
from the local machine. The user should not manually set DISPLAY.
Forwarding of X11 connections can be configured on the command line or
in configuration files.
The DISPLAY value set by ssh will point to the server machine, but with
a display number greater than zero. This is normal, and happens
because ssh creates a ``proxy'' X server on the server machine for for‐
warding the connections over the encrypted channel.
ssh will also automatically set up Xauthority data on the server
machine. For this purpose, it will generate a random authorization
cookie, store it in Xauthority on the server, and verify that any for‐
warded connections carry this cookie and replace it by the real cookie
when the connection is opened. The real authentication cookie is never
sent to the server machine (and no cookies are sent in the plain).
If the ForwardAgent variable is set to ``yes'' (or see the description
of the -A and -a options above) and the user is using an authentication
agent, the connection to the agent is automatically forwarded to the
remote side.
VERIFYING HOST KEYS
When connecting to a server for the first time, a fingerprint of the
server's public key is presented to the user (unless the option Stric‐
tHostKeyChecking has been disabled). Fingerprints can be determined
using ssh-keygen(1):
Dl $ ssh-keygen -l -f /usr/local/etc/ssh_host_rsa_key
If the fingerprint is already known, it can be matched and verified,
and the key can be accepted. If the fingerprint is unknown, an alter‐
native method of verification is available: SSH fingerprints verified
by DNS. An additional resource record (RR), SSHFP, is added to a zone‐
file and the connecting client is able to match the fingerprint with
that of the key presented.
In this example, we are connecting a client to a server, ``host.exam‐
ple.com''. The SSHFP resource records should first be added to the
zonefile for host.example.com:
$ ssh-keygen -f /usr/local/etc/ssh_host_rsa_key.pub -r host.exam‐
ple.com.
$ ssh-keygen -f /usr/local/etc/ssh_host_dsa_key.pub -r host.exam‐
ple.com.
The output lines will have to be added to the zonefile. To check that
the zone is answering fingerprint queries:
Dl $ dig -t SSHFP host.example.com
Finally the client connects:
$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?
See the VerifyHostKeyDNS option in ssh_config(5) for more information.
SSH-BASED VIRTUAL PRIVATE NETWORKS
ssh contains support for Virtual Private Network (VPN) tunnelling using
the tun(4) network pseudo-device, allowing two networks to be joined
securely. The sshd_config(5) configuration option PermitTunnel con‐
trols whether the server supports this, and at what level (layer 2 or 3
traffic).
The following example would connect client network 10.0.50.0/24 with
remote network 10.0.99.0/24, provided that the SSH server running on
the gateway to the remote network, at 192.168.1.15, allows it:
# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
Client access may be more finely tuned via the /root/.ssh/autho‐
rized_keys file (see below) and the PermitRootLogin server option. The
following entry would permit connections on the first tun(4) device
from user ``jane'' and on the second device from user ``john'', if Per‐
mitRootLogin is set to ``forced-commands-only :''
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun1" ssh-rsa ... john
Since a SSH-based setup entails a fair amount of overhead, it may be
more suited to temporary setups, such as for wireless VPNs. More per‐
manent VPNs are better provided by tools such as ipsecctl(8) and
isakmpd(8).
ENVIRONMENT
ssh will normally set the following environment variables:
DISPLAY
The DISPLAY variable indicates the location of the X11 server.
It is automatically set by ssh to point to a value of the form
``hostname:n'', where ``hostname'' indicates the host where the
shell runs, and `n' is an integer 1. ssh uses this special
value to forward X11 connections over the secure channel. The
user should normally not set DISPLAY explicitly, as that will
render the X11 connection insecure (and will require the user to
manually copy any required authorization cookies).
HOME Set to the path of the user's home directory.
LOGNAME
Synonym for USER; set for compatibility with systems that use
this variable.
MAIL Set to the path of the user's mailbox.
PATH Set to the default PATH, as specified when compiling .
SSH_ASKPASS
If ssh needs a passphrase, it will read the passphrase from the
current terminal if it was run from a terminal. If ssh does not
have a terminal associated with it but DISPLAY and SSH_ASKPASS
are set, it will execute the program specified by SSH_ASKPASS
and open an X11 window to read the passphrase. This is particu‐
larly useful when calling ssh from a .xsession or related
script. (Note that on some machines it may be necessary to re‐
direct the input from /dev/null to make this work.)
SSH_AUTH_SOCK
Identifies the path of a Ux-domain socket used to communicate
with the agent.
SSH_CONNECTION
Identifies the client and server ends of the connection. The
variable contains four space-separated values: client IP
address, client port number, server IP address, and server port
number.
SSH_ORIGINAL_COMMAND
This variable contains the original command line if a forced
command is executed. It can be used to extract the original
arguments.
SSH_TTY
This is set to the name of the tty (path to the device) associ‐
ated with the current shell or command. If the current session
has no tty, this variable is not set.
TZ This variable is set to indicate the present time zone if it was
set when the daemon was started (i.e., the daemon passes the
value on to new connections).
USER Set to the name of the user logging in.
Additionally, ssh reads ~/.ssh/environment, and adds lines of
the format ``VARNAME=value'' to the environment if the file
exists and users are allowed to change their environment. For
more information, see the PermitUserEnvironment option in
sshd_config(5).
FILES
~/.rhosts
This file is used for host-based authentication (see above). On
some machines this file may need to be world-readable if the
user's home directory is on an NFS partition, because sshd(8)
reads it as root. Additionally, this file must be owned by the
user, and must not have write permissions for anyone else. The
recommended permission for most machines is read/write for the
user, and not accessible by others.
~/.shosts
This file is used in exactly the same way as .rhosts, but allows
host-based authentication without permitting login with
rlogin/rsh.
~/.ssh/authorized_keys
Lists the public keys (RSA/DSA) that can be used for logging in
as this user. The format of this file is described in the
sshd(8) manual page. This file is not highly sensitive, but the
recommended permissions are read/write for the user, and not
accessible by others.
~/.ssh/config
This is the per-user configuration file. The file format and
configuration options are described in ssh_config(5). Because
of the potential for abuse, this file must have strict permis‐
sions: read/write for the user, and not accessible by others.
~/.ssh/environment
Contains additional definitions for environment variables; see
ENVIRONMENT , above.
~/.ssh/identity
~/.ssh/id_dsa
~/.ssh/id_rsa
Contains the private key for authentication. These files con‐
tain sensitive data and should be readable by the user but not
accessible by others (read/write/execute). ssh will simply
ignore a private key file if it is accessible by others. It is
possible to specify a passphrase when generating the key which
will be used to encrypt the sensitive part of this file using
3DES.
~/.ssh/identity.pub
~/.ssh/id_dsa.pub
~/.ssh/id_rsa.pub
Contains the public key for authentication. These files are not
sensitive and can (but need not) be readable by anyone.
~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged
into that are not already in the systemwide list of known host
keys. See sshd(8) for further details of the format of this
file.
~/.ssh/rc
Commands in this file are executed by ssh when the user logs in,
just before the user's shell (or command) is started. See the
sshd(8) manual page for more information.
/etc/hosts.equiv
This file is for host-based authentication (see above). It
should only be writable by root.
/usr/local/etc/shosts.equiv
This file is used in exactly the same way as hosts.equiv, but
allows host-based authentication without permitting login with
rlogin/rsh.
/usr/local/etc/ssh_config
Systemwide configuration file. The file format and configura‐
tion options are described in ssh_config(5).
/usr/local/etc/ssh_host_key
/usr/local/etc/ssh_host_dsa_key
/usr/local/etc/ssh_host_rsa_key
These three files contain the private parts of the host keys and
are used for host-based authentication. If protocol version 1
is used, ssh must be setuid root, since the host key is readable
only by root. For protocol version 2, ssh uses ssh-keysign(8)
to access the host keys, eliminating the requirement that ssh be
setuid root when host-based authentication is used. By default
ssh is not setuid root.
/usr/local/etc/ssh_known_hosts
Systemwide list of known host keys. This file should be pre‐
pared by the system administrator to contain the public host
keys of all machines in the organization. It should be world-
readable. See sshd(8) for further details of the format of this
file.
/etc/ssh/sshrc
Commands in this file are executed by ssh when the user logs in,
just before the user's shell (or command) is started. See the
sshd(8) manual page for more information.
SEE ALSOscp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-
keyscan(1), tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8),
sshd(8)
S. Lehtinen, T. Rinne, M. Saarinen, T. Kivinen and T. Ylonen, SSH Pro‐
tocol Architecture, draft-ietf-secsh-architecture-12.txt, January 2002,
work in progress material.
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.
September 25, 1999 SSH(1)
