SSH(1) BSD General Commands Manual SSH(1)
NAME
ssh -- OpenSSH SSH client (remote login program)
SYNOPSIS
ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
[-e escape_char] [-F configfile] [-i identity_file] [-L [bind_address:]port:host:hostport]
[-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-R
[bind_address:]port:host:hostport] [-S ctl_path] [-w local_tun[:remote_tun]] [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 arbitrary TCP ports can also be for-warded forwarded
warded 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 permis-sions permissions
sions 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 material from the agent, how-ever however
ever they can perform operations on the keys that enable them to authenticate using the identi-ties identities
ties 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 desir-able desirable
able on modem lines and other slow connections, but will only slow down things on fast net-works. networks.
works. 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 interoper-ability interoperability
ability 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, blowfish-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 chan-nel, channel,
nel, 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 speci-fied specified
fied 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 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 `*' 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 configuration file is given on the
command line, the system-wide configuration file (/etc/ssh_config) will be ignored. The
default for the per-user configuration file is ~/.ssh/config.
-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 smartcard 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
protocol version 2. Identity files may also be specified on a per-host basis in the configura-tion configuration
tion file. It is possible to have multiple -i options (and multiple identities specified in
configuration 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 configu-ration configuration
ration 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 `*' indicates 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 perhost
host basis in the configuration file.
-M Places the ssh client into ``master'' mode for connection sharing. 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 infor-mation. information.
mation.
-N Do not execute a remote command. This is useful for just forwarding 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 shad-ows.cs.hut.fi, shadows.cs.hut.fi,
ows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted chan-nel. channel.
nel. 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 configuration 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_config(5).
AddressFamily
BatchMode
BindAddress
ChallengeResponseAuthentication
CheckHostIP
Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
DynamicForward
EscapeChar
ExitOnForwardFailure
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 con-figuration configuration
figuration 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 connection 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 for-warded forwarded
warded 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 specifying a bind_address. An empty bind_address, or the address `*',
indicates that the remote socket should listen on all interfaces. 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 sharing. 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 fea-ture feature
ture of the SSH2 protocol which facilitate the use of SSH as a secure transport for other
applications (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 arbitrary 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, authentication, and configuration problems. Multiple -v options increase
the verbosity. The maximum is 3.
-w local_tun[:remote_tun]
Requests tunnel device forwarding with the specified tun(4) devices between the client
(local_tun) and the server (remote_tun).
The devices may be specified by numerical ID or the keyword ``any'', which uses the next avail-able available
able tunnel device. If remote_tun is not specified, it defaults to ``any''. See also the
Tunnel and TunnelDevice directives in ssh_config(5). If the Tunnel directive is unset, it is
set to the default tunnel mode, which is ``point-to-point''.
-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 permis-sions permissions
sions on the remote host (for the user's X authorization database) can access the local X11
display 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_config(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 mecha-nisms mechanisms
nisms 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: GSSAPI-based authentication, host-based authentication,
public key authentication, challenge-response authentication, and password authentication. Authentica-tion Authentication
tion 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 /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 description of /etc/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permit-ted. permitted.
ted. 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 protocol in gen-eral, general,
eral, are inherently insecure and should be disabled if security is desired.]
Public key authentication works as follows: The scheme is based on public-key cryptography, using cryp-tosystems cryptosystems
tosystems where encryption and decryption 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 permitted 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 public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 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/authorized_keys in his/her home directory on the remote machine. The authorized_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 "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 authentication include BSD Authentica-tion Authentication
tion (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; however, since all communications are encrypted, the password
cannot be seen by someone listening on the network.
ssh automatically maintains and checks a database containing identification for all hosts it has ever
been used with. Host keys are stored in ~/.ssh/known_hosts in the user's home directory. Addition-ally, Additionally,
ally, the file /etc/ssh_known_hosts is automatically checked for known hosts. Any new hosts are auto-matically automatically
matically 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 con-trol control
trol 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 con-nections connections
nections 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 ssh.
~# 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 sup-ports supports
ports 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[bind_address:]port. !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 sup-ports supports
ports 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
connects to the remote host using ssh, specifying a port to be used to forward connections to the
remote server. After that it is possible to start the service which is to be encrypted on the client
machine, connecting 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 services.
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 DISPLAY 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 con-nections connections
nections 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
forwarding 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 forwarded
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 StrictHostKeyChecking has been disabled). Fingerprints can be deter-mined determined
mined using ssh-keygen(1):
$ ssh-keygen -l -f /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 alternative method of verification is available: SSH fingerprints veri-fied verified
fied by DNS. An additional resource record (RR), SSHFP, is added to a zonefile 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.example.com''. The SSHFP resource
records should first be added to the zonefile for host.example.com:
$ ssh-keygen -r host.example.com.
The output lines will have to be added to the zonefile. To check that the zone is answering finger-print fingerprint
print queries:
$ 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, pseudodevice,
device, allowing two networks to be joined securely. The sshd_config(5) configuration option
PermitTunnel controls 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 using
a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the
gateway to the remote network, at 192.168.1.15, allows it.
On the client:
# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
# route add 10.0.99.0/24 10.1.1.2
On the server:
# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
# route add 10.0.50.0/24 10.1.1.1
Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the
PermitRootLogin server option. The following entry would permit connections on tun(4) device 1 from
user ``jane'' and on tun device 2 from user ``john'', if PermitRootLogin is set to
``forced-commands-only'':
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups,
such as for wireless VPNs. More permanent 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 automati-cally automatically
cally 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.
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 particularly
useful when calling ssh from a .xsession or related script. (Note that on some
machines it may be necessary to redirect the input from /dev/null to make this
work.)
SSH_AUTH_SOCK Identifies the path of a UNIX-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) associated with the cur-rent current
rent 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 envi-ronment environment
ronment 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
permissions: 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 contain 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.
/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.
/etc/ssh_config
Systemwide configuration file. The file format and configuration options are described in
ssh_config(5).
/etc/ssh_host_key
/etc/ssh_host_dsa_key
/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.
/etc/ssh_known_hosts
Systemwide list of known host keys. This file should be prepared by the system administrator
to contain the public host keys of all machines in the organization. It should be world-read-able. world-readable.
able. See sshd(8) for further details of the format of this file.
/etc/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 ALSO
scp(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)
The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006.
Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, 2006.
The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, 2006.
Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, 2006.
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.
BSD September 25, 1999 BSD
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