Windows PowerShell command on Get-command ssh

Manual Pages for UNIX Operating System command usage for man ssh

User Commands ssh(1)

NAME

ssh - secure shell client (remote login program)

SYNOPSIS

ssh [-l login_name] hostname | user@hostname [ command]

ssh [-afgknqstvxACNTX1246] [-b bind_address] [-m mac_spec]

[-c cipher_spec] [-e escape_char] [-i identity_file]

[-l login_name] [-F configfile] [-o option] [-p port]

[-L [bind_address:]port:host:hostport]

[-R [bind_address:]port:host:hostport]

[-D [bind_address:]port] hostname | user@hostname [command]

DESCRIPTION

ssh (Secure Shell) 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 to provide secure encrypted communications between two untrusted hosts over an

insecure network. X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.

ssh connects and logs into the specified hostname. The user

must prove his or her identity to the remote machine using one of several methods depending on the protocol version used: SSH Protocol Version 1 First, 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, the user is immediately permitted to log in. Second, if .rhosts or .shosts exists in the user's home directory on the remote machine and contains a line containing the name of the client machine and the name of the user on that machine, the user is permitted to log in. This form of authentication alone is normally not allowed by the server because it is not secure. The second (and primary) authentication method is the rhosts

or hosts.equiv method combined with RSA-based host authenti-

cation. It means that if the login would be permitted by

$HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or

/etc/shosts.equiv, and if additionally the server can verify

the client's host key (see /etc/ssh_known_hosts in the FILES

section), only then is login permitted. This authentication

method closes security holes due to IP spoofing, DNS spoof-

ing, and routing spoofing.

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User Commands ssh(1)

Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts,

and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.

As a third authentication method, ssh supports RSA-based

authentication. The scheme is based on public-key cryptogra-

phy. There are cryptosystems where encryption and decryption are done using separate keys, and it is not possible to derive the decryption key from the encryption key. RSA is one such system. 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. The file $HOME/.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 server checks if this key is permitted, and if so, sends the user (actually

the ssh program running on behalf of the user) a challenge

in the form of a random number, encrypted by the user's pub-

lic key. The challenge can only be decrypted using the proper private key. The user's client then decrypts the challenge using the private key, proving that he or she knows the private key but without disclosing it to the server.

ssh implements the RSA authentication protocol automati-

cally. The user creates his or her RSA key pair by running

ssh-keygen(1). This stores the private key in

$HOME/.ssh/identity and the public key in

$HOME/.ssh/identity.pub in the user's home directory. The

user should then copy the identity.pub to

$HOME/.ssh/authorized_keys in his or her home directory on

the remote machine (the authorized_keys file corresponds to

the conventional $HOME/.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. RSA authentica-

tion is much more secure than rhosts authentication. The most convenient way to use RSA authentication can be

with an authentication agent. See ssh-agent(1) for more

information.

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 net-

work.

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User Commands ssh(1)

SSH Protocol Version 2

The SSH version 2 protocol supports multiple user authenti-

cation methods, some of which are similar to those available with the SSH protocol version 1. These authentication mechanisms are negotiated by the client and server, with the

client trying methods in the order specified in the Prefer-

redAuthentications client configuration option. The server

decides when enough authentication methods have passed suc-

cessfully so as to complete the authentication phase of the protocol. When a user connects by using protocol version 2, similar authentication methods are available. Using the default values for PreferredAuthentications, the client tries to authenticate first by using the hostbased method. If this method fails, public key authentication is attempted.

Finally, if this method fails, keyboard-interactive and

password authentication are tried. The public key method is similar to RSA authentication described in the previous section and allows the RSA or DSA algorithm to be used: The client uses his or her private

key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to sign the

session identifier and sends the result to the server. The server checks whether the matching public key is listed in

$HOME/.ssh/authorized_keys and grants access if both the key

is found and the signature is correct. The session identif-

ier is derived from a shared Diffie-Hellman value and is

only known to the client and the server. If public key authentication fails or is not available, a

password can be sent encrypted to the remote host for prov-

ing the user's identity, or an extended prompt/reply proto-

col can be engaged.

Additionally, ssh supports hostbased or challenge response

authentication.

Protocol 2 provides additional mechanisms for confidential-

ity (the traffic is encrypted using 3DES, Blowfish, CAST128

or Arcfour) and integrity (hmac-sha1, hmac-md5). Protocol 1

lacks a strong mechanism for ensuring the integrity of the connection. Login Session and Remote Execution When the user's identity has been accepted by the server, the server either executes the specified command, or logs

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into the machine and gives the user a normal shell on the remote machine. All communication with the remote command or shell is automatically encrypted.

If a pseudo-terminal has been allocated (normal login ses-

sion), the user can use the escape characters noted below.

If a pseudo-terminal has been allocated (normal login ses-

sion), the user can disconnect with ~., and suspend ssh with

~^Z. All forwarded connections can be listed with ~#. If the

session blocks waiting for forwarded X11 or TCP/IP connec-

tions to terminate, ssh can be backgrounded with ~&,

although this should not be used while the user shell is active, as it can cause the shell to hang. All available escapes can be listed with ~?. A single tilde character can be sent as ~~, or by following the tilde with a character other than those described above. The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files or on the command line.

If no pseudo tty has been allocated, the session is tran-

sparent and can be used to reliably transfer binary data. On most systems, setting the escape character to "none" also makes 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/IP connections have been closed. The exit status of the remote program is

returned as the exit status of ssh.

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 with 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.

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~^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 supports it.

~C Open command line. Only useful for adding port for-

wardings using the -L and -R options).

~R Request rekeying of the connection. Only useful for SSH protocol version 2 and if the peer supports it. X11 and TCP Forwarding If the ForwardX11 variable is set to ``yes'' (or, see the

description of the -X and -x options described later) 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 pro-

grams started from the shell (or command) goes through the encrypted channel, and the connection to the real X server is 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 points to the server machine,

but with a display number greater than zero. This is normal

behavior, because ssh creates a "proxy" X11 server on the

server machine for forwarding the connections over the encrypted channel.

ssh also automatically sets up Xauthority data on the server

machine. For this purpose, it generates a random authoriza-

tion cookie, store it in Xauthority on the server, and ver-

ify that any forwarded 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).

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If the ForwardAgent variable is set to "yes" (or, see the

description of the -A and -a options described later) and

the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.

Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on the command line or in a

configuration file. One possible application of TCP/IP for-

warding is a secure connection to an electronic purse. Another possible application is firewall traversal. Server Authentication

ssh automatically maintains and checks a database containing

identifications for all hosts it has ever been used with.

Host keys are stored in $HOME/.ssh/known_hosts in the user's

home directory. Additionally, the file /etc/ssh_known_hosts

is automatically checked for known hosts. The behavior of

ssh with respect to unknown host keys is controlled by the

StrictHostKeyChecking parameter. If a host's identification

ever changes, ssh warns about this and disables password

authentication to prevent a trojan horse from getting the user's password. Another purpose of this mechanism is to prevent attacks by intermediaries which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to prevent logins to machines whose host key is not known or has changed.

However, when using key exchange protected by GSS-API, the

server can advertise a host key. The client automatically adds this host key to its known hosts file,

$HOME/.ssh/known_hosts, regardless of the setting of the

StrictHostKeyChecking option, unless the advertised host key collides with an existing known hosts entry.

When the user's GSS-API credentials expire, the client con-

tinues to be able to rekey the session using the server's public host key to protect the key exchanges.

GSS-API User and Server Authentication

ssh uses the user's GSS-API credentials to authenticate the

client to the server wherever possible, if GssKeyEx and/or GssAuthentication are set. With GssKeyEx, one can have an SSHv2 server that has no host public keys, so that only GssKeyEx can be used. With such servers, rekeying fails if the client's credentials are expired.

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GSS-API user authentication has the disadvantage that it

does not obviate the need for SSH host keys, but its failure

does not impact rekeying. ssh can try other authentication

methods (such as public key, password, and so on) if GSS-API

authentication fails.

Delegation of GSS-API credentials can be quite useful, but

is not without danger. As with passwords, users should not delegate GSS credentials to untrusted servers, since a

compromised server can use a user's delegated GSS creden-

tials to impersonate the user.

GSS-API user authorization is covered in gss_auth_rules(5).

Rekeying can be used to redelegate credentials when GssKeyEx is "yes". (See ~R under Escape Characters above.) OPTIONS The following options are supported:

-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

Disables forwarding of the authentication agent connec-

tion.

-A

Enables forwarding of the authentication agent

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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 material from the agent. However, the attacker can perform operations on the keys

that enable the attacker to authenticate using the iden-

tities loaded into the agent.

-b bind_address

Specifies the interface to transmit from on machines with multiple interfaces or aliased addresses.

-c cipher_spec

Selects the cipher specification for encrypting the ses-

sion.

For protocol version 1, cipher_spec is a single cipher.

See the Cipher option in ssh_config(4) for more informa-

tion.

For protocol version 2, cipher_spec is a comma-separated

list of ciphers listed in order of preference. See the

Ciphers option in ssh_config(4) for more information.

-C

Requests compression of all data (including stdin,

stdout, stderr, and data for forwarded X11 and TCP/IP connections). The compression algorithm is the same used by gzip(1). The gzip man page is available in the SUNWsfman package. The "level" can be controlled by the

CompressionLevel option (see ssh_config(4)). Compression

is desirable on modem lines and other slow connections, but only slows down things on fast networks. The default

value can be set on a host-by-host basis in the confi-

guration files. See the Compression option in

ssh_config(4).

-D [bind_address:]port

Specifies a local dynamic application-level port for-

warding. This works by allocating a socket to listen to

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port on the local side, optionally bound to the speci-

fied bind_address. Whenever a connection is made to this

port, the connection is forwarded over the secure chan-

nel. 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

acts as a SOCKS server. Only a user with enough privileges can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.

IPv6 addresses can be specified with an alternative syn-

tax: [bind_address/]port or by enclosing the address in

square brackets. By default, the local port is bound in accordance with the GatewayPorts setting. However, an

explicit bind_address can 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 ch | ^ch | none

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 fol-

lowed by a dot (.) closes the connection. If followed by

CTRL-z, the escape character suspends the connection. If

followed by itself, the escape character sends itself once. Setting the character to none disables any escapes and makes the session fully transparent.

-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 the -n option. The recommended

way to start X11 programs at a remote site is with some-

thing like ssh -f host xterm.

-F configfile

Specifies an alternative per-user configuration file. If

a configuration file is specified on the command line,

the system-wide configuration file, /etc/ssh_config, is

ignored. The default for the per-user configuration file

is $HOME/.ssh/config.

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-g

Allows remote hosts to connect to local forwarded ports.

-i identity_file

Selects a file from which the identity (private key) for RSA or DSA authentication is read. The default is

$HOME/.ssh/identity for protocol version 1, and

$HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for protocol

version 2. Identity files can 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 configuration files).

-l login_name

Specifies the user to log in as on the remote machine.

This also can be specified on a per-host basis in the

configuration file.

-L [bind_address:]port:host:hostport

Specifies that the specified port on the local (client) host is to be forwarded to the specified host and port on the remote side. This works by allocating a socket to listen to the port on the local side, optionally bound

to the specified bind_address. Then, whenever a connec-

tion 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. Only a user with enough privileges can forward privileged

ports. IPv6 addresses can be specified with an alterna-

tive syntax: [bind_address/]port/host/hostport or by

enclosing the address in square brackets. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit

bind_address can be used to bind the connection to a

specific address. The bind_address of localhost indi-

cates 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.

-m mac_spec

Additionally, for protocol version 2 a comma-separated

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list of MAC (message authentication code) algorithms can

be specified in order of preference. See the MACs key-

word for more information.

-n

Redirects stdin from /dev/null (actually, prevents read-

ing 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 &

starts an emacs on shadows.cs.hut.fi, and the X11 con-

nection is automatically forwarded over an encrypted

channel. The ssh program is put in the background. This

does not work if ssh needs to ask for a password or

passphrase. See also the -f option.

-N

Does not execute a remote command. This is useful if you just want to forward ports (protocol version 2 only).

-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. The option has the same format as a line in the configuration file.

-p port

Specifies the port to connect to on the remote host.

This can be specified on a per-host basis in the confi-

guration file.

-P

Obsoleted option. SSHv1 connections from privileged ports are not supported.

-q

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Quiet mode. Causes all warning and diagnostic messages to be suppressed. Only fatal errors are displayed.

-R [bind_address:]port:host:hostport

Specifies that the specified port on the remote (server) host is to be forwarded to the specified host and port on the local side. This works by allocating a socket to listen to the port on the remote side. Then, whenever a

connection is made to this port, the connection is for-

warded over the secure channel and a connection is made

to host port hostport from the local machine. Port for-

wardings can also be specified in the configuration

file. Privileged ports can be forwarded only when log-

ging in on the remote machine as a user with enough privileges. 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 is bound to the loopback interface only. This can be overridden

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 only succeeds if the server's GatewayPorts

option is enabled. See sshd_config(4).

-s

Can be used to request invocation of a subsystem on the

remote system. Subsystems are a feature of the SSH2 pro-

tocol which facilitate the use of SSH as a secure tran-

sport for other applications, for example, sftp. The subsystem is specified as the remote command.

-t

Forces pseudo-tty allocation. This can be used to exe-

cute arbitrary screen-based programs on a remote

machine, which can be very useful, for example, when

implementing menu services. Multiple -t options force

allocation, even if ssh has no local tty.

-T

Disables pseudo-tty allocation (protocol version 2

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only).

-v

Verbose mode. Causes ssh to print debugging messages

about its progress. This is helpful in debugging connec-

tion, authentication, and configuration problems. Multi-

ple -v options increase the verbosity. Maximum is 3.

-x

Disables X11 forwarding.

-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 display through the forwarded connection. An attacker can then be able to perform activities such as keystroke monitoring. For this reason, X11 forwarding might be subjected to X11 SECURITY extension restrictions. Refer to the

ForwardX11Trusted directive in ssh_config(4) for more

information. If X11 forwarding is enabled, remote X11 clients is trusted by default. This means that they have full access to the original X11 display. ENVIRONMENT VARIABLES

ssh normally sets the following environment variables:

DISPLAY

The DISPLAY variable must be set for X11 display for-

warding to work.

SSH_ASKPASS

If ssh needs a passphrase, it reads the passphrase from

the current terminal if it was run from a terminal. If

ssh does not have a terminal associated with it but

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DISPLAY and SSH_ASKPASS are set, it executes the program

specified by SSH_ASKPASS and opens an X11 window to read

the passphrase. This is particularly useful when calling

ssh from a .Xsession or related script. On some machines

it might be necessary to redirect the input from /dev/null to make this work. The system is shipped with

/usr/lib/ssh/ssh-askpass which is the default value for

SSH_ASKPASS

SSH_AUTH_SOCK

Indicates the path of a unix-domain socket used to com-

municate with the agent.

SSH_LANGS

A comma-separated list of IETF language tags (see

RFC3066) indicating the languages that the user can read and write. Used for negotiation of the locale on the server.

LANG, LC_ALL, LC_COLLATE, LC_CTYPE,

LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME

The values of these environment variables can be set in remote sessions according to the locale settings on the client side and availability of support for those locales on the server side. Environment Variable Passing (see RFC 4254) is used for passing them over to the server side.

See the ENVIRONMENT VARIABLES section in the sshd(1M) man

page for more information on how locale setting can be further changed depending on server side configuration. EXIT STATUS The status of the remote program is returned as the exit

status of ssh. 255 is returned if an error occurred at any-

time during the ssh connection, including the initial key

exchange. FILES

$HOME/.ssh/known_hosts Records host keys for all hosts

the user has logged into that are

not in /etc/ssh/ssh_known_hosts.

See sshd(1M).

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$HOME/.ssh/identity Contains the authentication iden-

$HOME/.ssh/id_dsa tity of the user. These files are

$HOME/.ssh/id_ssa for protocol 1 RSA, protocol 2

DSA, and protocol 2 RSA, respec-

tively. These files contain sensi-

tive data and should be readable by the user but not accessible by

others (read/write/execute). ssh

ignores a private key file if it is accessible by others. It is possible to specify a passphrase when generating the key. The passphrase is used to encrypt the sensitive part of this file using 3DES.

/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 sshd(1M) for more

information.

$HOME/.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 sshd(1M) for more

information.

$HOME/.ssh/environment Contains additional definitions

for environment variables. See ENVIRONMENT VARIABLES.

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes:

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____________________________________________________________

| ATTRIBUTE TYPE | ATTRIBUTE VALUE |

|_____________________________|_____________________________|

| Availability | network/ssh |

|_____________________________|_____________________________|

| Interface Stability | See below. |

|_____________________________|_____________________________|

The command line syntax is Committed. The remote locale

selection through passing LC_* environment variables is

Uncommitted.

rlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-

keygen(1), ssh-http-proxy-connect(1), ssh-socks5-proxy-

connect(1), telnet(1), sshd(1M), ssh_config(4),

sshd_config(4), attributes(5), gss_auth_rules(5), ker-

beros(5), privileges(5) RFC 1928 RFC 4254

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