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Diff for /src/usr.bin/ssh/ssh.1 between version 1.64.2.8 and 1.65

version 1.64.2.8, 2002/03/08 17:04:43

version 1.65, 2000/11/08 19:37:33

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.\" incompatible with the protocol description in the RFC file, it must be

.\" called by a name other than "ssh" or "Secure Shell".

.\"

.\" Redistribution and use in source and binary forms, with or without

.\" modification, are permitted provided that the following conditions

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.Os

.Sh NAME

.Nm ssh

.Nd OpenSSH SSH client (remote login program)

.Nd OpenSSH secure shell client (remote login program)

.Sh SYNOPSIS

.Nm ssh

.Op Fl l Ar login_name

.Ar hostname | user@hostname

.Op Ar hostname | user@hostname

.Op Ar command

.Pp

.Nm ssh

.Op Fl afgknqstvxACNPTX1246

.Op Fl afgknqtvxACNPTX246

.Op Fl b Ar bind_address

.Op Fl c Ar cipher_spec

.Op Fl e Ar escape_char

.Op Fl i Ar identity_file

.Op Fl l Ar login_name

.Op Fl m Ar mac_spec

.Op Fl o Ar option

.Op Fl p Ar port

.Op Fl F Ar configfile

.Oo Fl L Xo

.Sm off

.Ar port :

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.Sm on

.Xc

.Oc

.Op Fl D Ar port

.Op Ar hostname | user@hostname

.Ar hostname | user@hostname

.Op Ar command

.Sh DESCRIPTION

.Nm

(SSH client) is a program for logging into a remote machine and for

(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 provide secure encrypted communications between

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This form of authentication alone is normally not

allowed by the server because it is not secure.

.Pp

The second authentication method is the

The second (and primary) authentication method is the

.Pa rhosts

.Pa hosts.equiv

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.Pp

.Ss SSH protocol version 2

.Pp

When a user connects using protocol version 2

When a user connects using the protocol version 2

similar authentication methods are available.

different authentication methods are available:

Using the default values for

At first, the client attempts to authenticate using the public key method.

.Cm PreferredAuthentications ,

If this method fails password authentication is tried.

the client will try to authenticate first using the hostbased method;

if this method fails public key authentication is attempted,

and finally if this method fails keyboard-interactive and

password authentication are tried.

.Pp

The public key method is similar to RSA authentication described

in the previous section and allows the RSA or DSA algorithm to be used:

in the previous section except that the DSA algorithm is used

The client uses his private key,

instead of the patented RSA algorithm.

The client uses his private DSA key

.Pa $HOME/.ssh/id_dsa

.Pa $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

.Pa $HOME/.ssh/authorized_keys

.Pa $HOME/.ssh/authorized_keys2

and grants access if both the key is found and the signature is correct.

The session identifier is derived from a shared Diffie-Hellman value

and is only known to the client and the server.

.Pp

If public key authentication fails or is not available a password

can be sent encrypted to the remote host for proving the user's identity.

This protocol 2 implementation does not yet support Kerberos or

S/Key authentication.

.Pp

Additionally,

.Nm

supports hostbased or challenge response authentication.

.Pp

Protocol 2 provides additional mechanisms for confidentiality

(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)

and integrity (hmac-md5, hmac-sha1).

and integrity (hmac-sha1, hmac-md5).

Note that protocol 1 lacks a strong mechanism for ensuring the

integrity of the connection.

.Pp

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the remote command or shell will be automatically encrypted.

.Pp

If a pseudo-terminal has been allocated (normal login session), the

user may use the escape characters noted below.

user can disconnect with

.Ic ~. ,

and suspend

.Nm

with

.Ic ~^Z .

All forwarded connections can be listed with

.Ic ~#

and if

the session blocks waiting for forwarded X11 or TCP/IP

connections to terminate, it can be backgrounded with

.Ic ~&

(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

.Ic ~? .

.Pp

A single tilde character can be sent as

.Ic ~~

(or by following the tilde by 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.

.Pp

If no pseudo tty has been allocated, the

session is transparent and can be used to reliably transfer binary

data.

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.Dq none

will also make the session transparent even if a tty is used.

.Pp

The session terminates when the command or shell on the remote

The session terminates when the command or shell in on the remote

machine exits and all X11 and TCP/IP connections have been closed.

machine exists and all X11 and TCP/IP connections have been closed.

The exit status of the remote program is returned as the exit status

.Nm ssh .

.Pp

.Ss Escape Characters

.Pp

When a pseudo terminal has been requested, ssh supports a number of functions

through the use of an escape character.

.Pp

A single tilde character can be sent as

.Ic ~~

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

.Cm EscapeChar

configuration directive or on the command line by the

.Fl e

option.

.Pp

The supported escapes (assuming the default

.Ql ~ )

are:

.Bl -tag -width Ds

.It Cm ~.

Disconnect

.It Cm ~^Z

Background ssh

.It Cm ~#

List forwarded connections

.It Cm ~&

Background ssh at logout when waiting for forwarded connection / X11 sessions

to terminate

.It Cm ~?

Display a list of escape characters

.It Cm ~R

Request rekeying of the connection (only useful for SSH protocol version 2

and if the peer supports it)

.El

.Pp

.Ss X11 and TCP forwarding

.Pp

If the

If the user is using X11 (the

.Cm ForwardX11

variable is set to

.Dq yes

(or, see the description of the

.Fl X

and

.Fl x

options described later)

and the user is using X11 (the

.Ev 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

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.Pp

If the user is using an authentication agent, the connection to the agent

is automatically forwarded to the remote side unless disabled on

the command line or in a configuration file.

command line or in a configuration file.

.Pp

Forwarding of arbitrary TCP/IP connections over the secure channel can

be specified either on the command line or in a configuration file.

be specified either on command line or in a configuration file.

One possible application of TCP/IP forwarding is a secure connection to an

electronic purse; another is going through firewalls.

electronic purse; another is going trough firewalls.

.Pp

.Ss Server authentication

.Pp

.Nm

automatically maintains and checks a database containing

identifications for all hosts it has ever been used with.

Host keys are stored in

RSA host keys are stored in

.Pa $HOME/.ssh/known_hosts

and

DSA host keys are stored in

.Pa $HOME/.ssh/known_hosts2

in the user's home directory.

Additionally, the file

Additionally, the files

.Pa /etc/ssh_known_hosts

is automatically checked for known hosts.

and

.Pa /etc/ssh_known_hosts2

are automatically checked for known hosts.

Any new hosts are automatically added to the user's file.

If a host's identification

ever changes,

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.It Fl A

Enables forwarding of the authentication agent connection.

This can also be specified on a per-host basis in a configuration file.

.It Fl b Ar bind_address

.It Fl c Ar blowfish|3des

Specify the interface to transmit from on machines with multiple

interfaces or aliased addresses.

.It Fl c Ar blowfish|3des|des

Selects the cipher to use for encrypting the session.

.Ar 3des

is used by default.

It is believed to be secure.

.Ar 3des

(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.

It is presumably more secure than the

.Ar des

cipher which is no longer fully supported in

.Nm ssh .

.Ar blowfish

is a fast block cipher, it appears very secure and is much faster than

.Ar 3des .

.Ar des

.It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"

is only supported in the

.Nm

client for interoperability with legacy protocol 1 implementations

that do not support the

.Ar 3des

cipher. Its use is strongly discouraged due to cryptographic

weaknesses.

.It Fl c Ar cipher_spec

Additionally, for protocol version 2 a comma-separated list of ciphers can

be specified in order of preference.

See

Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode

.Cm Ciphers

and Arcfour.

for more information.

.It Fl e Ar ch|^ch|none

Sets the escape character for sessions with a pty (default:

.Ql ~ ) .

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.It Fl g

Allows remote hosts to connect to local forwarded ports.

.It Fl i Ar identity_file

Selects a file from which the identity (private key) for

Selects the file from which the identity (private key) for

RSA or DSA authentication is read.

RSA authentication is read.

The default is

Default is

.Pa $HOME/.ssh/identity

for protocol version 1, and

in the user's home directory.

.Pa $HOME/.ssh/id_rsa

and

.Pa $HOME/.ssh/id_dsa

for protocol version 2.

Identity files may also be specified on

a per-host basis in the configuration file.

It is possible to have multiple

.Fl i

options (and multiple identities specified in

configuration files).

.It Fl I Ar smartcard_device

Specifies which smartcard device to use. The argument is

the device

.Nm

should use to communicate with a smartcard used for storing the user's

private RSA key.

.It Fl k

Disables forwarding of Kerberos tickets and AFS tokens.

This may also be specified on a per-host basis in the configuration file.

.It Fl l Ar 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.

.It Fl m Ar 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

.Cm MACs

keyword for more information.

.It Fl n

Redirects stdin from

.Pa /dev/null

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option.)

.It Fl N

Do not execute a remote command.

This is useful for just forwarding ports

This is usefull if you just want to forward ports

(protocol version 2 only).

.It Fl o Ar option

Can be used to give options in the format used in the configuration file.

Can be used to give options in the format used in the config 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.

.It Fl p Ar port

Port to connect to on the remote host.

This can be specified on a

per-host basis in the configuration file.

.It Fl P

Use a non-privileged port for outgoing connections.

This can be used if a firewall does

This can be used if your firewall does

not permit connections from privileged ports.

Note that this option turns off

.Cm RhostsAuthentication

and

.Cm RhostsRSAAuthentication

.Cm RhostsRSAAuthentication .

for older servers.

.It Fl q

Quiet mode.

Causes all warning and diagnostic messages to be suppressed.

.It Fl s

Only fatal errors are displayed.

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 applications (eg. sftp). The

subsystem is specified as the remote command.

.It Fl 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

.Fl t

options force tty allocation, even if

.Nm

has no local tty.

.It Fl T

Disable pseudo-tty allocation.

Disable pseudo-tty allocation (protocol version 2 only).

.It Fl v

Verbose mode.

Causes

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to print debugging messages about its progress.

This is helpful in

debugging connection, authentication, and configuration problems.

Multiple

The verbose mode is also used to display

.Fl v

.Xr skey 1

options increases the verbosity.

challenges, if the user entered "s/key" as password.

Multiple -v options increases the verbosity.

Maximum is 3.

.It Fl x

Disables X11 forwarding.

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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

.Cm Compression

.Cm Compress

option below.

.It Fl F Ar configfile

Specifies an alternative per-user configuration file.

If a configuration file is given on the command line,

the system-wide configuration file

.Pq Pa /etc/ssh_config

will be ignored.

The default for the per-user configuration file is

.Pa $HOME/.ssh/config .

.It Fl L Ar 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.

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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 with an alternative syntax:

.Ar port/host/hostport

.It Fl D Ar port

Specifies a local

.Dq dynamic

application-level port forwarding.

This works by allocating a socket to listen to

.Ar port

on the local side, and 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 protocol is supported, and

.Nm

will act as a SOCKS4 server.

Only root can forward privileged ports.

Dynamic port forwardings can also be specified in the configuration file.

.It Fl 1

Forces

.Nm

to try protocol version 1 only.

.It Fl 2

Forces

.Nm

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.El

.Sh CONFIGURATION FILES

.Nm

obtains configuration data from the following sources in

obtains configuration data from the following sources (in this order):

the following order:

command line options, user's configuration file

.Pq Pa $HOME/.ssh/config ,

and system-wide configuration file

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.Pp

Otherwise a line is of the format

.Dq keyword arguments .

Configuration options may be separated by whitespace or

optional whitespace and exactly one

.Ql = ;

the latter format is useful to avoid the need to quote whitespace

when specifying configuration options using the

.Nm ssh ,

.Nm scp

and

.Nm sftp

.Fl o

option.

.Pp

The possible

keywords and their meanings are as follows (note that

keywords and their meanings are as follows (note that the

keywords are case-insensitive and arguments are case-sensitive):

configuration files are case-sensitive):

.Bl -tag -width Ds

.It Cm Host

Restricts the following declarations (up to the next

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.Dq yes

.Dq no .

This option applies to protocol version 1 only.

.It Cm BatchMode

If set to

.Dq yes ,

passphrase/password querying will be disabled.

This option is useful in scripts and other batch jobs where no user

This option is useful in scripts and other batch jobs where you have no

is present to supply the password.

user to supply the password.

The argument must be

.Dq yes

.Dq no .

The default is

.Dq no .

.It Cm BindAddress

Specify the interface to transmit from on machines with multiple

interfaces or aliased addresses.

Note that this option does not work if

.Cm UsePrivilegedPort

is set to

.Dq yes .

.It Cm CheckHostIP

If this flag is set to

.Dq yes ,

ssh will additionally check the host IP address in the

ssh will additionally check the host ip address in the

.Pa known_hosts

file.

This allows ssh to detect if a host key changed due to DNS spoofing.

If the option is set to

.Dq no ,

the check will not be executed.

The default is

.Dq yes .

.It Cm Cipher

Specifies the cipher to use for encrypting the session

in protocol version 1.

Currently,

.Dq blowfish ,

.Dq blowfish

.Dq 3des ,

and

.Dq des

.Dq 3des

are supported.

.Ar des

is only supported in the

.Nm

client for interoperability with legacy protocol 1 implementations

that do not support the

.Ar 3des

cipher. Its use is strongly discouraged due to cryptographic

weaknesses.

The default is

.Dq 3des .

.It Cm Ciphers

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in order of preference.

Multiple ciphers must be comma-separated.

The default is

.Pp

.Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour .

.Bd -literal

``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,

aes192-cbc,aes256-cbc''

.Ed

.It Cm ClearAllForwardings

Specifies that all local, remote and dynamic port forwardings

specified in the configuration files or on the command line be

cleared. This option is primarily useful when used from the

.Nm

command line to clear port forwardings set in

configuration files, and is automatically set by

.Xr scp 1

and

.Xr sftp 1 .

The argument must be

.Dq yes

.Dq no .

The default is

.Dq no .

.It Cm Compression

Specifies whether to use compression.

The argument must be

.Dq yes

.Dq no .

The default is

.Dq no .

.It Cm CompressionLevel

Specifies the compression level to use if compression is enabled.

Specifies the compression level to use if compression is enable.

The argument must be an integer from 1 (fast) to 9 (slow, best).

The default level is 6, which is good for most applications.

The meaning of the values is the same as in

.Xr gzip 1 .

Note that this option applies to protocol version 1 only.

.It Cm ConnectionAttempts

Specifies the number of tries (one per second) to make before falling

back to rsh or exiting.

The argument must be an integer.

This may be useful in scripts if the connection sometimes fails.

The default is 1.

.It Cm DSAAuthentication

.It Cm DynamicForward

Specifies whether to try DSA authentication.

Specifies that a TCP/IP port on the local machine be forwarded

The argument to this keyword must be

over the secure channel, and the application

.Dq yes

protocol is then used to determine where to connect to from the

remote machine. The argument must be a port number.

.Dq no .

Currently the SOCKS4 protocol is supported, and

DSA authentication will only be

.Nm

attempted if a DSA identity file exists.

will act as a SOCKS4 server.

Note that this option applies to protocol version 2 only.

Multiple forwardings may be specified, and

additional forwardings can be given on the command line. Only

the superuser can forward privileged ports.

.It Cm EscapeChar

Sets the escape character (default:

.Ql ~ ) .

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.Dq yes

.Dq no .

The default is

.Dq no .

.It Cm ForwardAgent

Specifies whether the connection to the authentication agent (if any)

will be forwarded to the remote machine.

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.It Cm GatewayPorts

Specifies whether remote hosts are allowed to connect to local

forwarded ports.

By default,

.Nm

binds local port forwardings to the loopback addresss. This

prevents other remote hosts from connecting to forwarded ports.

.Cm GatewayPorts

can be used to specify that

.Nm

should bind local port forwardings to the wildcard address,

thus allowing remote hosts to connect to forwarded ports.

The argument must be

.Dq yes

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The default is

.Dq no .

.It Cm GlobalKnownHostsFile

Specifies a file to use for the global

Specifies a file to use instead of

host key database instead of

.Pa /etc/ssh_known_hosts .

.It Cm HostbasedAuthentication

Specifies whether to try rhosts based authentication with public key

authentication.

The argument must be

.Dq yes

.Dq no .

The default is

.Dq no .

This option applies to protocol version 2 only and

is similar to

.Cm RhostsRSAAuthentication .

.It Cm HostKeyAlgorithms

Specifies the protocol version 2 host key algorithms

that the client wants to use in order of preference.

The default for this option is:

.Dq ssh-rsa,ssh-dss .

.It Cm HostKeyAlias

Specifies an alias that should be used instead of the

real host name when looking up or saving the host key

in the host key database files.

This option is useful for tunneling ssh connections

or for multiple servers running on a single host.

.It Cm HostName

Specifies the real host name to log into.

This can be used to specify nicknames or abbreviations for hosts.

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.Cm HostName

specifications).

.It Cm IdentityFile

Specifies a file from which the user's RSA or DSA authentication identity

Specifies the file from which the user's RSA authentication identity

is read. The default is

is read (default

.Pa $HOME/.ssh/identity

for protocol version 1, and

in the user's home directory).

.Pa $HOME/.ssh/id_rsa

and

.Pa $HOME/.ssh/id_dsa

for protocol version 2.

Additionally, any identities represented by the authentication agent

will be used for authentication.

The file name may use the tilde

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It is possible to have

multiple identity files specified in configuration files; all these

identities will be tried in sequence.

.It Cm IdentityFile2

Specifies the file from which the user's DSA authentication identity

is read (default

.Pa $HOME/.ssh/id_dsa

in the user's home directory).

The file name may use the tilde

syntax to refer to a user's home directory.

It is possible to have

multiple identity files specified in configuration files; all these

identities will be tried in sequence.

.It Cm KeepAlive

Specifies whether the system should send TCP keepalive messages to the

Specifies whether the system should send keepalive messages to the

other side.

If they are sent, death of the connection or crash of one

of the machines will be properly noticed.

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This is important in scripts, and many users want it too.

.Pp

To disable keepalives, the value should be set to

.Dq no .

.Dq no

in both the server and the client configuration files.

.It Cm KerberosAuthentication

Specifies whether Kerberos authentication will be used.

The argument to this keyword must be

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.Dq no .

.It Cm LocalForward

Specifies that a TCP/IP port on the local machine be forwarded over

the secure channel to the specified host and port from the remote machine.

the secure channel to given host:port from the remote machine.

The first argument must be a port number, and the second must be

.Ar host:port .

host:port.

IPv6 addresses can be specified with an alternative syntax:

.Ar host/port .

Multiple forwardings may be specified, and additional

forwardings can be given on the command line.

Only the superuser can forward privileged ports.

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Gives the verbosity level that is used when logging messages from

.Nm ssh .

The possible values are:

QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.

QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.

The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2

The default is INFO.

and DEBUG3 each specify higher levels of verbose output.

.It Cm MACs

Specifies the MAC (message authentication code) algorithms

in order of preference.

The MAC algorithm is used in protocol version 2

for data integrity protection.

Multiple algorithms must be comma-separated.

The default is

.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .

.It Cm NoHostAuthenticationForLocalhost

This option can be used if the home directory is shared across machines.

In this case localhost will refer to a different machine on each of

the machines and the user will get many warnings about changed host keys.

However, this option disables host authentication for localhost.

The argument to this keyword must be

.Dq yes

.Dq no .

The default is to check the host key for localhost.

.It Cm NumberOfPasswordPrompts

Specifies the number of password prompts before giving up.

The argument to this keyword must be an integer.

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.Dq yes

.Dq no .

The default is

Note that this option applies to both protocol version 1 and 2.

.Dq yes .

.It Cm Port

Specifies the port number to connect on the remote host.

Default is 22.

.It Cm PreferredAuthentications

Specifies the order in which the client should try protocol 2

authentication methods. This allows a client to prefer one method (e.g.

.Cm keyboard-interactive )

over another method (e.g.

.Cm password )

The default for this option is:

.Dq hostbased,publickey,keyboard-interactive,password .

.It Cm Protocol

Specifies the protocol versions

.Nm

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.Dq 2 .

Multiple versions must be comma-separated.

The default is

.Dq 2,1 .

.Dq 1,2 .

This means that

.Nm

tries version 2 and falls back to version 1

tries version 1 and falls back to version 2

if version 2 is not available.

if version 1 is not available.

.It Cm ProxyCommand

Specifies the command to use to connect to the server.

The command

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.Cm CheckHostIP

is not available for connects with a proxy command.

.Pp

.It Cm PubkeyAuthentication

Specifies whether to try public key authentication.

The argument to this keyword must be

.Dq yes

.Dq no .

The default is

.Dq yes .

This option applies to protocol version 2 only.

.It Cm RemoteForward

Specifies that a TCP/IP port on the remote machine be forwarded over

the secure channel to the specified host and port from the local machine.

the secure channel to given host:port from the local machine.

The first argument must be a port number, and the second must be

.Ar host:port .

host:port.

IPv6 addresses can be specified with an alternative syntax:

.Ar host/port .

Multiple forwardings may be specified, and additional

forwardings can be given on the command line.

Only the superuser can forward privileged ports.

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authentication time on slow connections when rhosts authentication is

not used.

Most servers do not permit RhostsAuthentication because it

is not secure (see

is not secure (see RhostsRSAAuthentication).

.Cm RhostsRSAAuthentication ) .

The argument to this keyword must be

.Dq yes

.Dq no .

The default is

.Dq yes .

This option applies to protocol version 1 only.

.It Cm RhostsRSAAuthentication

Specifies whether to try rhosts based authentication with RSA host

authentication.

This is the primary authentication method for most sites.

The argument must be

.Dq yes

.Dq no .

The default is

.Dq yes .

This option applies to protocol version 1 only.

.It Cm RSAAuthentication

Specifies whether to try RSA authentication.

The argument to this keyword must be

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RSA authentication will only be

attempted if the identity file exists, or an authentication agent is

running.

The default is

.Dq yes .

Note that this option applies to protocol version 1 only.

.It Cm ChallengeResponseAuthentication

.It Cm SkeyAuthentication

Specifies whether to use challenge response authentication.

Specifies whether to use

.Xr skey 1

authentication.

The argument to this keyword must be

.Dq yes

.Dq no .

The default is

.Dq yes .

.Dq no .

.It Cm SmartcardDevice

Specifies which smartcard device to use. The argument to this keyword is

the device

.Nm

should use to communicate with a smartcard used for storing the user's

private RSA key. By default, no device is specified and smartcard support

is not activated.

.It Cm StrictHostKeyChecking

If this flag is set to

.Dq yes ,

.Nm

will never automatically add host keys to the

ssh will never automatically add host keys to the

.Pa $HOME/.ssh/known_hosts

file, and refuses to connect to hosts whose host key has changed.

and

This provides maximum protection against trojan horse attacks,

.Pa $HOME/.ssh/known_hosts2

however, can be annoying when the

files, and refuses to connect hosts whose host key has changed.

This provides maximum protection against trojan horse attacks.

However, it can be somewhat annoying if you don't have good

.Pa /etc/ssh_known_hosts

file is poorly maintained, or connections to new hosts are

and

frequently made.

.Pa /etc/ssh_known_hosts2

This option forces the user to manually

files installed and frequently

add all new hosts.

connect new hosts.

If this flag is set to

Basically this option forces the user to manually

.Dq no ,

add any new hosts.

.Nm

Normally this option is disabled, and new hosts

will automatically add new host keys to the

will automatically be added to the known host files.

user known hosts files.

If this flag is set to

.Dq ask ,

new host keys

will be added to the user known host files only after the user

has confirmed that is what they really want to do, and

.Nm

will refuse to connect to hosts whose host key has changed.

The host keys of

known hosts will be verified automatically in all cases.

known hosts will be verified automatically in either case.

The argument must be

.Dq yes ,

.Dq yes

.Dq no

.Dq ask .

.Dq no .

The default is

.Dq ask .

.It Cm UsePrivilegedPort

Specifies whether to use a privileged port for outgoing connections.

The argument must be

Line 1177

Line 938

.Dq no .

The default is

.Dq no .

.Dq yes .

Note that this option must be set to

Note that setting this option to

.Dq yes

.Dq no

turns off

.Cm RhostsAuthentication

and

.Cm RhostsRSAAuthentication

.Cm RhostsRSAAuthentication .

authentications are needed with older servers.

.It Cm User

Specifies the user to log in as.

This can be useful when a different user name is used on different machines.

This can be useful if you have a different user name on different machines.

This saves the trouble of

having to remember to give the user name on the command line.

.It Cm UserKnownHostsFile

Specifies a file to use for the user

Specifies a file to use instead of

host key database instead of

.Pa $HOME/.ssh/known_hosts .

.It Cm UseRsh

Specifies that rlogin/rsh should be used for this host.

Line 1234

Line 993

.Nm

uses this special value to forward X11 connections over the secure

channel.

The user should normally not set

The user should normally not set DISPLAY explicitly, as that

.Ev DISPLAY

explicitly, as that

will render the X11 connection insecure (and will require the user to

manually copy any required authorization cookies).

.It Ev HOME

Line 1246

Line 1003

.Ev USER ;

set for compatibility with systems that use this variable.

.It Ev MAIL

Set to the path of the user's mailbox.

Set to point the user's mailbox.

.It Ev PATH

Set to the default

.Ev PATH ,

as specified when compiling

.Nm ssh .

.It Ev SSH_ASKPASS

.Nm

needs a passphrase, it will read the passphrase from the current

terminal if it was run from a terminal.

.Nm

does not have a terminal associated with it but

.Ev DISPLAY

and

.Ev SSH_ASKPASS

are set, it will execute the program specified by

.Ev SSH_ASKPASS

and open an X11 window to read the passphrase.

This is particularly useful when calling

.Nm

from a

.Pa .Xsession

or related script.

(Note that on some machines it

may be necessary to redirect the input from

.Pa /dev/null

to make this work.)

.It Ev SSH_AUTH_SOCK

Identifies the path of a unix-domain socket used to communicate with the

indicates the path of a unix-domain socket used to communicate with the

agent.

.It Ev SSH_CLIENT

Identifies the client end of the connection.

The variable contains

three space-separated values: client ip-address, client port number,

and server port number.

.It Ev SSH_ORIGINAL_COMMAND

The variable contains the original command line if a forced command

is executed.

It can be used to extract the original arguments.

.It Ev SSH_TTY

This is set to the name of the tty (path to the device) associated

with the current shell or command.

Line 1310

Line 1040

.Sh FILES

.Bl -tag -width Ds

.It Pa $HOME/.ssh/known_hosts

Records host keys for all hosts the user has logged into that are not

Records host keys for all hosts the user has logged into (that are not

.Pa /etc/ssh_known_hosts .

.Pa /etc/ssh_known_hosts ) .

See

.Xr sshd 8 .

.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa

.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa

Contains the authentication identity of the user.

Contains the RSA and the DSA authentication identity of the user.

They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.

These files

contain sensitive data and should be readable by the user but not

accessible by others (read/write/execute).

Line 1327

Line 1056

It is possible to specify a passphrase when

generating the key; the passphrase will be used to encrypt the

sensitive part of this file using 3DES.

.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub

.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub

Contains the public key for authentication (public part of the

identity file in human-readable form).

The contents of the

Line 1335

Line 1064

file should be added to

.Pa $HOME/.ssh/authorized_keys

on all machines

where the user wishes to log in using protocol version 1 RSA authentication.

where you wish to log in using RSA authentication.

The contents of the

.Pa $HOME/.ssh/id_dsa.pub

and

.Pa $HOME/.ssh/id_rsa.pub

file should be added to

.Pa $HOME/.ssh/authorized_keys

.Pa $HOME/.ssh/authorized_keys2

on all machines

where the user wishes to log in using protocol version 2 DSA/RSA authentication.

where you wish to log in using DSA authentication.

These files are not

sensitive and can (but need not) be readable by anyone.

These files are

never used automatically and are not necessary; they are only provided for

never used automatically and are not necessary; they is only provided for

the convenience of the user.

.It Pa $HOME/.ssh/config

This is the per-user configuration file.

Line 1359

Line 1086

but the recommended permissions are read/write for the user, and not

accessible by others.

.It Pa $HOME/.ssh/authorized_keys

Lists the public keys (RSA/DSA) that can be used for logging in as this user.

Lists the RSA keys that can be used for logging in as this user.

The format of this file is described in the

.Xr sshd 8

manual page.

In the simplest form the format is the same as the .pub

identity files.

identity files (that is, each line contains the number of bits in

modulus, public exponent, modulus, and comment fields, separated by

spaces).

This file is not highly sensitive, but the recommended

permissions are read/write for the user, and not accessible by others.

.It Pa /etc/ssh_known_hosts

.It Pa $HOME/.ssh/authorized_keys2

Lists the DSA keys that can be used for logging in as this user.

This file is not highly sensitive, but the recommended

permissions are read/write for the user, and not accessible by others.

.It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2

Systemwide list of known host keys.

This file should be prepared by the

.Pa /etc/ssh_known_hosts

contains RSA and

.Pa /etc/ssh_known_hosts2

contains DSA keys.

These files should be prepared by the

system administrator to contain the public host keys of all machines in the

organization.

This file should be world-readable.

This file contains

public keys, one per line, in the following format (fields separated

by spaces): system name, public key and optional comment field.

by spaces): system name, number of bits in modulus, public exponent,

modulus, and optional comment field.

When different names are used

for the same machine, all such names should be listed, separated by

commas.

Line 1396

Line 1134

values that are not specified in the user's configuration file, and

for those users who do not have a configuration file.

This file must be world-readable.

.It Pa /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

.Cm RhostsRSAAuthentication

and

.Cm HostbasedAuthentication .

Since they are readable only by root

.Nm

must be setuid root if these authentication methods are desired.

.It Pa $HOME/.rhosts

This file is used in

.Pa \&.rhosts

Line 1415

Line 1144

Each line of the file contains a host name (in the canonical form

returned by name servers), and then a user name on that host,

separated by a space.

On some machines this file may need to be

One some machines this file may need to be

world-readable if the user's home directory is on a NFS partition,

because

.Xr sshd 8

Line 1430

Line 1159

.Xr sshd 8

will be installed so that it requires successful RSA host

authentication before permitting \s+2.\s0rhosts authentication.

If the server machine does not have the client's host key in

If your server machine does not have the client's host key in

.Pa /etc/ssh_known_hosts ,

it can be stored in

you can store it in

.Pa $HOME/.ssh/known_hosts .

The easiest way to do this is to

connect back to the client from the server machine using ssh; this

Line 1487

Line 1216

Contains additional definitions for environment variables, see section

.Sx ENVIRONMENT

above.

.It Pa libcrypto.so.X.1

A version of this library which includes support for the RSA algorithm

is required for proper operation.

.El

.Sh DIAGNOSTICS

.Sh AUTHOR

.Nm

OpenSSH

exits with the exit status of the remote command or with 255

is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,

if an error occurred.

but with bugs removed and newer features re-added.

.Sh AUTHORS

Rapidly after the

OpenSSH is a derivative of the original and free

1.2.12 release, newer versions of the original ssh bore successively

ssh 1.2.12 release by Tatu Ylonen.

more restrictive licenses, and thus demand for a free version was born.

Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,

.Pp

Theo de Raadt and Dug Song

This version of OpenSSH

removed many bugs, re-added newer features and

.Bl -bullet

created OpenSSH.

.It

Markus Friedl contributed the support for SSH

has all components of a restrictive nature (i.e., patents, see

protocol versions 1.5 and 2.0.

.Xr ssl 8 )

directly removed from the source code; any licensed or patented components

are chosen from

external libraries.

.It

has been updated to support SSH protocol 1.5 and 2, making it compatible with

all other SSH clients and servers.

.It

contains added support for

.Xr kerberos 8

authentication and ticket passing.

.It

supports one-time password authentication with

.Xr skey 1 .

.El

.Pp

OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,

Niels Provos, Theo de Raadt, and Dug Song.

.Pp

The support for SSH protocol 2 was written by Markus Friedl.

.Sh SEE ALSO

.Xr rlogin 1 ,

.Xr rsh 1 ,

.Xr scp 1 ,

.Xr sftp 1 ,

.Xr ssh-add 1 ,

.Xr ssh-agent 1 ,

.Xr ssh-keygen 1 ,

.Xr telnet 1 ,

.Xr sshd 8

.Xr sshd 8 ,

.Rs

.Xr ssl 8

.%A T. Ylonen

.%A T. Kivinen

.%A M. Saarinen

.%A T. Rinne

.%A S. Lehtinen

.%T "SSH Protocol Architecture"

.%N draft-ietf-secsh-architecture-09.txt

.%D July 2001

.%O work in progress material

.Re