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version 1.1, 1999/09/26 20:53:37

version 1.2, 1999/09/26 22:30:06

Line 11

.\"

.\" $Id$

.\"

.TH SSH 1 "November 8, 1995" "SSH" "SSH"

.Dd September 25, 1999

.Dt SSH 1

.SH NAME

.Os

ssh \- secure shell client (remote login program)

.Sh NAME

.Nm ssh

.SH SYNOPSIS

.Nd secure shell client (remote login program)

.B ssh

.Sh SYNOPSIS

[\c

.Nm ssh

.BI \-l \ login_name\fR\c

.Op Fl l Ar login_name

]

.Ar hostname

.B hostname

.Op Ar command

[\c

.Pp

.IR command \c

.Nm ssh

]

.Op Fl knqtvxXC

.Op Fl c Ar blowfish|idea|des|3des|none

.B ssh

.Op Fl e Ar escape_char

[\c

.Op Fl i Ar identity_file

.BR \-k \c

.Op Fl l Ar login_name

]

.Op Fl o Ar option

[\c

.Op Fl p Ar port

.B \-c

.Op Fl L Ar port:host:hostport

\fIblowfish\fR\||\|\fIidea\fR\||\|\fIdes\fR\||\|\fI3des\fR\||\|\fInone\fR\c

.Op Fl R Ar port:host:hostport

]

.Ar hostname

[\c

.Op Ar command

.BI \-e \ escape_char\fR\c

.Sh DESCRIPTION

]

.Nm

[\c

.BI \-i \ identity_file\fR\c

]

[\c

.BI \-l \ login_name\fR\c

]

[\c

.BR \-n \c

]

[\c

.BI \-o \ option\fR\c

]

[\c

.BI \-p \ port\fR\c

]

[\c

.BR \-q \c

]

[\c

.BR \-t \c

]

[\c

.BR \-v \c

]

[\c

.BR \-x \c

]

[\c

.BR \-X \c

]

[\c

.BR \-C \c

]

[\c

.BI \-L \ port\fB:\fIhost\fB:\fIhostport\fR\c

]

[\c

.BI \-R \ port\fB:\fIhost\fB:\fIhostport\fR\c

]

.I hostname

[\c

.IR command \c

]

.SH DESCRIPTION

.LP

.B Ssh

(Secure Shell) a program for logging into a remote machine and for

executing commands in 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/IP ports can also be forwarded over the secure channel.

.LP

.Pp

.B Ssh

.Nm

connects and logs into the specified

.IR hostname .

.Ar hostname .

The user must prove

his/her identity to the remote machine using one of several methods.

.LP

.Pp

First, if the machine the user logs in from is listed in

.I /etc/hosts.equiv

.Pa /etc/hosts.equiv

.I /etc/shosts.equiv

.Pa /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

.I \&\s+2.\s0rhosts

.Pa \&.rhosts

.I \&\s+2.\s0shosts

.Pa \&.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.

.LP

.Pp

The second (and primary) authentication method is the

.B rhosts

.Pa rhosts

.B hosts.equiv

.Pa hosts.equiv

method combined with RSA-based host authentication. It

means that if the login would be permitted by

.I \&\s+2.\s0rhosts\c

.Pa \&.rhosts ,

\|,

.Pa \&.shosts ,

.I \&\s+2.\s0shosts\c

.Pa /etc/hosts.equiv ,

\|,

.IR /etc/hosts.equiv\c

\|,

.IR /etc/shosts.equiv ",

.Pa /etc/shosts.equiv ,

and additionally it can verify the client's

host key (see

.I \&$HOME/\s+2.\s0ssh/known_hosts

.Pa $HOME/.ssh/known_hosts

and

.I /etc/ssh_known_hosts

.Pa /etc/ssh_known_hosts

in the

.B \s-1FILES\s0

.Sx FILES

section), only then login is

permitted. This authentication method closes security holes due to IP

spoofing, DNS spoofing and routing spoofing. [Note to the

administrator:

.IR /etc/hosts.equiv ",

.Pa /etc/hosts.equiv ,

.IR \&\s+2.\s0rhosts ",

.Pa \&.rhosts ,

and the rlogin/rsh protocol in general, are inherently insecure and should be

disabled if security is desired.]

.LP

.Pp

As a third authentication method,

.B ssh

.Nm

supports RSA based authentication.

The scheme is based on public-key cryptography: there are cryptosystems

where encryption and decryption are done using separate keys, and it

Line 152

Line 102

key pair for authentication purposes. The

server knows the public key, and only the user knows the private key.

The file

.I \&$HOME/\s+2.\s0ssh/authorized_keys

.Pa $HOME/.ssh/authorized_keys

lists the public keys that are permitted for logging

in. When the user logs in, the

.B ssh

.Nm

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

.B ssh

.Nm

program running on behalf of the user) a challenge, a random number,

encrypted by the user's public 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/she knows the private

key but without disclosing it to the server.

.LP

.Pp

.B Ssh

.Nm

implements the RSA authentication protocol automatically. The user

creates his/her RSA key pair by running

.BR ssh-keygen (1).

.Xr ssh-keygen 1 .

This stores the private key in

.I \&\s+2.\s0ssh/identity

.Pa \&.ssh/identity

and the public key in

.I \&\s+2.\s0ssh/identity.pub

.Pa \&.ssh/identity.pub

in the user's home directory. The user should then

copy the

.I identity.pub

.Pa identity.pub

.I \&\s+2.\s0ssh/authorized_keys

.Pa \&.ssh/authorized_keys

in his/her home directory on the remote machine (the

.I authorized_keys

.Pa authorized_keys

file corresponds to the conventional

.I \&\s+2.\s0rhosts

.Pa \&.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 authentication is much

more secure than rhosts authentication.

.LP

.Pp

The most convenient way to use RSA authentication may be with an

authentication agent. See

.BR ssh-agent (1)

.Xr ssh-agent 1

for more information.

.LP

.Pp

If other authentication methods fail,

.B ssh

.Nm

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.

.LP

.Pp

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.

.LP

.Pp

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

user can disconnect with "~.", and suspend

user can disconnect with

.B ssh

.Ic ~. ,

with "~^Z". All forwarded connections can be listed with "~#", and if

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 "~&" (this

connections to terminate, it can be backgrounded with

should not be used while the user shell is active, as it can cause the

.Ic ~&

shell to hang). All available escapes can be listed with "~?".

(this should not be used while the user shell is active, as it can cause the

.LP

shell to hang). All available escapes can be listed with

A single tilde character can be sent as "~~" (or by

.Ic ~? .

following the tilde by a character other than those described above).

.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.

.LP

.Pp

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

data. On most systems, setting the escape character to

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

.Dq none

.LP

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

.Pp

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

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

.B ssh.

.Nm ssh .

.LP

.Pp

If the user is using X11 (the

.B \s-1DISPLAY\s0

.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

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

.BR \s-1DISPLAY\s0 ".

.Ev DISPLAY .

Forwarding of X11 connections can be

configured on the command line or in configuration files.

.LP

.Pp

The DISPLAY value set by

The

.B ssh

.Ev DISPLAY

value set by

.Nm

will point to the server machine, but with a display number greater

than zero. This is normal, and happens because

.B ssh

.Nm

creates a "proxy" X server on the server machine for forwarding the

creates a

.Dq proxy

X server on the server machine for forwarding the

connections over the encrypted channel.

.LP

.Pp

.B Ssh

.Nm

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 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).

.LP

.Pp

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

is automatically forwarded to the remote side unless disabled on

command line or in a configuration file.

.LP

.Pp

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

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 trough firewalls.

.LP

.Pp

.B Ssh

.Nm

automatically maintains and checks a database containing RSA-based

identifications for all hosts it has ever been used with. The

database is stored in

.I \&\s+2.\s0ssh/known_hosts

.Pa \&.ssh/known_hosts

in the user's home directory. Additionally, the file

.I /etc/ssh_known_hosts

.Pa /etc/ssh_known_hosts

is automatically checked for known hosts. Any new hosts are

automatically added to the user's file. If a host's identification

ever changes,

.B ssh

.Nm

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 man-in-the-middle attacks which could

otherwise be used to circumvent the encryption. The

.B StrictHostKeyChecking

.Cm StrictHostKeyChecking

option (see below) can be used to prevent logins to machines whose

host key is not known or has changed.

.Sh OPTIONS

.Bl -tag -width Ds

.ne 5

.It Fl c Ar blowfish|idea|des|3des|none

.SH OPTIONS

.TP

.BI \-c \ \fIblowfish\fR\||\|\fIidea\fR\||\|\fIdes\fR\||\|\fI3des\fR\||\|\fInone\fR

Selects the cipher to use for encrypting the session.

.B \s-13DES\s0

.Ar 3des

is used by default. It is believed to be secure.

.B \s-1DES\s0

.Ar des

is the data encryption standard, but is breakable by

governments, large corporations, and major criminal organizations.

.B \s-13DES\s0

.Ar 3des

(triple-des) is encrypt-decrypt-encrypt triple with three different

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

keys. It is presumably more secure than

It is presumably more secure than DES.

DES.

.Ar none

.B none

disables encryption entirely; it is only intended for debugging, and

it renders the connection insecure.

.ne 3

.It Fl e Ar ch|^ch|none

.TP

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

.B \-e \fIch\fR\||\|\fI^ch\fR\||\|\fInone\fR

.Ql ~ ) .

Sets the escape character for sessions with a pty (default: ~). The

The escape character is only recognized at the beginning of a line. The

escape character is only recognized at the beginning of a line. The

escape character followed by a dot

escape character followed by a dot (.) closes the connection, followed

.Pq Ql \&.

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

escape character once. Setting the character to

escapes and makes the session fully transparent.

.Dq none

.ne 3

disables any escapes and makes the session fully transparent.

.TP

.It Fl f

.B \-f

Requests

Requests ssh to go to background after authentication. This is useful

.Nm

if ssh is going to ask for passwords or passphrases, but the user

to go to background after authentication. This is useful

.Nm

is going to ask for passwords or passphrases, but the user

wants it in the background. This implies

.B \-n.

.Fl n .

The recommended way to start X11 programs at a remote site is with

something like "ssh -f host xterm".

something like

.ne 3

.Ic ssh -f host xterm .

.TP

.It Fl i Ar identity_file

.BI \-i \ identity_file

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

.B \s-1RSA\s0

RSA authentication is read. Default is

authentication is read. Default is

.Pa \&.ssh/identity

.I \&\s+2.\s0ssh/identity

in the user's home directory. Identity files may also be specified on

a per-host basis in the configuration file. It is possible to have

multiple \-i options (and multiple identities specified in

multiple

.Fl i

options (and multiple identities specified in

configuration files).

.ne 3

.It Fl k

.TP

.B \-k

Disables forwarding of Kerberos tickets / AFS tokens. This may

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

.ne 3

.It Fl l Ar login_name

.TP

.BI -l \ login_name

Specifies the user to log in as on the remote machine. This may also

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

.ne 3

.It Fl n

.TP

Redirects stdin from

.B \-n

.Pa /dev/null

Redirects stdin from /dev/null (actually, prevents reading from stdin).

(actually, prevents reading from stdin).

This must be used when

.B ssh

.Nm

is run in the background. A common trick is to use this to run X11

programs in a remote machine. For example, "ssh -n shadows.cs.hut.fi

programs in a remote machine. For example,

emacs &" will start an emacs on shadows.cs.hut.fi, and the X11

.Ic ssh -n shadows.cs.hut.fi emacs &

will start an emacs on shadows.cs.hut.fi, and the X11

connection will be automatically forwarded over an encrypted channel.

The

.B ssh

.Nm

program will be put in the background.

(This does not work if

.B ssh

.Nm

needs to ask for a password or passphrase; see also the -f option.)

needs to ask for a password or passphrase; see also the

.ne 3

.Fl f

.TP

option.)

.BI \-o "\ 'option'

.It Fl o Ar option

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.

.ne 3

.It Fl p Ar port

.TP

.BI \-p "\ port

Port to connect to on the remote host. This can be specified on a

per-host basis in the configuration file.

.ne 3

.It Fl q

.TP

.B \-q

Quiet mode. Causes all warning and diagnostic messages to be

suppressed. Only fatal errors are displayed.

.ne 3

.It Fl t

.TP

.B \-t

Force pseudo-tty allocation. This can be used to execute arbitary

screen-based programs on a remote machine, which can be very useful

e.g. when implementing menu services.

.ne 3

.It Fl v

.TP

.B \-v

Verbose mode. Causes

.B ssh

.Nm

to print debugging messages about its progress. This is helpful in

debugging connection, authentication, and configuration problems.

.ne 3

.It Fl x

.TP

.B \-x

Disables X11 forwarding. This can also be specified on a per-host

basis in a configuration file.

.ne 3

.It Fl X

.TP

.B \-X

Enables X11 forwarding.

.ne 3

.It Fl C

.TP

.B \-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, and the "level" can be controlled

algorithm is the same used by gzip, and the

by the

.Dq level

.B CompressionLevel

can be controlled by the

.Cm CompressionLevel

option (see below). Compression is desirable on modem lines and other

slow connections, but will only slow down things on fast networks.

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

configuration files; see the

.B Compress

.Cm Compress

option below.

.ne 3

.It Fl L Ar port:host:hostport

.TP

.BI \-L "\ 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

.B port

.Ar port

on the local side, and whenever a connection is made to this port, the

connection is forwarded over the secure channel, and a connection is

made to

.B host:hostport

.Ar host:hostport

from the remote machine. Port forwardings can also be specified in the

configuration file. Only root can forward privileged ports.

.ne 3

.It Fl R Ar port:host:hostport

.TP

.BI \-R "\ 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

.B port

.Ar 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

.B host:hostport

.Ar host:hostport

from the local machine. Port forwardings can also be specified in the

configuration file. Privileged ports can be forwarded only when

logging in as root on the remote machine.

.El

.SH CONFIGURATION FILES

.Sh CONFIGURATION FILES

.LP

.Nm

.B Ssh

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

command line options, user's configuration file

(\fI\&$HOME/\s+2.\s0ssh/config\fR), and system-wide configuration file

.Pq Pa $HOME/.ssh/config ,

(\fI/etc/ssh_config\fR). For each parameter, the first obtained value

and system-wide configuration file

.Pq Pa /etc/ssh_config .

For each parameter, the first obtained value

will be used. The configuration files contain sections bracketed by

"Host" specifications, and that section is only applied for hosts that

match one of the patterns given in the specification. The matched

host name is the one given on the command line.

.LP

.Pp

Since the first obtained value for each parameter is used, more

host-specific declarations should be given near the beginning of the

file, and general defaults at the end.

.LP

.Pp

The configuration file has the following format:

.IP

.Pp

Empty lines and lines starting with '#' are comments.

Empty lines and lines starting with

.IP

.Ql #

Otherwise a line is of the format "keyword arguments". The possible

are comments.

.Pp

Otherwise a line is of the format

.Dq keyword arguments .

The possible

keywords and their meanings are as follows (note that the

configuration files are case-sensitive):

.ne 3

.Bl -tag -width Ds

.TP

.It Cm Host

.de YN

"\fByes\fR" or "\fBno\fR".

.B Host

Restricts the following declarations (up to the next

.B Host

.Cm Host

keyword) to be only for those hosts that match one of the patterns

given after the keyword. '*' and '?' can be as wildcards in the

given after the keyword.

patterns. A single '*' as a pattern can be used to provide global

.Ql \&*

and

.Ql ?

can be used as wildcards in the

patterns. A single

.Ql \&*

as a pattern can be used to provide global

defaults for all hosts. The host is the

.IR hostname

.Ar hostname

argument given on the command line (i.e., the name is not converted to

a canonicalized host name before matching).

.ne 3

.It Cm AFSTokenPassing

.TP

.B AFSTokenPassing

Specifies whether to pass AFS tokens to remote host. The argument to

this keyword must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B BatchMode

.It Cm BatchMode

If set to "yes", passphrase/password querying will be disabled. This

If set to

.Dq yes ,

passphrase/password querying will be disabled. This

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

user to supply the password. The argument must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B Cipher

.It Cm Cipher

Specifies the cipher to use for encrypting the session. Currently,

.IR blowfish ",

.Dq blowfish ,

.IR idea ",

.Dq idea ,

.IR des ",

.Dq des ,

.IR 3des ",

.Dq 3des ,

and

.I none

.Dq none

are supported. The default is "3des". Using "none" (no encryption) is intended

are supported. The default is

only for debugging, and will render the connection insecure.

.Dq 3des .

.ne 3

Using

.TP

.Dq none

.B Compression

(no encryption) is intended only for debugging, and will render the connection

insecure.

.It Cm Compression

Specifies whether to use compression. The argument must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B CompressionLevel

.It Cm CompressionLevel

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 GNU GZIP.

.ne 3

.It Cm ConnectionAttempts

.TP

.B 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.

.ne 3

.It Cm EscapeChar

.TP

Sets the escape character (default:

.B EscapeChar

.Ql ~ ) .

Sets the escape character (default: ~). The escape character can also

The escape character can also

be set on the command line. The argument should be a single

character, '^' followed by a letter, or ``none'' to disable the escape

character,

.Ql ^

followed by a letter, or

.Dq none

to disable the escape

character entirely (making the connection transparent for binary

data).

.ne 3

.It Cm FallBackToRsh

.TP

.B FallBackToRsh

Specifies that if connecting via

.B ssh

.Nm

fails due to a connection refused error (there is no

.B sshd

.Xr sshd 8

listening on the remote host),

.B rsh

.Xr rsh 1

should automatically be used instead (after a suitable warning about

the session being unencrypted). The argument must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B ForwardAgent

.It Cm ForwardAgent

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

will be forwarded to the remote machine. The argument must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B ForwardX11

.It Cm ForwardX11

Specifies whether X11 connections will be automatically redirected

over the secure channel and

.B \s-1DISPLAY\s0

.Ev DISPLAY

set. The argument must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B GlobalKnownHostsFile

.It Cm GlobalKnownHostsFile

Specifies a file to use instead of

.IR /etc/ssh_known_hosts ".

.Pa /etc/ssh_known_hosts .

.ne 3

.It Cm HostName

.TP

.B HostName

Specifies the real host name to log into. This can be used to specify

nicnames or abbreviations for hosts. Default is the name given on the

command line. Numeric IP addresses are also permitted (both on the

command line and in

.B HostName

.Cm HostName

specifications).

.ne 3

.It Cm IdentityFile

.TP

.B IdentityFile

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

is read (default \fI\s+2.\s0ssh/identity\fR in the user's home directory).

is read (default

.Pa .ssh/identity

in the user's home directory).

Additionally, any identities represented by the authentication agent

will be used for authentication. 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.

.ne 3

.It Cm KeepAlive

.TP

.B KeepAlive

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. However, this means that

connections will die if the route is down temporarily, and some people

find it annoying.

.Pp

The default is "yes" (to send keepalives), and the client will notice

The default is

.Dq yes

(to send keepalives), and the client will notice

if the network goes down or the remote host dies. This is important

in scripts, and many users want it too.

.Pp

To disable keepalives, the value should be set to "no" in both the

To disable keepalives, the value should be set to

server and the client configuration files.

.Dq no

.ne 3

in both the server and the client configuration files.

.TP

.It Cm KerberosAuthentication

.B KerberosAuthentication

Specifies whether Kerberos authentication will be used.

.TP

.It Cm KerberosTgtPassing

.B KerberosTgtPassing

Specifies whether a Kerberos TGT will be forwarded to the server.

Note that TGT forwarding is normally not enabled in the server.

.TP

.It Cm LocalForward

.B LocalForward

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

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

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

host:port. Multiple forwardings may be specified, and additional

forwardings can be given on the command line. Only the root can

forward privileged ports.

.ne 3

.It Cm PasswordAuthentication

.TP

.B PasswordAuthentication

Specifies whether to use password authentication. The argument to

this keyword must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B Port

.It Cm Port

Specifies the port number to connect on the remote host. Default is

22.

.ne 3

.It Cm ProxyCommand

.TP

.B ProxyCommand

Specifies the command to use to connect to the server. The command

string extends to the end of the line, and is executed with /bin/sh.

In the command string, %h will be substituted by the host name to

connect and %p by the port. The command can be basically anything,

and should read from its stdin and write to its stdout. It should

eventually connect an

.B sshd

.Xr sshd 8

server running on some machine, or execute

"sshd -i" somewhere. Host key management will be done using the

.Ic sshd -i

somewhere. Host key management will be done using the

HostName of the host being connected (defaulting to the name typed by

the user).

.Pp

Note that

.B ssh

.Nm

can also be configured to support the SOCKS system using the

--with-socks compile-time configuration option.

.ne 3

.It Cm RemoteForward

.TP

.B RemoteForward

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

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

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

host:port. Multiple forwardings may be specified, and additional

forwardings can be given on the command line. Only the root can

forward privileged ports.

.ne 3

.It Cm RhostsAuthentication

.TP

.B RhostsAuthentication

Specifies whether to try rhosts based authentication. Note that this

declaration only affects the client side and has no effect whatsoever

on security. Disabling rhosts authentication may reduce

Line 659

Line 599

not used. Most servers do not permit RhostsAuthentication because it

is not secure (see RhostsRSAAuthentication). The argument to this

keyword must be

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B RhostsRSAAuthentication

.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

.YN

.Dq yes

.ne 3

.TP

.Dq no .

.B RSAAuthentication

.It Cm RSAAuthentication

Specifies whether to try RSA authentication. The argument to this

keyword must be

.YN

.Dq yes

.Dq no .

RSA authentication will only be

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

running.

.ne 3

.It Cm StrictHostKeyChecking

.TP

If this flag is set to

.B StrictHostKeyChecking

.Dq yes ,

If this flag is set to "yes",

.Nm

.B ssh

ssh will never automatically add host keys to the

.I $HOME/.ssh/known_hosts

.Pa $HOME/.ssh/known_hosts

file, 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

.I /etc/ssh_known_hosts

.Pa /etc/ssh_known_hosts

files installed and frequently

connect new hosts. Basically this option forces the user to manually

add any new hosts. Normally this option is disabled, and new hosts

will automatically be added to the known host files. The host keys of

known hosts will be verified automatically in either case. The

argument must be

.YN

.Dq yes

.ne3

.TP

.Dq no .

.B User

.It Cm User

Specifies the user to log in as. This can be useful if you have a

different user name in different machines. This saves the trouble of

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

.ne 3

.It Cm UserKnownHostsFile

.TP

Specifies a file to use instead of

.B UserKnownHostsFile

.Pa $HOME/.ssh/known_hosts .

Specifies a file to use instead of \fI$HOME/\s+2.\s0ssh/known_hosts\fR.

.It Cm UseRsh

.ne 3

.TP

.B UseRsh

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

possible that the host does not at all support the

.B ssh

.Nm

protocol. This causes

.B ssh

.Nm

to immediately exec

.B rsh.

.Xr rsh 1 .

All other options (except

.BR HostName )

.Cm HostName )

are ignored if this has been specified. The argument must be

.YN

.Dq yes

.SH ENVIRONMENT

.Dq no .

.LP

.Sh ENVIRONMENT

.B Ssh

.Nm

will normally set the following environment variables:

.TP

.Bl -tag -width Ds

.B DISPLAY

.It Ev DISPLAY

The DISPLAY variable indicates the location of the X11 server. It is

The

.Ev DISPLAY

variable indicates the location of the X11 server. It is

automatically set by

.B ssh

.Nm

to point to a value of the form "hostname:n" where hostname indicates

to point to a value of the form

.Dq 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).

.ne 3

.It Ev HOME

.TP

.B HOME

Set to the path of the user's home directory.

.ne 3

.It Ev LOGNAME

.TP

Synonym for

.B LOGNAME

.Ev USER ; set for compatibility with systems that use

Synonym for USER; set for compatibility with systems that use

this variable.

.ne 3

.It Ev MAIL

.TP

.B MAIL

Set to point the user's mailbox.

.ne 3

.It Ev PATH

.TP

Set to the default

.B PATH

.Ev PATH ,

Set to the default PATH, as specified when compiling

as specified when compiling

.B ssh

.Nm

or, on some systems,

.I /etc/environment

.Pa /etc/environment

.IR /etc/default/login ".

.Pa /etc/default/login .

.ne 3

.It Ev SSH_AUTHENTICATION_FD

.TP

.B SSH_AUTHENTICATION_FD

This is set to an integer value if you are using the authentication

agent and a connection to it has been forwarded. The value indicates

a file descriptor number used for communicating with the agent. On

some systems,

.B SSH_AUTHENTICATION_SOCKET

.Ev SSH_AUTHENTICATION_SOCKET

may be used instead to

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

agent (this method is less secure, and is only used on systems that

don't support the first method).

.ne 3

.It Ev SSH_CLIENT

.TP

.B 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.

.ne 3

.It Ev SSH_TTY

.TP

.B SSH_TTY

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

with the current shell or command. If the current session has no tty,

this variable is not set.

.ne 3

.It Ev TZ

.TP

.B TZ

The timezone variable is set to indicate the present timezone if it

was set when the daemon was started (e.i., the daemon passes the value

on to new connections).

.ne 3

.It Ev USER

.TP

.B USER

Set to the name of the user logging in.

.LP

.El

.RT

.Pp

Additionally,

.B ssh

.Nm

reads

.I /etc/environment

.Pa /etc/environment

and

.IR $HOME/.ssh/environment ",

.Pa $HOME/.ssh/environment ,

and adds lines of

and adds lines of the format

the format

.Dq VARNAME=value

.I VARNAME=value

to the environment. Some systems may have

still additional mechanisms for setting up the environment, such as

.I /etc/default/login

.Pa /etc/default/login

on Solaris.

.Sh FILES

.ne 3

.Bl -tag -width $HOME/.ssh/known_hosts

.SH FILES

.It Pa $HOME/.ssh/known_hosts

.TP

.I \&$HOME/\s+2.\s0ssh/known_hosts

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

in \fI/etc/ssh_known_hosts\fR). See

.B sshd

.Pa /etc/ssh_known_hosts ) .

manual page.

See

.ne 3

.Xr sshd 8 .

.TP

.It Pa $HOME/.ssh/random_seed

.I \&$HOME/\s+2.\s0ssh/random_seed

Used for seeding the random number generator. This file contains

sensitive data and should read/write for the user and not accessible

for others. This file is created the first time the program is run

and updated automatically. The user should never need to read or

modify this file.

.ne 5

.It Pa $HOME/.ssh/identity

.TP

.I \&$HOME/\s+2.\s0ssh/identity

Contains the RSA authentication identity of the user. This file

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

accessible by others. 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

sensitive part of this file using IDEA.

.BR \s-1IDEA\s0 ".

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

.ne 3

.TP

.I \&$HOME/\s+2.\s0ssh/identity.pub

Contains the public key for authentication (public part of the

identity file in human-readable form). The contents of this file

should be added to \fI$HOME/\s+2.\s0ssh/authorized_keys\fR on all machines

should be added to

.Pa $HOME/.ssh/authorized_keys

on all machines

where you wish to log in using RSA authentication. This file is not

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

never used automatically and is not necessary; it is only provided for

the convenience of the user.

.ne 3

.It Pa $HOME/.ssh/config

.TP

.I \&$HOME/\s+2.\s0ssh/config

This is the per-user configuration file. The format of this file is

described above. This file is used by the

.B ssh

.Nm

client. This file does not usually contain any sensitive information,

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

accessible by others.

.ne 3

.It Pa $HOME/.ssh/authorized_keys

.TP

.I \&$HOME/\s+2.\s0ssh/authorized_keys

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

format of this file is described in the

.B sshd

.Xr sshd 8

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

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.

.ne 3

.It Pa /etc/ssh_known_hosts

.TP

.I /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. This file should be world-readable. This file contains

Line 872

Line 786

modulus, and optional comment field. When different names are used

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

commas. The format is described on the

.B sshd

.Xr sshd 8

manual page.

.IP

.Pp

The canonical system name (as returned by name servers) is used by

.B sshd

.Xr sshd 8

to verify the client host when logging in; other names are needed because

.B ssh

.Nm

does not convert the user-supplied name to a canonical name before

checking the key, because someone with access to the name servers

would then be able to fool host authentication.

.ne 3

.It Pa /etc/ssh_config

.TP

.I /etc/ssh_config

Systemwide configuration file. This file provides defaults for those

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.

.ne 3

.It Pa $HOME/.rhosts

.TP

This file is used in

.I $HOME/\s+2.\s0rhosts

.Pa \&.rhosts

This file is used in \s+2.\s0rhosts authentication to list the

authentication to list the

host/user pairs that are permitted to log in. (Note that this file is

also used by rlogin and rsh, which makes using this file insecure.)

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

Line 900

Line 812

separated by a space. One some machines this file may need to be

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

because

.B sshd

.Xr 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.

.IP

.Pp

Note that by default

.B sshd

.Xr sshd 8

will be installed so that it requires successful RSA host

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

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

\fI/etc/ssh_known_hosts\fR, you can store it in

.Pa /etc/ssh_known_hosts ,

\fI$HOME/\s+2.\s0ssh/known_hosts\fR. The easiest way to do this is to

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

will automatically add the host key in \fI$HOME/\s+2.\s0ssh/known_hosts\fR.

will automatically add the host key inxi

.ne 3

.Pa $HOME/.ssh/known_hosts .

.TP

.It Pa $HOME/.shosts

.I $HOME/\s+2.\s0shosts

This file is used exactly the same way as

This file is used exactly the same way as \s+2.\s0rhosts. The purpose for

.Pa \&.rhosts .

The purpose for

having this file is to be able to use rhosts authentication with

.B ssh

.Nm

without permitting login with rlogin or rsh.

without permitting login with

.ne 3

.Xr rlogin 1

.TP

.I /etc/hosts.equiv

.Xr rsh 1 .

This file is used during \s+2.\s0rhosts authentication. It contains

.It Pa /etc/hosts.equiv

This file is used during

.Pa \&.rhosts authentication. It contains

canonical hosts names, one per line (the full format is described on

the

.B sshd

.Xr sshd 8

manual page). If the client host is found in this file, login is

automatically permitted provided client and server user names are the

same. Additionally, successful RSA host authentication is normally

required. This file should only be writable by root.

.TP

.It Pa /etc/shosts.equiv

.I /etc/shosts.equiv

This file is processed exactly as

.IR /etc/hosts.equiv ".

.Pa /etc/hosts.equiv .

This file may be useful to permit logins using

.B ssh

.Nm

but not using rsh/rlogin.

.ne 3

.It Pa /etc/sshrc

.TP

.I /etc/sshrc

Commands in this file are executed by

.B ssh

.Nm

when the user logs in just before the user's shell (or command) is started.

See the

.B sshd

.Xr sshd 8

manual page for more information.

.ne 3

.It Pa $HOME/.ssh/rc

.TP

.I $HOME/.ssh/rc

Commands in this file are executed by

.B ssh

.Nm

when the user logs in just before the user's shell (or command) is

started.

See the

.B sshd

.Xr sshd 8

manual page for more information.

.Sh INSTALLATION

.SH INSTALLATION

.Nm

.LP

.B Ssh

is normally installed as suid root. It needs root privileges only for

rhosts authentication (rhosts authentication requires that the

connection must come from a privileged port, and allocating such a

port requires root privileges). It also needs to be able to read

\fI/etc/ssh_host_key\fR to perform

.Pa /etc/ssh_host_key

.B \s-1RSA\s0

to perform RSA

host authentication. It is possible to use

.B ssh

.Nm

without root privileges, but rhosts authentication will then be

disabled.

.B Ssh

.Nm

drops any extra privileges immediately after the connection to the

remote host has been made.

.LP

.Pp

Considerable work has been put into making

.B sshd

.Xr sshd 8

secure. However, if you find a security problem, please report it

immediately to <ssh-bugs@cs.hut.fi>.

.Sh AUTHOR

.SH AUTHOR

.LP

Tatu Ylonen <ylo@cs.hut.fi>

.LP

.Pp

Information about new releases, mailing lists, and other related

Issues can be found from the SSH WWW home page:

issues can be found from the ssh WWW home page at

.Pp

http://www.cs.hut.fi/ssh.

.Dl http://www.cs.hut.fi/ssh

.Sh SEE ALSO

.SH SEE ALSO

.Xr make-ssh-known-hosts 1 ,

.BR sshd (8),

.Xr rlogin 1 ,

.BR ssh-keygen (1),

.Xr rsh 1 ,

.BR ssh-agent (1),

.Xr scp 1 ,

.BR ssh-add (1),

.Xr ssh-add 1 ,

.BR scp (1),

.Xr ssh-agent 1 ,

.BR make-ssh-known-hosts (1),

.Xr ssh-keygen 1 ,

.BR rlogin (1),

.Xr telnet 1 ,

.BR rsh (1),

.Xr sshd 8

.BR telnet (1)