995 lines
30 KiB
Groff
995 lines
30 KiB
Groff
.\" -*- nroff -*-
|
|
.\"
|
|
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
|
|
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
|
|
.\" All rights reserved
|
|
.\"
|
|
.\" As far as I am concerned, the code I have written for this software
|
|
.\" can be used freely for any purpose. Any derived versions of this
|
|
.\" software must be clearly marked as such, and if the derived work is
|
|
.\" incompatible with the protocol description in the RFC file, it must be
|
|
.\" called by a name other than "ssh" or "Secure Shell".
|
|
.\"
|
|
.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
|
|
.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
|
|
.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
|
|
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
|
|
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
|
|
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
|
|
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
|
|
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
|
|
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
|
|
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
|
|
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
|
|
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
.\"
|
|
.\" $OpenBSD: ssh.1,v 1.168 2003/03/28 10:11:43 jmc Exp $
|
|
.\" $FreeBSD$
|
|
.Dd September 25, 1999
|
|
.Dt SSH 1
|
|
.Os
|
|
.Sh NAME
|
|
.Nm ssh
|
|
.Nd OpenSSH SSH client (remote login program)
|
|
.Sh SYNOPSIS
|
|
.Nm ssh
|
|
.Op Fl l Ar login_name
|
|
.Ar hostname | user@hostname
|
|
.Op Ar command
|
|
.Pp
|
|
.Nm ssh
|
|
.Bk -words
|
|
.Op Fl afgknqstvxACNTX1246
|
|
.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 :
|
|
.Ar host :
|
|
.Ar hostport
|
|
.Sm on
|
|
.Xc
|
|
.Oc
|
|
.Ek
|
|
.Bk -words
|
|
.Oo Fl R Xo
|
|
.Sm off
|
|
.Ar port :
|
|
.Ar host :
|
|
.Ar hostport
|
|
.Sm on
|
|
.Xc
|
|
.Oc
|
|
.Op Fl D Ar port
|
|
.Ar hostname | user@hostname
|
|
.Op Ar command
|
|
.Ek
|
|
.Sh DESCRIPTION
|
|
.Nm
|
|
(SSH client) is a program for logging into a remote machine and for
|
|
executing commands on a remote machine.
|
|
It is intended to replace
|
|
rlogin and rsh, and provide secure encrypted communications between
|
|
two untrusted hosts over an insecure network.
|
|
X11 connections and
|
|
arbitrary TCP/IP ports can also be forwarded over the secure channel.
|
|
.Pp
|
|
.Nm
|
|
connects and logs into the specified
|
|
.Ar hostname .
|
|
The user must prove
|
|
his/her identity to the remote machine using one of several methods
|
|
depending on the protocol version used:
|
|
.Pp
|
|
.Ss SSH protocol version 1
|
|
.Pp
|
|
First, if the machine the user logs in from is listed in
|
|
.Pa /etc/hosts.equiv
|
|
or
|
|
.Pa /etc/ssh/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
|
|
.Pa \&.rhosts
|
|
or
|
|
.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.
|
|
.Pp
|
|
The second authentication method is the
|
|
.Pa rhosts
|
|
or
|
|
.Pa hosts.equiv
|
|
method combined with RSA-based host authentication.
|
|
It means that if the login would be permitted by
|
|
.Pa $HOME/.rhosts ,
|
|
.Pa $HOME/.shosts ,
|
|
.Pa /etc/hosts.equiv ,
|
|
or
|
|
.Pa /etc/ssh/shosts.equiv ,
|
|
and if additionally the server can verify the client's
|
|
host key (see
|
|
.Pa /etc/ssh/ssh_known_hosts
|
|
and
|
|
.Pa $HOME/.ssh/known_hosts
|
|
in the
|
|
.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:
|
|
.Pa /etc/hosts.equiv ,
|
|
.Pa $HOME/.rhosts ,
|
|
and the rlogin/rsh protocol in general, are inherently insecure and should be
|
|
disabled if security is desired.]
|
|
.Pp
|
|
As a third authentication method,
|
|
.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
|
|
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
|
|
.Pa $HOME/.ssh/authorized_keys
|
|
lists the public keys that are permitted for logging
|
|
in.
|
|
When the user logs in, the
|
|
.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
|
|
.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.
|
|
.Pp
|
|
.Nm
|
|
implements the RSA authentication protocol automatically.
|
|
The user creates his/her RSA key pair by running
|
|
.Xr ssh-keygen 1 .
|
|
This stores the private key in
|
|
.Pa $HOME/.ssh/identity
|
|
and the public key in
|
|
.Pa $HOME/.ssh/identity.pub
|
|
in the user's home directory.
|
|
The user should then copy the
|
|
.Pa identity.pub
|
|
to
|
|
.Pa $HOME/.ssh/authorized_keys
|
|
in his/her home directory on the remote machine (the
|
|
.Pa authorized_keys
|
|
file corresponds to the conventional
|
|
.Pa $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 authentication is much
|
|
more secure than rhosts authentication.
|
|
.Pp
|
|
The most convenient way to use RSA authentication may be with an
|
|
authentication agent.
|
|
See
|
|
.Xr ssh-agent 1
|
|
for more information.
|
|
.Pp
|
|
If other authentication methods fail,
|
|
.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.
|
|
.Pp
|
|
.Ss SSH protocol version 2
|
|
.Pp
|
|
When a user connects using protocol version 2
|
|
similar authentication methods are available.
|
|
Using the default values for
|
|
.Cm PreferredAuthentications ,
|
|
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:
|
|
The client uses his private key,
|
|
.Pa $HOME/.ssh/id_dsa
|
|
or
|
|
.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
|
|
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.
|
|
.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).
|
|
Note that protocol 1 lacks a strong mechanism for ensuring the
|
|
integrity of the connection.
|
|
.Pp
|
|
.Ss Login session and remote execution
|
|
.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.
|
|
.Pp
|
|
If a pseudo-terminal has been allocated (normal login session), the
|
|
user may use the escape characters noted below.
|
|
.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
|
|
.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
|
|
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
|
|
.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 ~C
|
|
Open command line (only useful for adding port forwardings using the
|
|
.Fl L
|
|
and
|
|
.Fl R
|
|
options)
|
|
.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
|
|
.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
|
|
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
|
|
.Ev DISPLAY .
|
|
Forwarding of X11 connections can be
|
|
configured on the command line or in configuration files.
|
|
Take note that X11 forwarding can represent a security hazard.
|
|
.Pp
|
|
The
|
|
.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
|
|
.Nm
|
|
creates a
|
|
.Dq proxy
|
|
X server on the server machine for forwarding the
|
|
connections over the encrypted channel.
|
|
.Pp
|
|
.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).
|
|
.Pp
|
|
If the
|
|
.Cm ForwardAgent
|
|
variable is set to
|
|
.Dq yes
|
|
(or, see the description of the
|
|
.Fl A
|
|
and
|
|
.Fl a
|
|
options described later) and
|
|
the user is using an authentication agent, the connection to the agent
|
|
is automatically forwarded to the remote side.
|
|
.Pp
|
|
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 forwarding is a secure connection to an
|
|
electronic purse; another is going through 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
|
|
.Pa $HOME/.ssh/known_hosts
|
|
in the user's home directory.
|
|
Additionally, the file
|
|
.Pa /etc/ssh/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,
|
|
.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
|
|
.Cm StrictHostKeyChecking
|
|
option can be used to prevent logins to machines whose
|
|
host key is not known or has changed.
|
|
.Pp
|
|
The options are as follows:
|
|
.Bl -tag -width Ds
|
|
.It Fl a
|
|
Disables forwarding of the authentication agent connection.
|
|
.It Fl A
|
|
Enables forwarding of the authentication agent connection.
|
|
This can also be specified on a per-host basis in a configuration file.
|
|
.Pp
|
|
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 they can perform operations on the keys that enable them to
|
|
authenticate using the identities loaded into the agent.
|
|
.It Fl b Ar bind_address
|
|
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.
|
|
.Ar blowfish
|
|
is a fast block cipher, it appears very secure and is much faster than
|
|
.Ar 3des .
|
|
.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.
|
|
.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
|
|
.Cm Ciphers
|
|
for more information.
|
|
.It Fl e Ar ch|^ch|none
|
|
Sets the escape character for sessions with a pty (default:
|
|
.Ql ~ ) .
|
|
The escape character is only recognized at the beginning of a line.
|
|
The escape character followed by a dot
|
|
.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
|
|
.Dq none
|
|
disables any escapes and makes the session fully transparent.
|
|
.It Fl f
|
|
Requests
|
|
.Nm
|
|
to go to background just before command execution.
|
|
This is useful if
|
|
.Nm
|
|
is going to ask for passwords or passphrases, but the user
|
|
wants it in the background.
|
|
This implies
|
|
.Fl n .
|
|
The recommended way to start X11 programs at a remote site is with
|
|
something like
|
|
.Ic ssh -f host xterm .
|
|
.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
|
|
RSA or DSA authentication is read.
|
|
The default is
|
|
.Pa $HOME/.ssh/identity
|
|
for protocol version 1, and
|
|
.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
|
|
(actually, prevents reading from stdin).
|
|
This must be used when
|
|
.Nm
|
|
is run in the background.
|
|
A common trick is to use this to run X11 programs on a remote machine.
|
|
For example,
|
|
.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
|
|
.Nm
|
|
program will be put in the background.
|
|
(This does not work if
|
|
.Nm
|
|
needs to ask for a password or passphrase; see also the
|
|
.Fl f
|
|
option.)
|
|
.It Fl N
|
|
Do not execute a remote command.
|
|
This is useful for just forwarding ports
|
|
(protocol version 2 only).
|
|
.It Fl o Ar 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.
|
|
.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 q
|
|
Quiet mode.
|
|
Causes all warning and diagnostic messages to be suppressed.
|
|
.It Fl s
|
|
May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
|
|
of SSH as a secure transport for other 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.
|
|
.It Fl v
|
|
Verbose mode.
|
|
Causes
|
|
.Nm
|
|
to print debugging messages about its progress.
|
|
This is helpful in
|
|
debugging connection, authentication, and configuration problems.
|
|
Multiple
|
|
.Fl v
|
|
options increases the verbosity.
|
|
Maximum is 3.
|
|
.It Fl x
|
|
Disables X11 forwarding.
|
|
.It Fl X
|
|
Enables X11 forwarding.
|
|
This can also be specified on a per-host basis in a configuration file.
|
|
.Pp
|
|
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 may then be able to perform activities such as keystroke monitoring.
|
|
.It Fl 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
|
|
.Xr gzip 1 ,
|
|
and the
|
|
.Dq level
|
|
can be controlled by the
|
|
.Cm CompressionLevel
|
|
option for protocol version 1.
|
|
Compression is desirable on modem lines and other
|
|
slow connections, but will only slow down things on fast networks.
|
|
The default value can be set on a host-by-host basis in the
|
|
configuration files; see the
|
|
.Cm Compression
|
|
option.
|
|
.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/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.
|
|
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 a connection is
|
|
made to
|
|
.Ar host
|
|
port
|
|
.Ar hostport
|
|
from the remote machine.
|
|
Port forwardings can also be specified in the configuration file.
|
|
Only root can forward privileged ports.
|
|
IPv6 addresses can be specified with an alternative syntax:
|
|
.Ar port/host/hostport
|
|
.It Fl R Ar 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
|
|
.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
|
|
.Ar host
|
|
port
|
|
.Ar hostport
|
|
from the local machine.
|
|
Port forwardings can also be specified in the configuration file.
|
|
Privileged ports can be forwarded only when
|
|
logging in as root on the remote machine.
|
|
IPv6 addresses can be specified 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
|
|
to try protocol version 2 only.
|
|
.It Fl 4
|
|
Forces
|
|
.Nm
|
|
to use IPv4 addresses only.
|
|
.It Fl 6
|
|
Forces
|
|
.Nm
|
|
to use IPv6 addresses only.
|
|
.El
|
|
.Sh CONFIGURATION FILES
|
|
.Nm
|
|
may additionally obtain configuration data from
|
|
a per-user configuration file and a system-wide configuration file.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.Sh ENVIRONMENT
|
|
.Nm
|
|
will normally set the following environment variables:
|
|
.Bl -tag -width Ds
|
|
.It Ev DISPLAY
|
|
The
|
|
.Ev DISPLAY
|
|
variable indicates the location of the X11 server.
|
|
It is automatically set by
|
|
.Nm
|
|
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.
|
|
.Nm
|
|
uses this special value to forward X11 connections over the secure
|
|
channel.
|
|
The user should normally not set
|
|
.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
|
|
Set to the path of the user's home directory.
|
|
.It Ev LOGNAME
|
|
Synonym for
|
|
.Ev USER ;
|
|
set for compatibility with systems that use this variable.
|
|
.It Ev MAIL
|
|
Set to the path of the user's mailbox.
|
|
.It Ev PATH
|
|
Set to the default
|
|
.Ev PATH ,
|
|
as specified when compiling
|
|
.Nm ssh .
|
|
.It Ev SSH_ASKPASS
|
|
If
|
|
.Nm
|
|
needs a passphrase, it will read the passphrase from the current
|
|
terminal if it was run from a terminal.
|
|
If
|
|
.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
|
|
agent.
|
|
.It Ev SSH_CONNECTION
|
|
Identifies the client and server ends of the connection.
|
|
The variable contains
|
|
four space-separated values: client ip-address, client port number,
|
|
server ip-address and server port number.
|
|
.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.
|
|
If the current session has no tty,
|
|
this variable is not set.
|
|
.It Ev TZ
|
|
The timezone variable is set to indicate the present timezone if it
|
|
was set when the daemon was started (i.e., the daemon passes the value
|
|
on to new connections).
|
|
.It Ev USER
|
|
Set to the name of the user logging in.
|
|
.El
|
|
.Pp
|
|
Additionally,
|
|
.Nm
|
|
reads
|
|
.Pa $HOME/.ssh/environment ,
|
|
and adds lines of the format
|
|
.Dq VARNAME=value
|
|
to the environment if the file exists and if users are allowed to
|
|
change their environment.
|
|
See the
|
|
.Cm PermitUserEnvironment
|
|
option in
|
|
.Xr sshd_config 5 .
|
|
.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
|
|
in
|
|
.Pa /etc/ssh/ssh_known_hosts .
|
|
See
|
|
.Xr sshd 8 .
|
|
.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
|
|
Contains the 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).
|
|
Note that
|
|
.Nm
|
|
ignores a private key file if it is 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 3DES.
|
|
.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
|
|
Contains the public key for authentication (public part of the
|
|
identity file in human-readable form).
|
|
The contents of the
|
|
.Pa $HOME/.ssh/identity.pub
|
|
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.
|
|
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
|
|
on all machines
|
|
where the user wishes to log in using protocol version 2 DSA/RSA 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
|
|
the convenience of the user.
|
|
.It Pa $HOME/.ssh/config
|
|
This is the per-user configuration file.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.It Pa $HOME/.ssh/authorized_keys
|
|
Lists the public keys (RSA/DSA) that can be used for logging in as this user.
|
|
The format of this file is described in the
|
|
.Xr sshd 8
|
|
manual page.
|
|
In the simplest form the format is the same as the .pub
|
|
identity files.
|
|
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/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
|
|
public keys, one per line, in the following format (fields separated
|
|
by spaces): system name, public key 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
|
|
.Xr sshd 8
|
|
manual page.
|
|
.Pp
|
|
The canonical system name (as returned by name servers) is used by
|
|
.Xr sshd 8
|
|
to verify the client host when logging in; other names are needed because
|
|
.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.
|
|
.It Pa /etc/ssh/ssh_config
|
|
Systemwide configuration file.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
|
|
These three files contain the private parts of the host keys
|
|
and are used for
|
|
.Cm RhostsRSAAuthentication
|
|
and
|
|
.Cm HostbasedAuthentication .
|
|
If the protocol version 1
|
|
.Cm RhostsRSAAuthentication
|
|
method is used,
|
|
.Nm
|
|
must be setuid root, since the host key is readable only by root.
|
|
For protocol version 2,
|
|
.Nm
|
|
uses
|
|
.Xr ssh-keysign 8
|
|
to access the host keys for
|
|
.Cm HostbasedAuthentication .
|
|
This eliminates the requirement that
|
|
.Nm
|
|
be setuid root when that authentication method is used.
|
|
By default
|
|
.Nm
|
|
is not setuid root.
|
|
.It Pa $HOME/.rhosts
|
|
This file is used in
|
|
.Pa \&.rhosts
|
|
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
|
|
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
|
|
world-readable if the user's home directory is on a NFS partition,
|
|
because
|
|
.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.
|
|
.Pp
|
|
Note that by default
|
|
.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
|
|
.Pa /etc/ssh/ssh_known_hosts ,
|
|
it can be stored 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 to
|
|
.Pa $HOME/.ssh/known_hosts .
|
|
.It Pa $HOME/.shosts
|
|
This file is used exactly the same way as
|
|
.Pa \&.rhosts .
|
|
The purpose for
|
|
having this file is to be able to use rhosts authentication with
|
|
.Nm
|
|
without permitting login with
|
|
.Nm rlogin
|
|
or
|
|
.Xr rsh 1 .
|
|
.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
|
|
.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.
|
|
.It Pa /etc/ssh/shosts.equiv
|
|
This file is processed exactly as
|
|
.Pa /etc/hosts.equiv .
|
|
This file may be useful to permit logins using
|
|
.Nm
|
|
but not using rsh/rlogin.
|
|
.It Pa /etc/ssh/sshrc
|
|
Commands in this file are executed by
|
|
.Nm
|
|
when the user logs in just before the user's shell (or command) is started.
|
|
See the
|
|
.Xr sshd 8
|
|
manual page for more information.
|
|
.It Pa $HOME/.ssh/rc
|
|
Commands in this file are executed by
|
|
.Nm
|
|
when the user logs in just before the user's shell (or command) is
|
|
started.
|
|
See the
|
|
.Xr sshd 8
|
|
manual page for more information.
|
|
.It Pa $HOME/.ssh/environment
|
|
Contains additional definitions for environment variables, see section
|
|
.Sx ENVIRONMENT
|
|
above.
|
|
.El
|
|
.Sh DIAGNOSTICS
|
|
.Nm
|
|
exits with the exit status of the remote command or with 255
|
|
if an error occurred.
|
|
.Sh AUTHORS
|
|
OpenSSH is a derivative of the original and free
|
|
ssh 1.2.12 release by Tatu Ylonen.
|
|
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
|
|
Theo de Raadt and Dug Song
|
|
removed many bugs, re-added newer features and
|
|
created OpenSSH.
|
|
Markus Friedl contributed the support for SSH
|
|
protocol versions 1.5 and 2.0.
|
|
.Sh SEE ALSO
|
|
.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 ssh_config 5 ,
|
|
.Xr ssh-keysign 8 ,
|
|
.Xr sshd 8
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A T. Kivinen
|
|
.%A M. Saarinen
|
|
.%A T. Rinne
|
|
.%A S. Lehtinen
|
|
.%T "SSH Protocol Architecture"
|
|
.%N draft-ietf-secsh-architecture-12.txt
|
|
.%D January 2002
|
|
.%O work in progress material
|
|
.Re
|