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Diff for /src/usr.bin/ssh/ssh.1 between version 1.182 and 1.182.2.2

version 1.182, 2004/03/05 10:53:58

version 1.182.2.2, 2005/03/10 17:15:05

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.Nd OpenSSH SSH client (remote login program)

.Sh SYNOPSIS

.Nm ssh

.Op Fl 1246AaCfgkNnqsTtVvXxY

.Bk -words

.Op Fl 1246AaCfgkMNnqsTtVvXxY

.Op Fl b Ar bind_address

.Op Fl c Ar cipher_spec

.Op Fl D Ar port

.Op Fl e Ar escape_char

.Op Fl F Ar configfile

.Op Fl i Ar identity_file

.Bk -words

.Oo Fl L\ \&

.Oo Fl L Xo

.Sm off

.Ar port :

.Oo Ar bind_address : Oc

.Ar host :

.Ar port : host : hostport

.Ar hostport

.Sm on

.Xc

.Oc

.Ek

.Op Fl l Ar login_name

.Op Fl m Ar mac_spec

.Op Fl O Ar ctl_cmd

.Op Fl o Ar option

.Bk -words

.Op Fl p Ar port

.Ek

.Oo Fl R\ \&

.Oo Fl R Xo

.Sm off

.Ar port :

.Oo Ar bind_address : Oc

.Ar host :

.Ar port : host : hostport

.Ar hostport

.Sm on

.Xc

.Oc

.Op Fl S Ar ctl_path

.Oo Ar user Ns @ Oc Ns Ar hostname

.Op Ar command

.Ek

.Sh DESCRIPTION

.Nm

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

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.Ar command

is executed on the remote host instead of a login shell.

.Ss SSH protocol version 1

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

The first authentication method is the

.Em rhosts

.Em hosts.equiv

method combined with RSA-based host authentication.

If the machine the user logs in from is listed in

.Pa /etc/hosts.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.

the same on both sides, or if the files

Second, if

.Pa $HOME/.rhosts

.Pa .rhosts

.Pa .shosts

.Pa $HOME/.shosts

exists in the user's home directory on the

exist in the user's home directory on the

remote machine and contains a line containing the name of the client

remote machine and contain a line containing the name of the client

machine and the name of the user on that machine, the user is

permitted to log in.

considered for log in.

This form of authentication alone is normally not

Additionally, if the server can verify the client's

allowed by the server because it is not secure.

.Pp

The second authentication method is the

.Em rhosts

.Em 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 ,

.Pa /etc/shosts.equiv ,

and if additionally the server can verify the client's

host key (see

.Pa /etc/ssh/ssh_known_hosts

and

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

disabled if security is desired.]

.Pp

As a third authentication method,

As a second authentication method,

.Nm

supports RSA based authentication.

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

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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

.Em rhosts

authentication.

.Pp

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

authentication agent.

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supports hostbased or challenge response authentication.

.Pp

Protocol 2 provides additional mechanisms for confidentiality

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

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

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

and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).

Note that protocol 1 lacks a strong mechanism for ensuring the

integrity of the connection.

.Ss Login session and remote execution

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Send a BREAK to the remote system

(only useful for SSH protocol version 2 and if the peer supports it).

.It Cm ~C

Open command line (only useful for adding port forwardings using the

Open command line.

Currently this allows the addition of port forwardings using the

.Fl L

and

.Fl R

options).

options (see below).

It also allows the cancellation of existing remote port-forwardings

using

.Fl KR Ar hostport .

Basic help is available, using the

.Fl h

option.

.It Cm ~R

Request rekeying of the connection

(only useful for SSH protocol version 2 and if the peer supports it).

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option can be used to prevent logins to machines whose

host key is not known or has changed.

.Pp

.Nm

can be configured to verify host identification using fingerprint resource

records (SSHFP) published in DNS.

The

.Cm VerifyHostKeyDNS

option can be used to control how DNS lookups are performed.

SSHFP resource records can be generated using

.Xr ssh-keygen 1 .

.Pp

The options are as follows:

.Bl -tag -width Ds

.It Fl 1

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configuration files; see the

.Cm Compression

option.

.It Fl c Ar blowfish | 3des | des

.It Fl c Ar cipher_spec

Selects the cipher to use for encrypting the session.

Selects the cipher specification for encrypting the session.

.Pp

Protocol version 1 allows specification of a single cipher.

The suported values are

.Dq 3des ,

.Dq blowfish

and

.Dq des .

.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 believed to be secure.

.Ar blowfish

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

.Ar 3des .

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.Ar 3des

cipher.

Its use is strongly discouraged due to cryptographic weaknesses.

.It Fl c Ar cipher_spec

The default is

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

.Dq 3des .

be specified in order of preference.

.Pp

See

For protocol version 2

.Cm Ciphers

.Ar cipher_spec

for more information.

is a comma-separated list of ciphers

listed in order of preference.

The supported ciphers are

.Dq 3des-cbc ,

.Dq aes128-cbc ,

.Dq aes192-cbc ,

.Dq aes256-cbc ,

.Dq aes128-ctr ,

.Dq aes192-ctr ,

.Dq aes256-ctr ,

.Dq arcfour ,

.Dq blowfish-cbc ,

and

.Dq cast128-cbc .

The default is

.Bd -literal

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

aes192-cbc,aes256-cbc''

.Ed

.It Fl D Ar port

Specifies a local

.Dq dynamic

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Disables forwarding (delegation) of GSSAPI credentials to the server.

.It Fl L Xo

.Sm off

.Oo Ar bind_address : Oc

.Ar port : host : hostport

.Sm on

.Xc

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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

on the local side, optionally bound to the specified

.Ar bind_address .

Whenever a connection is made to this port, the

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

made to

.Ar host

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.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:

.Sm off

.Xo

.Op Ar bind_address No /

.Ar port No / Ar host No /

.Ar hostport .

.Ar hostport

.Xc

.Sm on

or by enclosing the address in square brackets.

Only the superuser can forward privileged ports.

By default, the local port is bound in accordance with the

.Cm GatewayPorts

setting.

However, an explicit

.Ar bind_address

may be used to bind the connection to a specific address.

The

.Ar bind_address

.Dq localhost

indicates that the listening port be bound for local use only, while an

empty address or

.Sq *

indicates that the port should be available from all interfaces.

.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

Places the

.Nm

client into

.Dq master

mode for connection sharing.

Refer to the description of

.Cm ControlMaster

.Xr ssh_config 5

for details.

.It Fl m Ar mac_spec

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

(message authentication code) algorithms can

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

.Fl f

option.)

.It Fl O Ar ctl_cmd

Control an active connection multiplexing master process.

When the

.Fl O

option is specified, the

.Ar ctl_cmd

argument is interpreted and passed to the master process.

Valid commands are:

.Dq check

(check that the master process is running) and

.Dq exit

(request the master to exit).

.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

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.It Compression

.It CompressionLevel

.It ConnectionAttempts

.It ConnectionTimeout

.It ConnectTimeout

.It ControlMaster

.It ControlPath

.It DynamicForward

.It EscapeChar

.It ForwardAgent

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.It GlobalKnownHostsFile

.It GSSAPIAuthentication

.It GSSAPIDelegateCredentials

.It HashKnownHosts

.It Host

.It HostbasedAuthentication

.It HostKeyAlgorithms

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Line 702

.It HostName

.It IdentityFile

.It IdentitiesOnly

.It KbdInteractiveDevices

.It LocalForward

.It LogLevel

.It MACs

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.It RemoteForward

.It RhostsRSAAuthentication

.It RSAAuthentication

.It SendEnv

.It ServerAliveInterval

.It ServerAliveCountMax

.It SmartcardDevice

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Causes all warning and diagnostic messages to be suppressed.

.It Fl R Xo

.Sm off

.Oo Ar bind_address : Oc

.Ar port : host : hostport

.Sm on

.Xc

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port

.Ar hostport

from the local machine.

.Pp

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:

IPv6 addresses can be specified by enclosing the address in square braces or

using an alternative syntax:

.Sm off

.Xo

.Ar port No / Ar host No /

.Op Ar bind_address No /

.Ar hostport .

.Ar host No / Ar port No /

.Xc

.Ar hostport

.Xc .

.Sm on

.Pp

By default, the listening socket on the server will be bound to the loopback

interface only.

This may be overriden by specifying a

.Ar bind_address .

An empty

.Ar bind_address ,

or the address

.Ql * ,

indicates that the remote socket should listen on all interfaces.

Specifying a remote

.Ar bind_address

will only succeed if the server's

.Cm GatewayPorts

option is enabled (see

.Xr sshd_config 5 ) .

.It Fl S Ar ctl_path

Specifies the location of a control socket for connection sharing.

Refer to the description of

.Cm ControlPath

and

.Cm ControlMaster

.Xr ssh_config 5

for details.

.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

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(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.

.Pp

For this reason, X11 forwarding is subjected to X11 SECURITY extension

restrictions by default.

Please refer to the

.Nm

.Fl Y

option and the

.Cm ForwardX11Trusted

directive in

.Xr ssh_config 5

for more information.

.It Fl x

Disables X11 forwarding.

.It Fl Y

Enables trusted X11 forwarding.

Trusted X11 forwardings are not subjected to the X11 SECURITY extension

controls.

.El

.Sh CONFIGURATION FILES

.Nm

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This is particularly useful when calling

.Nm

from a

.Pa .Xsession

.Pa .xsession

or related script.

(Note that on some machines it

may be necessary to redirect the input from

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This is the per-user configuration file.

The file format and configuration options are described in

.Xr ssh_config 5 .

Because of the potential for abuse, this file must have strict permissions:

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

.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

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is not setuid root.

.It Pa $HOME/.rhosts

This file is used in

.Em rhosts

.Cm RhostsRSAAuthentication

and

.Cm HostbasedAuthentication

authentication to list the

host/user pairs that are permitted to log in.

(Note that this file is

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permission for most machines is read/write for the user, and not

accessible by others.

.Pp

Note that by default

Note that

.Xr sshd 8

will be installed so that it requires successful RSA host

allows authentication only in combination with client host key

authentication before permitting

authentication before permitting log in.

.Em rhosts

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

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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

having this file is to be able to use

.Nm

.Cm RhostsRSAAuthentication

without permitting login with

and

.Cm HostbasedAuthentication

authentication without permitting login with

.Xr rlogin

.Xr rsh 1 .

.It Pa /etc/hosts.equiv

This file is used during

.Em rhosts

.Cm RhostsRSAAuthentication

and

.Cm HostbasedAuthentication

authentication.

It contains

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

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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

Additionally, successful client host key authentication is required.

required.

This file should only be writable by root.

.It Pa /etc/shosts.equiv

This file is processed exactly as