ssh
NAME
SYNOPSIS
k -words
k -words
k -words
DESCRIPTION
(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 ports
can also be forwarded over the secure channel.
connects and logs into the specified
(with optional
name).
The user must prove
his/her identity to the remote machine using one of several methods
depending on the protocol version used (see below).
If
is specified,
it is executed on the remote host instead of a login shell.
The options are as follows:
- 1 Forces to try protocol version 1 only.
- 2 Forces to try protocol version 2 only.
- 4 Forces to use IPv4 addresses only.
- 6 Forces to use IPv6 addresses only.
- A Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file.
Agent forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the agent's Unix-domain socket)
can access the local agent through the forwarded connection.
An attacker cannot obtain key material from the agent,
however they can perform operations on the keys that enable them to
authenticate using the identities loaded into the agent.
a
Disables forwarding of the authentication agent connection.
bind_address
Use
on the local machine as the source address
of the connection.
Only useful on systems with more than one address.
C
Requests compression of all data (including stdin, stdout, stderr, and
data for forwarded X11 and TCP connections).
The compression algorithm is the same used by
gzip(1)
and the
can be controlled by the
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
option.
cipher_spec
Selects the cipher specification for encrypting the session.
Protocol version 1 allows specification of a single cipher.
The supported values are
and
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is believed to be secure.
is a fast block cipher; it appears very secure and is much faster than
is only supported in the
client for interoperability with legacy protocol 1 implementations
that do not support the
cipher.
Its use is strongly discouraged due to cryptographic weaknesses.
The default is
For protocol version 2,
is a comma-separated list of ciphers
listed in order of preference.
The supported ciphers are:
3des-cbc,
aes128-cbc,
aes192-cbc,
aes256-cbc,
aes128-ctr,
aes192-ctr,
aes256-ctr,
arcfour128,
arcfour256,
arcfour,
blowfish-cbc,
and
cast128-cbc.
The default is:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, aes192-ctr,aes256-ctr
IPv6 addresses can be specified with an alternative syntax:
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
setting.
However, an explicit
may be used to bind the connection to a specific address.
The
of
indicates that the listening port be bound for local use only, while an
empty address or
indicates that the port should be available from all interfaces.
escape_char
Sets the escape character for sessions with a pty (default:
The escape character is only recognized at the beginning of a line.
The escape character followed by a dot
closes the connection;
followed by control-Z suspends the connection;
and followed by itself sends the escape character once.
Setting the character to
disables any escapes and makes the session fully transparent.
configfile
Specifies an alternative per-user configuration file.
If a configuration file is given on the command line,
the system-wide configuration file
will be ignored.
The default for the per-user configuration file is
for protocol version 1, and
and
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
options (and multiple identities specified in
configuration files).
K
Enables forwarding (delegation) of GSSAPI credentials to the server.
k
Disables forwarding (delegation) of GSSAPI credentials to the server.
Xo
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
on the local side, optionally bound to the specified
Whenever a connection is made to this port, the
connection is forwarded over the secure channel, and a connection is
made to
port
from the remote machine.
Port forwardings can also be specified in the configuration file.
IPv6 addresses can be specified with an alternative syntax:
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
setting.
However, an explicit
may be used to bind the connection to a specific address.
The
of
indicates that the listening port be bound for local use only, while an
empty address or
indicates that the port should be available from all interfaces.
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.
M
Places the
client into
mode for connection sharing.
Multiple
options places
into
mode with confirmation required before slave connections are accepted.
Refer to the description of
in
ssh_config(5)
for details.
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
keyword for more information.
N
Do not execute a remote command.
This is useful for just forwarding ports
(protocol version 2 only).
n
Redirects stdin from
(actually, prevents reading from stdin).
This must be used when
is run in the background.
A common trick is to use this to run X11 programs on a remote machine.
For example,
c ssh -n shadows.cs.hut.fi emacs &
connection will be automatically forwarded over an encrypted channel.
The
program will be put in the background.
(This does not work if
needs to ask for a password or passphrase; see also the
option.)
ctl_cmd
Control an active connection multiplexing master process.
When the
option is specified, the
argument is interpreted and passed to the master process.
Valid commands are:
(check that the master process is running) and
(request the master to exit).
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.
For full details of the options listed below, and their possible values, see
ssh_config(5)
- AddressFamily
- BatchMode
- BindAddress
- ChallengeResponseAuthentication
- CheckHostIP
- Cipher
- Ciphers
- ClearAllForwardings
- Compression
- CompressionLevel
- ConnectionAttempts
- ConnectTimeout
- ControlMaster
- ControlPath
- DynamicForward
- EscapeChar
- ExitOnForwardFailure
- ForwardAgent
- ForwardX11
- ForwardX11Trusted
- GatewayPorts
- GlobalKnownHostsFile
- GSSAPIAuthentication
- GSSAPIDelegateCredentials
- HashKnownHosts
- Host
- HostbasedAuthentication
- HostKeyAlgorithms
- HostKeyAlias
- HostName
- IdentityFile
- IdentitiesOnly
- KbdInteractiveDevices
- LocalCommand
- LocalForward
- LogLevel
- MACs
- NoHostAuthenticationForLocalhost
- NumberOfPasswordPrompts
- PasswordAuthentication
- PermitLocalCommand
- Port
- PreferredAuthentications
- Protocol
- ProxyCommand
- PubkeyAuthentication
- RekeyLimit
- RemoteForward
- RhostsRSAAuthentication
- RSAAuthentication
- SendEnv
- ServerAliveInterval
- ServerAliveCountMax
- SmartcardDevice
- StrictHostKeyChecking
- TCPKeepAlive
- Tunnel
- TunnelDevice
- UsePrivilegedPort
- User
- UserKnownHostsFile
- VerifyHostKeyDNS
- XAuthLocation
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 by enclosing the address in square braces or
using an alternative syntax:
By default, the listening socket on the server will be bound to the loopback
interface only.
This may be overriden by specifying a
An empty
or the address
indicates that the remote socket should listen on all interfaces.
Specifying a remote
will only succeed if the server's
option is enabled (see
sshd_config(5)
ctl_path
Specifies the location of a control socket for connection sharing.
Refer to the description of
and
in
ssh_config(5)
for details.
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(1)
The subsystem is specified as the remote command.
T
Disable pseudo-tty allocation.
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
options force tty allocation, even if
has no local tty.
V
Display the version number and exit.
v
Verbose mode.
Causes
to print debugging messages about its progress.
This is helpful in
debugging connection, authentication, and configuration problems.
Multiple
options increase the verbosity.
The maximum is 3.
Xo
Requests
tunnel
device forwarding with the specified
tun(4)
devices between the client
and the server
The devices may be specified by numerical ID or the keyword
which uses the next available tunnel device.
If
is not specified, it defaults to
See also the
and
directives in
ssh_config(5)
If the
directive is unset, it is set to the default tunnel mode, which is
X
Enables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.
X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the user's X authorization database)
can access the local X11 display through the forwarded connection.
An attacker may then be able to perform activities such as keystroke monitoring.
For this reason, X11 forwarding is subjected to X11 SECURITY extension
restrictions by default.
Please refer to the
option and the
directive in
ssh_config(5)
for more information.
x
Disables X11 forwarding.
Y
Enables trusted X11 forwarding.
Trusted X11 forwardings are not subjected to the X11 SECURITY extension
controls.
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
ssh_config(5)
exits with the exit status of the remote command or with 255
if an error occurred.
AUTHENTICATION
The OpenSSH SSH client supports SSH protocols 1 and 2.
Protocol 2 is the default, with
falling back to protocol 1 if it detects protocol 2 is unsupported.
These settings may be altered using the
option in
ssh_config(5)
or enforced using the
and
options (see above).
Both protocols support similar authentication methods,
but protocol 2 is preferred since
it provides additional mechanisms for confidentiality
(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
Protocol 1 lacks a strong mechanism for ensuring the
integrity of the connection.
The methods available for authentication are:
GSSAPI-based authentication,
host-based authentication,
public key authentication,
challenge-response authentication,
and password authentication.
Authentication methods are tried in the order specified above,
though protocol 2 has a configuration option to change the default order:
Host-based authentication works as follows:
If the machine the user logs in from is listed in
or
on the remote machine, and the user names are
the same on both sides, or if the files
or
exist in the user's home directory on the
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
considered for login.
Additionally, the server
be able to verify the client's
host key (see the description of
and
below)
for login to be permitted.
This authentication method closes security holes due to IP
spoofing, DNS spoofing, and routing spoofing.
[Note to the administrator:
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
Public key authentication works as follows:
The scheme is based on public-key cryptography,
using cryptosystems
where encryption and decryption are done using separate keys,
and it is unfeasible to derive the decryption key from the encryption key.
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.
implements public key authentication protocol automatically,
using either the RSA or DSA algorithms.
Protocol 1 is restricted to using only RSA keys,
but protocol 2 may use either.
The
section of
ssl(8)
contains a brief discussion of the two algorithms.
The file
lists the public keys that are permitted for logging in.
When the user logs in, the
program tells the server which key pair it would like to use for
authentication.
The client proves that it has access to the private key
and the server checks that the corresponding public key
is authorized to accept the account.
The user creates his/her key pair by running
ssh-keygen(1)
This stores the private key in
(protocol 1),
(protocol 2 DSA),
or
(protocol 2 RSA)
and stores the public key in
(protocol 1),
(protocol 2 DSA),
or
(protocol 2 RSA)
in the user's home directory.
The user should then copy the public key
to
in his/her home directory on the remote machine.
The
file corresponds to the conventional
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.
The most convenient way to use public key authentication may be with an
authentication agent.
See
ssh-agent(1)
for more information.
Challenge-response authentication works as follows:
The server sends an arbitrary
text, and prompts for a response.
Protocol 2 allows multiple challenges and responses;
protocol 1 is restricted to just one challenge/response.
Examples of challenge-response authentication include
BSD Authentication (see
login.conf(5)
and PAM (some non-OpenBSD systems).
Finally, if other authentication methods fail,
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.
automatically maintains and checks a database containing
identification for all hosts it has ever been used with.
Host keys are stored in
in the user's home directory.
Additionally, the file
is automatically checked for known hosts.
Any new hosts are automatically added to the user's file.
If a host's identification ever changes,
warns about this and disables password authentication to prevent
server spoofing or man-in-the-middle attacks,
which could otherwise be used to circumvent the encryption.
The
option can be used to control logins to machines whose
host key is not known or has changed.
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.
If a pseudo-terminal has been allocated (normal login session), the
user may use the escape characters noted below.
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
will also make the session transparent even if a tty is used.
The session terminates when the command or shell on the remote
machine exits and all X11 and TCP connections have been closed.
ESCAPE CHARACTERS
When a pseudo-terminal has been requested,
supports a number of functions through the use of an escape character.
A single tilde character can be sent as
c ~~
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
configuration directive or on the command line by the
option.
The supported escapes (assuming the default
are:
- ~. Disconnect.
- ~^Z Background
- ~# List forwarded connections.
- ~& Background at logout when waiting for forwarded connection / X11 sessions to terminate.
- ~? Display a list of escape characters.
- ~B Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it).
- ~C Open command line. Currently this allows the addition of port forwardings using the and options (see above). It also allows the cancellation of existing remote port-forwardings using c ! Ns Ar command allows the user to execute a local command if the c PermitLocalCommand option is enabled in ssh_config(5) Basic help is available, using the option.
- ~R Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it).
TCP FORWARDING
Forwarding of arbitrary TCP connections over the secure channel can
be specified either on the command line or in a configuration file.
One possible application of TCP forwarding is a secure connection to a
mail server; another is going through firewalls.
In the example below, we look at encrypting communication between
an IRC client and server, even though the IRC server does not directly
support encrypted communications.
This works as follows:
the user connects to the remote host using
specifying a port to be used to forward connections
to the remote server.
After that it is possible to start the service which is to be encrypted
on the client machine,
connecting to the same local port,
and
will encrypt and forward the connection.
The following example tunnels an IRC session from client machine
(localhost)
to remote server
$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 $ irc -c '#users' -p 1234 pinky 127.0.0.1
This tunnels a connection to IRC server
joining channel
nickname
using port 1234.
It doesn't matter which port is used,
as long as it's greater than 1023
(remember, only root can open sockets on privileged ports)
and doesn't conflict with any ports already in use.
The connection is forwarded to port 6667 on the remote server,
since that's the standard port for IRC services.
The
option backgrounds
and the remote command
is specified to allow an amount of time
(10 seconds, in the example)
to start the service which is to be tunnelled.
If no connections are made within the time specified,
will exit.
X11 FORWARDING
If the
variable is set to
(or see the description of the
and
options above)
and the user is using X11 (the
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
Forwarding of X11 connections can be
configured on the command line or in configuration files.
The
value set by
will point to the server machine, but with a display number greater than zero.
This is normal, and happens because
creates a
X server on the server machine for forwarding the
connections over the encrypted channel.
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).
If the
variable is set to
(or see the description of the
and
options above) and
the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side.
VERIFYING HOST KEYS
When connecting to a server for the first time,
a fingerprint of the server's public key is presented to the user
(unless the option
has been disabled).
Fingerprints can be determined using
ssh-keygen(1)
If the fingerprint is already known,
it can be matched and verified,
and the key can be accepted.
If the fingerprint is unknown,
an alternative method of verification is available:
SSH fingerprints verified by DNS.
An additional resource record (RR),
SSHFP,
is added to a zonefile
and the connecting client is able to match the fingerprint
with that of the key presented.
In this example, we are connecting a client to a server,
The SSHFP resource records should first be added to the zonefile for
host.example.com:
$ ssh-keygen -r host.example.com.
The output lines will have to be added to the zonefile.
To check that the zone is answering fingerprint queries:
Finally the client connects:
$ ssh -o "VerifyHostKeyDNS ask" host.example.com [...] Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?
See the
option in
ssh_config(5)
for more information.
SSH-BASED VIRTUAL PRIVATE NETWORKS
contains support for Virtual Private Network (VPN) tunnelling
using the
tun(4)
network pseudo-device,
allowing two networks to be joined securely.
The
sshd_config(5)
configuration option
controls whether the server supports this,
and at what level (layer 2 or 3 traffic).
The following example would connect client network 10.0.50.0/24
with remote network 10.0.99.0/24 using a point-to-point connection
from 10.1.1.1 to 10.1.1.2,
provided that the SSH server running on the gateway to the remote network,
at 192.168.1.15, allows it.
On the client:
# ssh -f -w 0:1 192.168.1.15 true # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 # route add 10.0.99.0/24 10.1.1.2
On the server:
# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 # route add 10.0.50.0/24 10.1.1.1
Client access may be more finely tuned via the
file (see below) and the
server option.
The following entry would permit connections on
tun(4)
device 1 from user
and on tun device 2 from user
if
is set to
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Since an SSH-based setup entails a fair amount of overhead,
it may be more suited to temporary setups,
such as for wireless VPNs.
More permanent VPNs are better provided by tools such as
ipsecctl(8)
and
isakmpd(8)
ENVIRONMENT
will normally set the following environment variables:
- DISPLAY The variable indicates the location of the X11 server. It is automatically set by to point to a value of the form where indicates the host where the shell runs, and is an integer >= 1. uses this special value to forward X11 connections over the secure channel. The user should normally not set explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).
- HOME Set to the path of the user's home directory.
- LOGNAME Synonym for set for compatibility with systems that use this variable.
- MAIL Set to the path of the user's mailbox.
- PATH Set to the default as specified when compiling
- SSH_ASKPASS If needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If does not have a terminal associated with it but and are set, it will execute the program specified by and open an X11 window to read the passphrase. This is particularly useful when calling from a
or related script.
(Note that on some machines it
may be necessary to redirect the input from
to make this work.)
SSH_AUTH_SOCK
Identifies the path of a
socket used to communicate with the agent.
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.
SSH_ORIGINAL_COMMAND
This variable contains the original command line if a forced command
is executed.
It can be used to extract the original arguments.
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.
TZ
This variable is set to indicate the present time zone if it
was set when the daemon was started (i.e. the daemon passes the value
on to new connections).
USER
Set to the name of the user logging in.
Additionally,
reads
and adds lines of the format
to the environment if the file exists and users are allowed to
change their environment.
For more information, see the
option in
sshd_config(5)
FILES
- ~/.rhosts This file is used for host-based authentication (see above). On some machines this file may need to be world-readable if the user's home directory is on an NFS partition, because 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.
but allows host-based authentication without permitting login with
rlogin/rsh.
but allows host-based authentication without permitting login with
rlogin/rsh.
SEE ALSO
scp(1)
sftp(1)
ssh-add(1)
ssh-agent(1)
ssh-argv0(1)
ssh-keygen(1)
ssh-keyscan(1)
tun(4)
hosts.equiv(5)
ssh_config(5)
ssh-keysign(8)
sshd(8)
.%R RFC 4250
.%T "The Secure Shell (SSH) Protocol Assigned Numbers"
.%D 2006
.%R RFC 4251
.%T "The Secure Shell (SSH) Protocol Architecture"
.%D 2006
.%R RFC 4252
.%T "The Secure Shell (SSH) Authentication Protocol"
.%D 2006
.%R RFC 4253
.%T "The Secure Shell (SSH) Transport Layer Protocol"
.%D 2006
.%R RFC 4254
.%T "The Secure Shell (SSH) Connection Protocol"
.%D 2006
.%R RFC 4255
.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
.%D 2006
.%R RFC 4256
.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
.%D 2006
.%R RFC 4335
.%T "The Secure Shell (SSH) Session Channel Break Extension"
.%D 2006
.%R RFC 4344
.%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
.%D 2006
.%R RFC 4345
.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
.%D 2006
.%R RFC 4419
.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
.%D 2006
.%R RFC 4716
.%T "The Secure Shell (SSH) Public Key File Format"
.%D 2006
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.
ČLÁNKY DO MAILU