ssh connects and logs into the specified hostname (with optional user 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 command is specified, it is executed on the remote host instead of a login shell.
The options are as follows:
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.
Protocol version 1 allows specification of a single cipher. The supported values are ``3des'', ``blowfish'', and ``des''. 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. It is believed to be secure. blowfish is a fast block cipher; it appears very secure and is much faster than 3des. des is only supported in the ssh client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to cryptographic weaknesses. The default is ``3des''.
For protocol version 2, cipher_spec 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: [bind_address /] port 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 GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of ``localhost'' 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.
/etc/ssh/ssh_config
)
will be ignored.
The default for the per-user configuration file is
~/.ssh/config
.
~/.ssh/identity
for protocol version 1, and
~/.ssh/id_rsa
and
~/.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
-i
options (and multiple identities specified in
configuration files).
/dev/null
(actually, prevents reading from stdin).
This must be used when
ssh
is run in the background.
A common trick is to use this to run X11 programs on a remote machine.
For example,
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
ssh
program will be put in the background.
(This does not work if
ssh
needs to ask for a password or passphrase; see also the
-f
option.)
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: [bind_address /] host / port / hostport
By default, the listening socket on the server will be bound to the loopback interface only. This may be overriden by specifying a bind_address. An empty bind_address, or the address `*', indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config(5)).
The devices may be specified by numerical ID or the keyword ``any'', which uses the next available tunnel device. If remote_tun is not specified, it defaults to ``any''. See also the Tunnel and TunnelDevice directives in ssh_config(5). If the Tunnel directive is unset, it is set to the default tunnel mode, which is ``point-to-point''.
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 ssh -Y option and the ForwardX11Trusted directive in ssh_config(5) for more information.
ssh 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).
ssh exits with the exit status of the remote command or with 255 if an error occurred.
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: PreferredAuthentications.
Host-based authentication works as follows:
If the machine the user logs in from is listed in
/etc/hosts.equiv
or
/etc/shosts.equiv
on the remote machine, and the user names are
the same on both sides, or if the files
~/.rhosts
or
~/.shosts
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
must
be able to verify the client's
host key (see the description of
/etc/ssh/ssh_known_hosts
and
~/.ssh/known_hosts
,
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:
/etc/hosts.equiv
,
~/.rhosts
,
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. ssh 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 HISTORY section of ssl(8) contains a brief discussion of the two algorithms.
The file
~/.ssh/authorized_keys
lists the public keys that are permitted for logging in.
When the user logs in, the
ssh
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
~/.ssh/identity
(protocol 1),
~/.ssh/id_dsa
(protocol 2 DSA),
or
~/.ssh/id_rsa
(protocol 2 RSA)
and stores the public key in
~/.ssh/identity.pub
(protocol 1),
~/.ssh/id_dsa.pub
(protocol 2 DSA),
or
~/.ssh/id_rsa.pub
(protocol 2 RSA)
in the user's home directory.
The user should then copy the public key
to
~/.ssh/authorized_keys
in his/her home directory on the remote machine.
The
authorized_keys
file corresponds to the conventional
~/.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.
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 "challenge 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, ssh 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.
ssh
automatically maintains and checks a database containing
identification for all hosts it has ever been used with.
Host keys are stored in
~/.ssh/known_hosts
in the user's home directory.
Additionally, the file
/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,
ssh
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
StrictHostKeyChecking
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 ``none'' 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.
A single tilde character can be sent as ~~ 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 EscapeChar configuration directive or on the command line by the -e option.
The supported escapes (assuming the default `~') are:
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 ssh will encrypt and forward the connection.
The following example tunnels an IRC session from client machine ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
$ 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 ``server.example.com'', joining channel ``#users'', nickname ``pinky'', 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 -f option backgrounds ssh and the remote command ``sleep 10'' 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, ssh will exit.
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
DISPLAY
.
Forwarding of X11 connections can be
configured on the command line or in configuration files.
The
DISPLAY
value set by
ssh
will point to the server machine, but with a display number greater than zero.
This is normal, and happens because
ssh
creates a
``proxy''
X server on the server machine for forwarding the
connections over the encrypted channel.
ssh 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 ForwardAgent variable is set to ``yes'' (or see the description of the -A and -a options above) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.
$
ssh-keygen
-l
-f
/etc/ssh/ssh_host_rsa_key
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, ``host.example.com''. 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:
$
dig
-t
SSHFP
host.example.com
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 VerifyHostKeyDNS option in ssh_config(5) for more information.
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
/root/.ssh/authorized_keys
file (see below) and the
PermitRootLogin
server option.
The following entry would permit connections on
tun(4)
device 1 from user
``jane''
and on tun device 2 from user
``john'',
if
PermitRootLogin
is set to
``forced-commands-only'':
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).
DISPLAY
DISPLAY
variable indicates the location of the X11 server.
It is automatically set by
ssh
to point to a value of the form
``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).
HOME
LOGNAME
USER
;
set for compatibility with systems that use this variable.
MAIL
PATH
PATH
,
as specified when compiling
.
SSH_ASKPASS
DISPLAY
and
SSH_ASKPASS
are set, it will execute the program specified by
SSH_ASKPASS
and open an X11 window to read the passphrase.
This is particularly useful when calling
ssh
from a
.xsession
or related script.
(Note that on some machines it
may be necessary to redirect the input from
/dev/null
to make this work.)
SSH_AUTH_SOCK
SSH_CONNECTION
SSH_ORIGINAL_COMMAND
SSH_TTY
TZ
USER
Additionally,
ssh
reads
~/.ssh/environment
,
and adds lines of the format
``VARNAME=value''
to the environment if the file exists and users are allowed to
change their environment.
For more information, see the
PermitUserEnvironment
option in
sshd_config(5).
.rhosts
,
but allows host-based authentication without permitting login with
rlogin/rsh.
hosts.equiv
,
but allows host-based authentication without permitting login with
rlogin/rsh.
/etc/ssh/ssh_config