slogin(1)
NAME
ssh --- OpenSSH SSH client (remote login program)
SYNOPSIS
ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
[-D [bind_address:]port] [-E log_file] [-e escape_char]
[-F configfile] [-I pkcs11] [-i identity_file]
[-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec]
[-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
[-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
[user@]hostname [command]
DESCRIPTION
ssh (SSH client) is a program for logging into a remote machine and for
executing commands on a remote machine. It is intended to provide secure
encrypted communications between two untrusted hosts over an insecure
network. X11 connections, arbitrary TCP ports and UNIX-domain sockets
can also be forwarded over the secure channel.
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 (see below).
If command is specified, it is executed on the remote host instead of a
login shell.
The options are as follows:
-1 Forces ssh to try protocol version 1 only.
-2 Forces ssh to try protocol version 2 only.
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh 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.
-b bind_address
Use bind_address 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, TCP and UNIX-domain
connections). The compression algorithm is the same used by
gzip(1), and the "level" can be controlled by the
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
Compression option.
-c cipher_spec
Selects the cipher specification for encrypting the session.
Protocol version 1 allows specification of a single cipher. The
supported values are "3des", "blowfish", and "des". For protocol
version 2, cipher_spec is a comma-separated list of ciphers
listed in order of preference. See the Ciphers keyword in
ssh_config(5) for more information.
-D [bind_address:]port
Specifies a local "dynamic" application-level port forwarding.
This works by allocating a socket to listen to port on the local
side, optionally bound to the specified bind_address. 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 and SOCKS5 protocols are supported, and ssh will act
as a SOCKS server. Only root can forward privileged ports.
Dynamic port forwardings can also be specified in the
configuration file.
IPv6 addresses can be specified 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.
-E log_file
Append debug logs to log_file instead of standard error.
-e 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 "none" disables any escapes and makes the session
fully transparent.
-F configfile
Specifies an alternative per-user configuration file. If a
configuration file is given on the command line, the system-wide
configuration file (/etc/ssh/ssh_config) will be ignored. The
default for the per-user configuration file is ~/.ssh/config.
-f Requests ssh to go to background just before command execution.
This is useful if ssh is going to ask for passwords or
passphrases, but the user wants it in the background. This
implies -n. The recommended way to start X11 programs at a
remote site is with something like ssh -f host xterm.
If the ExitOnForwardFailure configuration option is set to "yes",
then a client started with -f will wait for all remote port
forwards to be successfully established before placing itself in
the background.
-G Causes ssh to print its configuration after evaluating Host and
Match blocks and exit.
-g Allows remote hosts to connect to local forwarded ports. If used
on a multiplexed connection, then this option must be specified
on the master process.
-I pkcs11
Specify the PKCS#11 shared library ssh should use to communicate
with a PKCS#11 token providing the user's private RSA key.
-i identity_file
Selects a file from which the identity (private key) for public
key authentication is read. The default is ~/.ssh/identity for
protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
~/.ssh/id_ed25519 and ~/.ssh/id_rsa 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). If
no certificates have been explicitly specified by the
CertificateFile directive, ssh will also try to load certificate
information from the filename obtained by appending -cert.pub to
identity filenames.
-J [user@]host[:port]
Connect to the target host by first making a ssh connection to
the jump host and then establishing a TCP forwarding to the
ultimate destination from there. Multiple jump hops may be
specified separated by comma characters. This is a shortcut to
specify a ProxyJump configuration directive.
-K Enables GSSAPI-based authentication and forwarding (delegation)
of GSSAPI credentials to the server.
-k Disables forwarding (delegation) of GSSAPI credentials to the
server.
-L [bind_address:]port:host:hostport
-L [bind_address:]port:remote_socket
-L local_socket:host:hostport
-L local_socket:remote_socket
Specifies that connections to the given TCP port or Unix socket
on the local (client) host are to be forwarded to the given host
and port, or Unix socket, on the remote side. This works by
allocating a socket to listen to either a TCP port on the local
side, optionally bound to the specified bind_address, or to a
Unix socket. Whenever a connection is made to the local port or
socket, the connection is forwarded over the secure channel, and
a connection is made to either host port hostport, or the Unix
socket remote_socket, from the remote machine.
Port forwardings can also be specified in the configuration file.
Only the superuser can forward privileged ports. IPv6 addresses
can be specified by enclosing the address in square brackets.
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.
-l 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 ssh client into "master" mode for connection sharing.
Multiple -M options places ssh into "master" mode with
confirmation required before slave connections are accepted.
Refer to the description of ControlMaster in ssh_config(5) for
details.
-m mac_spec
A comma-separated list of MAC (message authentication code)
algorithms, specified in order of preference. See the MACs
keyword for more information.
-N Do not execute a remote command. This is useful for just
forwarding ports.
-n Redirects stdin from /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.)
-O ctl_cmd
Control an active connection multiplexing master process. When
the -O option is specified, the ctl_cmd argument is interpreted
and passed to the master process. Valid commands are: "check"
(check that the master process is running), "forward" (request
forwardings without command execution), "cancel" (cancel
forwardings), "exit" (request the master to exit), and "stop"
(request the master to stop accepting further multiplexing
requests).
-o 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).
AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CertificateFile
ChallengeResponseAuthentication
CheckHostIP
Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
HashKnownHosts
Host
HostbasedAuthentication
HostbasedKeyTypes
HostKeyAlgorithms
HostKeyAlias
HostName
IdentitiesOnly
IdentityAgent
IdentityFile
Include
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
Protocol
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteForward
RequestTTY
RhostsRSAAuthentication
RSAAuthentication
SendEnv
ServerAliveInterval
ServerAliveCountMax
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
UsePrivilegedPort
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation
-p port
Port to connect to on the remote host. This can be specified on
a per-host basis in the configuration file.
-Q query_option
Queries ssh for the algorithms supported for the specified
version 2. The available features are: cipher (supported
symmetric ciphers), cipher-auth (supported symmetric ciphers that
support authenticated encryption), mac (supported message
integrity codes), kex (key exchange algorithms), key (key types),
key-cert (certificate key types), key-plain (non-certificate key
types), and protocol-version (supported SSH protocol versions).
-q Quiet mode. Causes most warning and diagnostic messages to be
suppressed.
-R [bind_address:]port:host:hostport
-R [bind_address:]port:local_socket
-R remote_socket:host:hostport
-R remote_socket:local_socket
Specifies that connections to the given TCP port or Unix socket
on the remote (server) host are to be forwarded to the given host
and port, or Unix socket, on the local side. This works by
allocating a socket to listen to either a TCP port or to a Unix
socket on the remote side. Whenever a connection is made to this
port or Unix socket, the connection is forwarded over the secure
channel, and a connection is made to either host port hostport,
or local_socket, 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 by enclosing
the address in square brackets.
By default, TCP listening sockets on the server will be bound to
the loopback interface only. This may be overridden 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)).
If the port argument is '0', the listen port will be dynamically
allocated on the server and reported to the client at run time.
When used together with -O forward the allocated port will be
printed to the standard output.
-S ctl_path
Specifies the location of a control socket for connection
sharing, or the string "none" to disable connection sharing.
Refer to the description of ControlPath and ControlMaster in
ssh_config(5) for details.
-s May be used to request invocation of a subsystem on the remote
system. Subsystems facilitate the use of SSH as a secure
transport for other applications (e.g. sftp(1)). The subsystem
is specified as the remote command.
-T Disable pseudo-terminal allocation.
-t Force pseudo-terminal 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 -t
options force tty allocation, even if ssh has no local tty.
-V Display the version number and exit.
-v Verbose mode. Causes ssh to print debugging messages about its
progress. This is helpful in debugging connection,
authentication, and configuration problems. Multiple -v options
increase the verbosity. The maximum is 3.
-W host:port
Requests that standard input and output on the client be
forwarded to host on port over the secure channel. Implies -N,
-T, ExitOnForwardFailure and ClearAllForwardings, though these
can be overridden in the configuration file or using -o command
line options.
-w local_tun[:remote_tun]
Requests tunnel device forwarding with the specified tun(4)
devices between the client (local_tun) and the server
(remote_tun).
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".
-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 ssh -Y
option and the ForwardX11Trusted directive in ssh_config(5) for
more information.
(Debian-specific: X11 forwarding is not subjected to X11 SECURITY
extension restrictions by default, because too many programs
currently crash in this mode. Set the ForwardX11Trusted option
to "no" to restore the upstream behaviour. This may change in
future depending on client-side improvements.)
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
subjected to the X11 SECURITY extension controls.
(Debian-specific: This option does nothing in the default
configuration: it is equivalent to "ForwardX11Trusted yes", which
is the default as described above. Set the ForwardX11Trusted
option to "no" to restore the upstream behaviour. This may
change in future depending on client-side improvements.)
-y Send log information using the syslog(3) system module. By
default this information is sent to stderr.
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).
AUTHENTICATION
The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
use protocol 2 only, though this can be changed via the Protocol option
in ssh_config(5) or the -1 and -2 options (see above). Protocol 1 should
not be used and is only offered to support legacy devices. It suffers
from a number of cryptographic weaknesses and doesn't support many of the
advanced features available for protocol 2.
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
PreferredAuthentications can be used to change the default order.
Host-based authentication works as follows: If the machine the user logs
in from is listed in /etc/hosts.equiv or /etc/ssh/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
one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of
ssl(8) (on non-OpenBSD systems, see
http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY)
contains a brief discussion of the DSA and RSA 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 (DSA),
~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa
(RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (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.
A variation on public key authentication is available in the form of
certificate authentication: instead of a set of public/private keys,
signed certificates are used. This has the advantage that a single
trusted certification authority can be used in place of many
public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
more information.
The most convenient way to use public key or certificate authentication
may be with an authentication agent. See ssh-agent(1) and (optionally)
the AddKeysToAgent directive in ssh_config(5) for more information.
Challenge-response authentication works as follows: The server sends an
arbitrary "challenge" text, and prompts for a 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 in a non-interactive session or, if no
command has been specified, logs into the machine and gives the user a
normal shell as an interactive session. All communication with the
remote command or shell will be automatically encrypted.
If an interactive session is requested ssh by default will only request a
pseudo-terminal (pty) for interactive sessions when the client has one.
The flags -T and -t can be used to override this behaviour.
If a pseudo-terminal has been allocated the user may use the escape
characters noted below.
If no pseudo-terminal 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.
ESCAPE CHARACTERS
When a pseudo-terminal has been requested, ssh supports a number of
functions through the use of an escape character.
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:
~. Disconnect.
~^Z Background ssh.
~# List forwarded connections.
~& Background ssh 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 if the peer
supports it).
~C Open command line. Currently this allows the addition of port
forwardings using the -L, -R and -D options (see above). It also
allows the cancellation of existing port-forwardings with
-KL[bind_address:]port for local, -KR[bind_address:]port for
remote and -KD[bind_address:]port for dynamic port-forwardings.
!command allows the user to execute a local command if the
PermitLocalCommand option is enabled in ssh_config(5). Basic
help is available, using the -h option.
~R Request rekeying of the connection (only useful if the peer
supports it).
~V Decrease the verbosity (LogLevel) when errors are being written
to stderr.
~v Increase the verbosity (LogLevel) when errors are being written
to stderr.
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 ssh, 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.
X11 FORWARDING
If the ForwardX11 variable is set to "yes" (or see the description of the
-X, -x, and -Y options above) and the user is using X11 (the 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.
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
StrictHostKeyChecking has been disabled). Fingerprints can be determined
using ssh-keygen(1):
$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
If the fingerprint is already known, it can be matched and the key can be
accepted or rejected. If only legacy (MD5) fingerprints for the server
are available, the ssh-keygen(1) -E option may be used to downgrade the
fingerprint algorithm to match.
Because of the difficulty of comparing host keys just by looking at
fingerprint strings, there is also support to compare host keys visually,
using random art. By setting the VisualHostKey option to "yes", a small
ASCII graphic gets displayed on every login to a server, no matter if the
session itself is interactive or not. By learning the pattern a known
server produces, a user can easily find out that the host key has changed
when a completely different pattern is displayed. Because these patterns
are not unambiguous however, a pattern that looks similar to the pattern
remembered only gives a good probability that the host key is the same,
not guaranteed proof.
To get a listing of the fingerprints along with their random art for all
known hosts, the following command line can be used:
$ ssh-keygen -lv -f ~/.ssh/known_hosts
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.
SSH-BASED VIRTUAL PRIVATE NETWORKS
ssh 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 PermitTunnel 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 /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).
ENVIRONMENT
ssh will normally set the following environment variables:
DISPLAY The 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 Set to the path of the user's home directory.
LOGNAME Synonym for USER; set for compatibility with
systems that use this variable.
MAIL Set to the path of the user's mailbox.
PATH Set to the default PATH, as specified when
compiling ssh.
SSH_ASKPASS If ssh needs a passphrase, it will read the
passphrase from the current terminal if it was run
from a terminal. If ssh does not have a terminal
associated with it but 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 Identifies the path of a UNIX-domain 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, 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).
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.
~/.shosts
This file is used in exactly the same way as .rhosts, but allows
host-based authentication without permitting login with
rlogin/rsh.
~/.ssh/
This directory is the default location for all user-specific
configuration and authentication information. There is no
general requirement to keep the entire contents of this directory
secret, but the recommended permissions are read/write/execute
for the user, and not accessible by others.
~/.ssh/authorized_keys
Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
for logging in as this user. The format of this file is
described in the sshd(8) manual page. This file is not highly
sensitive, but the recommended permissions are read/write for the
user, and not accessible by others.
~/.ssh/config
This is the per-user configuration file. The file format and
configuration options are described in ssh_config(5). Because of
the potential for abuse, this file must have strict permissions:
read/write for the user, and not writable by others. It may be
group-writable provided that the group in question contains only
the user.
~/.ssh/environment
Contains additional definitions for environment variables; see
ENVIRONMENT, above.
~/.ssh/identity
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
~/.ssh/id_rsa
Contains the private key for authentication. These files contain
sensitive data and should be readable by the user but not
accessible by others (read/write/execute). ssh will simply
ignore a private key file if it is accessible by others. It is
possible to specify a passphrase when generating the key which
will be used to encrypt the sensitive part of this file using
3DES.
~/.ssh/identity.pub
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_rsa.pub
Contains the public key for authentication. These files are not
sensitive and can (but need not) be readable by anyone.
~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged
into that are not already in the systemwide list of known host
keys. See sshd(8) for further details of the format of this
file.
~/.ssh/rc
Commands in this file are executed by ssh when the user logs in,
just before the user's shell (or command) is started. See the
sshd(8) manual page for more information.
/etc/hosts.equiv
This file is for host-based authentication (see above). It
should only be writable by root.
/etc/ssh/shosts.equiv
This file is used in exactly the same way as hosts.equiv, but
allows host-based authentication without permitting login with
rlogin/rsh.
/etc/ssh/ssh_config
Systemwide configuration file. The file format and configuration
options are described in ssh_config(5).
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys and are
used for host-based authentication.
/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. It should be world-readable.
See sshd(8) for further details of the format of this file.
/etc/ssh/sshrc
Commands in this file are executed by ssh when the user logs in,
just before the user's shell (or command) is started. See the
sshd(8) manual page for more information.
EXIT STATUS
ssh exits with the exit status of the remote command or with 255 if an
error occurred.
SEE ALSO
scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-argv0(1), ssh-keygen(1),
ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
STANDARDS
S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
Numbers, RFC 4250, January 2006.
T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
RFC 4251, January 2006.
T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
RFC 4252, January 2006.
T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
Protocol, RFC 4253, January 2006.
T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
4254, January 2006.
J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
(SSH) Key Fingerprints, RFC 4255, January 2006.
F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
Secure Shell Protocol (SSH), RFC 4256, January 2006.
J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
Extension, RFC 4335, January 2006.
M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
Layer Encryption Modes, RFC 4344, January 2006.
B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
Layer Protocol, RFC 4345, January 2006.
M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
Format, RFC 4716, November 2006.
D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer, RFC 5656, December 2009.
A. Perrig and D. Song, Hash Visualization: a New Technique to improve
Real-World Security, 1999, International Workshop on Cryptographic
Techniques and E-Commerce (CrypTEC '99).
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.
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