Provided by: openssh-client_9.0p1-1ubuntu8_amd64 
NAME
ssh — OpenSSH remote login client
SYNOPSIS
ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address] [-c cipher_spec]
[-D [bind_address:]port] [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11]
[-i identity_file] [-J destination] [-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]] destination [command [argument ...]]
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 destination, which may be specified as either
[user@]hostname or a URI of the form ssh://[user@]hostname[:port]. The user must prove
their identity to the remote machine using one of several methods (see below).
If a command is specified, it will be executed on the remote host instead of a login shell.
A complete command line may be specified as command, or it may have additional arguments.
If supplied, the arguments will be appended to the command, separated by spaces, before it
is sent to the server to be executed.
The options are as follows:
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
-A Enables forwarding of connections from an authentication agent such as ssh-agent(1).
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 safer
alternative may be to use a jump host (see -J).
-a Disables forwarding of the authentication agent connection.
-B bind_interface
Bind to the address of bind_interface before attempting to connect to the
destination host. This is only useful on systems with more than one address.
-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). 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. 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.
If set to “none”, no configuration files will be read.
-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. Refer to the description of
ForkAfterAuthentication in ssh_config(5) for details.
-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 keys for user authentication.
-i identity_file
Selects a file from which the identity (private key) for public key authentication
is read. You can also specify a public key file to use the corresponding private
key that is loaded in ssh-agent(1) when the private key file is not present locally.
The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk,
~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and ~/.ssh/id_dsa. 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 destination
Connect to the target host by first making a ssh connection to the jump host
described by destination 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.
Note that configuration directives supplied on the command-line generally apply to
the destination host and not any specified jump hosts. Use ~/.ssh/config to specify
configuration for jump hosts.
-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 but with confirmation required using
ssh-askpass(1) before each operation that changes the multiplexing state (e.g.
opening a new session). 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. Refer
to the description of SessionType in ssh_config(5) for details.
-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.) Refer to the description of StdinNull in ssh_config(5) for details.
-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
CASignatureAlgorithms
CertificateFile
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForkAfterAuthentication
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
HashKnownHosts
Host
HostbasedAcceptedAlgorithms
HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
SendEnv
ServerAliveInterval
ServerAliveCountMax
SessionType
SetEnv
StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
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 for the algorithms supported by one of the following features: cipher
(supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support
authenticated encryption), help (supported query terms for use with the -Q flag),
mac (supported message integrity codes), kex (key exchange algorithms), kex-gss
(GSSAPI key exchange algorithms), key (key types), key-cert (certificate key types),
key-plain (non-certificate key types), key-sig (all key types and signature
algorithms), protocol-version (supported SSH protocol versions), and sig (supported
signature algorithms). Alternatively, any keyword from ssh_config(5) or
sshd_config(5) that takes an algorithm list may be used as an alias for the
corresponding query_option.
-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
-R [bind_address:]port
Specifies that connections to the given TCP port or Unix socket on the remote
(server) host are to be forwarded to 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 from the
local machine to either an explicit destination specified by host port hostport, or
local_socket, or, if no explicit destination was specified, ssh will act as a SOCKS
4/5 proxy and forward connections to the destinations requested by the remote SOCKS
client.
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. Refer to the
description of SessionType in ssh_config(5) for details.
-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 will be set to the default tunnel mode, which
is “point-to-point”. If a different Tunnel forwarding mode it desired, then it
should be specified before -w.
-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. 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: In the default configuration, this option is equivalent to -X,
since ForwardX11Trusted defaults to “yes” 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 protocol 2.
The methods available for authentication are: GSSAPI-based authentication, host-based
authentication, public key authentication, keyboard-interactive 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, the user is non-root 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 server may inform the client of errors that prevented public key authentication from
succeeding after authentication completes using a different method. These may be viewed by
increasing the LogLevel to DEBUG or higher (e.g. by using the -v flag).
The user creates their key pair by running ssh-keygen(1). This stores the private key in
~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ecdsa_sk (authenticator-hosted
ECDSA), ~/.ssh/id_ed25519 (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA),
~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (authenticator-hosted 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 their 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.
Keyboard-interactive authentication works as follows: The server sends an arbitrary
"challenge" text and prompts for a response, possibly multiple times. Examples of keyboard-
interactive 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 a 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 for an IRC client, even though the
IRC server it connects to does not directly support encrypted communication. This works as
follows: the user connects to the remote host using ssh, specifying the ports to be used to
forward the connection. After that it is possible to start the program locally, and ssh
will encrypt and forward the connection to the remote server.
The following example tunnels an IRC session from the client to an IRC server at
“server.example.com”, joining channel “#users”, nickname “pinky”, using the standard IRC
port, 6667:
$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
$ irc -c '#users' pinky IRC/127.0.0.1
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 program which is going to use the
tunnel. 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_ASKPASS_REQUIRE Allows further control over the use of an askpass program. If this
variable is set to “never” then ssh will never attempt to use one. If
it is set to “prefer”, then ssh will prefer to use the askpass program
instead of the TTY when requesting passwords. Finally, if the
variable is set to “force”, then the askpass program will be used for
all passphrase input regardless of whether DISPLAY is set.
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.
SSH_TUNNEL Optionally set by sshd(8) to contain the interface names assigned if
tunnel forwarding was requested by the client.
SSH_USER_AUTH Optionally set by sshd(8), this variable may contain a pathname to a
file that lists the authentication methods successfully used when the
session was established, including any public keys that were used.
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/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ecdsa_sk
~/.ssh/id_ed25519
~/.ssh/id_ed25519_sk
~/.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 AES-128.
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ecdsa_sk.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_ed25519_sk.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.