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SSH(1) |
FreeBSD General Commands Manual |
SSH(1) |
ssh - OpenSSH remote login client
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...]
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
(/usr/local/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
- GSSAPIDelegateCredentials
- 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),
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.
- -x
- Disables X11 forwarding.
- -Y
- Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected
to the X11 SECURITY extension controls.
- -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).
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
/usr/local/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 /usr/local/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)
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 Qq challenge text and prompts for a response, possibly
multiple times. Examples of keyboard-interactive authentication include Bx
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 /usr/local/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.
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 .
- ~#
- 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.
! Ns 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.
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 , 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.
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.
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):
Dl $ ssh-keygen -l -f /usr/local/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 randomart. 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:
Dl $ 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:
Dl $ 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 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).
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_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).
- ~/.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.
- ~/.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.
- /usr/local/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.
- /usr/local/etc/ssh/ssh_config
- Systemwide configuration file. The file format and configuration options
are described in ssh_config(5).
- /usr/local/etc/ssh/ssh_host_key
- /usr/local/etc/ssh/ssh_host_dsa_key
- /usr/local/etc/ssh/ssh_host_ecdsa_key
- /usr/local/etc/ssh/ssh_host_ed25519_key
- /usr/local/etc/ssh/ssh_host_rsa_key
- These files contain the private parts of the host keys and are used for
host-based authentication.
- /usr/local/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.
- /usr/local/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.
ssh exits with the exit status of the remote command or with 255 if an
error occurred.
scp(1), sftp(1), ssh-add(1), ssh-agent(1),
ssh-keygen(1), ssh-keyscan(1), tun(4),
ssh_config(5), ssh-keysign(8), sshd(8)
C. Lonvick and S. Lehtinen, The Secure Shell (SSH) Protocol Assigned
Numbers, RFC 4250, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Protocol
Architecture, RFC 4251, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Authentication
Protocol, RFC 4252, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Transport
Layer Protocol, RFC 4253, January 2006.
C. Lonvick and T. Ylonen, The Secure Shell (SSH) Connection
Protocol, RFC 4254, January 2006.
W. Griffin and J. Schlyter, Using DNS to Securely Publish
Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
M. Forssen and F. Cusack, Generic Message Exchange
Authentication for the Secure Shell Protocol (SSH), RFC 4256, January
2006.
P. Remaker and J. Galbraith, The Secure Shell (SSH) Session
Channel Break Extension, RFC 4335, January 2006.
C. Namprempre, T. Kohno and M. Bellare, 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.
W. Simpson, N. Provos and M. Friedl, Diffie-Hellman Group
Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419,
March 2006.
R. Thayer and J. Galbraith, The Secure Shell (SSH) Public Key
File Format, RFC 4716, November 2006.
J. Green and D. Stebila, Elliptic Curve Algorithm Integration
in the Secure Shell Transport Layer, RFC 5656, December 2009.
D. Song and A. Perrig, Hash Visualization: a New Technique to
improve Real-World Security, 1999, International Workshop on
Cryptographic Techniques and E-Commerce (CrypTEC '99).
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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