ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
[-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]
-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
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.
with more than one address.
-C Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and
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.
“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.
listen to port on the local side, optionally bound to the specified bind_address. Whenever a connec‐
tion 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 privi‐
leged ports. Dynamic port forwardings can also be specified in the configuration file.
forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts set‐
ting. 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.
nized 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.
line, the system-wide configuration file (/etc/ssh/ssh_config) will be ignored. The default for the
per-user configuration file is ~/.ssh/config.
要求ssh在执行命令之前转到后台运行。这在SSH需要输入密码，但用户希望在后台运行命令时非常有用。该选项隐含了-n选项。推荐在远程站点启动X11程序的方式与ssh -f host xterm类似。
Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user’s private RSA key.
协议版本为1的默认值为：~/.ssh/identity；协议版本为2的默认值为：~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519, ~/.ssh/id_rsa；
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.
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 con‐
nection 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 priv‐
ileged 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.
in the configuration file.
-M Places the ssh client into “master” mode for connection sharing. Multiple -M options places ssh into
tion of ControlMaster in ssh_config(5) for details.
ence. See the MACs keyword for more information.
一个常见的技巧是使用它在远程机器上运行X11程序。比如： ssh -n shadows.cs.hut.fi emacs &将会在远程主机shadows.cs.hut.fi上运行emacs，并且X11连接将会被在一个加密的频道上自动被转发。ssh程序将会在后台运行。如果ssh需要提供密码，那么该做法不会起作用，参考-f选项。
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).
ing 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).
cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support authenti‐
cated 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).
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.
only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the
address in square brackets.
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)).
to the client at run time. When used together with -O forward the allocated port will be printed to
the standard output.
nection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config(5) for
-s May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use
-T Disable pseudo-terminal allocation.
-t Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a
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
The maximum is 3.
channel. Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings, though these can be overridden
in the configuration file or using -o command line options.
the server (remote_tun).
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.
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.
refer to the ssh -Y option and the ForwardX11Trusted directive in ssh_config(5) for more information.
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 exten‐
“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 improve‐
-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 configura‐
tion file. The file format and configuration options are described in ssh_config(5).
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 weak‐
nesses 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 dis‐
abled 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 pur‐
poses. 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
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 certi‐
fication 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 authenti‐
cation 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
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
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:
~^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
-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.
使用SSH进行TCP转发 – 本地到远程的TCP连接（-L）
下面的命令从本机 “127.0.0.1” (localhost)建立IRC会话隧道到远程服务器 “server.example.com”：
# irc -c ‘#users’ -p 1234 pinky 127.0.0.1
-f选项使得ssh在后台运行，远程命令“sleep 10”指定了启动要通过隧道的服务允许的总时间（在示例中为10秒）。 如果指定的时间内没有连接，ssh将退出。
使用SSH进行TCP转发 – 远程到本地的TCP连接（-R）
# ssh localhost -p 2345
ssh -N -R ‘:3456:localhost:22’ server.example.com。并且，还要在
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
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
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 fin‐
gerprint 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 guar‐
To get a listing of the fingerprints along with their random art for all known hosts, the following command
line can be used:
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:
The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint
Finally the client connects:
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:
# 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:
# 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
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
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
server port number.
SSH_ORIGINAL_COMMAND This variable contains the original command line if a forced command is executed. It can
SSH_TTY This is set to the name of the tty (path to the device) associated with the current shell
TZ This variable is set to indicate the present time zone if it was set when the daemon was
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).
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.
mitting login with rlogin/rsh.
tion. 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.
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.
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.
readable by anyone.
temwide list of known host keys. See sshd(8) for further details of the format of this file.
mand) is started. See the sshd(8) manual page for more information.
permitting login with rlogin/rsh.
tain 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.
mand) is started. See the sshd(8) manual page for more information.
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)
- Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, January 2006.
- Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January 2006.
- Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January 2006.
- Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.
- Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.
- Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
- Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, January 2006.
- Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, January 2006.
- Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, January 2006.
- Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, January 2006.
- Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
- Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.
- Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, December 2009.
- 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).