ssh 是 OpenSSH 的客户端，是一个远程登录程序，用于登录服务器并执行命令。它的目的是在两个不信任主机在非安全的网络上提供一个安全的加密通讯。也可以在安全隧道上进行 X11 连接、任意 TCP 端口、UNIX 域套接字的转发。

命令行语法格式

ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file]

用户登录远程主机时，需要指定主机地址及可选的用户名。同时，用户需要使用几种方法之一（见下文）提供身份信息。

如果指定了 command，则会在远程主机上执行 command，否则执行登录 SHELL。

命令行选项及含义

ssh 命令支持的命令行选项如下：

-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

-a Disables forwarding of the authentication agent connection.

-b bind_address

-C Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and

-c cipher_spec

-D [bind_address:]port

-E log_file

-e escape_char

-F configfile

-f
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。

如果 ExitOnForwardFailure 配置选项设置为“yes”，则客户端以-f 启动时将等待所有远程端口转发成功建立后，才会进入后台运行，否则会退出。

-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

-I pkcs11
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user’s private RSA key.

-i identity_file
指定私钥文件。
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；
也可以在$HOME/.ssh/config 文件中，以主机为单位进行指定。
可以有多个-i 选项（及在配置文件中指定多个身份文件）。
如果CertificateFile指令没有明确指定证书，ssh 也会尝试从通过将-cert.pub附加到 identity_file 后所获得的文件名中加载证书信息。

-J [user@]host[:port]

-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
端口转发，将到某个本地端口的 TCP 连接转发到远程主机上的某个端口。

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.

-l login_name

-M Places the ssh client into “master” mode for connection sharing. Multiple -M options places ssh into

-m mac_spec

-N
不要执行任何远程命令。在只做端口转发的时候非常有用。

-n
从 /dev/null 中重定向 stdin，实际上，是为了防止从 stdin 从读取。如果 ssh 运行在后台，必须使用该选项。

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。

-O ctl_cmd

-o option

-p port
远程主机 SSH 服务监听的端口号。可以在配置文件中基于主机进行地址，为不同主机指定不同的连接端口号。

-Q query_option

-q
安静模式。该选项会抑制大多数的警告和诊断信息的输出。

-R [bind_address:]port:host:hostport
-R [bind_address:]port:local_socket
-R remote_socket:host:hostport
-R remote_socket:local_socket
端口转发，将到某个远程主机上的某个端口的 TCP 连接转发到本地端口。

-S ctl_path

-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

-V Display the version number and exit.

-v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging

-W host:port

-w local_tun[:remote_tun]

-X Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.

-x Disables X11 forwarding.

-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY exten‐

-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).

身份验证

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
RSA algorithms.

The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs
in, the ssh program tells the server which key pair it would like to use for authentication. The client proves
that it has access to the private key and the server checks that the corresponding public key is authorized to
accept the account.

The user creates his/her key pair by running ssh-keygen(1). This stores the private key in ~/.ssh/identity
(protocol 1), ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA)
and stores the public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub
(ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user’s home directory. The user
should then copy the public key to ~/.ssh/authorized_keys in his/her home directory on the remote machine. The
authorized_keys file corresponds to the conventional ~/.rhosts file, and has one key per line, though the lines
can be very long. After this, the user can log in without giving the password.

A variation on public key authentication is available in the form of certificate authentication: instead of a
set of public/private keys, signed certificates are used. This has the advantage that a single trusted 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
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.

转义字符

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

~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）

TCP 转发可能的应用场景有：

• 与邮件服务器的安全连接。当然邮件服务器只是一个示例，其他的也可以。
• 另外一种场景是防火墙穿透；

在下面的示例中，我们创建了 IRC 服务器和 IRC 客户端之间的加密通讯（尽管 IRC 不支持直接支持加密连接）。它是这么工作的：

• 用户使用 SSH 连接远程服务器，并且指定一个本地的端口 P1，该端口号 P1 用于转发本地的连接到远程服务器。
• 之后，就可以启动客户端程序，并连接到那个本地端口号 P1，之后 SSH 将加密并转发该连接到远程服务器。

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”：

这将通过隧道连接到 IRC 服务器“server.example.com”，并加入“#users”频道，昵称为“pinky”，本地使用 1234 端口。使用哪个端口并不重要，只要它大于 1023（因为只有 root 可以打开特权端口上的套接字），并且不要与任何已经使用的端口冲突。连接被转发到远程主机的 6667 端口，因为这是 IRC 服务的标准端口。

-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。

通过安全隧道转发任意 TCP 连接的行为，既可以在命令行上指定，也可以在配置文件中指定。

使用 SSH 进行 TCP 转发 – 远程到本地的 TCP 连接（-R）

另外一种 TCP 转发是从远程到本地。可能的应用场景有：

• 穿透至内网。将内网里的某个服务端口映射到远程主机上的某个端口。
• 当然也包含了从远程到本地的加密连接。

下面的示例中，我们将演示如何从一个远程主机 R1 上访问某个内网里的一台主机 LM1。它是这么工作的：

• 首先，在内网主机 LM1 上执行 ssh 命令，将本地的端口 LP1 映射到远程主机端口 RP2。此时 SSH 会在远程主机上监听端口 RP2，任何到 RP2 的 TCP 连接，都会通过 SSH 隧道转发到内网主机的 LP1 端口。
• 之后，我们在远程主机上连接 RP2 端口，就可以访问到内网的主机。

以 SSH 为例：我们要实现在远程主机上访问本地内网的一台主机 SSH 服务。这样，我就能在远程服务器上连接上内网的一台主机了。需要执行的命令如下：

如上命令，第一条在内网主机上执行。内网主机将自己的 22 端口映射到了 server.example.com 上的 2345 端口。而第二条命令在远程主机上执行，访问 2345 端口，SSH 会将 2345 端口的 TCP 连接转发到内网主机的 22 端口。

通过安全隧道转发任意 TCP 连接的行为，既可以在命令行上指定，也可以在配置文件中指定。

题外话：如果我想使用内网 LAN2 中的主机访问内网 LAN1 中的主机怎么办？
这种情况下，两台机器在不同的网络中。当然，实现方法有很多。下面介绍的方法还是基于上面的示例：

动态端口转发（-D）

高能预警！！！使用动态端口转发可以科学上网。。。emmmmm。。。。对，科学上网，就是常说的使用 SSH 番羽土啬。当然，你依旧要有大陆以外的主机。

使用 SSH 进行 X11 转发

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):

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‐
anteed proof.

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
queries:

Finally the client connects:

See the VerifyHostKeyDNS option in ssh_config(5) for more information.

基于 SSH 的 VPN

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:

On the server:

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).

相关的环境变量

ssh will normally set the following environment variables:

DISPLAY The DISPLAY variable indicates the location of the X11 server. It is automatically set

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

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

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).

相关的配置文件

~/.rhosts

~/.shosts

~/.ssh/

~/.ssh/authorized_keys

~/.ssh/config
属于每个用户的配置文件。文件格式和配置选项查看ssh_config(5)。
为了避免滥用的问题，该文件必须具有严格的权限：当前用户必须可读、可写，其他用户是不能写的。如果该组只包含当前用户，则文件的权限可以是「组可写」的。

~/.ssh/environment

~/.ssh/identity
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
~/.ssh/id_rsa

~/.ssh/identity.pub
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_rsa.pub

~/.ssh/known_hosts

~/.ssh/rc

/etc/hosts.equiv

/etc/ssh/shosts.equiv

/etc/ssh/ssh_config

/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

/etc/ssh/ssh_known_hosts

/etc/ssh/sshrc

程序退出码

ssh 命令使用所执行远程命令的退出码为退出码，或者如果发生错误则返回 255。

相关文档

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)

标准

19. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, January 2006.

20. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January 2006.

20. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January 2006.

20. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.

20. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.

10. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.

6. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, January 2006.

10. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, January 2006.

13. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, January 2006.

2. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, January 2006.

13. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.

10. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.

4. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, December 2009.

1. 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).

参考文献

• man 1 ssh, version 7.5p1-5
• OpenSSH Config File Examples
• 实战 SSH 端口转发
• 利用 ssh 的端口转发实现 SOCKS5 代理