-4 强制ssh只使用IPv4地址
-6 强制ssh仅使用IPv6地址
-A 允许从认证代理(如ssh-agent)转发连接，慎用，有风险。最好使用下面的跳转主机方法(参见-J)
-a 禁止转发认证代理连接
-B bind_interface 在连接到目标主机之前，绑定到bind_interface地址，在系统有多个IP地址时有用。
-b bind_address 在本地机器上使用bind_address作为连接的源地址，在系统有多个IP地址时有用。
-E log_file 将调试日志追加到log_file文件而不是标准错误
-e escape_char Sets the escape character for sessions with a pty (default: ‘~’). The escape character is only recognized at the beginning of a line. The escape character followed by a dot (‘.’) closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to “none” disables any escapes and makes the session fully transparent.
-F configfile
Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide
configuration file (/etc/ssh/ssh_config) will be ignored. The default for the per-user configuration file is ~/.ssh/config.
-f Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or
passphrases, but the user wants it in the background. This implies -n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm.
If the ExitOnForwardFailure configuration option is set to “yes”, then a client started with -f will wait for all remote port forwards to be successfully established before placing itself in the background.
-G Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
-g Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.
-I pkcs11
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing keys for user authentication.
-i identity_file
Selects a file from which the identity (private key) for public key authentication is read. The default is ~/.ssh/id_dsa,
~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. 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.
-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.
-n Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background.
A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will
start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh
program will be put in the background. (This does not work if ssh needs to ask for a password or passphrase; see also the -f
option.)
-O ctl_cmd
Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted
and passed to the master process. Valid commands are: “check” (check that the master process is running), “forward” (request
forwardings without command execution), “cancel” (cancel forwardings), “exit” (request the master to exit), and “stop” (request
the master to stop accepting further multiplexing requests).
-o option
Can be used to give options in the format used in the configuration file. This is useful for specifying options for which
there is no separate command-line flag. For full details of the options listed below, and their possible values, see
ssh_config(5).
AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CASignatureAlgorithms
CertificateFile
ChallengeResponseAuthentication
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIDelegateCredentials
GSSAPIKexAlgorithms
GSSAPIRenewalForcesRekey
GSSAPIServerIdentity
GSSAPITrustDns
HashKnownHosts
Host
HostbasedAuthentication
HostbasedKeyTypes
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
SendEnv
ServerAliveInterval
ServerAliveCountMax
SetEnv
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation
-Q query_option
Queries ssh for the algorithms supported for the specified version 2. The available features are: cipher (supported symmetric
ciphers), cipher-auth (supported symmetric ciphers that support authenticated encryption), help (supported query terms for use
with the -Q flag), mac (supported message integrity codes), kex (key exchange algorithms), kex-gss (GSSAPI key exchange algo‐
rithms), 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). Alterna‐
tively, any keyword from ssh_config(5) or sshd_config(5) that takes an algorithm list may be used as an alias for the corre‐
sponding 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 lo‐
cal 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 speci‐
fying a bind_address. An empty bind_address, or the address ‘*’, indicates that the remote socket should listen on all inter‐
faces. 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 trans‐
port for other applications (e.g. sftp(1)). The subsystem is specified as the remote command.
-T Disable pseudo-terminal allocation.
-t Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can
be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.
-V Display the version number and exit.
-v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentica‐
tion, 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. Please refer to the ssh -Y op‐
tion 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 de‐
pending 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 for‐
mat 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, chal‐
lenge-response authentication, and password authentication. Authentication methods are tried in the order specified above, though
PreferredAuthentications can be used to change the default order.
Host-based authentication works as follows: If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv
on the remote machine, and the user names are the same on both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's home
directory on the remote machine and contain a line containing the name of the client machine and the name of the user on that machine,
the user is considered for login. Additionally, the server must be able to verify the client's host key (see the description of
/etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permitted. This authentication method closes security holes
due to IP spoofing, DNS spoofing, and routing spoofing. [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh
protocol in general, are inherently insecure and should be disabled if security is desired.]
Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where encryption and
decryption are done using separate keys, and it is unfeasible to derive the decryption key from the encryption key. The idea is that
each user creates a public/private key pair for authentication purposes. The server knows the public key, and only the user knows the
private key. ssh implements public key authentication protocol automatically, using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
The HISTORY section of ssl(8) (on non-OpenBSD systems, see http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) contains
a brief discussion of the DSA and RSA algorithms.
The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells
the server which key pair it would like to use for authentication. The client proves that it has access to the private key and the
server checks that the corresponding public key is authorized to accept the account.
The server may inform the client of errors that prevented public key authentication from succeeding after authentication completes us‐
ing 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 his/her 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 his/her home directory on the remote machine. The authorized_keys file corresponds to the conventional ~/.rhosts file, and has one
key per line, though the lines can be very long. After this, the user can log in without giving the password.
A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private
keys, signed certificates are used. This has the advantage that a single trusted certification authority can be used in place of many
public/private keys. See the CERTIFICATES section of ssh-keygen(1) for more information.
The most convenient way to use public key or certificate authentication may be with an authentication agent. See ssh-agent(1) and (op‐
tionally) 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. Exam‐
ples 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 check‐
ing; 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 sys‐
tems, 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 char‐
acter 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 re‐
mote 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 di‐
rectly 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 con‐
nection 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”, nick‐
name “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 hap‐
pens 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 authenti‐
cation 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 how‐
ever, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaran‐
teed 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 re‐
source record (RR), SSHFP, is added to a zonefile and the connecting client is able to match the fingerprint with that of the key pre‐
sented.
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 pro‐
gram specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is particularly useful when
calling ssh from a .xsession or related script. (Note that on some machines it may be necessary to redirect the
input from /dev/null to make this work.)
SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to communicate with the agent.
SSH_CONNECTION Identifies the client and server ends of the connection. The variable contains four space-separated values:
client IP address, client port number, server IP address, and server port number.
SSH_ORIGINAL_COMMAND This variable contains the original command line if a forced command is executed. It can be used to extract the
original arguments.
SSH_TTY This is set to the name of the tty (path to the device) associated with the current shell or command. If the
current session has no tty, this variable is not set.
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.
/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.
退出状态，如果成果返回远程命令的执行状态，出错的话返回255。
TODO
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-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.请求压缩所有数据(包括stdin, stdout, stderr，以及转发的X11, TCP和unix域连接的数据)。压缩算法与gzip(1)相同。压缩在调制解调器线路和其他慢速连接上是可取的，但只会在高速网络上减慢速度。可以在配置文件中对每个主机设置默认值;请参阅压缩选项。
- c cipher_spec 选择加密会话的密码规范。Cipher_spec是一个以逗号分隔的密码列表，按优先级排列，包括aes256，aes128等加密算法。
# 默认cp拷贝，一个1GB的文件，花费1.05秒 $ timecp a b cp a b 0.02s user 1.05s system 75% cpu 1.403 total
# 默认dd拷贝，一个1GB的文件，竟然花费了29.17秒 $ timeddif=a of=b 2048000+0 records in 2048000+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 34.7214 s, 30.2 MB/s ddif=a of=b 1.31s user 29.17s system 87% cpu 34.996 total
$ timeddif=a of=b bs=2M 500+0 records in 500+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 1.04747 s, 1.0 GB/s ddif=a of=b bs=2M 0.00s user 1.05s system 78% cpu 1.332 total $ timeddif=a of=b bs=4M 250+0 records in 250+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 1.00866 s, 1.0 GB/s ddif=a of=b bs=4M 0.00s user 1.00s system 76% cpu 1.304 total $ timeddif=a of=b bs=8M 125+0 records in 125+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.937974 s, 1.1 GB/s ddif=a of=b bs=8M 0.00s user 0.92s system 79% cpu 1.164 total $ timeddif=a of=b bs=10M 100+0 records in 100+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 1.01666 s, 1.0 GB/s ddif=a of=b bs=10M 0.00s user 1.03s system 82% cpu 1.257 total
测试硬盘速度
我最常使用的dd命令的用例是，测试硬盘的读写速度，比如很简单地写入1GB、10GB来看一下。
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$ ddif=/dev/zero of=tmp bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.7338 s, 1.4 GB/s
$ ddif=/dev/zero of=tmp bs=2M count=500 500+0 records in 500+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.611315 s, 1.7 GB/s
$ ddif=/dev/zero of=tmp bs=4M count=250 250+0 records in 250+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 0.602517 s, 1.7 GB/s