SSH(1) BSD General Commands Manual SSH(1)
NAME
ssh — OpenSSH SSH client (remote login program)
SYNOPSIS
ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
[-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file] [-L [bind_address:]port:host:hostport]
[-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-Q cipher | cipher-auth | mac | kex | key]
[-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
[user@]hostname [command]
DESCRIPTION
ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine.
It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted
hosts over an insecure network. X11 connections and arbitrary TCP ports can also be forwarded over the secure
channel.
ssh connects and logs into the specified hostname (with optional user name). The user must prove his/her iden‐
tity to the remote machine using one of several methods depending on the protocol version used (see below).
If command is specified, it is executed on the remote host instead of a login shell.
The options are as follows:
-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
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 Disables forwarding of the authentication agent connection.
-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 and TCP
connections). The compression algorithm is the same used by gzip(1), and the “level” can 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.
-c cipher_spec
Selects the cipher specification for encrypting the session.
Protocol version 1 allows specification of a single cipher. The supported values are “3des”,
“blowfish”, and “des”. 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different
keys. It is believed to be secure. blowfish is a fast block cipher; it appears very secure and is much
faster than 3des. des is only supported in the ssh client for interoperability with legacy protocol 1
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 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.
-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 recog‐
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.
-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 recom‐
mended 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 back‐
ground.
-g Allows remote hosts to connect to local forwarded ports.
-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
Selects a file from which the identity (private key) for public key authentication is read. The default
is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and
~/.ssh/id_rsa for protocol version 2. 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). ssh will also try to load certificate information from the filename obtained by
appending -cert.pub to identity filenames.
-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
Specifies that the given port on the local (client) host is to be forwarded to the given host and port
on the remote side. 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 for‐
warded over the secure channel, and a connection is made to host port hostport from the remote machine.
Port forwardings can also be specified in the configuration file. IPv6 addresses can be specified by
tion of ControlMaster in ssh_config(5) for details.
-m mac_spec
Additionally, for protocol version 2 a comma-separated list of MAC (message authentication code) algo‐
rithms can be 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 (protocol version 2 only).
-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 connec‐
tion 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 fur‐
ther 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).
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
ChallengeResponseAuthentication
CheckHostIP
Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
HostKeyAlgorithms
HostKeyAlias
HostName
IdentityFile
IdentitiesOnly
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
Protocol
ProxyCommand
ProxyUseFdpass
PubkeyAuthentication
RekeyLimit
RemoteForward
RequestTTY
RhostsRSAAuthentication
RSAAuthentication
SendEnv
ServerAliveInterval
ServerAliveCountMax
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UsePrivilegedPort
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 cipher | cipher-auth | mac | kex | key
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 authenti‐
cated encryption), mac (supported message integrity codes), kex (key exchange algorithms), key (key
types).
By default, the listening socket 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 con‐
nection 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 are a feature of the
SSH2 protocol which facilitate the use of SSH as a secure transport for other applications (eg.
sftp(1)). The subsystem is specified as the remote command.
-T Disable pseudo-tty allocation.
-t Force pseudo-tty 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 chan‐
nel. Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings. Works with Protocol version 2 only.
-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 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.
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 for‐
warded 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 option and the ForwardX11Trusted directive in ssh_config(5) for more information.
-x Disables X11 forwarding.
confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and integrity (hmac-
md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, umac-128, hmac-ripemd160). Protocol 1 lacks a strong
mechanism for ensuring the integrity of the connection.
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 protocol 2 has a configuration option to change the default order:
PreferredAuthentications.
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. Protocol 1 is
restricted to using only RSA keys, but protocol 2 may use any. 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 user creates his/her key pair by running ssh-keygen(1). This stores the private key in ~/.ssh/identity
(protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), ~/.ssh/id_ed25519 (protocol 2
ED25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), ~/.ssh/id_ed25519.pub (protocol 2
ED25519), or ~/.ssh/id_rsa.pub (protocol 2 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 certifi‐
cation 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) for more information.
Challenge-response authentication works as follows: The server sends an arbitrary "challenge" text, and prompts
for a response. Protocol 2 allows multiple challenges and responses; protocol 1 is restricted to just one chal‐
lenge/response. Examples of challenge-response authentication include BSD Authentication (see login.conf(5))
and PAM (some non-OpenBSD systems).
When the user's identity has been accepted by the server, the server either executes the given command, or logs
into the machine and gives the user a normal shell on the remote machine. All communication with the remote
command or shell will be automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the user may use the escape characters noted
below.
If no pseudo-tty has been allocated, the session is transparent and can be used to reliably transfer binary
data. On most systems, setting the escape character to “none” will also make the session transparent even if a
tty is used.
The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections
have been closed.
ESCAPE CHARACTERS
When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape char‐
acter.
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 for SSH protocol version 2 and if the peer supports it).
~C Open command line. Currently this allows the addition of port forwardings using the -L, -R and -D
options (see above). It also allows the cancellation of existing port-forwardings with
-KL[bind_address:]port for local, -KR[bind_address:]port for remote and -KD[bind_address:]port for
dynamic port-forwardings. !command allows the user to execute a local command if the PermitLocalCommand
option is enabled in ssh_config(5). Basic help is available, using the -h option.
~R Request rekeying of the connection (only useful for SSH protocol version 2 and 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 the 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.
This tunnels a connection to IRC server “server.example.com”, joining channel “#users”, nickname “pinky”, using
port 1234. It doesn't matter which port is used, as long as it's greater than 1023 (remember, only root can
open sockets on privileged ports) and doesn't conflict with any ports already in use. The connection is for‐
warded to port 6667 on the remote server, since that's the standard port for IRC services.
The -f option backgrounds ssh and the remote command “sleep 10” is specified to allow an amount of time (10 sec‐
onds, in the example) to start the service which is to be tunnelled. If no connections are made within the time
specified, ssh will exit.
X11 FORWARDING
If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above) and the
user is using X11 (the DISPLAY environment variable is set), the connection to the X11 display is automatically
forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go
through the encrypted channel, and the connection to the real X server will be made from the local machine. The
user should not manually set DISPLAY. Forwarding of X11 connections can be configured on the command line or in
configuration files.
The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero.
This is normal, and happens because ssh creates a “proxy” X server on the server machine for forwarding the con‐
nections over the encrypted channel.
ssh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a
random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections
carry this cookie and replace it by the real cookie when the connection is opened. The real authentication
cookie is never sent to the server machine (and no cookies are sent in the plain).
If the ForwardAgent variable is set to “yes” (or see the description of the -A and -a options above) and the
user is using an authentication agent, the connection to the agent is automatically forwarded to the remote
side.
VERIFYING HOST KEYS
When connecting to a server for the first time, a fingerprint of the server's public key is presented to the
user (unless the option StrictHostKeyChecking has been disabled). Fingerprints can be determined using
ssh-keygen(1):
$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
If the fingerprint is already known, it can be matched and the key can be accepted or rejected. Because of the
difficulty of comparing host keys just by looking at hex 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 pat‐
tern is displayed. Because these patterns are not unambiguous however, a pattern that looks similar to the pat‐
tern remembered only gives a good probability that the host key is the same, not guaranteed proof.
To get a listing of the fingerprints along with their random art for all known hosts, the following command line
can be used:
$ ssh-keygen -lv -f ~/.ssh/known_hosts
If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by
DNS. An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to
match the fingerprint with that of the key presented.
$ 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, allow‐
ing 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 explic‐
itly, as that will render the X11 connection insecure (and will require the user to manu‐
ally 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.
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_USER_AUTH This variable contains, for SSH2 only, a comma-separated list of authentication methods
that were successfuly used to authenticate. When possible, these methods are extended with
detailed information on the credential used.
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.
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).
ENVIRONMENT
SSH_USE_STRONG_RNG
The reseeding of the OpenSSL random generator is usually done from /dev/urandom. If the
SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL random generator is
reseeded from /dev/random. The number of bytes read is defined by the SSH_USE_STRONG_RNG value. Mini‐
mum is 14 bytes. This setting is not recommended on the computers without the hardware random generator
because insufficient entropy causes the connection to be blocked until enough entropy is available.
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 per‐
mitting login with rlogin/rsh.
~/.ssh/
This directory is the default location for all user-specific configuration and authentication informa‐
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.
~/.ssh/authorized_keys
Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be used for logging in as this user. The for‐
mat 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.
which will be used to encrypt the sensitive part of this file using 3DES.
~/.ssh/identity.pub
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ed25519.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 sys‐
temwide 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 com‐
mand) is started. See the sshd(8) manual page for more information.
/etc/hosts.equiv
This file is for host-based authentication (see above). It should only be writable by root.
/etc/ssh/shosts.equiv
This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without
permitting login with rlogin/rsh.
/etc/ssh/ssh_config
Systemwide configuration file. The file format and configuration options are described in
ssh_config(5).
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys and are used for host-based authentication. If
protocol version 1 is used, ssh must be setuid root, since the host key is readable only by root. For
protocol version 2, ssh uses ssh-keysign(8) to access the host keys, eliminating the requirement that
ssh be setuid root when host-based authentication is used. By default ssh is not setuid root.
/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 com‐
mand) is started. See the sshd(8) manual page for more information.
EXIT STATUS
ssh exits with the exit status of the remote command or with 255 if an error occurred.
IPV6
IPv6 address can be used everywhere where IPv4 address. In all entries must be the IPv6 address enclosed in
square brackets. Note: The square brackets are metacharacters for the shell and must be escaped in shell.
T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.
T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.
J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January
2006.
F. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256,
January 2006.
J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, January 2006.
M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344,
January 2006.
B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, January 2006.
M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
Protocol, RFC 4419, March 2006.
J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.
D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656,
December 2009.
A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security, 1999, International
Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck,
Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created
OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
BSD June 13, 2017 BSD
Back to main site | Back to man page index