ssh(1) obtains configuration data from the following sources in the fol-
1. command-line options
2. user's configuration file (~/.ssh/config)
3. system-wide configuration file (/etc/ssh/ssh_config)
For each parameter, the first obtained value will be used. The configu-
ration files contain sections separated by ``Host'' specifications, and
that section is only applied for hosts that match one of the patterns
given in the specification. The matched host name is the one given on
the command line.
Since the first obtained value for each parameter is used, more host-spe-
cific declarations should be given near the beginning of the file, and
general defaults at the end.
Note that the Debian openssh-client package sets several options as stan-
dard in /etc/ssh/ssh_config which are not the default in ssh(1):
o SendEnv LANG LC_*
o HashKnownHosts yes
o GSSAPIAuthentication yes
The configuration file has the following format:
Empty lines and lines starting with '#' are comments. Otherwise a line
is of the format ``keyword arguments''. Configuration options may be
separated by whitespace or optional whitespace and exactly one '='; the
latter format is useful to avoid the need to quote whitespace when speci-
fying configuration options using the ssh, scp, and sftp -o option.
Arguments may optionally be enclosed in double quotes (") in order to
represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that key-
words are case-insensitive and arguments are case-sensitive):
Host Restricts the following declarations (up to the next Host key-
word) to be only for those hosts that match one of the patterns
given after the keyword. If more than one pattern is provided,
they should be separated by whitespace. A single '*' as a pat-
tern can be used to provide global defaults for all hosts. The
host is the hostname argument given on the command line (i.e. the
name is not converted to a canonicalized host name before match-
A pattern entry may be negated by prefixing it with an exclama-
tion mark ('!'). If a negated entry is matched, then the Host
entry is ignored, regardless of whether any other patterns on the
line match. Negated matches are therefore useful to provide
seconds by default. This option is useful in scripts and other
batch jobs where no user is present to supply the password, and
where it is desirable to detect a broken network swiftly. The
argument must be ``yes'' or ``no''. The default is ``no''.
Use the specified address on the local machine as the source
address of the connection. Only useful on systems with more than
one address. Note that this option does not work if
UsePrivilegedPort is set to ``yes''.
Specifies whether to use challenge-response authentication. The
argument to this keyword must be ``yes'' or ``no''. The default
If this flag is set to ``yes'', ssh(1) will additionally check
the host IP address in the known_hosts file. This allows ssh to
detect if a host key changed due to DNS spoofing. If the option
is set to ``no'', the check will not be executed. The default is
Cipher Specifies the cipher to use for encrypting the session in proto-
col version 1. Currently, ``blowfish'', ``3des'', and ``des''
are supported. des is only supported in the ssh(1) client for
interoperability with legacy protocol 1 implementations that do
not support the 3des cipher. Its use is strongly discouraged due
to cryptographic weaknesses. The default is ``3des''.
Specifies the ciphers allowed for protocol version 2 in order of
preference. Multiple ciphers must be comma-separated. The sup-
ported ciphers are ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'',
``aes256-cbc'', ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'',
``arcfour128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'',
and ``cast128-cbc''. The default is:
Specifies that all local, remote, and dynamic port forwardings
specified in the configuration files or on the command line be
cleared. This option is primarily useful when used from the
ssh(1) command line to clear port forwardings set in configura-
tion files, and is automatically set by scp(1) and sftp(1). The
argument must be ``yes'' or ``no''. The default is ``no''.
Specifies whether to use compression. The argument must be
``yes'' or ``no''. The default is ``no''.
Specifies the timeout (in seconds) used when connecting to the
SSH server, instead of using the default system TCP timeout.
This value is used only when the target is down or really
unreachable, not when it refuses the connection.
Enables the sharing of multiple sessions over a single network
connection. When set to ``yes'', ssh(1) will listen for connec-
tions on a control socket specified using the ControlPath argu-
ment. Additional sessions can connect to this socket using the
same ControlPath with ControlMaster set to ``no'' (the default).
These sessions will try to reuse the master instance's network
connection rather than initiating new ones, but will fall back to
connecting normally if the control socket does not exist, or is
Setting this to ``ask'' will cause ssh to listen for control con-
nections, but require confirmation using the SSH_ASKPASS program
before they are accepted (see ssh-add(1) for details). If the
ControlPath cannot be opened, ssh will continue without connect-
ing to a master instance.
X11 and ssh-agent(1) forwarding is supported over these multi-
plexed connections, however the display and agent forwarded will
be the one belonging to the master connection i.e. it is not pos-
sible to forward multiple displays or agents.
Two additional options allow for opportunistic multiplexing: try
to use a master connection but fall back to creating a new one if
one does not already exist. These options are: ``auto'' and
``autoask''. The latter requires confirmation like the ``ask''
Specify the path to the control socket used for connection shar-
ing as described in the ControlMaster section above or the string
``none'' to disable connection sharing. In the path, '%L' will
be substituted by the first component of the local host name,
'%l' will be substituted by the local host name (including any
domain name), '%h' will be substituted by the target host name,
'%n' will be substituted by the original target host name speci-
fied on the command line, '%p' the port, '%r' by the remote login
username, and '%u' by the username of the user running ssh(1).
It is recommended that any ControlPath used for opportunistic
connection sharing include at least %h, %p, and %r. This ensures
that shared connections are uniquely identified.
When used in conjunction with ControlMaster, specifies that the
master connection should remain open in the background (waiting
for future client connections) after the initial client connec-
tion has been closed. If set to ``no'', then the master connec-
tion will not be placed into the background, and will close as
determine where to connect to from the remote machine.
The argument must be [bind_address:]port. IPv6 addresses can be
specified by enclosing addresses in square brackets. 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.
Currently the SOCKS4 and SOCKS5 protocols are supported, and
ssh(1) will act as a SOCKS server. Multiple forwardings may be
specified, and additional forwardings can be given on the command
line. Only the superuser can forward privileged ports.
Setting this option to ``yes'' in the global client configuration
file /etc/ssh/ssh_config enables the use of the helper program
ssh-keysign(8) during HostbasedAuthentication. The argument must
be ``yes'' or ``no''. The default is ``no''. This option should
be placed in the non-hostspecific section. See ssh-keysign(8)
for more information.
Sets the escape character (default: '~'). The escape character
can also be set on the command line. The argument should be a
single character, '^' followed by a letter, or ``none'' to dis-
able the escape character entirely (making the connection trans-
parent for binary data).
Specifies whether ssh(1) should terminate the connection if it
cannot set up all requested dynamic, tunnel, local, and remote
port forwardings. The argument must be ``yes'' or ``no''. The
default is ``no''.
Specifies whether the connection to the authentication agent (if
any) will be forwarded to the remote machine. The argument must
be ``yes'' or ``no''. The default is ``no''.
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
Specifies whether X11 connections will be automatically redi-
rected over the secure channel and DISPLAY set. The argument
must be ``yes'' or ``no''. The default is ``no''.
The default is to disable untrusted X11 forwarding after twenty
minutes has elapsed.
If this option is set to ``yes'', remote X11 clients will have
full access to the original X11 display.
If this option is set to ``no'', remote X11 clients will be con-
sidered untrusted and prevented from stealing or tampering with
data belonging to trusted X11 clients. Furthermore, the xauth(1)
token used for the session will be set to expire after 20 min-
utes. Remote clients will be refused access after this time.
The default is ``yes'' (Debian-specific).
See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.
Specifies whether remote hosts are allowed to connect to local
forwarded ports. By default, ssh(1) binds local port forwardings
to the loopback address. This prevents other remote hosts from
connecting to forwarded ports. GatewayPorts can be used to spec-
ify that ssh should bind local port forwardings to the wildcard
address, thus allowing remote hosts to connect to forwarded
ports. The argument must be ``yes'' or ``no''. The default is
Specifies one or more files to use for the global host key data-
base, separated by whitespace. The default is
Specifies whether user authentication based on GSSAPI is allowed.
The default is ``no''. Note that this option applies to protocol
version 2 only.
Specifies whether key exchange based on GSSAPI may be used. When
using GSSAPI key exchange the server need not have a host key.
The default is ``no''. Note that this option applies to protocol
version 2 only.
If set, specifies the GSSAPI client identity that ssh should use
when connecting to the server. The default is unset, which means
that the default identity will be used.
If set, specifies the GSSAPI server identity that ssh should
expect when connecting to the server. The default is unset, which
means that the expected GSSAPI server identity will be determined
from the target hostname.
Set to ``yes to indicate that the DNS is trusted to securely
canonicalize'' the name of the host being connected to. If ``no,
the hostname entered on the'' command line will be passed
untouched to the GSSAPI library. The default is ``no''. This
option only applies to protocol version 2 connections using GSS-
Indicates that ssh(1) should hash host names and addresses when
they are added to ~/.ssh/known_hosts. These hashed names may be
used normally by ssh(1) and sshd(8), but they do not reveal iden-
tifying information should the file's contents be disclosed. The
default is ``no''. Note that existing names and addresses in
known hosts files will not be converted automatically, but may be
manually hashed using ssh-keygen(1). Use of this option may
break facilities such as tab-completion that rely on being able
to read unhashed host names from ~/.ssh/known_hosts.
Specifies whether to try rhosts based authentication with public
key authentication. The argument must be ``yes'' or ``no''. The
default is ``no''. This option applies to protocol version 2
only and is similar to RhostsRSAAuthentication.
Specifies the protocol version 2 host key algorithms that the
client wants to use in order of preference. The default for this
If hostkeys are known for the destination host then this default
is modified to prefer their algorithms.
Specifies an alias that should be used instead of the real host
name when looking up or saving the host key in the host key data-
base files. This option is useful for tunneling SSH connections
or for multiple servers running on a single host.
Specifies the real host name to log into. This can be used to
specify nicknames or abbreviations for hosts. If the hostname
contains the character sequence '%h', then this will be replaced
with the host name specified on the command line (this is useful
for manipulating unqualified names). The default is the name
given on the command line. Numeric IP addresses are also permit-
tication identity is read. The default is ~/.ssh/identity for
protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
~/.ssh/id_rsa for protocol version 2. Additionally, any identi-
ties represented by the authentication agent will be used for
authentication. ssh(1) will try to load certificate information
from the filename obtained by appending -cert.pub to the path of
a specified IdentityFile.
The file name may use the tilde syntax to refer to a user's home
directory or one of the following escape characters: '%d' (local
user's home directory), '%u' (local user name), '%l' (local host
name), '%h' (remote host name) or '%r' (remote user name).
It is possible to have multiple identity files specified in con-
figuration files; all these identities will be tried in sequence.
Multiple IdentityFile directives will add to the list of identi-
ties tried (this behaviour differs from that of other configura-
IPQoS Specifies the IPv4 type-of-service or DSCP class for connections.
Accepted values are ``af11'', ``af12'', ``af13'', ``af14'',
``af22'', ``af23'', ``af31'', ``af32'', ``af33'', ``af41'',
``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'', ``cs4'',
``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'', ``throughput'',
``reliability'', or a numeric value. This option may take one or
two arguments, separated by whitespace. If one argument is spec-
ified, it is used as the packet class unconditionally. If two
values are specified, the first is automatically selected for
interactive sessions and the second for non-interactive sessions.
The default is ``lowdelay'' for interactive sessions and
``throughput'' for non-interactive sessions.
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be ``yes'' or ``no''. The
default is ``yes''.
Specifies the list of methods to use in keyboard-interactive
authentication. Multiple method names must be comma-separated.
The default is to use the server specified list. The methods
available vary depending on what the server supports. For an
OpenSSH server, it may be zero or more of: ``bsdauth'', ``pam'',
Specifies the available KEX (Key Exchange) algorithms. Multiple
algorithms must be comma-separated. The default is:
session of the ssh(1) that spawned it. It should not be used for
This directive is ignored unless PermitLocalCommand has been
Specifies that a TCP port on the local machine be forwarded over
the secure channel to the specified host and port from the remote
machine. The first argument must be [bind_address:]port and the
second argument must be host:hostport. IPv6 addresses can be
specified by enclosing addresses in square brackets. Multiple
forwardings may be specified, and additional forwardings can be
given on the command line. Only the superuser can forward privi-
leged ports. By default, the local port is bound in accordance
with the GatewayPorts setting. However, an explicit bind_address
may be used to bind the connection to a specific address. The
bind_address of ``localhost'' indicates that the listening port
be bound for local use only, while an empty address or '*' indi-
cates that the port should be available from all interfaces.
Gives the verbosity level that is used when logging messages from
ssh(1). The possible values are: QUIET, FATAL, ERROR, INFO, VER-
BOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
higher levels of verbose output.
MACs Specifies the MAC (message authentication code) algorithms in
order of preference. The MAC algorithm is used in protocol ver-
sion 2 for data integrity protection. Multiple algorithms must
be comma-separated. The default is:
This option can be used if the home directory is shared across
machines. In this case localhost will refer to a different
machine on each of the machines and the user will get many warn-
ings about changed host keys. However, this option disables host
authentication for localhost. The argument to this keyword must
be ``yes'' or ``no''. The default is to check the host key for
Specifies the number of password prompts before giving up. The
argument to this keyword must be an integer. The default is 3.
Specifies whether to use password authentication. The argument
to this keyword must be ``yes'' or ``no''. The default is
Port Specifies the port number to connect on the remote host. The
default is 22.
Specifies the order in which the client should try protocol 2
authentication methods. This allows a client to prefer one
method (e.g. keyboard-interactive) over another method (e.g.
password). The default is:
Specifies the protocol versions ssh(1) should support in order of
preference. The possible values are '1' and '2'. Multiple ver-
sions must be comma-separated. When this option is set to
``2,1'' ssh will try version 2 and fall back to version 1 if ver-
sion 2 is not available. The default is '2'.
Specifies the command to use to connect to the server. The com-
mand string extends to the end of the line, and is executed with
the user's shell. In the command string, any occurrence of '%h'
will be substituted by the host name to connect, '%p' by the
port, and '%r' by the remote user name. The command can be basi-
cally anything, and should read from its standard input and write
to its standard output. It should eventually connect an sshd(8)
server running on some machine, or execute sshd -i somewhere.
Host key management will be done using the HostName of the host
being connected (defaulting to the name typed by the user). Set-
ting the command to ``none'' disables this option entirely. Note
that CheckHostIP is not available for connects with a proxy com-
This directive is useful in conjunction with nc(1) and its proxy
support. For example, the following directive would connect via
an HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
Specifies whether to try public key authentication. The argument
to this keyword must be ``yes'' or ``no''. The default is
``yes''. This option applies to protocol version 2 only.
Specifies the maximum amount of data that may be transmitted
before the session key is renegotiated. The argument is the num-
ber of bytes, with an optional suffix of 'K', 'M', or 'G' to
indicate Kilobytes, Megabytes, or Gigabytes, respectively. The
default is between '1G' and '4G', depending on the cipher. This
option applies to protocol version 2 only.
If the bind_address is not specified, the default is to only bind
to loopback addresses. If the bind_address is '*' or an empty
string, then the forwarding is requested to 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)).
Specifies whether to request a pseudo-tty for the session. The
argument may be one of: ``no'' (never request a TTY), ``yes''
(always request a TTY when standard input is a TTY), ``force''
(always request a TTY) or ``auto'' (request a TTY when opening a
login session). This option mirrors the -t and -T flags for
Specifies whether to try rhosts based authentication with RSA
host authentication. The argument must be ``yes'' or ``no''.
The default is ``no''. This option applies to protocol version 1
only and requires ssh(1) to be setuid root.
Specifies whether to try RSA authentication. The argument to
this keyword must be ``yes'' or ``no''. RSA authentication will
only be attempted if the identity file exists, or an authentica-
tion agent is running. The default is ``yes''. Note that this
option applies to protocol version 1 only.
Specifies what variables from the local environ(7) should be sent
to the server. Note that environment passing is only supported
for protocol 2. The server must also support it, and the server
must be configured to accept these environment variables. Refer
to AcceptEnv in sshd_config(5) for how to configure the server.
Variables are specified by name, which may contain wildcard char-
acters. Multiple environment variables may be separated by
whitespace or spread across multiple SendEnv directives. The
default is not to send any environment variables.
See PATTERNS for more information on patterns.
Sets the number of server alive messages (see below) which may be
sent without ssh(1) receiving any messages back from the server.
If this threshold is reached while server alive messages are
being sent, ssh will disconnect from the server, terminating the
session. It is important to note that the use of server alive
messages is very different from TCPKeepAlive (below). The server
alive messages are sent through the encrypted channel and there-
fore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The server alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion has become inactive.
The default value is 3. If, for example, ServerAliveInterval
the server, or 300 if the BatchMode option is set. This option
applies to protocol version 2 only. ProtocolKeepAlives and
SetupTimeOut are Debian-specific compatibility aliases for this
If this flag is set to ``yes'', ssh(1) will never automatically
add host keys to the ~/.ssh/known_hosts file, and refuses to con-
nect to hosts whose host key has changed. This provides maximum
protection against trojan horse attacks, though it can be annoy-
ing when the /etc/ssh/ssh_known_hosts file is poorly maintained
or when connections to new hosts are frequently made. This
option forces the user to manually add all new hosts. If this
flag is set to ``no'', ssh will automatically add new host keys
to the user known hosts files. If this flag is set to ``ask'',
new host keys will be added to the user known host files only
after the user has confirmed that is what they really want to do,
and ssh will refuse to connect to hosts whose host key has
changed. The host keys of known hosts will be verified automati-
cally in all cases. The argument must be ``yes'', ``no'', or
``ask''. The default is ``ask''.
Specifies whether the system should send TCP keepalive messages
to the other side. If they are sent, death of the connection or
crash of one of the machines will be properly noticed. This
option only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection
dies. As such, you probably want the ServerAliveInterval option
as well. However, this means that connections will die if the
route is down temporarily, and some people find it annoying.
The default is ``yes'' (to send TCP keepalive messages), and the
client will notice if the network goes down or the remote host
dies. This is important in scripts, and many users want it too.
To disable TCP keepalive messages, the value should be set to
Tunnel Request tun(4) device forwarding between the client and the
server. The argument must be ``yes'', ``point-to-point'' (layer
3), ``ethernet'' (layer 2), or ``no''. Specifying ``yes''
requests the default tunnel mode, which is ``point-to-point''.
The default is ``no''.
Specifies the tun(4) devices to open on the client (local_tun)
and the server (remote_tun).
The argument must be local_tun[: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''. The default is ``any:any''.
tions. The argument must be ``yes'' or ``no''. The default is
``no''. If set to ``yes'', ssh(1) must be setuid root. Note
that this option must be set to ``yes'' for
RhostsRSAAuthentication with older servers.
User Specifies the user to log in as. This can be useful when a dif-
ferent user name is used on different machines. This saves the
trouble of having to remember to give the user name on the com-
Specifies one or more files to use for the user host key data-
base, separated by whitespace. The default is
Specifies whether to verify the remote key using DNS and SSHFP
resource records. If this option is set to ``yes'', the client
will implicitly trust keys that match a secure fingerprint from
DNS. Insecure fingerprints will be handled as if this option was
set to ``ask''. If this option is set to ``ask'', information on
fingerprint match will be displayed, but the user will still need
to confirm new host keys according to the StrictHostKeyChecking
option. The argument must be ``yes'', ``no'', or ``ask''. The
default is ``no''. Note that this option applies to protocol
version 2 only.
See also VERIFYING HOST KEYS in ssh(1).
If this flag is set to ``yes'', an ASCII art representation of
the remote host key fingerprint is printed in addition to the hex
fingerprint string at login and for unknown host keys. If this
flag is set to ``no'', no fingerprint strings are printed at
login and only the hex fingerprint string will be printed for
unknown host keys. The default is ``no''.
Specifies the full pathname of the xauth(1) program. The default
A pattern consists of zero or more non-whitespace characters, '*' (a
wildcard that matches zero or more characters), or '?' (a wildcard that
matches exactly one character). For example, to specify a set of decla-
rations for any host in the ``.co.uk'' set of domains, the following pat-
tern could be used:
The following pattern would match any host in the 192.168.0.[0-9] network
is described above. This file is used by the SSH client.
Because of the potential for abuse, this file must have strict
permissions: read/write for the user, and not accessible by oth-
ers. It may be group-writable provided that the group in ques-
tion contains only the user.
Systemwide configuration file. This file provides defaults for
those values that are not specified in the user's configuration
file, and for those users who do not have a configuration file.
This file must be world-readable.
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 cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
BSD April 27, 2017 BSD
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