ssh_config
~/.ssh/config
/etc/ssh/ssh_config
DESCRIPTION
ssh(1) obtains configuration data from the following sources in the fol-
lowing order:
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-
ing).
See PATTERNS for more information on patterns.
AddressFamily
Specifies which address family to use when connecting. Valid
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''.
ChallengeResponseAuthentication
Specifies whether to use challenge-response authentication. The
argument to this keyword must be ``yes'' or ``no''. The default
is ``yes''.
CheckHostIP
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
``yes''.
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''.
Ciphers
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:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
ClearAllForwardings
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''.
Compression
Specifies whether to use compression. The argument must be
``yes'' or ``no''. The default is ``no''.
CompressionLevel
Specifies the compression level to use if compression is enabled.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications. The
meaning of the values is the same as in gzip(1). Note that this
option applies to protocol version 1 only.
ControlMaster
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
not listening.
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''
option.
ControlPath
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 local host name, '%h' will be substituted
by the target host name, '%p' the port, and '%r' by the remote
login username. 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.
DynamicForward
Specifies that a TCP port on the local machine be forwarded over
the secure channel, and the application protocol is then used to
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 or by using
an alternative syntax: [bind_address/]port. By default, the
local port is bound in accordance with the GatewayPorts setting.
However, an explicit bind_address may be used to bind the connec-
tion 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
EscapeChar
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).
ExitOnForwardFailure
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''.
ForwardAgent
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
the agent.
ForwardX11
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''.
X11 forwarding should be enabled with caution. Users with the
ability to bypass file permissions on the remote host (for the
user's X11 authorization database) can access the local X11 dis-
play through the forwarded connection. An attacker may then be
able to perform activities such as keystroke monitoring if the
ForwardX11Trusted option is also enabled.
ForwardX11Trusted
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.
GatewayPorts
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is ``no''. Note that this option applies to protocol
version 2 only.
GSSAPIKeyExchange
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.
GSSAPIClientIdentity
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.
GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is
``no''. Note that this option applies to protocol version 2 con-
nections using GSSAPI.
GSSAPIRenewalForcesRekey
If set to ``yes'' then renewal of the client's GSSAPI credentials
will force the rekeying of the ssh connection. With a compatible
server, this can delegate the renewed credentials to a session on
the server. The default is ``no''.
GSSAPITrustDns
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-
API.
HashKnownHosts
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.
HostbasedAuthentication
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.
HostKeyAlgorithms
specify nicknames or abbreviations for hosts. The default is the
name given on the command line. Numeric IP addresses are also
permitted (both on the command line and in HostName specifica-
tions).
IdentitiesOnly
Specifies that ssh(1) should only use the authentication identity
files configured in the ssh_config files, even if ssh-agent(1)
offers more identities. The argument to this keyword must be
``yes'' or ``no''. This option is intended for situations where
ssh-agent offers many different identities. The default is
``no''.
IdentityFile
Specifies a file from which the user's RSA or DSA authentication
identity is read. The default is ~/.ssh/identity for protocol
version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol ver-
sion 2. Additionally, any identities represented by the authen-
tication agent will be used for authentication.
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.
KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be ``yes'' or ``no''. The
default is ``yes''.
KbdInteractiveDevices
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'',
and ``skey''.
LocalCommand
Specifies a command to execute on the local machine after suc-
cessfully connecting to the server. The command string extends
to the end of the line, and is executed with the user's shell.
The following escape character substitutions will be performed:
'%d' (local user's home directory), '%h' (remote host name), '%l'
(local host name), '%n' (host name as provided on the command
line), '%p' (remote port), '%r' (remote user name) or '%u' (local
user name). This directive is ignored unless PermitLocalCommand
has been enabled.
LocalForward
Specifies that a TCP port on the local machine be forwarded over
address or '*' indicates that the port should be available from
all interfaces.
LogLevel
Gives the verbosity level that is used when logging messages from
ssh(1). The possible values are: SILENT, QUIET, FATAL, ERROR,
INFO, VERBOSE, 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:
hmac-md5,hmac-sha1,umac-64@openssh.com,
hmac-ripemd160,hmac-sha1-96,hmac-md5-96
NoHostAuthenticationForLocalhost
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
localhost.
NumberOfPasswordPrompts
Specifies the number of password prompts before giving up. The
argument to this keyword must be an integer. The default is 3.
PasswordAuthentication
Specifies whether to use password authentication. The argument
to this keyword must be ``yes'' or ``no''. The default is
``yes''.
PermitLocalCommand
Allow local command execution via the LocalCommand option or
using the !command escape sequence in ssh(1). The argument must
be ``yes'' or ``no''. The default is ``no''.
Port Specifies the port number to connect on the remote host. The
default is 22.
PreferredAuthentications
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 for this option is: ``gssapi-with-mic,
hostbased, publickey, keyboard-interactive, password''.
Protocol
Specifies the protocol versions ssh(1) should support in order of
preference. The possible values are '1' and '2'. Multiple ver-
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). Setting the command to ``none'' disables
this option entirely. Note that CheckHostIP is not available for
connects with a proxy command.
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
PubkeyAuthentication
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.
RekeyLimit
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.
RemoteForward
Specifies that a TCP port on the remote machine be forwarded over
the secure channel to the specified host and port from the local
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 or by using
an alternative syntax: [bind_address/]port and host/hostport.
Multiple forwardings may be specified, and additional forwardings
can be given on the command line. Privileged ports can be for-
warded only when logging in as root on the remote machine.
If the port argument is '0', the listen port will be dynamically
allocated on the server and reported to the client at run time.
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)).
RhostsRSAAuthentication
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.
RSAAuthentication
Specifies whether to try RSA authentication. The argument to
this keyword must be ``yes'' or ``no''. RSA authentication will
whitespace or spread across multiple SendEnv directives. The
default is not to send any environment variables.
See PATTERNS for more information on patterns.
ServerAliveCountMax
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
(see below) is set to 15 and ServerAliveCountMax is left at the
default, if the server becomes unresponsive, ssh will disconnect
after approximately 45 seconds. This option applies to protocol
version 2 only; in protocol version 1 there is no mechanism to
request a response from the server to the server alive messages,
so disconnection is the responsibility of the TCP stack.
ServerAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the server, ssh(1) will send a message through
the encrypted channel to request a response from the server. The
default is 0, indicating that these messages will not be sent to
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
option.
SmartcardDevice
Specifies which smartcard device to use. The argument to this
keyword is the device ssh(1) should use to communicate with a
smartcard used for storing the user's private RSA key. By
default, no device is specified and smartcard support is not
activated.
StrictHostKeyChecking
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
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
``no''.
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''.
TunnelDevice
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''.
UseBlacklistedKeys
Specifies whether ssh(1) should use keys recorded in its black-
list of known-compromised keys (see ssh-vulnkey(1)) for authenti-
cation. If ``yes'', then attempts to use compromised keys for
authentication will be logged but accepted. It is strongly rec-
ommended that this be used only to install new authorized keys on
the remote system, and even then only with the utmost care. If
``no'', then attempts to use compromised keys for authentication
will be prevented. The default is ``no''.
UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connec-
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-
mand line.
UserKnownHostsFile
Specifies a file to use for the user host key database instead of
~/.ssh/known_hosts.
VerifyHostKeyDNS
Specifies whether to verify the remote key using DNS and SSHFP
VisualHostKey
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''.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default
is /usr/bin/xauth.
PATTERNS
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:
Host *.co.uk
The following pattern would match any host in the 192.168.0.[0-9] network
range:
Host 192.168.0.?
A pattern-list is a comma-separated list of patterns. Patterns within
pattern-lists may be negated by preceding them with an exclamation mark
('!'). For example, to allow a key to be used from anywhere within an
organisation except from the ``dialup'' pool, the following entry (in
authorized_keys) could be used:
from="!*.dialup.example.com,*.example.com"
FILES
~/.ssh/config
This is the per-user configuration file. The format of this file
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.
/etc/ssh/ssh_config
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.
SEE ALSO
ssh(1)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
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