SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
NAME
ssh_config - OpenSSH SSH client configuration files
SYNOPSIS
~/.ssh/config
/etc/ssh/ssh_config
DESCRIPTION
ssh(1) obtains configuration data from the following sources
in the following 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 configuration files contain sections
separated by ``Host'' specifications, and that section
is only applied for hosts that match one of the pat-
terns given in the specification. The matched host
name is usually the one given on the command line (see
the CanonicalizeHostname option for exceptions.)
Since the first obtained value for each parameter is
used, more host-specific declarations should be given
near the beginning of the file, and general defaults at
the end.
The configuration file has the following format:
Empty lines and lines starting with `#' are comments.
Otherwise a line is of the format ``keyword argu-
ments''. 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 specifying 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 keywords are case-insensitive and arguments
are case-sensitive):
Host Restricts the following declarations (up to the next
Host or Match keyword) 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 pattern can
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be used to provide global defaults for all hosts. The
host is usually the hostname argument given on the com-
mand line (see the CanonicalizeHostname option for
exceptions.)
A pattern entry may be negated by prefixing it with an
exclamation 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 exceptions for wildcard
matches.
See PATTERNS for more information on patterns.
Match
Restricts the following declarations (up to the next
Host or Match keyword) to be used only when the condi-
tions following the Match keyword are satisfied. Match
conditions are specified using one or more criteria or
the single token all which always matches. The avail-
able criteria keywords are: canonical, exec, host,
originalhost, user, and localuser. The all criteria
must appear alone or immediately after canonical.
Other criteria may be combined arbitrarily. All cri-
teria but all and canonical require an argument. Cri-
teria may be negated by prepending an exclamation mark
(`!'.)
The canonical keyword matches only when the configura-
tion file is being re-parsed after hostname canonicali-
zation (see the CanonicalizeHostname option.) This may
be useful to specify conditions that work with canoni-
cal host names only. The exec keyword executes the
specified command under the user's shell. If the com-
mand returns a zero exit status then the condition is
considered true. Commands containing whitespace char-
acters must be quoted. The following character
sequences in the command will be expanded prior to exe-
cution: `%L' will be substituted by the first com-
ponent 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
specified on the command-line, `%p' the destination
port, `%r' by the remote login username, and `%u' by
the username of the user running ssh(1).
The other keywords' criteria must be single entries or
comma-separated lists and may use the wildcard and
negation operators described in the PATTERNS section.
The criteria for the host keyword are matched against
the target hostname, after any substitution by the
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Hostname or CanonicalizeHostname options. The ori-
ginalhost keyword matches against the hostname as it
was specified on the command-line. The user keyword
matches against the target username on the remote host.
The localuser keyword matches against the name of the
local user running ssh(1) (this keyword may be useful
in system-wide ssh_config files).
AddKeysToAgent
Specifies whether keys should be automatically added to
a running ssh-agent(1). If this option is set to
``yes'' and a key is loaded from a file, the key and
its passphrase are added to the agent with the default
lifetime, as if by ssh-add(1). If this option is set
to ``ask'', ssh will require confirmation using the
SSH_ASKPASS program before adding a key (see ssh-add(1)
for details). If this option is set to ``confirm'',
each use of the key must be confirmed, as if the -c
option was specified to ssh-add(1). If this option is
set to ``no'', no keys are added to the agent. The
argument must be ``yes'', ``confirm'', ``ask'', or
``no''. The default is ``no''.
AddressFamily
Specifies which address family to use when connecting.
Valid arguments are ``any'', ``inet'' (use IPv4 only),
or ``inet6'' (use IPv6 only). The default is ``any''.
BatchMode
If set to ``yes'', passphrase/password querying will be
disabled. This option is useful in scripts and other
batch jobs where no user is present to supply the pass-
word. The argument must be ``yes'' or ``no''. The
default is ``no''.
BindAddress
Use the specified address on the local machine as the
source address of the connection. Only useful on sys-
tems with more than one address. Note that this option
does not work if UsePrivilegedPort is set to ``yes''.
CanonicalDomains
When CanonicalizeHostname is enabled, this option
specifies the list of domain suffixes in which to
search for the specified destination host.
CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname
canonicalization fails. The default, ``yes'', will
attempt to look up the unqualified hostname using the
system resolver's search rules. A value of ``no'' will
cause ssh(1) to fail instantly if CanonicalizeHostname
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is enabled and the target hostname cannot be found in
any of the domains specified by CanonicalDomains.
CanonicalizeHostname
Controls whether explicit hostname canonicalization is
performed. The default, ``no'', is not to perform any
name rewriting and let the system resolver handle all
hostname lookups. If set to ``yes'' then, for connec-
tions that do not use a ProxyCommand, ssh(1) will
attempt to canonicalize the hostname specified on the
command line using the CanonicalDomains suffixes and
CanonicalizePermittedCNAMEs rules. If Canonical-
izeHostname is set to ``always'', then canonicalization
is applied to proxied connections too.
If this option is enabled, then the configuration files
are processed again using the new target name to pick
up any new configuration in matching Host and Match
stanzas.
CanonicalizeMaxDots
Specifies the maximum number of dot characters in a
hostname before canonicalization is disabled. The
default, ``1'', allows a single dot (i.e.
hostname.subdomain).
CanonicalizePermittedCNAMEs
Specifies rules to determine whether CNAMEs should be
followed when canonicalizing hostnames. The rules con-
sist of one or more arguments of source_domain_list
:target_domain_list, where source_domain_list is a
pattern-list of domains that may follow CNAMEs in
canonicalization, and target_domain_list is a pattern-
list of domains that they may resolve to.
For example,
``*.a.example.com:*.b.example.com,*.c.example.com''
will allow hostnames matching ``*.a.example.com'' to be
canonicalized to names in the ``*.b.example.com'' or
``*.c.example.com'' domains.
CertificateFile
Specifies a file from which the user's certificate is
read. A corresponding private key must be provided
separately in order to use this certificate either from
an IdentityFile directive or -i flag to ssh(1), via
ssh-agent(1), or via a PKCS11Provider.
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
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host name) or `%r' (remote user name).
It is possible to have multiple certificate files
specified in configuration files; these certificates
will be tried in sequence. Multiple CertificateFile
directives will add to the list of certificates used
for authentication.
ChallengeResponseAuthentication
Specifies whether to use challenge-response authentica-
tion. 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 addition-
ally check the host IP address in the known_hosts file.
This allows ssh to detect if a host key changed due to
DNS spoofing and will add addresses of destination
hosts to ~/.ssh/known_hosts in the process, regardless
of the setting of StrictHostKeyChecking. 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 protocol version 1. Currently, ``blowfish'',
``3des'', and ``des'' are supported. des is only sup-
ported 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. If the specified value begins with a `+'
character, then the specified ciphers will be appended
to the default set instead of replacing them.
The supported ciphers are:
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
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aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
chacha20-poly1305@openssh.com
The default is:
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc
The list of available ciphers may also be obtained
using the -Q option of ssh(1) with an argument of
``cipher''.
ClearAllForwardings
Specifies that all local, remote, and dynamic port for-
wardings specified in the configuration files or on the
command line be cleared. This option is primarily use-
ful when used from the ssh(1) command line to clear
port forwardings set in configuration 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.
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ConnectionAttempts
Specifies the number of tries (one per second) to make
before exiting. The argument must be an integer. This
may be useful in scripts if the connection sometimes
fails. The default is 1.
ConnectTimeout
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.
ControlMaster
Enables the sharing of multiple sessions over a single
network connection. When set to ``yes'', ssh(1) will
listen for connections on a control socket specified
using the ControlPath argument. 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 net-
work 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 connections, but require confirmation using
ssh-askpass(1). If the ControlPath cannot be opened,
ssh will continue without connecting to a master
instance.
X11 and ssh-agent(1) forwarding is supported over these
multiplexed connections, however the display and agent
forwarded will be the one belonging to the master con-
nection i.e. it is not possible to forward multiple
displays or agents.
Two additional options allow for opportunistic multi-
plexing: 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 connec-
tion sharing 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
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target host name specified on the command line, `%p'
the destination port, `%r' by the remote login user-
name, `%u' by the username and `%i' by the numeric user
ID (uid) of the user running ssh(1), and `%C' by a hash
of the concatenation: %l%h%p%r. It is recommended that
any ControlPath used for opportunistic connection shar-
ing include at least %h, %p, and %r (or alternatively
%C) and be placed in a directory that is not writable
by other users. This ensures that shared connections
are uniquely identified.
ControlPersist
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 connection has been closed.
If set to ``no'', then the master connection will not
be placed into the background, and will close as soon
as the initial client connection is closed. If set to
``yes'' or ``0'', then the master connection will
remain in the background indefinitely (until killed or
closed via a mechanism such as the ssh(1) ``Fl O No
exit'' option). If set to a time in seconds, or a time
in any of the formats documented in sshd_config(5),
then the backgrounded master connection will automati-
cally terminate after it has remained idle (with no
client connections) for the specified time.
DynamicForward
Specifies that a TCP port on the local machine be for-
warded 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. By default, the local port is bound
in accordance with the GatewayPorts setting. However,
an explicit bind_address may be used to bind the con-
nection 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 sup-
ported, and ssh(1) will act as a SOCKS server. Multi-
ple forwardings may be specified, and additional for-
wardings can be given on the command line. Only the
superuser can forward privileged ports.
EnableSSHKeysign
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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 Host-
basedAuthentication. 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.
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 disable the escape charac-
ter entirely (making the connection transparent for
binary data).
ExitOnForwardFailure
Specifies whether ssh(1) should terminate the connec-
tion if it cannot set up all requested dynamic, tunnel,
local, and remote port forwardings, (e.g. if either end
is unable to bind and listen on a specified port).
Note that ExitOnForwardFailure does not apply to con-
nections made over port forwardings and will not, for
example, cause ssh(1) to exit if TCP connections to the
ultimate forwarding destination fail. The argument
must be ``yes'' or ``no''. The default is ``no''.
FingerprintHash
Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: ``md5'' and
``sha256''. The default is ``md5''.
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 connec-
tion. 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
redirected over the secure channel and DISPLAY set.
The argument must be ``yes'' or ``no''. The default is
``no''.
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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 display through the forwarded
connection. An attacker may then be able to perform
activities such as keystroke monitoring if the
ForwardX11Trusted option is also enabled.
ForwardX11Timeout
Specify a timeout for untrusted X11 forwarding using
the format described in the TIME FORMATS section of
sshd_config(5). X11 connections received by ssh(1)
after this time will be refused. The default is to
disable untrusted X11 forwarding after twenty minutes
has elapsed.
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 considered untrusted and prevented from steal-
ing 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 minutes. Remote
clients will be refused access after this time.
The default is ``no''.
See the X11 SECURITY extension specification for full
details on the restrictions imposed on untrusted
clients.
GatewayPorts
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 for-
warded ports. GatewayPorts can be used to specify that
ssh should bind local port forwardings to the wildcard
address, thus allowing remote hosts to connect to for-
warded ports. The argument must be ``yes'' or ``no''.
The default is ``no''.
GlobalKnownHostsFile
Specifies one or more files to use for the global host
key database, separated by whitespace. The default is
/etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI
is allowed. The default is ``no''.
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GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The
default is ``no''.
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 identifying 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).
HostbasedAuthentication
Specifies whether to try rhosts based authentication
with public key authentication. The argument must be
``yes'' or ``no''. The default is ``no''.
HostbasedKeyTypes
Specifies the key types that will be used for hostbased
authentication as a comma-separated pattern list.
Alternately if the specified value begins with a `+'
character, then the specified key types will be
appended to the default set instead of replacing them.
The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-
nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported
key types.
HostKeyAlgorithms
Specifies the host key algorithms that the client wants
to use in order of preference. Alternately if the
specified value begins with a `+' character, then the
specified key types will be appended to the default set
instead of replacing them. The default for this option
is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
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ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-
nistp521,
ssh-ed25519,ssh-rsa
If hostkeys are known for the destination host then
this default is modified to prefer their algorithms.
The list of available key types may also be obtained
using the -Q option of ssh(1) with an argument of
``key''.
HostKeyAlias
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 database files. This option is useful
for tunneling SSH connections or for multiple servers
running on a single host.
HostName
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 character sequence `%%' will
be replaced by a single `%' character, which may be
used when specifying IPv6 link-local addresses.
The default is the name given on the command line.
Numeric IP addresses are also permitted (both on the
command line and in HostName specifications).
IdentitiesOnly
Specifies that ssh(1) should only use the authentica-
tion identity and certificate files explicitly config-
ured in the ssh_config files or passed on the ssh(1)
command-line, even if ssh-agent(1) or a PKCS11Provider
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 iden-
tities. The default is ``no''.
IdentityAgent
Specifies the UNIX-domain socket used to communicate
with the authentication agent.
This option overrides the ``SSH_AUTH_SOCK'' environment
variable and can be used to select a specific agent.
Setting the socket name to ``none'' disables the use of
an authentication agent. If the string
``SSH_AUTH_SOCK'' is specified, the location of the
socket will be read from the SSH_AUTH_SOCK environment
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variable.
The socket 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).
IdentityFile
Specifies a file from which the user's DSA, ECDSA,
Ed25519 or RSA authentication identity 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. Additionally,
any identities represented by the authentication agent
will be used for authentication unless IdentitiesOnly
is set. If no certificates have been explicitly speci-
fied by CertificateFile, ssh(1) will try to load certi-
ficate information from the filename obtained by
appending -cert.pub to the path of a specified Identi-
tyFile.
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 speci-
fied in configuration files; all these identities will
be tried in sequence. Multiple IdentityFile directives
will add to the list of identities tried (this
behaviour differs from that of other configuration
directives).
IdentityFile may be used in conjunction with Identitie-
sOnly to select which identities in an agent are
offered during authentication. IdentityFile may also
be used in conjunction with CertificateFile in order to
provide any certificate also needed for authentication
with the identity.
IgnoreUnknown
Specifies a pattern-list of unknown options to be
ignored if they are encountered in configuration pars-
ing. This may be used to suppress errors if ssh_config
contains options that are unrecognised by ssh(1). It
is recommended that IgnoreUnknown be listed early in
the configuration file as it will not be applied to
unknown options that appear before it.
Include
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Include the specified configuration file(s). Multiple
pathnames may be specified and each pathname may con-
tain glob(3) wildcards and, for user configurations,
shell-like ``~'' references to user home directories.
Files without absolute paths are assumed to be in
~/.ssh if included in a user configuration file or
/etc/ssh if included from the system configuration
file. Include directive may appear inside a Match or
Host block to perform conditional inclusion.
IPQoS
Specifies the IPv4 type-of-service or DSCP class for
connections. Accepted values are ``af11'', ``af12'',
``af13'', ``af21'', ``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 specified, 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.
KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authenti-
cation. 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''.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated. Alter-
nately if the specified value begins with a `+' charac-
ter, then the specified methods will be appended to the
default set instead of replacing them. The default is:
curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-
nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group-exchange-sha1,
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
diffie-hellman-group14-sha1
The list of available key exchange algorithms may also
be obtained using the -Q option of ssh(1) with an argu-
ment of ``kex''.
LocalCommand
Specifies a command to execute on the local machine
after successfully connecting to the server. The com-
mand string extends to the end of the line, and is exe-
cuted 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) or `%C' by a hash of
the concatenation: %l%h%p%r.
The command is run synchronously and does not have
access to the session of the ssh(1) that spawned it.
It should not be used for interactive commands.
This directive is ignored unless PermitLocalCommand has
been enabled.
LocalForward
Specifies that a TCP port on the local machine be for-
warded 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 speci-
fied by enclosing addresses in square brackets. Multi-
ple forwardings may be specified, and additional for-
wardings can be given on the command line. Only the
superuser can forward privileged ports. By default,
the local port is bound in accordance with the Gateway-
Ports 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.
LogLevel
Gives the verbosity level that is used when logging
messages from ssh(1). The possible values are: 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 lev-
els of verbose output.
MACs Specifies the MAC (message authentication code)
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
algorithms in order of preference. The MAC algorithm
is used for data integrity protection. Multiple algo-
rithms must be comma-separated. If the specified value
begins with a `+' character, then the specified algo-
rithms will be appended to the default set instead of
replacing them.
The algorithms that contain ``-etm'' calculate the MAC
after encryption (encrypt-then-mac). These are con-
sidered safer and their use recommended.
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-
etm@openssh.com,
hmac-sha1-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be
obtained using the -Q option of ssh(1) with an argument
of ``mac''.
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 warnings 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 ! Ns command escape sequence in
ssh(1). The argument must be ``yes'' or ``no''. The
default is ``no''.
PKCS11Provider
Specifies which PKCS#11 provider to use. The argument
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
to this keyword is the PKCS#11 shared library ssh(1)
should use to communicate with a PKCS#11 token provid-
ing the user's private RSA key.
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
authentication methods. This allows a client to prefer
one method (e.g. keyboard-interactive) over another
method (e.g. password). The default 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 versions must be comma-separated.
When this option is set to ``2,1'' ssh will try version
2 and fall back to version 1 if version 2 is not avail-
able. The default is `2'. Protocol 1 suffers from a
number of cryptographic weaknesses and should not be
used. It is only offered to support legacy devices.
ProxyCommand
Specifies the command to use to connect to the server.
The command string extends to the end of the line, and
is executed using the user's shell `exec' directive to
avoid a lingering shell process.
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 basically 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). Setting the command to ``none'' disables this
option entirely. Note that CheckHostIP is not avail-
able for connects with a proxy command.
This directive is useful in conjunction with nc(1) and
its proxy support. For example, the following direc-
tive 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
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
ProxyJump
Specifies one or more jump proxies as
[user@] host [:port] Multiple proxies may be separated
by comma characters and will be visited sequentially.
Setting this option will cause ssh(1) to connect to the
target host by first making a ssh(1) connection to the
specified ProxyJump host and then establishing a TCP
forwarding to the ultimate target from there.
Note that this option will compete with the ProxyCom-
mand option - whichever is specified first will prevent
later instances of the other from taking effect.
ProxyUseFdpass
Specifies that ProxyCommand will pass a connected file
descriptor back to ssh(1) instead of continuing to exe-
cute and pass data. The default is ``no''.
PubkeyAcceptedKeyTypes
Specifies the key types that will be used for public
key authentication as a comma-separated pattern list.
Alternately if the specified value begins with a `+'
character, then the key types after it will be appended
to the default instead of replacing it. The default
for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-
nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported
key types.
PubkeyAuthentication
Specifies whether to try public key authentication.
The argument to this keyword must be ``yes'' or ``no''.
The default is ``yes''.
RekeyLimit
Specifies the maximum amount of data that may be
transmitted before the session key is renegotiated,
optionally followed a maximum amount of time that may
pass before the session key is renegotiated. The first
argument is specified in bytes and may have a suffix of
`K', `M', or `G' to indicate Kilobytes, Megabytes, or
Gigabytes, respectively. The default is between `1G'
and `4G', depending on the cipher. The optional second
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
value is specified in seconds and may use any of the
units documented in the TIME FORMATS section of
sshd_config(5). The default value for RekeyLimit is
``default none'', which means that rekeying is per-
formed after the cipher's default amount of data has
been sent or received and no time based rekeying is
done.
RemoteForward
Specifies that a TCP port on the remote machine be for-
warded 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 speci-
fied by enclosing addresses in square brackets. Multi-
ple forwardings may be specified, and additional for-
wardings can be given on the command line. Privileged
ports can be forwarded 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 interfaces. Specifying a
remote bind_address will only succeed if the server's
GatewayPorts option is enabled (see sshd_config(5)) .
RequestTTY
Specifies whether to request a pseudo-tty for the ses-
sion. 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
ssh(1).
RevokedHostKeys
Specifies revoked host public keys. Keys listed in
this file will be refused for host authentication.
Note that if this file does not exist or is not read-
able, then host authentication will be refused for all
hosts. Keys may be specified as a text file, listing
one public key per line, or as an OpenSSH Key Revoca-
tion List (KRL) as generated by ssh-keygen(1). For
more information on KRLs, see the KEY REVOCATION LISTS
section in ssh-keygen(1).
RhostsRSAAuthentication
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
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 argu-
ment to this keyword must be ``yes'' or ``no''. RSA
authentication will only be attempted if the identity
file exists, or an authentication agent is running.
The default is ``yes''. Note that this option applies
to protocol version 1 only.
SendEnv
Specifies what variables from the local environ(7)
should be sent to the server. The server must also
support it, and the server must be configured to accept
these environment variables. Note that the TERM
environment variable is always sent whenever a pseudo-
terminal is requested as it is required by the proto-
col. Refer to AcceptEnv in sshd_config(5) for how to
configure the server. Variables are specified by name,
which may contain wildcard characters. 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.
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 therefore will not be spoofable.
The TCP keepalive option enabled by TCPKeepAlive is
spoofable. The server alive mechanism is valuable when
the client or server depend on knowing when a connec-
tion has become inactive.
The default value is 3. If, for example, Ser-
verAliveInterval (see below) is set to 15 and Ser-
verAliveCountMax is left at the default, if the server
becomes unresponsive, ssh will disconnect after approx-
imately 45 seconds.
ServerAliveInterval
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
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, indicat-
ing that these messages will not be sent to the server.
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used
when creating a Unix-domain socket file for local or
remote port forwarding. This option is only used for
port forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain
socket file that is readable and writable only by the
owner. Note that not all operating systems honor the
file mode on Unix-domain socket files.
StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain
socket file for local or remote port forwarding before
creating a new one. If the socket file already exists
and StreamLocalBindUnlink is not enabled, ssh will be
unable to forward the port to the Unix-domain socket
file. This option is only used for port forwarding to
a Unix-domain socket file.
The argument must be ``yes'' or ``no''. The default is
``no''.
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 connect to hosts whose host key
has changed. This provides maximum protection against
trojan horse attacks, though it can be annoying 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 automatically in all cases. The argument must
be ``yes'', ``no'', or ``ask''. The default is
``ask''.
TCPKeepAlive
Specifies whether the system should send TCP keepalive
messages to the other side. If they are sent, death of
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
the connection or crash of one of the machines will be
properly noticed. 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 mes-
sages), 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 dev-
ices 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''.
UpdateHostKeys
Specifies whether ssh(1) should accept notifications of
additional hostkeys from the server sent after authen-
tication has completed and add them to UserK-
nownHostsFile. The argument must be ``yes'', ``no''
(the default) or ``ask''. Enabling this option allows
learning alternate hostkeys for a server and supports
graceful key rotation by allowing a server to send
replacement public keys before old ones are removed.
Additional hostkeys are only accepted if the key used
to authenticate the host was already trusted or expli-
citly accepted by the user. If UpdateHostKeys is set
to ``ask'', then the user is asked to confirm the
modifications to the known_hosts file. Confirmation is
currently incompatible with ControlPersist, and will be
disabled if it is enabled.
Presently, only sshd(8) from OpenSSH 6.8 and greater
support the ``hostkeys@openssh.com'' protocol extension
used to inform the client of all the server's hostkeys.
UsePrivilegedPort
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
Specifies whether to use a privileged port for outgoing
connections. 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 different user name is used on different
machines. This saves the trouble of having to remember
to give the user name on the command line.
UserKnownHostsFile
Specifies one or more files to use for the user host
key database, separated by whitespace. The default is
~/.ssh/known_hosts, ~/.ssh/known_hosts2.
VerifyHostKeyDNS
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 finger-
prints 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''.
See also VERIFYING HOST KEYS in ssh(1).
VisualHostKey
If this flag is set to ``yes'', an ASCII art represen-
tation of the remote host key fingerprint is printed in
addition to the 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
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/X/bin/xauth.
PATTERNS
A pattern consists of zero or more non-whitespace charac-
ters, `*' (a wildcard that matches zero or more characters),
or `?' (a wildcard that matches exactly one character). For
example, to specify a set of declarations for any host in
the ``.co.uk'' set of domains, the following pattern could
be used:
Dl Host *.co.uk
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SSH_CONFIG(5) FILE FORMATS SSH_CONFIG(5)
The following pattern would match any host in the
192.168.0.[0-9] network range:
Dl Host 192.168.0.?
A pattern-list is a comma-separated list of patterns. Pat-
terns 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 organization except from
the ``dialup'' pool, the following entry (in
authorized_keys) could be used:
Dl 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 others.
/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 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 ver-
sions 1.5 and 2.0.
Last change: July 22 2016 24
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