ssh_config —
OpenSSH client configuration file
ssh(1) obtains configuration data from the
following sources in the following order:
- command-line options
- user's configuration file
(~/.ssh/config)
- 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 patterns 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.
Note that the Debian
openssh-client package sets
several options as standard in
/etc/ssh/ssh_config which are not the default in
ssh(1):
/etc/ssh/ssh_config.d/*.conf files are included at
the start of the system-wide configuration file, so options set there will
override those in
/etc/ssh/ssh_config.
The file contains keyword-argument pairs, one per line. Lines starting with
‘
#
’ and empty lines are interpreted as
comments. Arguments may optionally be enclosed in double quotes (") in
order to represent arguments containing spaces. 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.
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 be used to
provide global defaults for all hosts. The host is usually the
hostname argument given on the command
line (see the CanonicalizeHostname keyword
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 conditions following the
Match keyword are satisfied. Match conditions
are specified using one or more criteria or the single token
all which always matches. The available
criteria keywords are: canonical,
final, exec,
host,
originalhost,
user, and
localuser. The
all criteria must appear alone or immediately
after canonical or
final. Other criteria may be combined
arbitrarily. All criteria but all,
canonical, and
final require an argument. Criteria may be
negated by prepending an exclamation mark (‘!’).
The canonical keyword matches only when the
configuration file is being re-parsed after hostname canonicalization (see
the CanonicalizeHostname option). This may be
useful to specify conditions that work with canonical host names only.
The final keyword requests that the
configuration be re-parsed (regardless of whether
CanonicalizeHostname is enabled), and matches
only during this final pass. If
CanonicalizeHostname is enabled, then
canonical and
final match during the same pass.
The exec keyword executes the specified command
under the user's shell. If the command returns a zero exit status then the
condition is considered true. Commands containing whitespace characters
must be quoted. Arguments to exec accept the
tokens described in the TOKENS
section.
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
Hostname or
CanonicalizeHostname options. The
originalhost 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(1) 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.
Alternately, this option may be specified as a time interval using the
format described in the TIME
FORMATS section of sshd_config(5) to
specify the key's lifetime in ssh-agent(1),
after which it will automatically be removed. The argument must be
no (the default),
yes, confirm
(optionally followed by a time interval), ask
or a time interval.
- AddressFamily
- Specifies which address family to use when connecting.
Valid arguments are any (the default),
inet (use IPv4 only), or
inet6 (use IPv6 only).
- BatchMode
- If set to yes, user
interaction such as password prompts and host key confirmation requests
will be disabled. In addition, the
ServerAliveInterval option will be set to 300
seconds by default (Debian-specific). This option is useful in scripts and
other batch jobs where no user is present to interact with
ssh(1), and where it is desirable to detect a
broken network swiftly. The argument must be
yes or no (the
default).
- BindAddress
- Use the specified address on the local machine as the
source address of the connection. Only useful on systems with more than
one address.
- BindInterface
- Use the address of the specified interface on the local
machine as the source address of the connection.
- 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 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 connections
that do not use a ProxyCommand or
ProxyJump,
ssh(1) will attempt to canonicalize the
hostname specified on the command line using the
CanonicalDomains suffixes and
CanonicalizePermittedCNAMEs rules. If
CanonicalizeHostname 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. A value of none disables the use of
a ProxyJump host.
- 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 consist 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.
A single argument of “none” causes no CNAMEs to be considered
for canonicalization. This is the default behaviour.
- CASignatureAlgorithms
- Specifies which algorithms are allowed for signing of
certificates by certificate authorities (CAs). The default is:
If the specified list begins with a ‘+’ character, then the
specified algorithms will be appended to the default set instead of
replacing them. If the specified list begins with a ‘-’
character, then the specified algorithms (including wildcards) will be
removed from the default set instead of replacing them.
ssh(1) will not accept host certificates signed
using algorithms other than those specified.
- 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 or
SecurityKeyProvider.
Arguments to CertificateFile may use the tilde
syntax to refer to a user's home directory, the tokens described in the
TOKENS section and environment
variables as described in the
ENVIRONMENT
VARIABLES section.
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.
- CheckHostIP
- If set to yes,
ssh(1) will additionally check the host IP
address in the known_hosts file. This allows
it 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 default), the
check will not be executed.
- Ciphers
- Specifies the ciphers allowed and their order of
preference. Multiple ciphers must be comma-separated. If the specified
list begins with a ‘+’ character, then the specified ciphers
will be appended to the default set instead of replacing them. If the
specified list begins with a ‘-’ character, then the
specified ciphers (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with a
‘^’ character, then the specified ciphers will be placed at
the head of the default set.
The supported ciphers are:
The default is:
The list of available ciphers may also be obtained using “ssh -Q
cipher”.
- 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 configuration files, and is automatically set by
scp(1) and
sftp(1). The argument must be
yes or no (the
default).
- Compression
- Specifies whether to use compression. The argument must be
yes or no (the
default).
- 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
timeout is applied both to establishing the connection and to performing
the initial SSH protocol handshake and key exchange.
- 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 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(1) to listen for control connections, but
require confirmation using ssh-askpass(1). If
the ControlPath cannot be opened,
ssh(1) 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 connection i.e. it is
not possible 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
sharing as described in the ControlMaster
section above or the string none to disable
connection sharing. Arguments to ControlPath
may use the tilde syntax to refer to a user's home directory, the tokens
described in the TOKENS
section and environment variables as described in the
ENVIRONMENT
VARIABLES section. It is recommended that any
ControlPath used for opportunistic connection
sharing 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 (the default), 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 -O exit”). 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 automatically 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 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.
By default, the local port is bound in accordance with the
GatewayPorts setting. However, an explicit
bind_address may be used to bind the
connection to a specific address. The
bind_address of
localhost indicates that the listening port
be bound for local use only, while an empty address or ‘*’
indicates that the port should be available from all interfaces.
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.
- EnableEscapeCommandline
- Enables the command line option in the
EscapeChar menu for interactive sessions
(default ‘
~C
’). By default, the
command line is disabled.
- EnableSSHKeysign
- 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). 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 character entirely
(making the connection transparent 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, (e.g. if either end is unable
to bind and listen on a specified port). Note that
ExitOnForwardFailure does not apply to
connections 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).
- FingerprintHash
- Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: md5 and
sha256 (the default).
- ForkAfterAuthentication
- Requests ssh to go to
background just before command execution. This is useful if
ssh is going to ask for passwords or
passphrases, but the user wants it in the background. This implies the
StdinNull configuration option being set to
“yes”. The recommended way to start X11 programs at a remote
site is with something like ssh -f host
xterm, which is the same as ssh host
xterm if the ForkAfterAuthentication
configuration option is set to “yes”.
If the ExitOnForwardFailure configuration
option is set to “yes”, then a client started with the
ForkAfterAuthentication configuration option
being set to “yes” will wait for all remote port forwards to
be successfully established before placing itself in the background. The
argument to this keyword must be yes (same as
the -f option) or
no (the default).
- ForwardAgent
- Specifies whether the connection to the authentication
agent (if any) will be forwarded to the remote machine. The argument may
be yes, no (the
default), an explicit path to an agent socket or the name of an
environment variable (beginning with ‘$’) in which to find
the path.
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
redirected over the secure channel and
DISPLAY
set. The argument must be
yes or no (the
default).
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. Setting
ForwardX11Timeout to zero will disable the
timeout and permit X11 forwarding for the life of the connection. The
default is to disable untrusted X11 forwarding after twenty minutes has
elapsed.
- ForwardX11Trusted
- If this option is set to yes,
(the Debian-specific default), remote X11 clients will have full access to
the original X11 display.
If this option is set to no (the upstream
default), remote X11 clients will be considered 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 minutes. Remote clients will be
refused access after this time.
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 forwarded 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 forwarded ports. The argument must be
yes or no (the
default).
- 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.
- 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.
- 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”.
- 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 will delegate the renewed credentials to a session
on the server.
Checks are made to ensure that credentials are only propagated when the new
credentials match the old ones on the originating client and where the
receiving server still has the old set in its cache.
The default is “no”.
For this to work GSSAPIKeyExchange needs to be
enabled in the server and also used by the client.
- GSSAPIServerIdentity
- 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.
- 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”.
- GSSAPIKexAlgorithms
- The list of key exchange algorithms that are offered for
GSSAPI key exchange. Possible values are
The default is
“gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-”.
This option only applies to connections using GSSAPI.
- 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 visually reveal
identifying information if the file's contents are 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.
- HostbasedAcceptedAlgorithms
- Specifies the signature algorithms that will be used for
hostbased authentication as a comma-separated list of patterns.
Alternately if the specified list begins with a ‘+’
character, then the specified signature algorithms will be appended to the
default set instead of replacing them. If the specified list begins with a
‘-’ character, then the specified signature algorithms
(including wildcards) will be removed from the default set instead of
replacing them. If the specified list begins with a ‘^’
character, then the specified signature algorithms will be placed at the
head of the default set. The default for this option is:
The -Q option of
ssh(1) may be used to list supported
signature algorithms. This was formerly named HostbasedKeyTypes.
- HostbasedAuthentication
- Specifies whether to try rhosts based authentication with
public key authentication. The argument must be
yes or no (the
default).
- HostKeyAlgorithms
- Specifies the host key signature algorithms that the client
wants to use in order of preference. Alternately if the specified list
begins with a ‘+’ character, then the specified signature
algorithms will be appended to the default set instead of replacing them.
If the specified list begins with a ‘-’ character, then the
specified signature algorithms (including wildcards) will be removed from
the default set instead of replacing them. If the specified list begins
with a ‘^’ character, then the specified signature
algorithms will be placed at the head of the default set. The default for
this option is:
If hostkeys are known for the destination host then this default is modified
to prefer their algorithms.
The list of available signature algorithms may also be obtained using
“ssh -Q HostKeyAlgorithms”.
- 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 and when validating host certificates. 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. Arguments to
Hostname accept the tokens described in the
TOKENS section. Numeric IP
addresses are also permitted (both on the command line and in
Hostname specifications). The default is the
name given on the command line.
- IdentitiesOnly
- Specifies that ssh(1) should
only use the configured authentication identity and certificate files
(either the default files, or those explicitly configured in the
ssh_config files or passed on the
ssh(1) command-line), even if
ssh-agent(1) or a
PKCS11Provider or
SecurityKeyProvider offers more identities.
The argument to this keyword must be yes or
no (the default). This option is intended for
situations where ssh-agent offers many different identities.
- 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 variable.
Otherwise if the specified value begins with a ‘$’
character, then it will be treated as an environment variable containing
the location of the socket.
Arguments to IdentityAgent may use the tilde
syntax to refer to a user's home directory, the tokens described in the
TOKENS section and environment
variables as described in the
ENVIRONMENT
VARIABLES section.
- IdentityFile
- Specifies a file from which the user's DSA, ECDSA,
authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA
authentication identity is read. You can also specify a public key file to
use the corresponding private key that is loaded in
ssh-agent(1) when the private key file is not
present locally. The default is
~/.ssh/id_rsa,
~/.ssh/id_ecdsa,
~/.ssh/id_ecdsa_sk,
~/.ssh/id_ed25519,
~/.ssh/id_ed25519_sk and
~/.ssh/id_dsa. Additionally, any identities
represented by the authentication agent will be used for authentication
unless IdentitiesOnly is set. If no
certificates have been explicitly specified by
CertificateFile,
ssh(1) will try to load certificate
information from the filename obtained by appending
-cert.pub to the path of a specified
IdentityFile.
Arguments to IdentityFile may use the tilde
syntax to refer to a user's home directory or the tokens described in the
TOKENS section.
It is possible to have multiple identity files specified 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
IdentitiesOnly 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 parsing. 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
- Include the specified configuration file(s). Multiple
pathnames may be specified and each pathname may contain
glob(7) wildcards and, for user
configurations, shell-like ‘~’ references to user home
directories. Wildcards will be expanded and processed in lexical order.
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,
le, lowdelay,
throughput,
reliability, a numeric value, or
none to use the operating system default.
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
authentication. The argument to this keyword must be
yes (the default) or
no.
ChallengeResponseAuthentication is a
deprecated alias for this.
- 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 and
pam.
- KexAlgorithms
- Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated. If the specified list begins
with a ‘+’ character, then the specified algorithms will be
appended to the default set instead of replacing them. If the specified
list begins with a ‘-’ character, then the specified
algorithms (including wildcards) will be removed from the default set
instead of replacing them. If the specified list begins with a
‘^’ character, then the specified algorithms will be placed
at the head of the default set. The default is:
The list of available key exchange algorithms may also be obtained using
“ssh -Q kex”.
- KnownHostsCommand
- Specifies a command to use to obtain a list of host keys,
in addition to those listed in
UserKnownHostsFile and
GlobalKnownHostsFile. This command is
executed after the files have been read. It may write host key lines to
standard output in identical format to the usual files (described in the
VERIFYING HOST
KEYS section in ssh(1)). Arguments to
KnownHostsCommand accept the tokens described
in the TOKENS section. The
command may be invoked multiple times per connection: once when preparing
the preference list of host key algorithms to use, again to obtain the
host key for the requested host name and, if
CheckHostIP is enabled, one more time to
obtain the host key matching the server's address. If the command exits
abnormally or returns a non-zero exit status then the connection is
terminated.
- LocalCommand
- Specifies a command to execute on the local machine after
successfully connecting to the server. The command string extends to the
end of the line, and is executed with the user's shell. Arguments to
LocalCommand accept the tokens described in
the TOKENS section.
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 forwarded
over the secure channel to the specified host and port from the remote
machine. The first argument specifies the listener and may be
[bind_address:]port
or a Unix domain socket path. The second argument is the destination and
may be
host:hostport
or a Unix domain socket path if the remote host supports it.
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 privileged
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 ‘*’
indicates that the port should be available from all interfaces. Unix
domain socket paths may use the tokens described in the
TOKENS section and environment
variables as described in the
ENVIRONMENT
VARIABLES section.
- 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 levels of verbose output.
- LogVerbose
- Specify one or more overrides to LogLevel. An override
consists of a pattern lists that matches the source file, function and
line number to force detailed logging for. For example, an override
pattern of:
would enable detailed logging for line 1000 of
kex.c, everything in the
kex_exchange_identification() function, and
all code in the packet.c file. This option is
intended for debugging and no overrides are enabled by default.
- MACs
- Specifies the MAC (message authentication code) algorithms
in order of preference. The MAC algorithm is used for data integrity
protection. Multiple algorithms must be comma-separated. If the specified
list begins with a ‘+’ character, then the specified
algorithms will be appended to the default set instead of replacing them.
If the specified list begins with a ‘-’ character, then the
specified algorithms (including wildcards) will be removed from the
default set instead of replacing them. If the specified list begins with a
‘^’ character, then the specified algorithms will be placed
at the head of the default set.
The algorithms that contain “-etm” calculate the MAC after
encryption (encrypt-then-mac). These are considered safer and their use
recommended.
The default is:
The list of available MAC algorithms may also be obtained using “ssh
-Q mac”.
- NoHostAuthenticationForLocalhost
- Disable host authentication for localhost (loopback
addresses). The argument to this keyword must be
yes or no (the
default).
- 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 (the
default) or no.
- 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).
- PermitRemoteOpen
- Specifies the destinations to which remote TCP port
forwarding is permitted when RemoteForward is
used as a SOCKS proxy. The forwarding specification must be one of the
following forms:
Multiple forwards may be specified by separating them with whitespace. An
argument of any can be used to remove all
restrictions and permit any forwarding requests. An argument of
none can be used to prohibit all forwarding
requests. The wildcard ‘*’ can be used for host or port to
allow all hosts or ports respectively. Otherwise, no pattern matching or
address lookups are performed on supplied names.
- PKCS11Provider
- Specifies which PKCS#11 provider to use or
none to indicate that no provider should be
used (the default). The argument to this keyword is a path to the PKCS#11
shared library ssh(1) should use to
communicate with a PKCS#11 token providing keys for user
authentication.
- 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:
- 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.
Arguments to ProxyCommand accept the tokens
described in the TOKENS
section. 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 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:
- ProxyJump
- Specifies one or more jump proxies as either
[user@]host[:port]
or an ssh URI. 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. Setting the host to
none disables this option entirely.
Note that this option will compete with the
ProxyCommand option - whichever is specified
first will prevent later instances of the other from taking effect.
Note also that the configuration for the destination host (either supplied
via the command-line or the configuration file) is not generally applied
to jump hosts. ~/.ssh/config should be used
if specific configuration is required for jump hosts.
- ProxyUseFdpass
- Specifies that ProxyCommand
will pass a connected file descriptor back to
ssh(1) instead of continuing to execute and
pass data. The default is no.
- PubkeyAcceptedAlgorithms
- Specifies the signature algorithms that will be used for
public key authentication as a comma-separated list of patterns. If the
specified list begins with a ‘+’ character, then the
algorithms after it will be appended to the default instead of replacing
it. If the specified list begins with a ‘-’ character, then
the specified algorithms (including wildcards) will be removed from the
default set instead of replacing them. If the specified list begins with a
‘^’ character, then the specified algorithms will be placed
at the head of the default set. The default for this option is:
The list of available signature algorithms may also be obtained using
“ssh -Q PubkeyAcceptedAlgorithms”.
- PubkeyAuthentication
- Specifies whether to try public key authentication. The
argument to this keyword must be yes (the
default), no,
unbound or
host-bound. The final two options enable
public key authentication while respectively disabling or enabling the
OpenSSH host-bound authentication protocol extension required for
restricted ssh-agent(1) forwarding.
- RekeyLimit
- Specifies the maximum amount of data that may be
transmitted or received before the session key is renegotiated, optionally
followed by 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 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
performed after the cipher's default amount of data has been sent or
received and no time based rekeying is done.
- RemoteCommand
- Specifies a command to execute on the remote machine after
successfully connecting to the server. The command string extends to the
end of the line, and is executed with the user's shell. Arguments to
RemoteCommand accept the tokens described in
the TOKENS section.
- RemoteForward
- Specifies that a TCP port on the remote machine be
forwarded over the secure channel. The remote port may either be forwarded
to a specified host and port from the local machine, or may act as a SOCKS
4/5 proxy that allows a remote client to connect to arbitrary destinations
from the local machine. The first argument is the listening specification
and may be
[bind_address:]port
or, if the remote host supports it, a Unix domain socket path. If
forwarding to a specific destination then the second argument must be
host:hostport
or a Unix domain socket path, otherwise if no destination argument is
specified then the remote forwarding will be established as a SOCKS proxy.
When acting as a SOCKS proxy, the destination of the connection can be
restricted by PermitRemoteOpen.
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. Privileged ports can be forwarded only when
logging in as root on the remote machine. Unix domain socket paths may use
the tokens described in the
TOKENS section and environment
variables as described in the
ENVIRONMENT
VARIABLES section.
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 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 ssh(1).
- RequiredRSASize
- Specifies the minimum RSA key size (in bits) that
ssh(1) will accept. User authentication keys
smaller than this limit will be ignored. Servers that present host keys
smaller than this limit will cause the connection to be terminated. The
default is 1024 bits. Note that this limit
may only be raised from the default.
- 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 readable, 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 Revocation List (KRL) as generated by
ssh-keygen(1). For more information on KRLs,
see the KEY REVOCATION LISTS section in
ssh-keygen(1).
- SecurityKeyProvider
- Specifies a path to a library that will be used when
loading any FIDO authenticator-hosted keys, overriding the default of
using the built-in USB HID support.
If the specified value begins with a ‘$’ character, then it
will be treated as an environment variable containing the path to the
library.
- 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
protocol. 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.
See PATTERNS for more
information on patterns.
It is possible to clear previously set SendEnv
variable names by prefixing patterns with -.
The default is not to send any environment variables.
- 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
connection has become unresponsive.
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.
- 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 (Debian-specific).
ProtocolKeepAlives and
SetupTimeOut are Debian-specific
compatibility aliases for this option.
- SessionType
- May be used to either request invocation of a subsystem on
the remote system, or to prevent the execution of a remote command at all.
The latter is useful for just forwarding ports. The argument to this
keyword must be none (same as the
-N option),
subsystem (same as the
-s option) or
default (shell or command execution).
- SetEnv
- Directly specify one or more environment variables and
their contents to be sent to the server. Similarly to
SendEnv, with the exception of the
TERM
variable, the server must be
prepared to accept the environment variable.
- StdinNull
- Redirects stdin from /dev/null
(actually, prevents reading from stdin). Either this or the equivalent
-n option must be used when
ssh is run in the background. The argument to
this keyword must be yes (same as the
-n option) or no
(the default).
- 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).
- 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 man-in-the-middle (MITM) 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 accept-new then ssh will
automatically add new host keys to the user's
known_hosts file, but will not permit
connections to hosts with changed host keys. If this flag is set to
no or off, ssh
will automatically add new host keys to the user known hosts files and
allow connections to hosts with changed hostkeys to proceed, subject to
some restrictions. If this flag is set to ask
(the default), 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.
- SyslogFacility
- Gives the facility code that is used when logging messages
from ssh(1). The possible values are: DAEMON,
USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6,
LOCAL7. The default is USER.
- TCPKeepAlive
- 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
no. See also
ServerAliveInterval for protocol-level
keepalives.
- 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 (the default). Specifying
yes requests the default tunnel mode, which
is point-to-point.
- 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.
- UpdateHostKeys
- Specifies whether ssh(1)
should accept notifications of additional hostkeys from the server sent
after authentication has completed and add them to
UserKnownHostsFile. The argument must be
yes, no or
ask. 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 explicitly accepted by the user, the host was
authenticated via UserKnownHostsFile (i.e.
not GlobalKnownHostsFile) and the host was
authenticated using a plain key and not a certificate.
UpdateHostKeys is enabled by default if the
user has not overridden the default
UserKnownHostsFile setting and has not
enabled VerifyHostKeyDNS, otherwise
UpdateHostKeys will be set to
no.
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 “[email protected]” protocol
extension used to inform the client of all the server's hostkeys.
- 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. Each filename may use tilde notation to
refer to the user's home directory, the tokens described in the
TOKENS section and environment
variables as described in the
ENVIRONMENT
VARIABLES section. A value of none causes
ssh(1) to ignore any user-specific known
hosts files. 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 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 default is
no.
See also VERIFYING HOST
KEYS in ssh(1).
- VisualHostKey
- If this flag is set to yes, an
ASCII art representation 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 (the default), no
fingerprint strings are printed at login and only the fingerprint string
will be printed for unknown host keys.
- XAuthLocation
- Specifies the full pathname of the
xauth(1) program. The default is
/usr/bin/xauth.
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 declarations for any host in the
“.co.uk” set of domains, the following pattern 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 organization except from the
“dialup” pool, the following entry (in authorized_keys) could be
used:
from="!*.dialup.example.com,*.example.com"
Note that a negated match will never produce a positive result by itself. For
example, attempting to match “host3” against the following
pattern-list will fail:
from="!host1,!host2"
The solution here is to include a term that will yield a positive match, such as
a wildcard:
from="!host1,!host2,*"
Arguments to some keywords can make use of tokens, which are expanded at
runtime:
- %%
- A literal ‘%’.
- %C
- Hash of %l%h%p%r.
- %d
- Local user's home directory.
- %f
- The fingerprint of the server's host key.
- %H
- The known_hosts hostname or
address that is being searched for.
- %h
- The remote hostname.
- %I
- A string describing the reason for a
KnownHostsCommand execution: either
ADDRESS when looking up a host by address
(only when CheckHostIP is enabled),
HOSTNAME when searching by hostname, or
ORDER when preparing the host key algorithm
preference list to use for the destination host.
- %i
- The local user ID.
- %K
- The base64 encoded host key.
- %k
- The host key alias if specified, otherwise the original
remote hostname given on the command line.
- %L
- The local hostname.
- %l
- The local hostname, including the domain name.
- %n
- The original remote hostname, as given on the command
line.
- %p
- The remote port.
- %r
- The remote username.
- %T
- The local tun(4) or
tap(4) network interface assigned if tunnel
forwarding was requested, or “NONE” otherwise.
- %t
- The type of the server host key, e.g.
ssh-ed25519.
- %u
- The local username.
CertificateFile,
ControlPath,
IdentityAgent,
IdentityFile,
KnownHostsCommand,
LocalForward,
Match
exec,
RemoteCommand,
RemoteForward, and
UserKnownHostsFile accept the tokens %%, %C, %d,
%h, %i, %k, %L, %l, %n, %p, %r, and %u.
KnownHostsCommand additionally accepts the tokens
%f, %H, %I, %K and %t.
Hostname accepts the tokens %% and %h.
LocalCommand accepts all tokens.
ProxyCommand and
ProxyJump accept the tokens %%, %h, %n, %p, and
%r.
Arguments to some keywords can be expanded at runtime from environment variables
on the client by enclosing them in
${}, for
example
${HOME}/.ssh would refer to the user's
.ssh directory. If a specified environment variable does not exist then an
error will be returned and the setting for that keyword will be ignored.
The keywords
CertificateFile,
ControlPath,
IdentityAgent,
IdentityFile,
KnownHostsCommand, and
UserKnownHostsFile support environment variables.
The keywords
LocalForward and
RemoteForward support environment variables only
for Unix domain socket paths.
- ~/.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 writable by others. It may be
group-writable provided that the group in question 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.
ssh(1)
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen.
Aaron Campbell,
Bob Beck,
Markus Friedl,
Niels Provos,
Theo de Raadt and
Dug Song removed many bugs, re-added newer
features and created OpenSSH.
Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.