(1)
obtains configuration data from the following sources in
the following order:
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.
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
Sx 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
Sx 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
Sx 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
files).
- AddKeysToAgent
-
Specifies whether keys should be automatically added to a running
ssh-agent1.
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-add1.
If this option is set to
ask
ssh(1)
will require confirmation using the
SSH_ASKPASS
program before adding a key (see
ssh-add1
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-add1.
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
Sx TIME FORMATS
section of
sshd_config5
to specify the key's lifetime in
ssh-agent1,
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.
This option is useful in scripts and other batch jobs where no user
is present to interact with
ssh(1).
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.
- 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,
Qq *.a.example.com:*.b.example.com,*.c.example.com
will allow hostnames matching
Qq *.a.example.com
to be canonicalized to names in the
Qq *.b.example.com
or
Qq *.c.example.com
domains.
- CASignatureAlgorithms
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies which algorithms are allowed for signing of certificates
by certificate authorities (CAs).
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-agent1,
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
Sx TOKENS
section and environment variables as described in the
Sx 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.
- ChallengeResponseAuthentication
-
Specifies whether to use challenge-response authentication.
The argument to this keyword must be
yes
(the default)
or
no
- 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
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
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 built-in
openssh 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 built-in openssh 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
built-in openssh default set.
The supported ciphers are:
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com
The list of available ciphers may also be obtained using
Qq 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-askpass1.
If the
ControlPath
cannot be opened,
ssh(1)
will continue without connecting to a master instance.
X11 and
ssh-agent1
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
Sx TOKENS
section and environment variables as described in the
Sx 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
Qq ssh -O exit ) .
If set to a time in seconds, or a time in any of the formats documented in
sshd_config5,
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.
- EnableSSHKeysign
-
Setting this option to
yes
in the global client configuration file
/etc/ssh/ssh_config
enables the use of the helper program
ssh-keysign8
during
HostbasedAuthentication
The argument must be
yes
or
no
(the default).
This option should be placed in the non-hostspecific section.
See
ssh-keysign8
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).
- 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
Sx TIME FORMATS
section of
sshd_config5.
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
remote X11 clients will have full access to the original X11 display.
If this option is set to
no
(the 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 default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
The list of key exchange algorithms that are offered for GSSAPI
key exchange. Possible values are
gss-gex-sha1-,
gss-group1-sha1-,
gss-group14-sha1-,
gss-group14-sha256-,
gss-group16-sha512-,
gss-nistp256-sha256-,
gss-curve25519-sha256-
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-keygen1.
- 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:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256,ssh-rsa
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:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256,ssh-rsa
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
Qq 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
Sx 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
files
or passed on the
ssh(1)
command-line),
even if
ssh-agent1
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
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
Qq 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
Sx TOKENS
section and environment variables as described in the
Sx 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.
The default is
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ecdsa_sk
~/.ssh/id_ed25519
~/.ssh/id_ed25519_sk
and
~/.ssh/id_rsa
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
Sx 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.
The authentication identity can be also specified in a form of PKCS#11 URI
starting with a string
pkcs11:
There is supported a subset of the PKCS#11 URI as defined
in RFC 7512 (implemented path arguments
id
manufacturer
object
token
and query arguments
module-path
and
pin-value
). The URI can not be in quotes.
- 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
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
af21
(Low-Latency Data)
for interactive sessions and
cs1
(Lower Effort)
for non-interactive sessions.
- KbdInteractiveAuthentication
-
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be
yes
(the default)
or
no
- 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
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
If the specified list begins with a
`+'
character, then the specified methods will be appended to the built-in
openssh default set instead of replacing them.
If the specified list begins with a
`-'
character, then the specified methods (including wildcards) will be removed
from the built-in openssh default set instead of replacing them.
If the specified list begins with a
`^'
character, then the specified methods will be placed at the head of the
built-in openssh default set.
The list of available key exchange algorithms may also be obtained using
Qq 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
Sx VERIFYING HOST KEYS
section in
ssh(1)).
Arguments to
KnownHostsCommand
accept the tokens described in the
Sx 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
Sx 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
Sx TOKENS
section and environment variables as described in the
Sx 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:
kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
would enable detailed logging for line 1000 of
kex.c
everything in the
Fn 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 The default is handled system-wide by
-
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
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 built-in
openssh 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 built-in openssh 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
built-in openssh default set.
The algorithms that contain
Qq -etm
calculate the MAC after encryption (encrypt-then-mac).
These are considered safer and their use recommended.
The list of available MAC algorithms may also be obtained using
Qq 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:
- PermitRemoteOpen
-
host : port
- PermitRemoteOpen
-
IPv4_addr : port
- PermitRemoteOpen
-
[ IPv6_addr ] : port
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:
gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password
- 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
Sx 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:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
- 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
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
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 built-in
openssh default instead of replacing it.
If the specified list begins with a
`-'
character, then the specified algorithms (including wildcards) will be removed
from the built-in openssh 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
built-in openssh default set.
The list of available signature algorithms may also be obtained using
Qq ssh -Q PubkeyAcceptedAlgorithms .
- PubkeyAuthentication
-
Specifies whether to try public key authentication.
The argument to this keyword must be
yes
(the default)
or
no
- RekeyLimit
-
Specifies the maximum amount of data that may be transmitted 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_config5.
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
Sx 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
Sx TOKENS
section and environment variables as described in the
Sx 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_config5).
- 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).
- 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-keygen1.
For more information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen1.
- 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_config5
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
Sx 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.
- SetEnv
-
Directly specify one or more environment variables and their contents to
be sent to the server.
Similarly to
SendEnv
the server must be prepared to accept the environment variable.
- 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
known hosts files, 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.
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
Qq hostkeys@openssh.com
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
Sx TOKENS
section and environment variables as described in the
Sx ENVIRONMENT VARIABLES
section.
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
Sx 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
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
Qq .co.uk
set of domains,
the following pattern could be used:
The following pattern
would match any host in the 192.168.0.[0-9] network range:
Note that a negated match will never produce a positive result by itself.
For example, attempting to match
Qq host3
against the following pattern-list will fail:
The solution here is to include a term that will yield a positive match,
such as a wildcard:
Arguments to some keywords can make use of tokens,
which are expanded at runtime:
Arguments to some keywords can be expanded at runtime from environment
variables on the client by enclosing them in
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.
An -nosplit
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by
An Tatu Ylonen .
An Aaron Campbell , Bob Beck , Markus Friedl ,
An Niels Provos , Theo de Raadt
and
An Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
An Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.