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ssh_config.5
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.\"
.\" Author: Tatu Ylonen <[email protected]>
.\" Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
.\" All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose. Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $OpenBSD: ssh_config.5,v 1.380 2023/03/27 03:56:11 dtucker Exp $
.Dd $Mdocdate: March 27 2023 $
.Dt SSH_CONFIG 5
.Os
.Sh NAME
.Nm ssh_config
.Nd OpenSSH client configuration file
.Sh DESCRIPTION
.Xr ssh 1
obtains configuration data from the following sources in
the following order:
.Pp
.Bl -enum -offset indent -compact
.It
command-line options
.It
user's configuration file
.Pq Pa ~/.ssh/config
.It
system-wide configuration file
.Pq Pa /etc/ssh/ssh_config
.El
.Pp
Unless noted otherwise, for each parameter, the first obtained value
will be used.
The configuration files contain sections separated by
.Cm 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
.Cm CanonicalizeHostname
option for exceptions).
.Pp
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.
.Pp
The file contains keyword-argument pairs, one per line.
Lines starting with
.Ql #
and empty lines are interpreted as comments.
Arguments may optionally be enclosed in double quotes
.Pq \&"
in order to represent arguments containing spaces.
Configuration options may be separated by whitespace or
optional whitespace and exactly one
.Ql = ;
the latter format is useful to avoid the need to quote whitespace
when specifying configuration options using the
.Nm ssh ,
.Nm scp ,
and
.Nm sftp
.Fl o
option.
.Pp
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
.It Cm Host
Restricts the following declarations (up to the next
.Cm Host
or
.Cm 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
.Ql *
as a pattern can be used to provide global
defaults for all hosts.
The host is usually the
.Ar hostname
argument given on the command line
(see the
.Cm CanonicalizeHostname
keyword for exceptions).
.Pp
A pattern entry may be negated by prefixing it with an exclamation mark
.Pq Sq !\& .
If a negated entry is matched, then the
.Cm 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.
.Pp
See
.Sx PATTERNS
for more information on patterns.
.It Cm Match
Restricts the following declarations (up to the next
.Cm Host
or
.Cm Match
keyword) to be used only when the conditions following the
.Cm Match
keyword are satisfied.
Match conditions are specified using one or more criteria
or the single token
.Cm all
which always matches.
The available criteria keywords are:
.Cm canonical ,
.Cm final ,
.Cm exec ,
.Cm host ,
.Cm originalhost ,
.Cm user ,
and
.Cm localuser .
The
.Cm all
criteria must appear alone or immediately after
.Cm canonical
or
.Cm final .
Other criteria may be combined arbitrarily.
All criteria but
.Cm all ,
.Cm canonical ,
and
.Cm final
require an argument.
Criteria may be negated by prepending an exclamation mark
.Pq Sq !\& .
.Pp
The
.Cm canonical
keyword matches only when the configuration file is being re-parsed
after hostname canonicalization (see the
.Cm CanonicalizeHostname
option).
This may be useful to specify conditions that work with canonical host
names only.
.Pp
The
.Cm final
keyword requests that the configuration be re-parsed (regardless of whether
.Cm CanonicalizeHostname
is enabled), and matches only during this final pass.
If
.Cm CanonicalizeHostname
is enabled, then
.Cm canonical
and
.Cm final
match during the same pass.
.Pp
The
.Cm 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
.Cm exec
accept the tokens described in the
.Sx TOKENS
section.
.Pp
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
.Cm host
keyword are matched against the target hostname, after any substitution
by the
.Cm Hostname
or
.Cm CanonicalizeHostname
options.
The
.Cm originalhost
keyword matches against the hostname as it was specified on the command-line.
The
.Cm user
keyword matches against the target username on the remote host.
The
.Cm localuser
keyword matches against the name of the local user running
.Xr ssh 1
(this keyword may be useful in system-wide
.Nm
files).
.It Cm AddKeysToAgent
Specifies whether keys should be automatically added to a running
.Xr ssh-agent 1 .
If this option is set to
.Cm 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
.Xr ssh-add 1 .
If this option is set to
.Cm ask ,
.Xr ssh 1
will require confirmation using the
.Ev SSH_ASKPASS
program before adding a key (see
.Xr ssh-add 1
for details).
If this option is set to
.Cm confirm ,
each use of the key must be confirmed, as if the
.Fl c
option was specified to
.Xr ssh-add 1 .
If this option is set to
.Cm 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
.Xr sshd_config 5
to specify the key's lifetime in
.Xr ssh-agent 1 ,
after which it will automatically be removed.
The argument must be
.Cm no
(the default),
.Cm yes ,
.Cm confirm
(optionally followed by a time interval),
.Cm ask
or a time interval.
.It Cm AddressFamily
Specifies which address family to use when connecting.
Valid arguments are
.Cm any
(the default),
.Cm inet
(use IPv4 only), or
.Cm inet6
(use IPv6 only).
.It Cm BatchMode
If set to
.Cm 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
.Xr ssh 1 .
The argument must be
.Cm yes
or
.Cm no
(the default).
.It Cm 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.
.It Cm BindInterface
Use the address of the specified interface on the local machine as the
source address of the connection.
.It Cm CanonicalDomains
When
.Cm CanonicalizeHostname
is enabled, this option specifies the list of domain suffixes in which to
search for the specified destination host.
.It Cm CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canonicalization fails.
The default,
.Cm yes ,
will attempt to look up the unqualified hostname using the system resolver's
search rules.
A value of
.Cm no
will cause
.Xr ssh 1
to fail instantly if
.Cm CanonicalizeHostname
is enabled and the target hostname cannot be found in any of the domains
specified by
.Cm CanonicalDomains .
.It Cm CanonicalizeHostname
Controls whether explicit hostname canonicalization is performed.
The default,
.Cm no ,
is not to perform any name rewriting and let the system resolver handle all
hostname lookups.
If set to
.Cm yes
then, for connections that do not use a
.Cm ProxyCommand
or
.Cm ProxyJump ,
.Xr ssh 1
will attempt to canonicalize the hostname specified on the command line
using the
.Cm CanonicalDomains
suffixes and
.Cm CanonicalizePermittedCNAMEs
rules.
If
.Cm CanonicalizeHostname
is set to
.Cm always ,
then canonicalization is applied to proxied connections too.
.Pp
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
.Cm Host
and
.Cm Match
stanzas.
A value of
.Cm none
disables the use of a
.Cm ProxyJump
host.
.It Cm 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).
.It Cm CanonicalizePermittedCNAMEs
Specifies rules to determine whether CNAMEs should be followed when
canonicalizing hostnames.
The rules consist of one or more arguments of
.Ar source_domain_list : Ns Ar target_domain_list ,
where
.Ar source_domain_list
is a pattern-list of domains that may follow CNAMEs in canonicalization,
and
.Ar target_domain_list
is a pattern-list of domains that they may resolve to.
.Pp
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.
.Pp
A single argument of
.Qq none
causes no CNAMEs to be considered for canonicalization.
This is the default behaviour.
.It Cm CASignatureAlgorithms
Specifies which algorithms are allowed for signing of certificates
by certificate authorities (CAs).
The default is:
.Bd -literal -offset indent
ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
rsa-sha2-512,rsa-sha2-256
.Ed
.Pp
If the specified list begins with a
.Sq +
character, then the specified algorithms will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
character, then the specified algorithms (including wildcards) will be removed
from the default set instead of replacing them.
.Pp
.Xr ssh 1
will not accept host certificates signed using algorithms other than those
specified.
.It Cm 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
.Cm IdentityFile
directive or
.Fl i
flag to
.Xr ssh 1 ,
via
.Xr ssh-agent 1 ,
or via a
.Cm PKCS11Provider
or
.Cm SecurityKeyProvider .
.Pp
Arguments to
.Cm 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.
.Pp
It is possible to have multiple certificate files specified in
configuration files; these certificates will be tried in sequence.
Multiple
.Cm CertificateFile
directives will add to the list of certificates used for
authentication.
.It Cm CheckHostIP
If set to
.Cm yes ,
.Xr ssh 1
will additionally check the host IP address in the
.Pa 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
.Pa ~/.ssh/known_hosts
in the process, regardless of the setting of
.Cm StrictHostKeyChecking .
If the option is set to
.Cm no
(the default),
the check will not be executed.
.It Cm Ciphers
Specifies the ciphers allowed and their order of preference.
Multiple ciphers must be comma-separated.
If the specified list begins with a
.Sq +
character, then the specified ciphers will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
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
.Sq ^
character, then the specified ciphers will be placed at the head of the
default set.
.Pp
The supported ciphers are:
.Bd -literal -offset indent
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
.Ed
.Pp
The default is:
.Bd -literal -offset indent
aes128-ctr,aes192-ctr,aes256-ctr,
.Ed
.Pp
The list of available ciphers may also be obtained using
.Qq ssh -Q cipher .
.It Cm 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
.Xr ssh 1
command line to clear port forwardings set in
configuration files, and is automatically set by
.Xr scp 1
and
.Xr sftp 1 .
The argument must be
.Cm yes
or
.Cm no
(the default).
.It Cm Compression
Specifies whether to use compression.
The argument must be
.Cm yes
or
.Cm no
(the default).
.It Cm 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.
.It Cm 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.
.It Cm ControlMaster
Enables the sharing of multiple sessions over a single network connection.
When set to
.Cm yes ,
.Xr ssh 1
will listen for connections on a control socket specified using the
.Cm ControlPath
argument.
Additional sessions can connect to this socket using the same
.Cm ControlPath
with
.Cm ControlMaster
set to
.Cm 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.
.Pp
Setting this to
.Cm ask
will cause
.Xr ssh 1
to listen for control connections, but require confirmation using
.Xr ssh-askpass 1 .
If the
.Cm ControlPath
cannot be opened,
.Xr ssh 1
will continue without connecting to a master instance.
.Pp
X11 and
.Xr 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.
.Pp
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:
.Cm auto
and
.Cm autoask .
The latter requires confirmation like the
.Cm ask
option.
.It Cm ControlPath
Specify the path to the control socket used for connection sharing as described
in the
.Cm ControlMaster
section above or the string
.Cm none
to disable connection sharing.
Arguments to
.Cm 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
.Cm 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.
.It Cm ControlPersist
When used in conjunction with
.Cm 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
.Cm 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
.Cm 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
.Xr sshd_config 5 ,
then the backgrounded master connection will automatically terminate
after it has remained idle (with no client connections) for the
specified time.
.It Cm 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.
.Pp
The argument must be
.Sm off
.Oo Ar bind_address : Oc Ar port .
.Sm on
IPv6 addresses can be specified by enclosing addresses in square brackets.
By default, the local port is bound in accordance with the
.Cm GatewayPorts
setting.
However, an explicit
.Ar bind_address
may be used to bind the connection to a specific address.
The
.Ar bind_address
of
.Cm localhost
indicates that the listening port be bound for local use only, while an
empty address or
.Sq *
indicates that the port should be available from all interfaces.
.Pp
Currently the SOCKS4 and SOCKS5 protocols are supported, and
.Xr 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.
.It Cm EnableEscapeCommandline
Enables the command line option in the
.Cm EscapeChar
menu for interactive sessions (default
.Ql ~C ) .
By default, the command line is disabled.
.It Cm EnableSSHKeysign
Setting this option to
.Cm yes
in the global client configuration file
.Pa /etc/ssh/ssh_config
enables the use of the helper program
.Xr ssh-keysign 8
during
.Cm HostbasedAuthentication .
The argument must be
.Cm yes
or
.Cm no
(the default).
This option should be placed in the non-hostspecific section.
See
.Xr ssh-keysign 8
for more information.
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
The escape character can also
be set on the command line.
The argument should be a single character,
.Ql ^
followed by a letter, or
.Cm none
to disable the escape
character entirely (making the connection transparent for binary
data).
.It Cm ExitOnForwardFailure
Specifies whether
.Xr 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
.Cm ExitOnForwardFailure
does not apply to connections made over port forwardings and will not,
for example, cause
.Xr ssh 1
to exit if TCP connections to the ultimate forwarding destination fail.
The argument must be
.Cm yes
or
.Cm no
(the default).
.It Cm FingerprintHash
Specifies the hash algorithm used when displaying key fingerprints.
Valid options are:
.Cm md5
and
.Cm sha256
(the default).
.It Cm ForkAfterAuthentication
Requests
.Nm ssh
to go to background just before command execution.
This is useful if
.Nm ssh
is going to ask for passwords or passphrases, but the user
wants it in the background.
This implies the
.Cm StdinNull
configuration option being set to
.Dq yes .
The recommended way to start X11 programs at a remote site is with
something like
.Ic ssh -f host xterm ,
which is the same as
.Ic ssh host xterm
if the
.Cm ForkAfterAuthentication
configuration option is set to
.Dq yes .
.Pp
If the
.Cm ExitOnForwardFailure
configuration option is set to
.Dq yes ,
then a client started with the
.Cm ForkAfterAuthentication
configuration option being set to
.Dq 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
.Cm yes
(same as the
.Fl f
option) or
.Cm no
(the default).
.It Cm ForwardAgent
Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine.
The argument may be
.Cm yes ,
.Cm no
(the default),
an explicit path to an agent socket or the name of an environment variable
(beginning with
.Sq $ )
in which to find the path.
.Pp
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.
.It Cm ForwardX11
Specifies whether X11 connections will be automatically redirected
over the secure channel and
.Ev DISPLAY
set.
The argument must be
.Cm yes
or
.Cm no
(the default).
.Pp
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
.Cm ForwardX11Trusted
option is also enabled.
.It Cm ForwardX11Timeout
Specify a timeout for untrusted X11 forwarding
using the format described in the
.Sx TIME FORMATS
section of
.Xr sshd_config 5 .
X11 connections received by
.Xr ssh 1
after this time will be refused.
Setting
.Cm 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.
.It Cm ForwardX11Trusted
If this option is set to
.Cm yes ,
remote X11 clients will have full access to the original X11 display.
.Pp
If this option is set to
.Cm 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
.Xr xauth 1
token used for the session will be set to expire after 20 minutes.
Remote clients will be refused access after this time.
.Pp
See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports.
By default,
.Xr ssh 1
binds local port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm 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
.Cm yes
or
.Cm no
(the default).
.It Cm GlobalKnownHostsFile
Specifies one or more files to use for the global
host key database, separated by whitespace.
The default is
.Pa /etc/ssh/ssh_known_hosts ,
.Pa /etc/ssh/ssh_known_hosts2 .
.It Cm GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is
.Cm no .
.It Cm GSSAPIDelegateCredentials
Forward (delegate) credentials to the server.
The default is
.Cm no .
.It Cm HashKnownHosts
Indicates that
.Xr ssh 1
should hash host names and addresses when they are added to
.Pa ~/.ssh/known_hosts .
These hashed names may be used normally by
.Xr ssh 1
and
.Xr sshd 8 ,
but they do not visually reveal identifying information if the
file's contents are disclosed.
The default is
.Cm no .
Note that existing names and addresses in known hosts files
will not be converted automatically,
but may be manually hashed using
.Xr ssh-keygen 1 .
.It Cm 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
.Sq +
character, then the specified signature algorithms will be appended
to the default set instead of replacing them.
If the specified list begins with a
.Sq -
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
.Sq ^
character, then the specified signature algorithms will be placed
at the head of the default set.
The default for this option is:
.Bd -literal -offset 3n
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
rsa-sha2-512,rsa-sha2-256
.Ed
.Pp
The
.Fl Q
option of
.Xr ssh 1
may be used to list supported signature algorithms.
This was formerly named HostbasedKeyTypes.
.It Cm HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key
authentication.
The argument must be
.Cm yes
or
.Cm no
(the default).
.It Cm 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
.Sq +
character, then the specified signature algorithms will be appended to
the default set instead of replacing them.
If the specified list begins with a
.Sq -
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
.Sq ^
character, then the specified signature algorithms will be placed
at the head of the default set.
The default for this option is:
.Bd -literal -offset 3n
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
rsa-sha2-512,rsa-sha2-256
.Ed
.Pp
If hostkeys are known for the destination host then this default is modified
to prefer their algorithms.
.Pp
The list of available signature algorithms may also be obtained using
.Qq ssh -Q HostKeyAlgorithms .
.It Cm 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.
.It Cm Hostname
Specifies the real host name to log into.
This can be used to specify nicknames or abbreviations for hosts.
Arguments to
.Cm Hostname
accept the tokens described in the
.Sx TOKENS
section.
Numeric IP addresses are also permitted (both on the command line and in
.Cm Hostname
specifications).
The default is the name given on the command line.
.It Cm IdentitiesOnly
Specifies that
.Xr ssh 1
should only use the configured authentication identity and certificate files
(either the default files, or those explicitly configured in the
.Nm
files
or passed on the
.Xr ssh 1
command-line),
even if
.Xr ssh-agent 1
or a
.Cm PKCS11Provider
or
.Cm SecurityKeyProvider
offers more identities.
The argument to this keyword must be
.Cm yes
or
.Cm no
(the default).
This option is intended for situations where ssh-agent
offers many different identities.
.It Cm IdentityAgent
Specifies the
.Ux Ns -domain
socket used to communicate with the authentication agent.
.Pp
This option overrides the
.Ev SSH_AUTH_SOCK
environment variable and can be used to select a specific agent.
Setting the socket name to
.Cm none
disables the use of an authentication agent.
If the string
.Qq SSH_AUTH_SOCK