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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: sshd_config.5,v 1.332 2021/07/02 05:11:21 dtucker Exp $
.Dd $Mdocdate: July 2 2021 $
.Dt SSHD_CONFIG 5
.Os
.Sh NAME
.Nm sshd_config
.Nd OpenSSH daemon configuration file
.Sh DESCRIPTION
.Xr sshd 8
reads configuration data from
.Pa /etc/ssh/sshd_config
(or the file specified with
.Fl f
on the command line).
The file contains keyword-argument pairs, one per line.
For each keyword, the first obtained value will be used.
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.
.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 AcceptEnv
Specifies what environment variables sent by the client will be copied into
the session's
.Xr environ 7 .
See
.Cm SendEnv
and
.Cm SetEnv
in
.Xr ssh_config 5
for how to configure the client.
The
.Ev TERM
environment variable is always accepted whenever the client
requests a pseudo-terminal as it is required by the protocol.
Variables are specified by name, which may contain the wildcard characters
.Ql *
and
.Ql \&? .
Multiple environment variables may be separated by whitespace or spread
across multiple
.Cm AcceptEnv
directives.
Be warned that some environment variables could be used to bypass restricted
user environments.
For this reason, care should be taken in the use of this directive.
The default is not to accept any environment variables.
.It Cm AddressFamily
Specifies which address family should be used by
.Xr sshd 8 .
Valid arguments are
.Cm any
(the default),
.Cm inet
(use IPv4 only), or
.Cm inet6
(use IPv6 only).
.It Cm AllowAgentForwarding
Specifies whether
.Xr ssh-agent 1
forwarding is permitted.
The default is
.Cm yes .
Note that disabling agent forwarding does not improve security
unless users are also denied shell access, as they can always install
their own forwarders.
.It Cm AllowGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
If specified, login is allowed only for users whose primary
group or supplementary group list matches one of the patterns.
Only group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.
The allow/deny groups directives are processed in the following order:
.Cm DenyGroups ,
.Cm AllowGroups .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
The available options are
.Cm yes
(the default)
or
.Cm all
to allow StreamLocal forwarding,
.Cm no
to prevent all StreamLocal forwarding,
.Cm local
to allow local (from the perspective of
.Xr ssh 1 )
forwarding only or
.Cm remote
to allow remote forwarding only.
Note that disabling StreamLocal forwarding does not improve security unless
users are also denied shell access, as they can always install their
own forwarders.
.It Cm AllowTcpForwarding
Specifies whether TCP forwarding is permitted.
The available options are
.Cm yes
(the default)
or
.Cm all
to allow TCP forwarding,
.Cm no
to prevent all TCP forwarding,
.Cm local
to allow local (from the perspective of
.Xr ssh 1 )
forwarding only or
.Cm remote
to allow remote forwarding only.
Note that disabling TCP forwarding does not improve security unless
users are also denied shell access, as they can always install their
own forwarders.
.It Cm AllowUsers
This keyword can be followed by a list of user name patterns, separated
by spaces.
If specified, login is allowed only for user names that
match one of the patterns.
Only user names are valid; a numerical user ID is not recognized.
By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
HOST criteria may additionally contain addresses to match in CIDR
address/masklen format.
The allow/deny users directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm AuthenticationMethods
Specifies the authentication methods that must be successfully completed
for a user to be granted access.
This option must be followed by one or more lists of comma-separated
authentication method names, or by the single string
.Cm any
to indicate the default behaviour of accepting any single authentication
method.
If the default is overridden, then successful authentication requires
completion of every method in at least one of these lists.
.Pp
For example,
.Qq publickey,password publickey,keyboard-interactive
would require the user to complete public key authentication, followed by
either password or keyboard interactive authentication.
Only methods that are next in one or more lists are offered at each stage,
so for this example it would not be possible to attempt password or
keyboard-interactive authentication before public key.
.Pp
For keyboard interactive authentication it is also possible to
restrict authentication to a specific device by appending a
colon followed by the device identifier
.Cm bsdauth
or
.Cm pam .
depending on the server configuration.
For example,
.Qq keyboard-interactive:bsdauth
would restrict keyboard interactive authentication to the
.Cm bsdauth
device.
.Pp
If the publickey method is listed more than once,
.Xr sshd 8
verifies that keys that have been used successfully are not reused for
subsequent authentications.
For example,
.Qq publickey,publickey
requires successful authentication using two different public keys.
.Pp
Note that each authentication method listed should also be explicitly enabled
in the configuration.
.Pp
The available authentication methods are:
.Qq gssapi-with-mic ,
.Qq hostbased ,
.Qq keyboard-interactive ,
.Qq none
(used for access to password-less accounts when
.Cm PermitEmptyPasswords
is enabled),
.Qq password
and
.Qq publickey .
.It Cm AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys.
The program must be owned by root, not writable by group or others and
specified by an absolute path.
Arguments to
.Cm AuthorizedKeysCommand
accept the tokens described in the
.Sx TOKENS
section.
If no arguments are specified then the username of the target user is used.
.Pp
The program should produce on standard output zero or
more lines of authorized_keys output (see
.Sx AUTHORIZED_KEYS
in
.Xr sshd 8 ) .
.Cm AuthorizedKeysCommand
is tried after the usual
.Cm AuthorizedKeysFile
files and will not be executed if a matching key is found there.
By default, no
.Cm AuthorizedKeysCommand
is run.
.It Cm AuthorizedKeysCommandUser
Specifies the user under whose account the
.Cm AuthorizedKeysCommand
is run.
It is recommended to use a dedicated user that has no other role on the host
than running authorized keys commands.
If
.Cm AuthorizedKeysCommand
is specified but
.Cm AuthorizedKeysCommandUser
is not, then
.Xr sshd 8
will refuse to start.
.It Cm AuthorizedKeysFile
Specifies the file that contains the public keys used for user authentication.
The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
.Xr sshd 8 .
Arguments to
.Cm AuthorizedKeysFile
accept the tokens described in the
.Sx TOKENS
section.
After expansion,
.Cm AuthorizedKeysFile
is taken to be an absolute path or one relative to the user's home
directory.
Multiple files may be listed, separated by whitespace.
Alternately this option may be set to
.Cm none
to skip checking for user keys in files.
The default is
.Qq .ssh/authorized_keys .ssh/authorized_keys2 .
.It Cm AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of allowed
certificate principals as per
.Cm AuthorizedPrincipalsFile .
The program must be owned by root, not writable by group or others and
specified by an absolute path.
Arguments to
.Cm AuthorizedPrincipalsCommand
accept the tokens described in the
.Sx TOKENS
section.
If no arguments are specified then the username of the target user is used.
.Pp
The program should produce on standard output zero or
more lines of
.Cm AuthorizedPrincipalsFile
output.
If either
.Cm AuthorizedPrincipalsCommand
or
.Cm AuthorizedPrincipalsFile
is specified, then certificates offered by the client for authentication
must contain a principal that is listed.
By default, no
.Cm AuthorizedPrincipalsCommand
is run.
.It Cm AuthorizedPrincipalsCommandUser
Specifies the user under whose account the
.Cm AuthorizedPrincipalsCommand
is run.
It is recommended to use a dedicated user that has no other role on the host
than running authorized principals commands.
If
.Cm AuthorizedPrincipalsCommand
is specified but
.Cm AuthorizedPrincipalsCommandUser
is not, then
.Xr sshd 8
will refuse to start.
.It Cm AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for
certificate authentication.
When using certificates signed by a key listed in
.Cm TrustedUserCAKeys ,
this file lists names, one of which must appear in the certificate for it
to be accepted for authentication.
Names are listed one per line preceded by key options (as described in
.Sx AUTHORIZED_KEYS FILE FORMAT
in
.Xr sshd 8 ) .
Empty lines and comments starting with
.Ql #
are ignored.
.Pp
Arguments to
.Cm AuthorizedPrincipalsFile
accept the tokens described in the
.Sx TOKENS
section.
After expansion,
.Cm AuthorizedPrincipalsFile
is taken to be an absolute path or one relative to the user's home directory.
The default is
.Cm none ,
i.e. not to use a principals file \(en in this case, the username
of the user must appear in a certificate's principals list for it to be
accepted.
.Pp
Note that
.Cm AuthorizedPrincipalsFile
is only used when authentication proceeds using a CA listed in
.Cm TrustedUserCAKeys
and is not consulted for certification authorities trusted via
.Pa ~/.ssh/authorized_keys ,
though the
.Cm principals=
key option offers a similar facility (see
.Xr sshd 8
for details).
.It Cm Banner
The contents of the specified file are sent to the remote user before
authentication is allowed.
If the argument is
.Cm none
then no banner is displayed.
By default, no banner is displayed.
.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
Certificates signed using other algorithms will not be accepted for
public key or host-based authentication.
.It Cm ChrootDirectory
Specifies the pathname of a directory to
.Xr chroot 2
to after authentication.
At session startup
.Xr sshd 8
checks that all components of the pathname are root-owned directories
which are not writable by any other user or group.
After the chroot,
.Xr sshd 8
changes the working directory to the user's home directory.
Arguments to
.Cm ChrootDirectory
accept the tokens described in the
.Sx TOKENS
section.
.Pp
The
.Cm ChrootDirectory
must contain the necessary files and directories to support the
user's session.
For an interactive session this requires at least a shell, typically
.Xr sh 1 ,
and basic
.Pa /dev
nodes such as
.Xr null 4 ,
.Xr zero 4 ,
.Xr stdin 4 ,
.Xr stdout 4 ,
.Xr stderr 4 ,
and
.Xr tty 4
devices.
For file transfer sessions using SFTP
no additional configuration of the environment is necessary if the in-process
sftp-server is used,
though sessions which use logging may require
.Pa /dev/log
inside the chroot directory on some operating systems (see
.Xr sftp-server 8
for details).
.Pp
For safety, it is very important that the directory hierarchy be
prevented from modification by other processes on the system (especially
those outside the jail).
Misconfiguration can lead to unsafe environments which
.Xr sshd 8
cannot detect.
.Pp
The default is
.Cm none ,
indicating not to
.Xr chroot 2 .
.It Cm Ciphers
Specifies the ciphers allowed.
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:
.Pp
.Bl -item -compact -offset indent
.It
3des-cbc
.It
aes128-cbc
.It
aes192-cbc
.It
aes256-cbc
.It
aes128-ctr
.It
aes192-ctr
.It
aes256-ctr
.It
.It
.It
.El
.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 ClientAliveCountMax
Sets the number of client alive messages which may be sent without
.Xr sshd 8
receiving any messages back from the client.
If this threshold is reached while client alive messages are being sent,
sshd will disconnect the client, terminating the session.
It is important to note that the use of client alive messages is very
different from
.Cm TCPKeepAlive .
The client alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
.Cm TCPKeepAlive
is spoofable.
The client alive mechanism is valuable when the client or
server depend on knowing when a connection has become unresponsive.
.Pp
The default value is 3.
If
.Cm ClientAliveInterval
is set to 15, and
.Cm ClientAliveCountMax
is left at the default, unresponsive SSH clients
will be disconnected after approximately 45 seconds.
Setting a zero
.Cm ClientAliveCountMax
disables connection termination.
.It Cm ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the client,
.Xr sshd 8
will send a message through the encrypted
channel to request a response from the client.
The default
is 0, indicating that these messages will not be sent to the client.
.It Cm Compression
Specifies whether compression is enabled after
the user has authenticated successfully.
The argument must be
.Cm yes ,
.Cm delayed
(a legacy synonym for
.Cm yes )
or
.Cm no .
The default is
.Cm yes .
.It Cm DenyGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
Login is disallowed for users whose primary group or supplementary
group list matches one of the patterns.
Only group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.
The allow/deny groups directives are processed in the following order:
.Cm DenyGroups ,
.Cm AllowGroups .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm DenyUsers
This keyword can be followed by a list of user name patterns, separated
by spaces.
Login is disallowed for user names that match one of the patterns.
Only user names are valid; a numerical user ID is not recognized.
By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
HOST criteria may additionally contain addresses to match in CIDR
address/masklen format.
The allow/deny users directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm DisableForwarding
Disables all forwarding features, including X11,
.Xr ssh-agent 1 ,
TCP and StreamLocal.
This option overrides all other forwarding-related options and may
simplify restricted configurations.
.It Cm ExposeAuthInfo
Writes a temporary file containing a list of authentication methods and
public credentials (e.g. keys) used to authenticate the user.
The location of the file is exposed to the user session through the
.Ev SSH_USER_AUTH
environment variable.
The default is
.Cm no .
.It Cm FingerprintHash
Specifies the hash algorithm used when logging key fingerprints.
Valid options are:
.Cm md5
and
.Cm sha256 .
The default is
.Cm sha256 .
.It Cm ForceCommand
Forces the execution of the command specified by
.Cm ForceCommand ,
ignoring any command supplied by the client and
.Pa ~/.ssh/rc
if present.
The command is invoked by using the user's login shell with the -c option.
This applies to shell, command, or subsystem execution.
It is most useful inside a
.Cm Match
block.
The command originally supplied by the client is available in the
.Ev SSH_ORIGINAL_COMMAND
environment variable.
Specifying a command of
.Cm internal-sftp
will force the use of an in-process SFTP server that requires no support
files when used with
.Cm ChrootDirectory .
The default is
.Cm none .
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client.
By default,
.Xr sshd 8
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that sshd
should allow remote port forwardings to bind to non-loopback addresses, thus
allowing other hosts to connect.
The argument may be
.Cm no
to force remote port forwardings to be available to the local host only,
.Cm yes
to force remote port forwardings to bind to the wildcard address, or
.Cm clientspecified
to allow the client to select the address to which the forwarding is bound.
The default is
.Cm no .
.It Cm GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is
.Cm no .
.It Cm GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache
on logout.
The default is
.Cm yes .
.It Cm GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor
a client authenticates against.
If set to
.Cm yes
then the client must authenticate against the host
service on the current hostname.
If set to
.Cm no
then the client may authenticate against any service key stored in the
machine's default store.
This facility is provided to assist with operation on multi homed machines.
The default is
.Cm yes .
.It Cm HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be accepted for hostbased
authentication as a list of comma-separated 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,ssh-rsa
.Ed
.Pp
The list of available signature algorithms may also be obtained using
.Qq ssh -Q HostbasedAcceptedAlgorithms .
This was formerly named HostbasedAcceptedKeyTypes.
.It Cm HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful public key client host authentication is allowed
(host-based authentication).
The default is
.Cm no .
.It Cm HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a reverse
name lookup when matching the name in the
.Pa ~/.shosts ,
.Pa ~/.rhosts ,
and
.Pa /etc/hosts.equiv
files during
.Cm HostbasedAuthentication .
A setting of
.Cm yes
means that
.Xr sshd 8
uses the name supplied by the client rather than
attempting to resolve the name from the TCP connection itself.
The default is
.Cm no .
.It Cm HostCertificate
Specifies a file containing a public host certificate.
The certificate's public key must match a private host key already specified
by
.Cm HostKey .
The default behaviour of
.Xr sshd 8
is not to load any certificates.
.It Cm HostKey
Specifies a file containing a private host key
used by SSH.
The defaults are
.Pa /etc/ssh/ssh_host_ecdsa_key ,
.Pa /etc/ssh/ssh_host_ed25519_key
and
.Pa /etc/ssh/ssh_host_rsa_key .
.Pp
Note that
.Xr sshd 8
will refuse to use a file if it is group/world-accessible
and that the
.Cm HostKeyAlgorithms
option restricts which of the keys are actually used by
.Xr sshd 8 .
.Pp
It is possible to have multiple host key files.
It is also possible to specify public host key files instead.
In this case operations on the private key will be delegated
to an
.Xr ssh-agent 1 .
.It Cm HostKeyAgent
Identifies the UNIX-domain socket used to communicate
with an agent that has access to the private host keys.
If the string
.Qq SSH_AUTH_SOCK
is specified, the location of the socket will be read from the
.Ev SSH_AUTH_SOCK
environment variable.
.It Cm HostKeyAlgorithms
Specifies the host key signature algorithms
that the server offers.
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,ssh-rsa
.Ed
.Pp
The list of available signature algorithms may also be obtained using
.Qq ssh -Q HostKeyAlgorithms .
.It Cm IgnoreRhosts
Specifies whether to ignore per-user
.Pa .rhosts
and
.Pa .shosts
files during
.Cm HostbasedAuthentication .
The system-wide
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv
are still used regardless of this setting.
.Pp
Accepted values are
.Cm yes
(the default) to ignore all per-user files,
.Cm shosts-only
to allow the use of
.Pa .shosts
but to ignore
.Pa .rhosts
or
.Cm no
to allow both
.Pa .shosts
and
.Pa rhosts .
.It Cm IgnoreUserKnownHosts
Specifies whether
.Xr sshd 8
should ignore the user's
.Pa ~/.ssh/known_hosts
during
.Cm HostbasedAuthentication
and use only the system-wide known hosts file
.Pa /etc/ssh/known_hosts .
The default is
.Dq no .
.It Cm Include
Include the specified configuration file(s).
Multiple pathnames may be specified and each pathname may contain
.Xr glob 7
wildcards that will be expanded and processed in lexical order.
Files without absolute paths are assumed to be in
.Pa /etc/ssh .
An
.Cm Include
directive may appear inside a
.Cm Match
block
to perform conditional inclusion.
.It Cm IPQoS
Specifies the IPv4 type-of-service or DSCP class for the connection.
Accepted values are
.Cm af11 ,
.Cm af12 ,
.Cm af13 ,
.Cm af21 ,
.Cm af22 ,
.Cm af23 ,
.Cm af31 ,
.Cm af32 ,
.Cm af33 ,
.Cm af41 ,
.Cm af42 ,
.Cm af43 ,
.Cm cs0 ,
.Cm cs1 ,
.Cm cs2 ,
.Cm cs3 ,
.Cm cs4 ,
.Cm cs5 ,
.Cm cs6 ,
.Cm cs7 ,
.Cm ef ,
.Cm le ,
.Cm lowdelay ,
.Cm throughput ,
.Cm reliability ,
a numeric value, or
.Cm 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
.Cm af21
(Low-Latency Data)
for interactive sessions and
.Cm cs1
(Lower Effort)
for non-interactive sessions.
.It Cm KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication.
All authentication styles from
.Xr login.conf 5
are supported.
The default is
.Cm yes .
The argument to this keyword must be
.Cm yes
or
.Cm no .
.Cm ChallengeResponseAuthentication
is a deprecated alias for this.
.It Cm KerberosAuthentication
Specifies whether the password provided by the user for
.Cm PasswordAuthentication
will be validated through the Kerberos KDC.
To use this option, the server needs a
Kerberos servtab which allows the verification of the KDC's identity.
The default is
.Cm no .
.It Cm KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
an AFS token before accessing the user's home directory.
The default is
.Cm no .
.It Cm KerberosOrLocalPasswd
If password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
such as
.Pa /etc/passwd .
The default is
.Cm yes .
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
The default is
.Cm yes .
.It Cm KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
Alternately if the specified list begins with a
.Sq +
character, then the specified methods will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
character, then the specified methods (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 methods will be placed at the head of the
default set.
The supported algorithms are:
.Pp
.Bl -item -compact -offset indent
.It
curve25519-sha256
.It
.It
diffie-hellman-group1-sha1
.It
diffie-hellman-group14-sha1
.It
diffie-hellman-group14-sha256
.It
diffie-hellman-group16-sha512
.It
diffie-hellman-group18-sha512
.It
diffie-hellman-group-exchange-sha1
.It
diffie-hellman-group-exchange-sha256
.It
ecdh-sha2-nistp256
.It
ecdh-sha2-nistp384
.It
ecdh-sha2-nistp521
.It
.El
.Pp
The default is:
.Bd -literal -offset indent
curve25519-sha256,[email protected],
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256
.Ed
.Pp
The list of available key exchange algorithms may also be obtained using
.Qq ssh -Q KexAlgorithms .
.It Cm ListenAddress
Specifies the local addresses
.Xr sshd 8
should listen on.
The following forms may be used:
.Pp
.Bl -item -offset indent -compact
.It
.Cm ListenAddress
.Sm off
.Ar hostname | address
.Sm on
.Op Cm rdomain Ar domain
.It
.Cm ListenAddress
.Sm off
.Ar hostname : port
.Sm on
.Op Cm rdomain Ar domain
.It
.Cm ListenAddress
.Sm off
.Ar IPv4_address : port
.Sm on
.Op Cm rdomain Ar domain
.It
.Cm ListenAddress
.Sm off
.Oo Ar hostname | address Oc : Ar port
.Sm on
.Op Cm rdomain Ar domain
.El
.Pp
The optional
.Cm rdomain
qualifier requests
.Xr sshd 8