illumos: manual page: sshd

SSHD_CONFIG(5) File Formats and Configurations SSHD_CONFIG(5)

NAME

sshd_config - OpenSSH daemon configuration file

DESCRIPTION

sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-argument
pairs, one per line. Unless noted otherwise, for each keyword, the first
obtained value will be used. 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.

The possible keywords and their meanings are as follows (note that keywords
are case-insensitive and arguments are case-sensitive):

AcceptEnv
Specifies what environment variables sent by the client will be
copied into the session's environ(7). See SendEnv and SetEnv in
ssh_config(5) for how to configure the client. The 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 `*' and `?'. Multiple environment variables may be
separated by whitespace or spread across multiple 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 to
accept only LANG and the LC_* family of environment variables. If
you wish to remove one of these default variables, you may either
use the argument "none" first to remove all of them, or prefix
specific variable names with a "-" character (e.g. "-LANG" ).
Otherwise any specified directives will add to this default set.

AddressFamily
Specifies which address family should be used by sshd(8). Valid
arguments are any (the default), inet (use IPv4 only), or inet6
(use IPv6 only).

AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted. The
default is 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.

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:
DenyGroups, AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.

AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is
permitted. The available options are yes (the default) or all to
allow StreamLocal forwarding, no to prevent all StreamLocal
forwarding, local to allow local (from the perspective of ssh(1))
forwarding only or 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.

AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available
options are yes (the default) or all to allow TCP forwarding, no to
prevent all TCP forwarding, local to allow local (from the
perspective of ssh(1)) forwarding only or 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.

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:
DenyUsers, AllowUsers.

See PATTERNS in ssh_config(5) for more information on patterns.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.

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 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.

For example, "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.

For keyboard interactive authentication it is also possible to
restrict authentication to a specific device by appending a colon
followed by the device identifier bsdauth or pam. depending on the
server configuration. For example, "keyboard-interactive:bsdauth"
would restrict keyboard interactive authentication to the bsdauth
device.

If the publickey method is listed more than once, sshd(8) verifies
that keys that have been used successfully are not reused for
subsequent authentications. For example, "publickey,publickey"
requires successful authentication using two different public keys.

Note that each authentication method listed should also be
explicitly enabled in the configuration.

The available authentication methods are: "gssapi-with-mic",
"hostbased", "keyboard-interactive", "none" (used for access to
password-less accounts when PermitEmptyPasswords is enabled),
"password" and "publickey".

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
AuthorizedKeysCommand accept the tokens described in the TOKENS
section. If no arguments are specified then the username of the
target user is used.

The program should produce on standard output zero or more lines of
authorized_keys output (see AUTHORIZED_KEYS in sshd(8)).
AuthorizedKeysCommand is tried after the usual AuthorizedKeysFile
files and will not be executed if a matching key is found there.
By default, no AuthorizedKeysCommand is run.

AuthorizedKeysCommandUser
Specifies the user under whose account the 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
AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser is
not, then sshd(8) will refuse to start.

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 sshd(8). Arguments to AuthorizedKeysFile
accept the tokens described in the TOKENS section. After
expansion, 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 none to skip checking for user keys in files. The default
is ".ssh/authorized_keys .ssh/authorized_keys2".

AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of allowed
certificate principals as per AuthorizedPrincipalsFile. The
program must be owned by root, not writable by group or others and
specified by an absolute path. Arguments to
AuthorizedPrincipalsCommand accept the tokens described in the
TOKENS section. If no arguments are specified then the username of
the target user is used.

The program should produce on standard output zero or more lines of
AuthorizedPrincipalsFile output. If either
AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is
specified, then certificates offered by the client for
authentication must contain a principal that is listed. By
default, no AuthorizedPrincipalsCommand is run.

AuthorizedPrincipalsCommandUser
Specifies the user under whose account the
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 AuthorizedPrincipalsCommand is
specified but AuthorizedPrincipalsCommandUser is not, then sshd(8)
will refuse to start.

AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for
certificate authentication. When using certificates signed by a
key listed in 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 AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
Empty lines and comments starting with `#' are ignored.

Arguments to AuthorizedPrincipalsFile accept the tokens described
in the TOKENS section. After expansion, AuthorizedPrincipalsFile
is taken to be an absolute path or one relative to the user's home
directory. The default is none, i.e. not to use a principals file
- in this case, the username of the user must appear in a
certificate's principals list for it to be accepted.

Note that AuthorizedPrincipalsFile is only used when authentication
proceeds using a CA listed in TrustedUserCAKeys and is not
consulted for certification authorities trusted via
~/.ssh/authorized_keys, though the principals= key option offers a
similar facility (see sshd(8) for details).

Banner The contents of the specified file are sent to the remote user
before authentication is allowed. If the argument is none then no
banner is displayed. By default, no banner is displayed.

CASignatureAlgorithms
Specifies which algorithms are allowed for signing of certificates
by certificate authorities (CAs). The default is:

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

If the specified list begins with a `+' character, then the
specified algorithms will be appended to the default set instead of
replacing them. If the specified list begins with a `-' character,
then the specified algorithms (including wildcards) will be removed
from the default set instead of replacing them.

Certificates signed using other algorithms will not be accepted for
public key or host-based authentication.

ChannelTimeout
Specifies whether and how quickly sshd(8) should close inactive
channels. Timeouts are specified as one or more "type=interval"
pairs separated by whitespace, where the "type" must be a channel
type name (as described in the table below), optionally containing
wildcard characters.

The timeout value "interval" is specified in seconds or may use any
of the units documented in the TIME FORMATS section. For example,
"session:*=5m" would cause all sessions to terminate after five
minutes of inactivity. Specifying a zero value disables the
inactivity timeout.

The available channel types include:

agent-connection
Open connections to ssh-agent(1).

direct-tcpip, direct-streamlocal@openssh.com
Open TCP or Unix socket (respectively) connections that
have been established from a ssh(1) local forwarding, i.e.
LocalForward or DynamicForward.

forwarded-tcpip, forwarded-streamlocal@openssh.com
Open TCP or Unix socket (respectively) connections that
have been established to a sshd(8) listening on behalf of a
ssh(1) remote forwarding, i.e. RemoteForward.

session:command
Command execution sessions.

session:shell
Interactive shell sessions.

session:subsystem:...
Subsystem sessions, e.g. for sftp(1), which could be
identified as session:subsystem:sftp.

x11-connection
Open X11 forwarding sessions.

Note that in all the above cases, terminating an inactive session
does not guarantee to remove all resources associated with the
session, e.g. shell processes or X11 clients relating to the
session may continue to execute.

Moreover, terminating an inactive channel or session does not
necessarily close the SSH connection, nor does it prevent a client
from requesting another channel of the same type. In particular,
expiring an inactive forwarding session does not prevent another
identical forwarding from being subsequently created. See also
UnusedConnectionTimeout, which may be used in conjunction with this
option.

The default is not to expire channels of any type for inactivity.

ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after
authentication. At session startup 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, sshd(8)
changes the working directory to the user's home directory.
Arguments to ChrootDirectory accept the tokens described in the
TOKENS section.

The 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 sh(1), and basic
/dev nodes such as null(4), zero(4), stdin(4), stdout(4),
stderr(4), and 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 /dev/log inside the chroot directory on some
operating systems (see sftp-server(8) for details).

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 sshd(8) cannot detect.

The default is none, indicating not to chroot(2).

Ciphers
Specifies the ciphers allowed. Multiple ciphers must be comma-
separated. If the specified list begins with a `+' character, then
the specified ciphers will be appended to the default set instead
of replacing them. If the specified list begins with a `-'
character, then the specified ciphers (including wildcards) will be
removed from the default set instead of replacing them. If the
specified list begins with a `^' character, then the specified
ciphers will be placed at the head of the default set.

The supported ciphers are:

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 default is:

chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com

The list of available ciphers may also be obtained using "ssh -Q
cipher".

ClientAliveCountMax
Sets the number of client alive messages which may be sent without
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 TCPKeepAlive. The client alive messages are sent
through the encrypted channel and therefore will not be spoofable.
The TCP keepalive option enabled by TCPKeepAlive is spoofable. The
client alive mechanism is valuable when the client or server depend
on knowing when a connection has become unresponsive.

The default value is 3. If ClientAliveInterval is set to 15, and
ClientAliveCountMax is left at the default, unresponsive SSH
clients will be disconnected after approximately 45 seconds.
Setting a zero ClientAliveCountMax disables connection termination.

ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been
received from the client, 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.

Compression
Specifies whether compression is enabled after the user has
authenticated successfully. The argument must be yes, delayed (a
legacy synonym for yes) or no. The default is yes.

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: DenyGroups,
AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.

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: DenyUsers,
AllowUsers.

See PATTERNS in ssh_config(5) for more information on patterns.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.

DisableForwarding
Disables all forwarding features, including X11, ssh-agent(1), TCP
and StreamLocal. This option overrides all other forwarding-
related options and may simplify restricted configurations.

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
SSH_USER_AUTH environment variable. The default is no.

FingerprintHash
Specifies the hash algorithm used when logging key fingerprints.
Valid options are: md5 and sha256. The default is sha256.

ForceCommand
Forces the execution of the command specified by ForceCommand,
ignoring any command supplied by the client and ~/.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 Match block. The command
originally supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment variable. Specifying a command of
internal-sftp will force the use of an in-process SFTP server that
requires no support files when used with ChrootDirectory. The
default is none.

GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client. By default, sshd(8) binds remote port
forwardings to the loopback address. This prevents other remote
hosts from connecting to forwarded ports. 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 no to force remote port forwardings to be
available to the local host only, yes to force remote port
forwardings to bind to the wildcard address, or clientspecified to
allow the client to select the address to which the forwarding is
bound. The default is no.

GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default on Solaris is yes.

GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials
cache on logout. The default is yes.

GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI
acceptor a client authenticates against. If set to yes then the
client must authenticate against the host service on the current
hostname. If set to 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 yes.

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 `+' 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-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

The list of available signature algorithms may also be obtained
using "ssh -Q HostbasedAcceptedAlgorithms". This was formerly
named HostbasedAcceptedKeyTypes.

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 no.

HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a
reverse name lookup when matching the name in the ~/.shosts,
~/.rhosts, and /etc/hosts.equiv files during
HostbasedAuthentication. A setting of yes means that sshd(8) uses
the name supplied by the client rather than attempting to resolve
the name from the TCP connection itself. The default is no.

HostCertificate
Specifies a file containing a public host certificate. The
certificate's public key must match a private host key already
specified by HostKey. The default behaviour of sshd(8) is not to
load any certificates.

HostKey
Specifies a file containing a private host key used by SSH. The
defaults are /var/ssh/ssh_host_ecdsa_key,
/var/ssh/ssh_host_ed25519_key and /var/ssh/ssh_host_rsa_key.

Note that sshd(8) will refuse to use a file if it is group/world-
accessible and that the HostKeyAlgorithms option restricts which of
the keys are actually used by sshd(8).

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 ssh-agent(1).

HostKeyAgent
Identifies the UNIX-domain socket used to communicate with an agent
that has access to the private host keys. If the string
"SSH_AUTH_SOCK" is specified, the location of the socket will be
read from the SSH_AUTH_SOCK environment variable.

HostKeyAlgorithms
Specifies the host key signature algorithms that the server offers.
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-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

The list of available signature algorithms may also be obtained
using "ssh -Q HostKeyAlgorithms".

IgnoreRhosts
Specifies whether to ignore per-user .rhosts and .shosts files
during HostbasedAuthentication. The system-wide /etc/hosts.equiv
and /etc/ssh/shosts.equiv are still used regardless of this
setting.

Accepted values are yes (the default) to ignore all per-user files,
shosts-only to allow the use of .shosts but to ignore .rhosts or no
to allow both .shosts and rhosts.

IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during HostbasedAuthentication and use only the
system-wide known hosts file /etc/ssh/ssh_known_hosts. The default
is "no".

Include
Include the specified configuration file(s). Multiple pathnames
may be specified and each pathname may contain glob(7) wildcards
that will be expanded and processed in lexical order. Files
without absolute paths are assumed to be in /etc/ssh. An Include
directive may appear inside a Match block to perform conditional
inclusion.

IPQoS Specifies the IPv4 type-of-service or DSCP class for the
connection. 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 allow keyboard-interactive authentication.
All authentication styles from login.conf(5) are supported. The
default is yes. The argument to this keyword must be yes or no.
ChallengeResponseAuthentication is a deprecated alias for this.

KerberosAuthentication
Specifies whether the password provided by the user for
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 no.

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 no.

KerberosOrLocalPasswd
If password authentication through Kerberos fails then the password
will be validated via any additional local mechanism such as
/etc/passwd. The default is yes.

KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout. The default is yes.

KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple
algorithms must be comma-separated. Alternately if the specified
list begins with a `+' character, then the specified algorithms
will be appended to the default set instead of replacing them. If
the specified list begins with a `-' character, then the specified
algorithms (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with a
`^' character, then the specified algorithms will be placed at the
head of the default set. The supported algorithms are:

curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
sntrup761x25519-sha512@openssh.com

The default is:

sntrup761x25519-sha512@openssh.com,
curve25519-sha256,curve25519-sha256@libssh.org,
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

The list of available key exchange algorithms may also be obtained
using "ssh -Q KexAlgorithms".

ListenAddress
Specifies the local addresses sshd(8) should listen on. The
following forms may be used:

ListenAddress hostname|address [rdomain domain]
ListenAddress hostname:port [rdomain domain]
ListenAddress IPv4_address:port [rdomain domain]
ListenAddress [hostname|address]:port [rdomain domain]

The optional rdomain qualifier requests sshd(8) listen in an
explicit routing domain. If port is not specified, sshd will
listen on the address and all Port options specified. The default
is to listen on all local addresses on the current default routing
domain. Multiple ListenAddress options are permitted. For more
information on routing domains, see rdomain(4).

LoginGraceTime
The server disconnects after this time if the user has not
successfully logged in. If the value is 0, there is no time limit.
The default is 120 seconds.

LogLevel
Gives the verbosity level that is used when logging messages from
sshd(8). 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 debugging output. Logging with a DEBUG level
violates the privacy of users and is not recommended.

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 kex_exchange_identification() function, and all code in the
packet.c file. This option is intended for debugging and no
overrides are enabled by default.

MACs Specifies the available MAC (message authentication code)
algorithms. The MAC algorithm is used for data integrity
protection. Multiple algorithms must be comma-separated. If the
specified list begins with a `+' character, then the specified
algorithms will be appended to the default set instead of replacing
them. If the specified list begins with a `-' character, then the
specified algorithms (including wildcards) will be removed from the
default set instead of replacing them. If the specified list
begins with a `^' character, then the specified algorithms will be
placed at the head of the default set.

The algorithms that contain "-etm" calculate the MAC after
encryption (encrypt-then-mac). These are considered safer and
their use recommended. The supported MACs are:

hmac-md5
hmac-md5-96
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
umac-64@openssh.com
umac-128@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com

The default is:

umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
hmac-sha1-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1

The list of available MAC algorithms may also be obtained using
"ssh -Q mac".

Match Introduces a conditional block. If all of the criteria on the
Match line are satisfied, the keywords on the following lines
override those set in the global section of the config file, until
either another Match line or the end of the file. If a keyword
appears in multiple Match blocks that are satisfied, only the first
instance of the keyword is applied.

The arguments to Match are one or more criteria-pattern pairs or
the single token All which matches all criteria. The available
criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
and Address (with RDomain representing the rdomain(4) on which the
connection was received).

The match patterns may consist of single entries or comma-separated
lists and may use the wildcard and negation operators described in
the PATTERNS section of ssh_config(5).

The patterns in an Address criteria may additionally contain
addresses to match in CIDR address/masklen format, such as
192.0.2.0/24 or 2001:db8::/32. Note that the mask length provided
must be consistent with the address - it is an error to specify a
mask length that is too long for the address or one with bits set
in this host portion of the address. For example, 192.0.2.0/33 and
192.0.2.0/8, respectively.

Only a subset of keywords may be used on the lines following a
Match keyword. Available keywords are AcceptEnv,
AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
AllowTcpForwarding, AllowUsers, AuthenticationMethods,
AuthorizedKeysCommand, AuthorizedKeysCommandUser,
AuthorizedKeysFile, AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile, Banner,
CASignatureAlgorithms, ChannelTimeout, ChrootDirectory,
ClientAliveCountMax, ClientAliveInterval, DenyGroups, DenyUsers,
DisableForwarding, ExposeAuthInfo, ForceCommand, GatewayPorts,
GSSAPIAuthentication, HostbasedAcceptedAlgorithms,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
IgnoreRhosts, Include, IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, LogLevel, MaxAuthTries, MaxSessions,
PasswordAuthentication, PermitEmptyPasswords, PermitListen,
PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel, PermitUserRC,
PubkeyAcceptedAlgorithms, PubkeyAuthentication, PubkeyAuthOptions,
RekeyLimit, RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
StreamLocalBindUnlink, TrustedUserCAKeys, UnusedConnectionTimeout,
X11DisplayOffset, X11Forwarding and X11UseLocalhost.

MaxAuthTries
Specifies the maximum number of authentication attempts permitted
per connection. Once the number of failures reaches half this
value, additional failures are logged. The default is 6, or the
value given by "RETRIES=" in the file "/etc/default/login", if
available (see login(1) ).

MaxSessions
Specifies the maximum number of open shell, login or subsystem
(e.g. sftp) sessions permitted per network connection. Multiple
sessions may be established by clients that support connection
multiplexing. Setting MaxSessions to 1 will effectively disable
session multiplexing, whereas setting it to 0 will prevent all
shell, login and subsystem sessions while still permitting
forwarding. The default is 10.

MaxStartups
Specifies the maximum number of concurrent unauthenticated
connections to the SSH daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime expires
for a connection. The default is 10:30:100.

Alternatively, random early drop can be enabled by specifying the
three colon separated values start:rate:full (e.g. "10:30:60").
sshd(8) will refuse connection attempts with a probability of
rate/100 (30%) if there are currently start (10) unauthenticated
connections. The probability increases linearly and all connection
attempts are refused if the number of unauthenticated connections
reaches full (60).

ModuliFile
Specifies the moduli(5) file that contains the Diffie-Hellman
groups used for the "diffie-hellman-group-exchange-sha1" and
"diffie-hellman-group-exchange-sha256" key exchange methods. The
default is /usr/share/lib/ssh/moduli.

PAMServiceName
Specifies the PAM service name for the PAM session. The
PAMServiceName and PAMServicePrefix options are mutually exclusive
and if both set, sshd does not start. If this option is set the
service name is the same for all user authentication methods. The
option has no default value. See PAMServicePrefix for more
information.

PAMServicePrefix
Specifies the PAM service name prefix for service names used for
individual user authentication methods. The default is sshd. The
PAMServiceName and PAMServicePrefix options are mutually exclusive
and if both set, sshd does not start.

For example, if this option is set to admincli, the service name
for the keyboard-interactive authentication method is admincli-
kbdint instead of the default sshd-kbdint.

PasswordAuthentication
Specifies whether password authentication is allowed. The default
is yes.

PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings. The
default is no. unless "PASSREQ=YES" is present in
"/etc/default/login" (see login(1) ).

PermitListen
Specifies the addresses/ports on which a remote TCP port forwarding
may listen. The listen specification must be one of the following
forms:

PermitListen port
PermitListen host:port

Multiple permissions may be specified by separating them with
whitespace. An argument of any can be used to remove all
restrictions and permit any listen requests. An argument of none
can be used to prohibit all listen requests. The host name may
contain wildcards as described in the PATTERNS section in
ssh_config(5). The wildcard `*' can also be used in place of a
port number to allow all ports. By default all port forwarding
listen requests are permitted. Note that the GatewayPorts option
may further restrict which addresses may be listened on. Note also
that ssh(1) will request a listen host of "localhost" if no listen
host was specifically requested, and this name is treated
differently to explicit localhost addresses of "127.0.0.1" and
"::1".

PermitOpen
Specifies the destinations to which TCP port forwarding is
permitted. The forwarding specification must be one of the
following forms:

PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [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. By default all port forwarding
requests are permitted.

PermitRootLogin
Specifies whether root can log in using ssh(1). The argument must
be yes, prohibit-password, forced-commands-only, or no. The
default is prohibit-password.

If this option is set to prohibit-password (or its deprecated
alias, without-password), password and keyboard-interactive
authentication are disabled for root.

If this option is set to forced-commands-only, root login with
public key authentication will be allowed, but only if the command
option has been specified (which may be useful for taking remote
backups even if root login is normally not allowed). All other
authentication methods are disabled for root.

If this option is set to no, root is not allowed to log in.

PermitTTY
Specifies whether pty(4) allocation is permitted. The default is
yes.

PermitTunnel
Specifies whether tun(4) device forwarding is allowed. The
argument must be yes, point-to-point (layer 3), ethernet (layer 2),
or no. Specifying yes permits both point-to-point and ethernet.
The default is no.

Independent of this setting, the permissions of the selected tun(4)
device must allow access to the user.

PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in
~/.ssh/authorized_keys are processed by sshd(8). Valid options are
yes, no or a pattern-list specifying which environment variable
names to accept (for example "LANG,LC_*"). The default is no.
Enabling environment processing may enable users to bypass access
restrictions in some configurations using mechanisms such as
LD_PRELOAD.

PermitUserRC
Specifies whether any ~/.ssh/rc file is executed. The default is
yes.

PerSourceMaxStartups
Specifies the number of unauthenticated connections allowed from a
given source address, or "none" if there is no limit. This limit
is applied in addition to MaxStartups, whichever is lower. The
default is none.

PerSourceNetBlockSize
Specifies the number of bits of source address that are grouped
together for the purposes of applying PerSourceMaxStartups limits.
Values for IPv4 and optionally IPv6 may be specified, separated by
a colon. The default is 32:128, which means each address is
considered individually.

PidFile
Specifies the file that contains the process ID of the SSH daemon,
or none to not write one. The default is /var/run/sshd.pid.

Port Specifies the port number that sshd(8) listens on. The default is
22. Multiple options of this type are permitted. See also
ListenAddress.

PrintLastLog
Specifies whether sshd(8) should print the date and time of the
last user login when a user logs in interactively. On Solaris this
option is always ignored since pam_unix_session(5) reports the last
login time.

PrintMotd
Specifies whether sshd(8) should print /etc/motd when a user logs
in interactively. (On some systems it is also printed by the
shell, /etc/profile, or equivalent.) The default is yes.

PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be accepted for public
key authentication as a list of comma-separated patterns.
Alternately if the specified list begins with a `+' character, then
the specified algorithms will be appended to the default set
instead of replacing them. If the specified list begins with a `-'
character, then the specified algorithms (including wildcards) will
be removed from the default set instead of replacing them. If the
specified list begins with a `^' character, then the specified
algorithms will be placed at the head of the default set. The
default 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-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

The list of available signature algorithms may also be obtained
using "ssh -Q PubkeyAcceptedAlgorithms".

PubkeyAuthOptions
Sets one or more public key authentication options. The supported
keywords are: none (the default; indicating no additional options
are enabled), touch-required and verify-required.

The touch-required option causes public key authentication using a
FIDO authenticator algorithm (i.e. ecdsa-sk or ed25519-sk) to
always require the signature to attest that a physically present
user explicitly confirmed the authentication (usually by touching
the authenticator). By default, sshd(8) requires user presence
unless overridden with an authorized_keys option. The
touch-required flag disables this override.

The verify-required option requires a FIDO key signature attest
that the user was verified, e.g. via a PIN.

Neither the touch-required or verify-required options have any
effect for other, non-FIDO, public key types.

PubkeyAuthentication
Specifies whether public key authentication is allowed. The
default is yes.

RekeyLimit
Specifies the maximum amount of data that may be transmitted or
received before the session key is renegotiated, optionally
followed by a maximum amount of time that may pass before the
session key is renegotiated. The first argument is specified in
bytes and may have a suffix of `K', `M', or `G' to indicate
Kilobytes, Megabytes, or Gigabytes, respectively. The default is
between `1G' and `4G', depending on the cipher. The optional
second value is specified in seconds and may use any of the units
documented in the TIME FORMATS section. 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.

RequiredRSASize
Specifies the minimum RSA key size (in bits) that sshd(8) will
accept. User and host-based authentication keys smaller than this
limit will be refused. The default is 1024 bits. Note that this
limit may only be raised from the default.

RevokedKeys
Specifies revoked public keys file, or none to not use one. Keys
listed in this file will be refused for public key authentication.
Note that if this file is not readable, then public key
authentication will be refused for all users. Keys may be
specified as a text file, listing one public key per line, or as an
OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
For more information on KRLs, see the KEY REVOCATION LISTS section
in ssh-keygen(1).

RDomain
Specifies an explicit routing domain that is applied after
authentication has completed. The user session, as well as any
forwarded or listening IP sockets, will be bound to this
rdomain(4). If the routing domain is set to %D, then the domain in
which the incoming connection was received will be applied.

SecurityKeyProvider
Specifies a path to a library that will be used when loading FIDO
authenticator-hosted keys, overriding the default of using the
built-in USB HID support.

SetEnv Specifies one or more environment variables to set in child
sessions started by sshd(8) as "NAME=VALUE". The environment value
may be quoted (e.g. if it contains whitespace characters).
Environment variables set by SetEnv override the default
environment and any variables specified by the user via AcceptEnv
or PermitUserEnvironment.

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, sshd will be unable to forward the port to the Unix-domain
socket file. This option is only used for port forwarding to a
Unix-domain socket file.

The argument must be yes or no. The default is no.

StrictModes
Specifies whether sshd(8) should check file modes and ownership of
the user's files and home directory before accepting login. This
is normally desirable because novices sometimes accidentally leave
their directory or files world-writable. The default is yes. Note
that this does not apply to ChrootDirectory, whose permissions and
ownership are checked unconditionally.

Subsystem
Configures an external subsystem (e.g. file transfer daemon).
Arguments should be a subsystem name and a command (with optional
arguments) to execute upon subsystem request.

The command sftp-server implements the SFTP file transfer
subsystem.

Alternately the name internal-sftp implements an in-process SFTP
server. This may simplify configurations using ChrootDirectory to
force a different filesystem root on clients.

By default no subsystems are defined.

SyslogFacility
Gives the facility code that is used when logging messages from
sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
default is AUTH.

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. On the other hand, if TCP
keepalives are not sent, sessions may hang indefinitely on the
server, leaving "ghost" users and consuming server resources.

The default is yes (to send TCP keepalive messages), and the server
will notice if the network goes down or the client host crashes.
This avoids infinitely hanging sessions.

To disable TCP keepalive messages, the value should be set to no.

TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities
that are trusted to sign user certificates for authentication, or
none to not use one. Keys are listed one per line; empty lines and
comments starting with `#' are allowed. If a certificate is
presented for authentication and has its signing CA key listed in
this file, then it may be used for authentication for any user
listed in the certificate's principals list. Note that
certificates that lack a list of principals will not be permitted
for authentication using TrustedUserCAKeys. For more details on
certificates, see the CERTIFICATES section in ssh-keygen(1).

UnusedConnectionTimeout
Specifies whether and how quickly sshd(8) should close client
connections with no open channels. Open channels include active
shell, command execution or subsystem sessions, connected network,
socket, agent or X11 forwardings. Forwarding listeners, such as
those from the ssh(1) -R flag, are not considered as open channels
and do not prevent the timeout. The timeout value is specified in
seconds or may use any of the units documented in the TIME FORMATS
section.

Note that this timeout starts when the client connection completes
user authentication but before the client has an opportunity to
open any channels. Caution should be used when using short timeout
values, as they may not provide sufficient time for the client to
request and open its channels before terminating the connection.

The default none is to never expire connections for having no open
channels. This option may be useful in conjunction with
ChannelTimeout.

UseDNS Specifies whether sshd(8) should look up the remote host name, and
to check that the resolved host name for the remote IP address maps
back to the very same IP address.

If this option is set to no (the default) then only addresses and
not host names may be used in ~/.ssh/authorized_keys from and
sshd_config Match Host directives.

UsePAM Enables the Pluggable Authentication Module interface. If set to
yes this will enable PAM authentication using
KbdInteractiveAuthentication and PasswordAuthentication in addition
to PAM account and session module processing for all authentication
types.

Because PAM keyboard-interactive authentication usually serves an
equivalent role to password authentication, you should disable
either PasswordAuthentication or KbdInteractiveAuthentication.

If UsePAM is enabled, you will not be able to run sshd(8) as a non-
root user. On Solaris, the option is always enabled.

VersionAddendum
Optionally specifies additional text to append to the SSH protocol
banner sent by the server upon connection. The default is none.

X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11
forwarding. This prevents sshd from interfering with real X11
servers. The default is 10.

X11Forwarding
Specifies whether X11 forwarding is permitted. The argument must
be yes or no. The default on Solaris is yes.

When X11 forwarding is enabled, there may be additional exposure to
the server and to client displays if the sshd(8) proxy display is
configured to listen on the wildcard address (see X11UseLocalhost),
though this is not the default. Additionally, the authentication
spoofing and authentication data verification and substitution
occur on the client side. The security risk of using X11
forwarding is that the client's X11 display server may be exposed
to attack when the SSH client requests forwarding (see the warnings
for ForwardX11 in ssh_config(5)). A system administrator may have
a stance in which they want to protect clients that may expose
themselves to attack by unwittingly requesting X11 forwarding,
which can warrant a no setting.

Note that disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install their own
forwarders.

X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding server to
the loopback address or to the wildcard address. By default, sshd
binds the forwarding server to the loopback address and sets the
hostname part of the DISPLAY environment variable to localhost.
This prevents remote hosts from connecting to the proxy display.
However, some older X11 clients may not function with this
configuration. X11UseLocalhost may be set to no to specify that
the forwarding server should be bound to the wildcard address. The
argument must be yes or no. The default is yes.

XAuthLocation
Specifies the full pathname of the xauth(1) program, or none to not
use one. The default is /opt/local/bin/xauth.

TIME FORMATS

sshd(8) command-line arguments and configuration file options that specify
time may be expressed using a sequence of the form: time[qualifier], where
time is a positive integer value and qualifier is one of the following:

<none> seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks

Each member of the sequence is added together to calculate the total time
value.

Time format examples:

600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)

TOKENS

Arguments to some keywords can make use of tokens, which are expanded at
runtime:

%% A literal `%'.
%C Identifies the connection endpoints, containing four space-
separated values: client address, client port number, server
address, and server port number.
%D The routing domain in which the incoming connection was
received.
%F The fingerprint of the CA key.
%f The fingerprint of the key or certificate.
%h The home directory of the user.
%i The key ID in the certificate.
%K The base64-encoded CA key.
%k The base64-encoded key or certificate for authentication.
%s The serial number of the certificate.
%T The type of the CA key.
%t The key or certificate type.
%U The numeric user ID of the target user.
%u The username.

AuthorizedKeysCommand accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U,
and %u.

AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.

AuthorizedPrincipalsCommand accepts the tokens %%, %C, %D, %F, %f, %h, %i,
%K, %k, %s, %T, %t, %U, and %u.

AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.

ChrootDirectory accepts the tokens %%, %h, %U, and %u.

RoutingDomain accepts the token %D.

FILES

/etc/ssh/sshd_config
Contains configuration data for sshd(8). This file should be
writable by root only, but it is recommended (though not necessary)
that it be world-readable.

AUTHORS