(8)
reads configuration data from
/etc/ssh/sshd_config
(or the file specified with
-
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
`#'
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_config5
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 not to accept any environment variables.
- 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-agent1
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_config5
for more information on patterns.
- 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_config5
for more information on patterns.
- 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,
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.
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,
Qq 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,
Qq 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:
Qq gssapi-with-mic ,
Qq hostbased ,
Qq keyboard-interactive ,
Qq none
(used for access to password-less accounts when
PermitEmptyPasswords
is enabled),
Qq password
and
Qq 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
Sx 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
Sx 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
Sx 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
Qq .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
Sx 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
Sx AUTHORIZED_KEYS FILE FORMAT
in
sshd(8)).
Empty lines and comments starting with
`#'
are ignored.
Arguments to
AuthorizedPrincipalsFile
accept the tokens described in the
Sx 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
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies which algorithms are allowed for signing of certificates
by certificate authorities (CAs).
Certificates signed using other algorithms will not be accepted for
public key or host-based authentication.
- ChallengeResponseAuthentication
-
Specifies whether challenge-response authentication is allowed (e.g. via
PAM or through authentication styles supported in
login.conf5)
The default is
yes
- 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
Sx 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-server8
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
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies the ciphers allowed.
Multiple ciphers must be comma-separated.
If the specified list begins with a
`+'
character, then the specified ciphers will be appended to the built-in
openssh default set instead of replacing them.
If the specified list begins with a
`-'
character, then the specified ciphers (including wildcards) will be removed
from the built-in openssh default set instead of replacing them.
If the specified list begins with a
`^'
character, then the specified ciphers will be placed at the head of the
built-in openssh default set.
The supported ciphers are:
- 3des-cbc
-
- aes128-cbc
-
- aes192-cbc
-
- aes256-cbc
-
- aes128-ctr
-
- aes192-ctr
-
- aes256-ctr
-
- aes128-gcm@openssh.com
-
- aes256-gcm@openssh.com
-
- chacha20-poly1305@openssh.com
-
The list of available ciphers may also be obtained using
Qq ssh -Q cipher .
- 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_config5
for more information on patterns.
- 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_config5
for more information on patterns.
- DisableForwarding
-
Disables all forwarding features, including X11,
ssh-agent1,
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 is
no
- GSSAPICleanupCredentials
-
Specifies whether to automatically destroy the user's credentials cache
on logout.
The default is
yes
- GSSAPIEnablek5users
-
Specifies whether to look at .k5users file for GSSAPI authentication
access control. Further details are described in
ksu(1).
The default is
no
- GSSAPIKeyExchange
-
Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
doesn't rely on ssh keys to verify host identity.
The default is
no
- 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
- GSSAPIStoreCredentialsOnRekey
-
Controls whether the user's GSSAPI credentials should be updated following a
successful connection rekeying. This option can be used to accepted renewed
or updated credentials from a compatible client. The default is
``no''
For this to work
GSSAPIKeyExchange
needs to be enabled in the server and also used by the client.
- GSSAPIKexAlgorithms
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
The list of key exchange algorithms that are accepted by GSSAPI
key exchange. Possible values are
gss-gex-sha1-
gss-group1-sha1-
gss-group14-sha1-
gss-group14-sha256-
gss-group16-sha512-
gss-nistp256-sha256-
gss-curve25519-sha256-
This option only applies to connections using GSSAPI.
- 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-rsa-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available signature algorithms may also be obtained using
Qq 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
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
and
/etc/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-agent1.
- 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
SSH_AUTH_SOCK
environment variable.
- HostKeyAlgorithms
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies the host key signature algorithms
that the server offers.
The list of available signature algorithms may also be obtained using
Qq 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/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.
The argument to this keyword must be
yes
or
no
The default is to use whatever value
ChallengeResponseAuthentication
is set to
(by default
yes )
- 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
- KerberosUniqueCCache
-
Specifies whether to store the acquired tickets in the per-session credential
cache under /tmp/ or whether to use per-user credential cache as configured in
/etc/krb5.conf
The default value
no
can lead to overwriting previous tickets by subseqent connections to the same
user account.
- KerberosUseKuserok
-
Specifies whether to look at .k5login file for user's aliases.
The default is
yes
- KexAlgorithms
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
Alternately if the specified list begins with a
`+'
character, then the specified methods will be appended to the built-in
openssh default set instead of replacing them.
If the specified list begins with a
`-'
character, then the specified methods (including wildcards) will be removed
from the built-in openssh default set instead of replacing them.
If the specified list begins with a
`^'
character, then the specified methods will be placed at the head of the
built-in openssh default set.
The 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 list of available key exchange algorithms may also be obtained using
Qq 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
Fn kex_exchange_identification
function, and all code in the
packet.c
file.
This option is intended for debugging and no overrides are enabled by default.
- MACs The default is handled system-wide by
-
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies the 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 built-in
openssh default set instead of replacing them.
If the specified list begins with a
`-'
character, then the specified algorithms (including wildcards) will be removed
from the built-in openssh default set instead of replacing them.
If the specified list begins with a
`^'
character, then the specified algorithms will be placed at the head of the
built-in openssh default set.
The algorithms that contain
Qq -etm
calculate the MAC after encryption (encrypt-then-mac).
These are considered safer and their use recommended.
The 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 list of available MAC algorithms may also be obtained using
Qq 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
Sx PATTERNS
section of
ssh_config5.
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
ChrootDirectory
ClientAliveCountMax
ClientAliveInterval
DenyGroups
DenyUsers
DisableForwarding
ForceCommand
GatewayPorts
GSSAPIAuthentication
HostbasedAcceptedAlgorithms
HostbasedAuthentication
HostbasedUsesNameFromPacketOnly
IgnoreRhosts
Include
IPQoS
KbdInteractiveAuthentication
KerberosAuthentication
KerberosUseKuserok
LogLevel
MaxAuthTries
MaxSessions
PasswordAuthentication
PermitEmptyPasswords
PermitListen
PermitOpen
PermitRootLogin
PermitTTY
PermitTunnel
PermitUserRC
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RevokedKeys
RDomain
SetEnv
StreamLocalBindMask
StreamLocalBindUnlink
TrustedUserCAKeys
X11DisplayOffset
X11MaxDisplays
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.
- 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).
- 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
- 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_config5.
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
Qq 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.
The default is
yes
- 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
-
The default is handled system-wide by
crypto-policies7.
To see the defaults and how to modify this default, see manual page
update-crypto-policies8.
Specifies the signature algorithms that will be 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 built-in
openssh default set instead of replacing them.
If the specified list begins with a
`-'
character, then the specified algorithms (including wildcards) will be removed
from the built-in openssh default set instead of replacing them.
If the specified list begins with a
`^'
character, then the specified algorithms will be placed at the head of the
built-in openssh default set.
The list of available signature algorithms may also be obtained using
Qq 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 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
Sx 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.
- 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-keygen1.
For more information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen1.
- RDomain
-
Specifies an explicit routing domain that is applied after authentication
has completed.
The user session, as well and 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, AUTHPRIV, 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
Qq 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-keygen1.
- 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
Match
Host
directives.
- UsePAM
-
Enables the Pluggable Authentication Module interface.
If set to
yes
this will enable PAM authentication using
ChallengeResponseAuthentication
and
PasswordAuthentication
in addition to PAM account and session module processing for all
authentication types.
Because PAM challenge-response authentication usually serves an equivalent
role to password authentication, you should disable either
PasswordAuthentication
or
ChallengeResponseAuthentication.
If
UsePAM
is enabled, you will not be able to run
sshd(8)
as a non-root user.
The default is
no
- 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)Ns's
X11 forwarding.
This prevents sshd from interfering with real X11 servers.
The default is 10.
- X11MaxDisplays
-
Specifies the maximum number of displays available for
sshd(8)Ns's
X11 forwarding.
This prevents sshd from exhausting local ports.
The default is 1000.
- X11Forwarding
-
Specifies whether X11 forwarding is permitted.
The argument must be
yes
or
no
The default is
no
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_config5).
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
/usr/bin/xauth
(8)
command-line arguments and configuration file options that specify time
may be expressed using a sequence of the form:
Each member of the sequence is added together to calculate
the total time value.
Arguments to some keywords can make use of tokens,
which are expanded at runtime:
An -nosplit
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by
An Tatu Ylonen .
An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
An Theo de Raadt
and
An Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
An Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.
An Niels Provos
and
An Markus Friedl
contributed support for privilege separation.