sshd_config —
OpenSSH daemon configuration file
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. 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.
Note that the Debian
openssh-server package sets
several options as standard in
/etc/ssh/sshd_config which are not the default in
sshd(8):
/etc/ssh/sshd_config.d/*.conf files are included at
the start of the configuration file, so options set there will override those
in
/etc/ssh/sshd_config.
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 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-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.
- 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.
- 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:
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,
[email protected]
- Open TCP or Unix socket (respectively) connections that
have been established from a ssh(1) local
forwarding, i.e. LocalForward or
DynamicForward.
-
forwarded-tcpip,
[email protected]
- 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:
The default is:
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.
- DebianBanner
- Specifies whether the distribution-specified extra version
suffix is included during initial protocol handshake. 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.
- 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.
- 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 is no.
- GSSAPICleanupCredentials
- Specifies whether to automatically destroy the user's
credentials cache on logout. The default is
yes.
- 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 list of key exchange algorithms that are accepted by
GSSAPI key exchange. Possible values are
The default is
“gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-”.
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:
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 /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-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:
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 lowdelay for interactive sessions
and throughput for non-interactive
sessions.
- KbdInteractiveAuthentication
- Specifies whether to allow keyboard-interactive
authentication. 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
- [email protected]
- 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
- [email protected]
The default is:
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:
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. Multiple
ListenAddress options are permitted.
- 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:
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:
The default is:
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, and
Address.
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,
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.
- 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 /etc/ssh/moduli.
- 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:
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:
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 /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
- 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:
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).
- 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.
This option was formerly called KeepAlive.
- 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. 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)'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 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_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
/usr/bin/xauth.
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)
Arguments to some keywords can make use of tokens, which are expanded at
runtime:
- %%
- A literal ‘%’.
- %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 %%, %f,
%h, %k, %t, %U, and %u.
AuthorizedKeysFile accepts the tokens %%, %h, %U,
and %u.
AuthorizedPrincipalsCommand accepts the tokens %%,
%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.
- /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.
sftp-server(8),
sshd(8)
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen.
Aaron Campbell,
Bob Beck,
Markus Friedl,
Niels Provos,
Theo de Raadt and
Dug Song removed many bugs, re-added newer
features and created OpenSSH.
Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.
Niels Provos and
Markus Friedl contributed support for
privilege separation.