sshd_config(5) - Man pages

SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)

NAME
sshd_config — OpenSSH daemon configuration file

DESCRIPTION
sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-ar-
gument pairs, one per line. Unless noted otherwise, for each keyword,
the first obtained value will be used. Lines starting with ‘#’ and
empty lines are interpreted as comments. Arguments may optionally be
enclosed in double quotes (") in order to represent arguments contain-
ing 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):

• Include /etc/ssh/sshd_config.d/*.conf
• KbdInteractiveAuthentication no
• X11Forwarding yes
• PrintMotd no
• AcceptEnv LANG LC_*
• Subsystem sftp /usr/lib/openssh/sftp-server
• UsePAM yes

/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 key-
words 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 en-
vironment variable is always accepted whenever the client re-
quests 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 di-
rectives. 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.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.

AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding
is permitted. The available options are yes (the default) or
all to allow StreamLocal forwarding, no to prevent all Stream-
Local 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 im-
prove 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, lo-
gin is allowed for all users. If the pattern takes the form
USER@HOST then USER and HOST are separately checked, restrict-
ing logins to particular users from particular hosts. HOST
criteria may additionally contain addresses to match in CIDR
address/masklen format. The allow/deny users directives are
processed in the following order: DenyUsers, AllowUsers.

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

AuthenticationMethods
Specifies the authentication methods that must be successfully
completed for a user to be granted access. This option must be
followed by one or more lists of comma-separated authentication
method names, or by the single string any to indicate the de-
fault 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 com-
plete 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. de-
pending on the server configuration. For example,
"keyboard-interactive:bsdauth" would restrict keyboard interac-
tive authentication to the bsdauth device.

If the publickey method is listed more than once, sshd(8) veri-
fies 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 ex-
plicitly 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 user-
name 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 dedi-
cated user that has no other role on the host than running au-
thorized 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. Alter-
nately 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 user-
name 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 authen-
tication 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 ac-
cepted for authentication. Names are listed one per line pre-
ceded 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 de-
scribed 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 authenti-
cation proceeds using a CA listed in TrustedUserCAKeys and is
not consulted for certification authorities trusted via
~/.ssh/authorized_keys, though the principals= key option of-
fers 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 certifi-
cates by certificate authorities (CAs). The default is:

ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

If the specified list begins with a ‘+’ character, then the
specified algorithms will be appended to the default set in-
stead of replacing them. If the specified list begins with a
‘-’ character, then the specified algorithms (including wild-
cards) will be removed from the default set instead of replac-
ing 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 dis-
ables the inactivity timeout.

The available channel types include:

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

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

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

session:command
Command execution sessions.

session:shell
Interactive shell sessions.

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

x11-connection
Open X11 forwarding sessions.

Note that in all the above cases, terminating an inactive ses-
sion 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 inactiv-
ity.

ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after au-
thentication. At session startup sshd(8) checks that all com-
ponents 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 direc-
tory. Arguments to ChrootDirectory accept the tokens described
in the “TOKENS” section.

The ChrootDirectory must contain the necessary files and direc-
tories to support the user's session. For an interactive ses-
sion 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 us-
ing SFTP no additional configuration of the environment is nec-
essary if the in-process sftp-server is used, though sessions
which use logging may require /dev/log inside the chroot direc-
tory on some operating systems (see sftp-server(8) for de-
tails).

For safety, it is very important that the directory hierarchy
be prevented from modification by other processes on the system
(especially those outside the jail). Misconfiguration can lead
to unsafe environments which sshd(8) cannot detect.

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

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

The supported ciphers are:

3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com

The default is:

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

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

ClientAliveCountMax
Sets the number of client alive messages which may be sent
without sshd(8) receiving any messages back from the client.
If this threshold is reached while client alive messages are
being sent, sshd will disconnect the client, terminating the
session. It is important to note that the use of client alive
messages is very different from TCPKeepAlive. The client alive
messages are sent through the encrypted channel and therefore
will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The client alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion 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 sec-
onds. 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 au-
thenticated 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 suf-
fix 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 pri-
mary group or supplementary group list matches one of the pat-
terns. 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 or-
der: DenyGroups, AllowGroups.

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

DenyUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. Login is disallowed for user names that
match one of the patterns. Only user names are valid; a numer-
ical 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 ad-
ditionally contain addresses to match in CIDR address/masklen
format. The allow/deny users directives are processed in the
following order: DenyUsers, AllowUsers.

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

DisableForwarding
Disables all forwarding features, including X11, ssh-agent(1),
TCP and StreamLocal. This option overrides all other forward-
ing-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 ses-
sion through the SSH_USER_AUTH environment variable. The de-
fault is no.

FingerprintHash
Specifies the hash algorithm used when logging key finger-
prints. 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 for-
wardings 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 al-
lowed. The default is no.

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

GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI is allowed. GSS-
API key exchange doesn't rely on ssh keys to verify host iden-
tity. The default is no.

GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSS-
API 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 authen-
ticate 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 up-
dated 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

gss-gex-sha1-,
gss-group1-sha1-,
gss-group14-sha1-,
gss-group14-sha256-,
gss-group16-sha512-,
gss-nistp256-sha256-,
gss-curve25519-sha256-

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 algo-
rithms (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins
with a ‘^’ character, then the specified signature algorithms
will be placed at the head of the default set. The default for
this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

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

HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication to-
gether 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 of-
fers. 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 re-
gardless 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 path-
names 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 con-
nection. 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 white-
space. 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 sec-
ond for non-interactive sessions. The default is lowdelay for
interactive sessions and throughput for non-interactive ses-
sions.

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 de-
fault 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 direc-
tory. 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. Multi-
ple 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 re-
placing them. If the specified list begins with a ‘-’ charac-
ter, 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 speci-
fied algorithms will be placed at the head of the default set.
The supported algorithms are:

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

The default is:

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

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

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

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

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 suc-
cessfully 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 recom-
mended.

LogVerbose
Specify one or more overrides to LogLevel. An override con-
sists 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, every-
thing in the kex_exchange_identification() function, and all
code in the packet.c file. This option is intended for debug-
ging and no overrides are enabled by default.

MACs Specifies the available MAC (message authentication code) algo-
rithms. The MAC algorithm is used for data integrity protec-
tion. 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 re-
placing them. If the specified list begins with a ‘-’ charac-
ter, 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 speci-
fied algorithms will be placed at the head of the default set.

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

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

The default is:

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

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

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

The arguments to Match are one or more criteria-pattern pairs
or the single token All which matches all criteria. The avail-
able criteria are User, Group, Host, LocalAddress, LocalPort,
and Address.

The match patterns may consist of single entries or comma-sepa-
rated lists and may use the wildcard and negation operators de-
scribed 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 pro-
vided 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 exam-
ple, 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 permit-
ted 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. Multi-
ple sessions may be established by clients that support connec-
tion multiplexing. Setting MaxSessions to 1 will effectively
disable session multiplexing, whereas setting it to 0 will pre-
vent all shell, login and subsystem sessions while still per-
mitting forwarding. The default is 10.

MaxStartups
Specifies the maximum number of concurrent unauthenticated con-
nections to the SSH daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime ex-
pires 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 lin-
early 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 de-
fault 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 for-
warding 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 re-
strictions 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 op-
tion 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 per-
mitted. The forwarding specification must be one of the fol-
lowing 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 re-
strictions 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 au-
thentication 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 tak-
ing 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 ar-
gument 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 op-
tions are yes, no or a pattern-list specifying which environ-
ment 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 us-
ing 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 lim-
its. 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 dae-
mon, 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 pat-
terns. 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 in-
stead 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 sup-
ported keywords are: none (the default; indicating no addi-
tional options are enabled), touch-required and
verify-required.

The touch-required option causes public key authentication us-
ing 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 autho-
rized_keys option. The touch-required flag disables this over-
ride.

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 op-
tional second value is specified in seconds and may use any of
the units documented in the “TIME FORMATS” section. The de-
fault 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 authen-
tication. 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 REVO-
CATION 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 charac-
ters). Environment variables set by SetEnv override the de-
fault environment and any variables specified by the user via
AcceptEnv or PermitUserEnvironment.

StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creat-
ing a Unix-domain socket file for local or remote port forward-
ing. 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-do-
main 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 acciden-
tally leave their directory or files world-writable. The de-
fault 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 op-
tional arguments) to execute upon subsystem request.

The command sftp-server implements the SFTP file transfer sub-
system.

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. How-
ever, 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 indefi-
nitely 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 authori-
ties that are trusted to sign user certificates for authentica-
tion, 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 authentica-
tion 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 ac-
tive shell, command execution or subsystem sessions, connected
network, socket, agent or X11 forwardings. Forwarding listen-
ers, 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 docu-
mented in the “TIME FORMATS” section.

Note that this timeout starts when the client connection com-
pletes user authentication but before the client has an oppor-
tunity 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 termi-
nating 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 ad-
dress 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 ad-
dition to PAM account and session module processing for all au-
thentication types.

Because PAM keyboard-interactive authentication usually serves
an equivalent role to password authentication, you should dis-
able 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 pro-
tocol 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 expo-
sure 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. Addition-
ally, the authentication spoofing and authentication data veri-
fication and substitution occur on the client side. The secu-
rity risk of using X11 forwarding is that the client's X11 dis-
play server may be exposed to attack when the SSH client re-
quests 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 de-
fault, 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 de-
fault is yes.

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

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

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

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

Time format examples:

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

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

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

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

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

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

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

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

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