sshd_config(5)

SSHD_CONFIG(5) BSD 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-argu-
ment 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 stan-
dard in /etc/ssh/sshd_config which are not the default in sshd(8):

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

/etc/ssh/sshd_config.d/*.conf files are included at the start of the con-
figuration 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 environ-
ment 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 de-
fault is yes. Note that disabling agent forwarding does not im-
prove 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 for-
warding, 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 in-
stall their own forwarders.

AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available op-
tions are yes (the default) or all to allow TCP forwarding, no to
prevent all TCP forwarding, local to allow local (from the per-
spective of ssh(1)) forwarding only or remote to allow remote
forwarding only. Note that disabling TCP forwarding does not im-
prove 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 ad-
dress/masklen format. The allow/deny users directives are pro-
cessed 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 authentica-
tion. Only methods that are next in one or more lists are of-
fered 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 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 oth-
ers 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 expan-
sion, 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 pro-
gram 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 speci-
fied, 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 princi-
pals 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 authentica-
tion 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 certifi-
cates by certificate authorities (CAs). The default is:

ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa

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

ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed
(e.g. via PAM). The default is yes.

ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after au-
thentication. At session startup sshd(8) checks that all compo-
nents 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. Ar-
guments to ChrootDirectory accept the tokens described in the
TOKENS section.

The ChrootDirectory must contain the necessary files and directo-
ries 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 ad-
ditional 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 operat-
ing 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 (es-
pecially 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 spec-
ified 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 with-
out 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 spoofa-
ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa-
ble. The client alive mechanism is valuable when the client or
server depend on knowing when a connection has become unrespon-
sive.

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 termina-
tion.

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 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 recog-
nized. By default, login is allowed for all groups. The al-
low/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 numeri-
cal 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 con-
tain addresses to match in CIDR address/masklen format. The al-
low/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 meth-
ods 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 con-
nect. 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 fa-
cility is provided to assist with operation on multi homed ma-
chines. 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

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.

HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for hostbased au-
thentication as a list of comma-separated patterns. Alternately
if the specified list begins with a '+' character, then the spec-
ified key types will be appended to the default set instead of
replacing them. If the specified list begins with a '-' charac-
ter, then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. If the
specified list begins with a '^' character, then the specified
key types will be placed at the head of the default set. The de-
fault for this option is:

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

The list of available key types may also be obtained using "ssh
-Q 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 re-
verse 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 cer-
tificate's public key must match a private host key already spec-
ified 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 pos-
sible to specify public host key files instead. In this case op-
erations 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 algorithms that the server offers. The
default for this option is:

The list of available key types 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 set-
ting.

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

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

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

IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec-
tion. 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 uncondi-
tionally. If two values are specified, the first is automati-
cally 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 argument to this keyword must be yes or no. The default is
to use whatever value ChallengeResponseAuthentication is set to
(by default yes).

KerberosAuthentication
Specifies whether the password provided by the user for
PasswordAuthentication will be validated through the Kerberos
KDC. To use this option, the server needs a Kerberos servtab
which allows the verification of the KDC's identity. The default
is no.

KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to
acquire an AFS token before accessing the user's home directory.
The default is no.

KerberosOrLocalPasswd
If password authentication through Kerberos fails then the pass-
word 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 methods will
be appended to the default set instead of replacing them. If the
specified list begins with a '-' character, then the specified
methods (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with
a '^' character, then the specified methods will be placed at the
head of the 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
sntrup4591761x25519-sha512@tinyssh.org

The default is:

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 fol-
lowing forms may be used:

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

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

LoginGraceTime
The server disconnects after this time if the user has not 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 recommended.

MACs Specifies the available MAC (message authentication code) algo-
rithms. 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 spec-
ified algorithms (including wildcards) will be removed from the
default set instead of replacing them. If the specified list be-
gins 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 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, un-
til either another Match line or the end of the file. If a key-
word appears in multiple Match blocks that are satisfied, only
the first instance of the keyword is applied.

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

The match patterns may consist of single entries or comma-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 ad-
dresses 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 example,
192.0.2.0/33 and 192.0.2.0/8, respectively.

Only a subset of keywords may be used on the lines following a
Match keyword. Available keywords are AcceptEnv,
AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
AllowTcpForwarding, AllowUsers, AuthenticationMethods,
AuthorizedKeysCommand, AuthorizedKeysCommandUser,
AuthorizedKeysFile, AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
Banner, ChrootDirectory, ClientAliveCountMax,
ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
IgnoreRhosts, Include, IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, LogLevel, MaxAuthTries, MaxSessions,
PasswordAuthentication, PermitEmptyPasswords, PermitListen,
PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication,
RekeyLimit, RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
StreamLocalBindUnlink, TrustedUserCAKeys, 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 for-
warding. 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 linearly and all connec-
tion attempts are refused if the number of unauthenticated con-
nections reaches full (60).

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 forward-
ing may listen. The listen specification must be one of the fol-
lowing 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 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 per-
mitted. The forwarding specification must be one of the follow-
ing forms:

PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port

Multiple forwards may be specified by separating them with white-
space. 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 respec-
tively. Otherwise, no pattern matching or address lookups are
performed on supplied names. By default all port forwarding re-
quests 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 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 argu-
ment 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 vari-
able 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.

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.

PubkeyAcceptedKeyTypes
Specifies the key types that will be accepted for public key au-
thentication as a list of comma-separated patterns. Alternately
if the specified list begins with a '+' character, then the spec-
ified key types will be appended to the default set instead of
replacing them. If the specified list begins with a '-' charac-
ter, then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. If the
specified list begins with a '^' character, then the specified
key types will be placed at the head of the default set. The de-
fault for this option is:

The list of available key types may also be obtained using "ssh
-Q PubkeyAcceptedKeyTypes".

PubkeyAuthOptions
Sets one or more public key authentication options. Two option
keywords are currently supported: none (the default; indicating
no additional options are enabled) and touch-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. This option has no
effect for other, non-authenticator public key types.

PubkeyAuthentication
Specifies whether public key authentication is allowed. The de-
fault is yes.

RekeyLimit
Specifies the maximum amount of data that may be transmitted be-
fore the session key is renegotiated, optionally followed a maxi-
mum amount of time that may pass before the session key is rene-
gotiated. 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.

RevokedKeys
Specifies revoked public keys file, or none to not use one. Keys
listed in this file will be refused for public key authentica-
tion. Note that if this file is not readable, then public key
authentication will be refused for all users. Keys may be speci-
fied 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 sec-
tion in ssh-keygen(1).

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

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

SetEnv Specifies one or more environment variables to set in child ses-
sions started by sshd(8) as "NAME=VALUE". The environment value
may be quoted (e.g. if it contains whitespace characters). Envi-
ronment 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 subsys-
tem.

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 de-
fault 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 tempo-
rarily, 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 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).

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
ChallengeResponseAuthentication and PasswordAuthentication in ad-
dition to PAM account and session module processing for all au-
thentication types.

Because PAM challenge-response authentication usually serves an
equivalent role to password authentication, you should disable
either PasswordAuthentication or ChallengeResponseAuthentication.

If UsePAM is enabled, you will not be able to run sshd(8) as a
non-root user. The default is no.

VersionAddendum
Optionally specifies additional text to append to the SSH proto-
col 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 adminis-
trator 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 wild-
card 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.

TIME FORMATS
sshd(8) command-line arguments and configuration file options that spec-
ify 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 fol-
lowing:

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 '%'.
%D The routing domain in which the incoming connection was re-
ceived.
%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.

RoutingDomain accepts the token %D.

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