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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-argument pairs, one per line. For each keyword,
the first obtained value will be used. Lines starting with ‘#’ and empty lines are interpreted
as comments. Arguments may optionally be enclosed in double quotes (") in order to represent
arguments containing spaces.

Note that the Debian openssh-server package sets several options as standard in
/etc/ssh/sshd_config which are not the default in sshd(8):

•• 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 op‐
tions set there will override those in /etc/ssh/sshd_config.

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

AcceptEnv
Specifies what environment variables sent by the client will be copied into the ses‐
sion's environ(7). See SendEnv and SetEnv in ssh_config(5) for how to configure the
client. The TERM environment variable is always accepted whenever the client requests
a pseudo-terminal as it is required by the protocol. Variables are specified by name,
which may contain the wildcard characters ‘*’ and ‘?’. Multiple environment variables
may be separated by whitespace or spread across multiple AcceptEnv directives. Be
warned that some environment variables could be used to bypass restricted user environ‐
ments. For this reason, care should be taken in the use of this directive. The de‐
fault is not to accept any environment variables.

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

AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted. The default is yes. Note that
disabling agent forwarding does not improve security unless users are also denied shell
access, as they can always install their own forwarders.

AllowGroups
This keyword can be followed by a list of group name patterns, separated by spaces. If
specified, login is allowed only for users whose primary group or supplementary group
list matches one of the patterns. Only group names are valid; a numerical group ID is
not recognized. By default, login is allowed for all groups. The allow/deny groups
directives are processed in the following order: DenyGroups, AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.

AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The avail‐
able options are yes (the default) or all to allow StreamLocal forwarding, no to pre‐
vent all StreamLocal forwarding, local to allow local (from the perspective of ssh(1))
forwarding only or remote to allow remote forwarding only. Note that disabling Stream‐
Local forwarding does not improve security unless users are also denied shell access,
as they can always install their own forwarders.

AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available options are yes (the de‐
fault) 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 for‐
warding only. Note that disabling TCP forwarding does not improve security unless
users are also denied shell access, as they can always install their own forwarders.

AllowUsers
This keyword can be followed by a list of user name patterns, separated by spaces. If
specified, login is allowed only for user names that match one of the patterns. Only
user names are valid; a numerical user ID is not recognized. By default, login is al‐
lowed for all users. If the pattern takes the form USER@HOST then USER and HOST are
separately checked, restricting logins to particular users from particular hosts. HOST
criteria may additionally contain addresses to match in CIDR address/masklen format.
The allow/deny users directives are processed in the following order: DenyUsers,
AllowUsers.

See PATTERNS in ssh_config(5) for more information on patterns.

AuthenticationMethods
Specifies the authentication methods that must be successfully completed for a user to
be granted access. This option must be followed by one or more lists of comma-sepa‐
rated 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 interac‐
tive 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 key‐
board-interactive authentication before public key.

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

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

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

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

AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys. The program must be
owned by root, not writable by group or others and specified by an absolute path. Ar‐
guments 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 recom‐
mended to use a dedicated user that has no other role on the host than running autho‐
rized 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 program must be owned by root, not writable by
group or others and specified by an absolute path. Arguments to
AuthorizedPrincipalsCommand accept the tokens described in the TOKENS section. If no
arguments are specified then the username of the target user is used.

The program should produce on standard output zero or more lines of
AuthorizedPrincipalsFile output. If either AuthorizedPrincipalsCommand or
AuthorizedPrincipalsFile is specified, then certificates offered by the client for au‐
thentication 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 au‐
thorized 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 authenti‐
cation. 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 au‐
thentication. 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 sec‐
tion. 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 prin‐
cipals list for it to be accepted.

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

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

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

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

If the specified list begins with a ‘+’ character, then the specified algorithms will
be appended to the default set instead of replacing them. If the specified list begins
with a ‘-’ character, then the specified algorithms (including wildcards) will be re‐
moved from the default set instead of replacing them.

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

ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after authentication. At session
startup sshd(8) checks that all components of the pathname are root-owned directories
which are not writable by any other user or group. After the chroot, sshd(8) changes
the working directory to the user's home directory. Arguments to ChrootDirectory ac‐
cept the tokens described in the TOKENS section.

The ChrootDirectory must contain the necessary files and directories to support the
user's session. For an interactive session this requires at least a shell, typically
sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
and tty(4) devices. For file transfer sessions using SFTP no additional configuration
of the environment is necessary if the in-process sftp-server is used, though sessions
which use logging may require /dev/log inside the chroot directory on some operating
systems (see sftp-server(8) for details).

For safety, it is very important that the directory hierarchy be prevented from modifi‐
cation by other processes on the system (especially those outside the jail). Miscon‐
figuration 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 spec‐
ified 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 ‘^’ charac‐
ter, 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 AT openssh.com
aes256-gcm AT openssh.com
chacha20-poly1305 AT openssh.com

The default is:

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

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

ClientAliveCountMax
Sets the number of client alive messages which may be sent without sshd(8) receiving
any messages back from the client. If this threshold is reached while client alive
messages are being sent, sshd will disconnect the client, terminating the session. It
is important to note that the use of client alive messages is very different from
TCPKeepAlive. The client alive messages are sent through the encrypted channel and
therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is
spoofable. The client alive mechanism is valuable when the client or server depend on
knowing when a connection has become unresponsive.

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

ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received from the
client, sshd(8) will send a message through the encrypted channel to request a response
from the client. The default is 0, indicating that these messages will not be sent to
the client.

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

DebianBanner
Specifies whether the distribution-specified extra version suffix is included during
initial protocol handshake. The default is yes.

DenyGroups
This keyword can be followed by a list of group name patterns, separated by spaces.
Login is disallowed for users whose primary group or supplementary group list matches
one of the patterns. Only group names are valid; a numerical group ID is not recog‐
nized. By default, login is allowed for all groups. The allow/deny groups directives
are processed in the following order: DenyGroups, AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.

DenyUsers
This keyword can be followed by a list of user name patterns, separated by spaces. Lo‐
gin is disallowed for user names that match one of the patterns. Only user names are
valid; a numerical user ID is not recognized. By default, login is allowed for all
users. If the pattern takes the form USER@HOST then USER and HOST are separately
checked, restricting logins to particular users from particular hosts. HOST criteria
may additionally contain addresses to match in CIDR address/masklen format. The 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 methods and public creden‐
tials (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-loop‐
back addresses, thus allowing other hosts to connect. The argument may be no to force
remote port forwardings to be available to the local host only, yes to force remote
port forwardings to bind to the wildcard address, or clientspecified to allow the
client to select the address to which the forwarding is bound. The default is no.

GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed. The default is no.

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

GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't
rely on ssh keys to verify host identity. The default is no.

GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor a client au‐
thenticates against. If set to yes then the client must authenticate against the host
service on the current hostname. If set to no then the client may authenticate against
any service key stored in the machine's default store. This facility is provided to
assist with operation on multi homed machines. The default is yes.

GSSAPIStoreCredentialsOnRekey
Controls whether the user's GSSAPI credentials should be updated following a successful
connection rekeying. This option can be used to accepted renewed or updated credentials
from a compatible client. The default is “no”.

For this to work GSSAPIKeyExchange needs to be enabled in the server and also used by
the client.

GSSAPIKexAlgorithms
The list of key exchange algorithms that are accepted by GSSAPI key exchange. Possible
values are

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 algorithms (including wildcards) will be removed from the de‐
fault set instead of replacing them. If the specified list begins with a ‘^’ charac‐
ter, 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 AT openssh.com,
ecdsa-sha2-nistp256-cert-v01 AT openssh.com,
ecdsa-sha2-nistp384-cert-v01 AT openssh.com,
ecdsa-sha2-nistp521-cert-v01 AT openssh.com,
sk-ssh-ed25519-cert-v01 AT openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01 AT openssh.com,
rsa-sha2-512-cert-v01 AT openssh.com,
rsa-sha2-256-cert-v01 AT openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519 AT openssh.com,
sk-ecdsa-sha2-nistp256 AT openssh.com,
rsa-sha2-512,rsa-sha2-256

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

HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together with successful
public key client host authentication is allowed (host-based authentication). The de‐
fault is no.

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

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

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

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

It is possible to have multiple host key files. It is also possible to specify public
host key files instead. In this case operations on the private key will be delegated
to an ssh-agent(1).

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

HostKeyAlgorithms
Specifies the host key signature algorithms that the server offers. The default for
this option is:

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

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

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

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

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

IPQoS Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values
are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1,
cs2, cs3, cs4, cs5, cs6, cs7, ef, le, lowdelay, throughput, reliability, a numeric
value, or none to use the operating system default. This option may take one or two
arguments, separated by whitespace. If one argument is specified, it is used as the
packet class unconditionally. If two values are specified, the first is automatically
selected for interactive sessions and the second for non-interactive sessions. The de‐
fault is lowdelay for interactive sessions and throughput for non-interactive sessions.

KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication. The default is yes.
The argument to this keyword must be yes or no. ChallengeResponseAuthentication is a
deprecated alias for this.

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

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

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

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

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

curve25519-sha256
curve25519-sha256 AT 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 AT openssh.com

The default is:

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

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

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

ListenAddress hostname|address
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 speci‐
fied. The default is to listen on all local addresses. Multiple ListenAddress options
are permitted.

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

LogLevel
Gives the verbosity level that is used when logging messages from sshd(8). The possi‐
ble values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
higher levels of debugging output. Logging with a DEBUG level violates the privacy of
users and is not recommended.

LogVerbose
Specify one or more overrides to LogLevel. An override consists of a pattern lists
that matches the source file, function and line number to force detailed logging for.
For example, an override pattern of:

kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

would enable detailed logging for line 1000 of kex.c, everything in the
kex_exchange_identification() function, and all code in the packet.c file. This option
is intended for debugging and no overrides are enabled by default.

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

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

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

The default is:

umac-64-etm AT openssh.com,umac-128-etm AT openssh.com,
hmac-sha2-256-etm AT openssh.com,hmac-sha2-512-etm AT openssh.com,
hmac-sha1-etm AT openssh.com,
umac-64 AT openssh.com,umac-128 AT 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 satis‐
fied, the keywords on the following lines override those set in the global section of
the config file, until either another Match line or the end of the file. If a keyword
appears in multiple Match blocks that are satisfied, only the first instance of the
keyword is applied.

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

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

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

Only a subset of keywords may be used on the lines following a Match keyword. Avail‐
able keywords are AcceptEnv, AllowAgentForwarding, AllowGroups,
AllowStreamLocalForwarding, AllowTcpForwarding, AllowUsers, AuthenticationMethods,
AuthorizedKeysCommand, AuthorizedKeysCommandUser, AuthorizedKeysFile,
AuthorizedPrincipalsCommand, AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
Banner, CASignatureAlgorithms, 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, 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 subsys‐
tem sessions while still permitting forwarding. The default is 10.

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

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

ModuliFile
Specifies the moduli(5) file that contains the Diffie-Hellman groups used for the
“diffie-hellman-group-exchange-sha1” and “diffie-hellman-group-exchange-sha256” key ex‐
change methods. The default is /etc/ssh/moduli.

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

PermitEmptyPasswords
When password authentication is allowed, it specifies whether the server allows login
to accounts with empty password strings. The default is no.

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

PermitListen port
PermitListen host:port

Multiple permissions may be specified by separating them with whitespace. An argument
of any can be used to remove all restrictions and permit any listen requests. An argu‐
ment 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 for‐
warding 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 lis‐
ten host of “localhost” if no listen host was specifically requested, and this name is
treated differently to explicit localhost addresses of “127.0.0.1” and “::1”.

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

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

Multiple forwards may be specified by separating them with whitespace. An argument of
any can be used to remove all restrictions and permit any forwarding requests. An ar‐
gument 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 pat‐
tern matching or address lookups are performed on supplied names. By default all port
forwarding requests are permitted.

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

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

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

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

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

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

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

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

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

PerSourceMaxStartups
Specifies the number of unauthenticated connections allowed from a given source ad‐
dress, 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 pur‐
poses of applying PerSourceMaxStartups limits. Values for IPv4 and optionally IPv6 may
be specified, separated by a colon. The default is 32:128, which means each address is
considered individually.

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

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

PrintLastLog
Specifies whether sshd(8) should print the date and time of the last user login when a
user logs in interactively. The default is yes.

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

PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be accepted for public key authentication
as a list of comma-separated patterns. Alternately if the specified list begins with a
‘+’ character, then the specified algorithms will be appended to the default set in‐
stead of replacing them. If the specified list begins with a ‘-’ character, then the
specified algorithms (including wildcards) will be removed from the default set instead
of replacing them. If the specified list begins with a ‘^’ character, then the speci‐
fied algorithms will be placed at the head of the default set. The default for this
option is:

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

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

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

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

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

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

RekeyLimit
Specifies the maximum amount of data that may be transmitted before the session key is
renegotiated, optionally followed by a maximum amount of time that may pass before the
session key is renegotiated. The first argument is specified in bytes and may have a
suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respec‐
tively. 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 authentication. Note that if this file is not readable,
then public key authentication will be refused for all users. Keys may be specified as
a text file, listing one public key per line, or as an OpenSSH Key Revocation List
(KRL) as generated by ssh-keygen(1). For more information on KRLs, see the KEY REVOCA‐
TION LISTS section in ssh-keygen(1).

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

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

StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket
file for local or remote port forwarding. This option is only used for port forwarding
to a Unix-domain socket file.

The default value is 0177, which creates a Unix-domain socket file that is readable and
writable only by the owner. Note that not all operating systems honor the file mode on
Unix-domain socket files.

StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain socket file for local or remote
port forwarding before creating a new one. If the socket file already exists and
StreamLocalBindUnlink is not enabled, sshd will be unable to forward the port to the
Unix-domain socket file. This option is only used for port forwarding to a Unix-domain
socket file.

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

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

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

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

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

By default no subsystems are defined.

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

TCPKeepAlive
Specifies whether the system should send TCP keepalive messages to the other side. If
they are sent, death of the connection or crash of one of the machines will be properly
noticed. However, this means that connections will die if the route is down temporar‐
ily, and some people find it annoying. On the other hand, if TCP keepalives are not
sent, sessions may hang indefinitely on the server, leaving "ghost" users and consuming
server resources.

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

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

This option was formerly called KeepAlive.

TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are trusted to
sign user certificates for authentication, or none to not use one. Keys are listed one
per line; empty lines and comments starting with ‘#’ are allowed. If a certificate is
presented for authentication and has its signing CA key listed in this file, then it
may be used for authentication for any user listed in the certificate's principals
list. Note that certificates that lack a list of principals will not be permitted for
authentication using TrustedUserCAKeys. For more details on certificates, see the CER‐
TIFICATES 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 address maps back to the very same IP address.

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

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

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

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

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

X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11 forwarding. This pre‐
vents 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 au‐
thentication spoofing and authentication data verification and substitution occur on
the client side. The security risk of using X11 forwarding is that the client's X11
display server may be exposed to attack when the SSH client requests forwarding (see
the warnings for ForwardX11 in ssh_config(5)). A system administrator may have a
stance in which they want to protect clients that may expose themselves to attack by
unwittingly requesting X11 forwarding, which can warrant a no setting.

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

X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address
or to the wildcard address. By default, sshd binds the forwarding server to the loop‐
back 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 ad‐
dress. 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 de‐
fault is /usr/bin/xauth.

TIME FORMATS
sshd(8) command-line arguments and configuration file options that specify time may be ex‐
pressed 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 ‘%’.
%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.

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