Ubuntu Manpage: Config::Model::models::Sshd

Provided by: libconfig-model-openssh-perl_2.9.0.2-1_all

NAME

       Config::Model::models::Sshd - Configuration class Sshd

DESCRIPTION

       Configuration classes used by Config::Model

       This configuration class was generated from sshd_system documentation.  by parse-man.pl
       <https://github.com/dod38fr/config-model-openssh/contrib/parse-man.pl>

Elements

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

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. Optional. Type boolean.

upstream_default value :
yes

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. Optional. Type list of
uniline.

AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The available
options are yes (the default) or all to allow StreamLocal forwarding, no to prevent all
StreamLocal forwarding, local to allow local (from the perspective of ssh(1)) forwarding
only or remote to allow remote forwarding only. Note that disabling StreamLocal forwarding
does not improve security unless users are also denied shell access, as they can always
install their own forwarders. Optional. Type enum. choice: 'yes', 'all', 'no', 'local',
'remote'.

upstream_default value :
yes

AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available options are yes (the default)
or all to allow TCP forwarding, no to prevent all TCP forwarding, local to allow local
(from the perspective of ssh(1)) forwarding only or remote to allow remote forwarding
only. Note that disabling TCP forwarding does not improve security unless users are also
denied shell access, as they can always install their own forwarders. Optional. Type
enum. choice: 'yes', 'all', 'no', 'local', 'remote'.

upstream_default value :
yes

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

See PATTERNS in ssh_config(5) for more information on patterns. Optional. Type list of
uniline.

AuthenticationMethods
Specifies the authentication methods that must be successfully completed for a user to be
granted access. This option must be followed by one or more lists of comma-separated
authentication method names, or by the single string any to indicate the default behaviour
of accepting any single authentication method. If the default is overridden, then
successful authentication requires completion of every method in at least one of these
lists.

For example, "publickey, password publickey, keyboard-interactive" would require the user
to complete public key authentication, followed by either password or keyboard interactive
authentication. Only methods that are next in one or more lists are offered at each stage,
so for this example it would not be possible to attempt password or keyboard-interactive
authentication before public key.

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

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

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

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

AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys. The program must be
owned by root, not writable by group or others and specified by an absolute path.
Arguments to AuthorizedKeysCommand accept the tokens described in the TOKENS section. If
no arguments are specified then the username of the target user is used.

The program should produce on standard output zero or more lines of authorized_keys output
(see AUTHORIZED_KEYS in sshd(8)). AuthorizedKeysCommand is tried after the usual
AuthorizedKeysFile files and will not be executed if a matching key is found there. By
default, no AuthorizedKeysCommand is run. Optional. Type uniline.

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. Optional. Type uniline.

AuthorizedKeysFile
Specifies the file that contains the public keys used for user authentication. The format
is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Arguments to
AuthorizedKeysFile accept the tokens described in the TOKENS section. After expansion,
AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home
directory. Multiple files may be listed, separated by whitespace. Alternately this option
may be set to none to skip checking for user keys in files. The default is
".ssh/authorized_keys .ssh/authorized_keys2". Optional. Type list of uniline.

Note: AuthorizedKeysFile values are migrated from '- AuthorizedKeysFile2'

AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of allowed certificate principals as
per AuthorizedPrincipalsFile. The program must be owned by root, not writable by group or
others and specified by an absolute path. Arguments to AuthorizedPrincipalsCommand accept
the tokens described in the TOKENS section. If no arguments are specified then the
username of the target user is used.

The program should produce on standard output zero or more lines of
AuthorizedPrincipalsFile output. If either AuthorizedPrincipalsCommand or
AuthorizedPrincipalsFile is specified, then certificates offered by the client for
authentication must contain a principal that is listed. By default, no
AuthorizedPrincipalsCommand is run. Optional. Type uniline.

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. Optional. Type
uniline.

AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for certificate
authentication. When using certificates signed by a key listed in TrustedUserCAKeys, this
file lists names, one of which must appear in the certificate for it to be accepted for
authentication. Names are listed one per line preceded by key options (as described in
AUTHORIZED_KEYS FILE FORMAT in sshd(8)). Empty lines and comments starting with '#' are
ignored.

Arguments to AuthorizedPrincipalsFile accept the tokens described in the TOKENS section.
After expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative
to the user's home directory. The default is none, i.e. not to use a principals file X in
this case, the username of the user must appear in a certificate's principals list for it
to be accepted.

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

upstream_default value :
none

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. Optional. Type uniline.

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

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

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

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

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

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

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

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

upstream_default value :
none

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

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

upstream_default value :
3

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. Optional. Type integer.

upstream_default value :
0

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

See PATTERNS in ssh_config(5) for more information on patterns. Optional. Type list of
uniline.

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

See PATTERNS in ssh_config(5) for more information on patterns. Optional. Type list of
uniline.

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. Optional. Type uniline.

ExposeAuthInfo
Writes a temporary file containing a list of authentication methods and public credentials
(e.g. keys) used to authenticate the user. The location of the file is exposed to the user
session through the SSH_USER_AUTH environment variable. The default is no. Optional. Type
boolean.

upstream_default value :
no

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. Optional. Type uniline.

upstream_default value :
none

GatewayPorts
Specifies whether remote hosts are allowed to connect to ports forwarded for the client.
By default, sshd(8) binds remote port forwardings to the loopback address. This prevents
other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify
that sshd should allow remote port forwardings to bind to non-loopback addresses, thus
allowing other hosts to connect. The argument may be no to force remote port forwardings
to be available to the local host only, yes to force remote port forwardings to bind to
the wildcard address, or clientspecified to allow the client to select the address to
which the forwarding is bound. The default is no. Optional. Type enum. choice: 'no',
'yes', 'clientspecified'.

upstream_default value :
no

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

upstream_default value :
no

HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be accepted for hostbased authentication as a
list of comma-separated patterns. Alternately if the specified list begins with a '+'
character, then the specified signature algorithms will be appended to the default set
instead of replacing them. If the specified list begins with a '-' character, then the
specified signature algorithms (including wildcards) will be removed from the default set
instead of replacing them. If the specified list begins with a '^' character, then the
specified signature algorithms will be placed at the head of the default set. The default
for this option is:

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

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

HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together with successful
public key client host authentication is allowed (host-based authentication). The default
is no. Optional. Type boolean.

upstream_default value :
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. Optional. Type boolean.

upstream_default value :
no

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

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

upstream_default value :
yes

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

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

upstream_default value :
af21 cs1

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. Optional. Type boolean.

upstream_default value :
yes

Note: KbdInteractiveAuthentication is migrated with '$old' and with:

• $old => "- ChallengeResponseAuthentication"

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. Optional.
Type boolean.

upstream_default value :
no

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. Optional. Type enum. choice: 'QUIET', 'FATAL', 'ERROR', 'INFO',
'VERBOSE', 'DEBUG', 'DEBUG1', 'DEBUG2', 'DEBUG3'.

upstream_default value :
INFO

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. Optional. Type integer.

upstream_default value :
6

MaxSessions
Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions
permitted per network connection. Multiple sessions may be established by clients that
support connection multiplexing. Setting MaxSessions to 1 will effectively disable
session multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem
sessions while still permitting forwarding. The default is 10. Optional. Type integer.

upstream_default value :
10

PasswordAuthentication
Specifies whether password authentication is allowed. The default is yes. Optional. Type
boolean.

upstream_default value :
yes

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

upstream_default value :
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:

PermitListenport PermitListen host:port

Multiple permissions may be specified by separating them with whitespace. An argument of
any can be used to remove all restrictions and permit any listen requests. An argument of
none can be used to prohibit all listen requests. The host name may contain wildcards as
described in the PATTERNS section in ssh_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 X127.0.0.1X and X::1X. Optional. Type list of uniline.

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

PermitOpenhost:port PermitOpen IPv4_addr:port PermitOpen [IPv6_addr]:port

Multiple forwards may be specified by separating them with whitespace. An argument of any
can be used to remove all restrictions and permit any forwarding requests. An argument of
none can be used to prohibit all forwarding requests. The wildcard '*' can be used for
host or port to allow all hosts or ports respectively. Otherwise, no pattern matching or
address lookups are performed on supplied names. By default all port forwarding requests
are permitted. Optional. Type list of uniline.

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

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

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

If this option is set to no, root is not allowed to log in. Optional. Type enum. choice:
'yes', 'prohibit-password', 'forced-commands-only', 'no'.

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

upstream_default value :
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. Optional. Type enum. choice: 'yes', 'point-to-point', 'ethernet',
'no'.

upstream_default value :
no

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

upstream_default value :
yes

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

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

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

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

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

Neither the touch-required or verify-required options have any effect for other, non-FIDO,
public key types. Optional. Type enum. choice: 'none', 'touch-required',
'verify-required'.

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

upstream_default value :
yes

RekeyLimit
Specifies the maximum amount of data that may be transmitted before the session key is
renegotiated, optionally followed by a maximum amount of time that may pass before the
session key is renegotiated. The first argument is specified in bytes and may have a
suffix of 'K', 'M', or 'G' to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
The default is between '1G' and '4G', depending on the cipher. The optional second value
is specified in seconds and may use any of the units documented in the 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. Optional. Type uniline.

RevokedKeys
Specifies revoked public keys file, or none to not use one. Keys listed in this file will
be refused for public key authentication. Note that if this file is not readable, then
public key authentication will be refused for all users. Keys may be specified as a text
file, listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as
generated by ssh-keygen(1). For more information on KRLs, see the KEY REVOCATION LISTS
section in ssh-keygen(1). Optional. Type uniline.

SetEnv
Specifies one or more environment variables to set in child sessions started by sshd(8) as
XNAME=VALUEX. 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. Optional. Type
uniline.

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. Optional. Type uniline.

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. Optional. Type boolean.

upstream_default value :
no

TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are trusted to
sign user certificates for authentication, or none to not use one. Keys are listed one per
line; empty lines and comments starting with '#' are allowed. If a certificate is
presented for authentication and has its signing CA key listed in this file, then it may
be used for authentication for any user listed in the certificate's principals list. Note
that certificates that lack a list of principals will not be permitted for authentication
using TrustedUserCAKeys. For more details on certificates, see the CERTIFICATES section in
ssh-keygen(1). Optional. Type uniline.

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. Optional. Type uniline.

X11Forwarding
Specifies whether X11 forwarding is permitted. The argument must be yes or no. The default
is no.

When X11 forwarding is enabled, there may be additional exposure to the server and to
client displays if the sshd(8) proxy display is configured to listen on the wildcard
address (see X11UseLocalhost), though this is not the default. Additionally, the
authentication spoofing and authentication data verification and substitution occur on the
client side. The security risk of using X11 forwarding is that the client's X11 display
server may be exposed to attack when the SSH client requests forwarding (see the warnings
for ForwardX11 in ssh_config(5)). A system administrator may have a stance in which they
want to protect clients that may expose themselves to attack by unwittingly requesting X11
forwarding, which can warrant a no setting.

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

upstream_default value :
no

X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or
to the wildcard address. By default, sshd binds the forwarding server to the loopback
address and sets the hostname part of the DISPLAY environment variable to localhost. This
prevents remote hosts from connecting to the proxy display. However, some older X11
clients may not function with this configuration. X11UseLocalhost may be set to no to
specify that the forwarding server should be bound to the wildcard address. The argument
must be yes or no. The default is yes. Optional. Type boolean.

upstream_default value :
yes

AuthorizedKeysFile2
This parameter is now ignored by Ssh. Deprecated Optional. Type list of uniline.

ChallengeResponseAuthentication
Deprecated Optional. Type boolean.

KeyRegenerationInterval
Deprecated Optional. Type uniline.

Protocol
Deprecated Optional. Type uniline.

RDomain
Deprecated Optional. Type uniline.

RSAAuthentication
Deprecated Optional. Type uniline.

RhostsRSAAuthentication
Deprecated Optional. Type uniline.

UsePrivilegeSeparation
Deprecated Optional. Type uniline.

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). Optional. Type enum. choice:
'any', 'inet', 'inet6'.

upstream_default value :
any

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

The supported ciphers are:

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

The default is:

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

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

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.
Optional. Type enum. choice: 'yes', 'delayed', 'no'.

upstream_default value :
yes

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

upstream_default value :
yes

FingerprintHash
Specifies the hash algorithm used when logging key fingerprints. Valid options are: md5
and sha256. The default is sha256. Optional. Type enum. choice: 'md5', 'sha256'.

upstream_default value :
sha256

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

upstream_default value :
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. Optional. Type boolean.

upstream_default value :
no

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

upstream_default value :
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. Optional. Type boolean.

upstream_default value :
no

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 Xgss-group14-sha256-, gss-group16-sha512-, gss-nistp256-sha256-,
gss-curve25519-sha256-, gss-gex-sha1-, gss-group14-sha1-X. This option only applies to
connections using GSSAPI. Optional. Type uniline.

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. Optional. Type uniline.

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). Optional. Type uniline.

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. Optional. Type uniline.

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". Optional. Type uniline.

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. Optional. Type boolean.

upstream_default value :
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. Optional. Type boolean.

upstream_default value :
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. Optional. Type
boolean.

upstream_default value :
yes

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

upstream_default value :
yes

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

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

The default is:

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

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

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

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

hostname|address ]:port

If port is not specified, sshd will listen on the address and all Port options specified.
The default is to listen on all local addresses. Multiple ListenAddress options are
permitted. Optional. Type uniline.

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. Optional. Type uniline.

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. Optional. Type uniline.

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

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

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". Optional.
Type uniline.

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

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

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

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

Only a subset of keywords may be used on the lines following a Match keyword. Available
keywords are AcceptEnv, AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
AllowTcpForwarding, AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
AuthorizedKeysCommandUser, AuthorizedKeysFile, AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile, Banner, CASignatureAlgorithms,
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. Optional. Type list of node of class
Sshd::MatchBlock .

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

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

upstream_default value :
10

ModuliFile
Specifies the moduli(5) file that contains the Diffie-Hellman groups used for the
Xdiffie-hellman-group-exchange-sha1X and Xdiffie-hellman-group-exchange-sha256X key
exchange methods. The default is /etc/ssh/moduli. Optional. Type uniline.

upstream_default value :
/etc/ssh/moduli

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. Optional. Type boolean.

upstream_default value :
no

PerSourceMaxStartups
Specifies the number of unauthenticated connections allowed from a given source address,
or none if there is no limit. This limit is applied in addition to MaxStartups, whichever
is lower. The default is none. Optional. Type uniline.

upstream_default value :
none

PerSourceNetBlockSize
Specifies the number of bits of source address that are grouped together for the purposes
of applying PerSourceMaxStartups limits. Values for IPv4 and optionally IPv6 may be
specified, separated by a colon. The default is 32:128, which means each address is
considered individually. Optional. Type uniline.

upstream_default value :
32:128

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. Optional. Type uniline.

Port
Specifies the port number that sshd(8) listens on. The default is 22. Multiple options of
this type are permitted. See also ListenAddress. Optional. Type uniline.

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. Optional. Type boolean.

upstream_default value :
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. Optional. Type boolean.

upstream_default value :
yes

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. Optional. Type
uniline.

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. Optional. Type boolean.

upstream_default value :
yes

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

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

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

By default no subsystems are defined. Optional. Type hash of uniline.

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. Optional. Type enum. choice: 'DAEMON', 'USER', 'AUTH',
'LOCAL0', 'LOCAL1', 'LOCAL2', 'LOCAL3', 'LOCAL4', 'LOCAL5', 'LOCAL6', 'LOCAL7'.

upstream_default value :
AUTH

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

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

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

This option was formerly called KeepAlive. Optional. Type boolean.

upstream_default value :
yes

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_keysfrom and sshd_config Match Host directives. Optional. Type
boolean.

upstream_default value :
no

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

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

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

upstream_default value :
no

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

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

upstream_default value :
/usr/bin/xauth

LICENSE

       LGPL2

NAME

DESCRIPTION

Elements

SEE ALSO

LICENSE