Ubuntu Manpage: sshd_config — OpenSSH SSH daemon configuration file

Provided by: openssh-server_7.6p1-4ubuntu0.7_amd64

NAME

     sshd_config — OpenSSH SSH 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. 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):

• ChallengeResponseAuthentication no
• X11Forwarding yes
• PrintMotd no
• AcceptEnv LANG LC_*
• Subsystem sftp /usr/lib/openssh/sftp-server
• UsePAM yes

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

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

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

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

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

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

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

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

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

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 comma-separated
lists of 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, pam, or skey, 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
PermitEmptyPassword is enabled), "password" and "publickey".

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

The program should produce on standard output zero or more lines of authorized_keys
output (see AUTHORIZED_KEYS in sshd(8)). If a key supplied by AuthorizedKeysCommand
does not successfully authenticate and authorize the user then public key
authentication continues using the usual AuthorizedKeysFile files. By default, no
AuthorizedKeysCommand is run.

AuthorizedKeysCommandUser
Specifies the user under whose account the AuthorizedKeysCommand is run. It is
recommended to use a dedicated user that has no other role on the host than running
authorized keys commands. If AuthorizedKeysCommand is specified but
AuthorizedKeysCommandUser is not, then sshd(8) will refuse to start.

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

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

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

AuthorizedPrincipalsCommandUser
Specifies the user under whose account the AuthorizedPrincipalsCommand is run. It
is recommended to use a dedicated user that has no other role on the host than
running authorized principals commands. If AuthorizedPrincipalsCommand is specified
but AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start.

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

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

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

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

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

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

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

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

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

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

The supported ciphers are:

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

The default is:

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

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

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

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.

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

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

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

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

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

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

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

DisableForwarding
Disables all forwarding features, including X11, ssh-agent(1), TCP and StreamLocal.
This option overrides all other forwarding-related options and may simplify
restricted configurations.

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

FingerprintHash
Specifies the hash algorithm used when logging key fingerprints. Valid options are:
md5 and sha256. The default is sha256.

ForceCommand
Forces the execution of the command specified by ForceCommand, ignoring any command
supplied by the client and ~/.ssh/rc if present. The command is invoked by using
the user's login shell with the -c option. This applies to shell, command, or
subsystem execution. It is most useful inside a Match block. The command
originally supplied by the client is available in the SSH_ORIGINAL_COMMAND
environment variable. Specifying a command of internal-sftp will force the use of
an in-process SFTP server that requires no support files when used with
ChrootDirectory. The default is none.

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

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

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.

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

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

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

HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for hostbased authentication as a
comma-separated pattern list. Alternately if the specified value begins with a ‘+’
character, then the specified key types will be appended to the default set instead
of replacing them. If the specified value begins with a ‘-’ character, then the
specified key types (including wildcards) will be removed from the default set
instead of replacing them. The default for this option is:

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

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

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

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

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

HostKey
Specifies a file containing a private host key used by SSH. The defaults are
/etc/ssh/ssh_host_rsa_key, /etc/ssh/ssh_host_ecdsa_key and
/etc/ssh/ssh_host_ed25519_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 algorithms that the server offers. The default for this
option is:

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

IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
HostbasedAuthentication.

/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is yes.

IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts during
HostbasedAuthentication. The default is no.

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, lowdelay, throughput, reliability, a
numeric value, or none to use the operating system default. This option may take
one or two arguments, separated by whitespace. If one argument is specified, it is
used as the packet class unconditionally. If two values are specified, the first is
automatically selected for interactive sessions and the second for non-interactive
sessions. The default is lowdelay for interactive sessions and throughput for non-
interactive sessions.

KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication. The argument to
this keyword must be yes or no. The default is to use whatever value
ChallengeResponseAuthentication is set to (by default yes).

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

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

KerberosOrLocalPasswd
If password authentication through Kerberos fails then the 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 value begins with a ‘+’ character,
then the specified methods will be appended to the default set instead of replacing
them. If the specified value begins with a ‘-’ character, then the specified
methods (including wildcards) will be removed from the default set instead of
replacing them. The supported algorithms are:

curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521

The default is:

curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1

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

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

ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port

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

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

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

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

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".

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, ChrootDirectory, ClientAliveCountMax,
ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand, GatewayPorts,
GSSAPIAuthentication, HostbasedAcceptedKeyTypes, HostbasedAuthentication,
HostbasedUsesNameFromPacketOnly, IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, LogLevel, MaxAuthTries, MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication, RekeyLimit, RevokedKeys,
StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
X11Forwarding and X11UseLocalHost.

MaxAuthTries
Specifies the maximum number of authentication attempts permitted per connection.
Once the number of failures reaches half this value, additional failures are logged.
The default is 6.

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

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

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

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.

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 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. 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, without-password, forced-commands-only, or no. The default is
prohibit-password.

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

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

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

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

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

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

PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in
~/.ssh/authorized_keys are processed by sshd(8). The default is no. Enabling
environment processing may enable users to bypass access restrictions in some
configurations using mechanisms such as LD_PRELOAD.

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

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

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

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

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

PubkeyAcceptedKeyTypes
Specifies the key types that will be accepted for public key authentication as a
comma-separated pattern list. Alternately if the specified value begins with a ‘+’
character, then the specified key types will be appended to the default set instead
of replacing them. If the specified value begins with a ‘-’ character, then the
specified key types (including wildcards) will be removed from the default set
instead of replacing them. The default for this option is:

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

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 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.

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).

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

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

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

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

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

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

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

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

By default no subsystems are defined.

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

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

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

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

This option was formerly called KeepAlive.

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

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 ChallengeResponseAuthentication and
PasswordAuthentication in addition to PAM account and session module processing for
all authentication types.

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

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

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

X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11 forwarding. This
prevents sshd from interfering with real X11 servers. The default is 10.

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

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

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

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

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

TIME FORMATS

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

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

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

     Time format examples:

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

TOKENS

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

           %%    A literal ‘%’.
           %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 username.

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

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

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

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

     ChrootDirectory accepts the tokens %%, %h, 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.

AUTHORS

     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron
     Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many
     bugs, re-added newer features and created OpenSSH.  Markus Friedl contributed the support
     for SSH protocol versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
     for privilege separation.

NAME

DESCRIPTION

TIME FORMATS

TOKENS

FILES

SEE ALSO

AUTHORS