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sshd_config.0
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SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)
NAME
sshd_config M-bM-^@M-^S OpenSSH daemon configuration file
DESCRIPTION
sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-
argument pairs, one per line. Unless noted otherwise, for each keyword,
the first obtained value will be used. Lines starting with M-bM-^@M-^X#M-bM-^@M-^Y and empty
lines are interpreted as comments. Arguments may optionally be enclosed
in double quotes (") in order to represent arguments containing spaces.
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 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 M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. 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 groups directives are processed in the
following order: DenyGroups, AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.
AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is
permitted. The available options are yes (the default) or all to
allow StreamLocal forwarding, no to prevent all 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 users directives are
processed in the following order: DenyUsers, AllowUsers.
See PATTERNS in ssh_config(5) for more information on patterns.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.
AuthenticationMethods
Specifies the authentication methods that must be successfully
completed for a user to be granted access. This option must be
followed by one or more lists of comma-separated authentication
method names, or by the single string any to indicate the 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".
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.
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 M-bM-^@M-^X#M-bM-^@M-^Y 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 M-bM-^@M-^S 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.
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,
rsa-sha2-512,rsa-sha2-256
If the specified list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
specified algorithms will be appended to the default set instead
of replacing them. If the specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y
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.
ChannelTimeout
Specifies whether and how quickly sshd(8) should close inactive
channels. Timeouts are specified as one or more M-bM-^@M-^\type=intervalM-bM-^@M-^]
pairs separated by whitespace, where the M-bM-^@M-^\typeM-bM-^@M-^] must be the
special keyword M-bM-^@M-^\globalM-bM-^@M-^] or a channel type name from the list
below, optionally containing wildcard characters.
The timeout value M-bM-^@M-^\intervalM-bM-^@M-^] is specified in seconds or may use
any of the units documented in the TIME FORMATS section. For
example, M-bM-^@M-^\session=5mM-bM-^@M-^] would cause interactive sessions to
terminate after five minutes of inactivity. Specifying a zero
value disables the inactivity timeout.
The special timeout M-bM-^@M-^\globalM-bM-^@M-^] applies to all active channels,
taken together. Traffic on any active channel will reset the
timeout, but when the timeout expires then all open channels will
be closed. Note that this global timeout is not matched by
wildcards and must be specified explicitly.
The available channel type names include:
agent-connection
Open connections to ssh-agent(1).
direct-tcpip, [email protected]
Open TCP or Unix socket (respectively) connections that
have been established from a ssh(1) local forwarding,
i.e. LocalForward or DynamicForward.
forwarded-tcpip, [email protected]
Open TCP or Unix socket (respectively) connections that
have been established to a sshd(8) listening on behalf of
a ssh(1) remote forwarding, i.e. RemoteForward.
session
The interactive main session, including shell session,
command execution, scp(1), sftp(1), etc.
tun-connection
Open TunnelForward connections.
x11-connection
Open X11 forwarding sessions.
Note that in all the above cases, terminating an inactive session
does not guarantee to remove all resources associated with the
session, e.g. shell processes or X11 clients relating to the
session may continue to execute.
Moreover, terminating an inactive channel or session does not
necessarily close the SSH connection, nor does it prevent a
client from requesting another channel of the same type. In
particular, expiring an inactive forwarding session does not
prevent another identical forwarding from being subsequently
created.
The default is not to expire channels of any type for inactivity.
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 group or others. 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 list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
then the specified ciphers will be appended to the default set
instead of replacing them. If the specified list begins with a
M-bM-^@M-^X-M-bM-^@M-^Y 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 M-bM-^@M-^X^M-bM-^@M-^Y 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
The default is:
aes128-ctr,aes192-ctr,aes256-ctr,
The list of available ciphers may also be obtained using "ssh -Q
cipher".
ClientAliveCountMax
Sets the number of client alive messages which may be sent
without sshd(8) receiving any messages back from the client. If
this threshold is reached while client alive messages are being
sent, sshd will disconnect the client, terminating the session.
It is important to note that the use of client alive messages is
very different from TCPKeepAlive. The client alive messages are
sent through the encrypted channel and therefore will not be
spoofable. The TCP keepalive option enabled by TCPKeepAlive is
spoofable. The client alive mechanism is valuable when the
client or server depend on knowing when a connection has become
unresponsive.
The default value is 3. If ClientAliveInterval is set to 15, and
ClientAliveCountMax is left at the default, unresponsive SSH
clients will be disconnected after approximately 45 seconds.
Setting a zero ClientAliveCountMax disables connection
termination.
ClientAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the client, sshd(8) will send a message
through the encrypted channel to request a response from the
client. The default is 0, indicating that these messages will
not be sent to the client.
Compression
Specifies whether compression is enabled after the user has
authenticated successfully. The argument must be yes, delayed (a
legacy synonym for yes) or no. The default is yes.
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.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.
DenyUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. Login is disallowed for user names that
match one of the patterns. Only user names are valid; a
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.
This keyword may appear multiple times in sshd_config with each
instance appending to the list.
DisableForwarding
Disables all forwarding features, including X11, ssh-agent(1),
TCP and StreamLocal. This option overrides all other 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.
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.
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 M-bM-^@M-^X+M-bM-^@M-^Y character,
then the specified signature algorithms will be appended to the
default set instead of replacing them. If the specified list
begins with a M-bM-^@M-^X-M-bM-^@M-^Y 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 M-bM-^@M-^X^M-bM-^@M-^Y character, then the specified signature algorithms will be
placed at the head of the default set. The default for this
option is:
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be obtained
using "ssh -Q HostbasedAcceptedAlgorithms". This was formerly
named HostbasedAcceptedKeyTypes.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful public key client host authentication is
allowed (host-based authentication). The default is no.
HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a
reverse name lookup when matching the name in the ~/.shosts,
~/.rhosts, and /etc/hosts.equiv files during
HostbasedAuthentication. A setting of yes means that sshd(8)
uses the name supplied by the client rather than attempting to
resolve the name from the TCP connection itself. The default is
no.
HostCertificate
Specifies a file containing a public host certificate. The
certificate's public key must match a private host key already
specified by HostKey. The default behaviour of sshd(8) is not to
load any certificates.
HostKey
Specifies a file containing a private host key used by SSH. The
defaults are /etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.
Note that sshd(8) will refuse to use a file if it is group/world-
accessible and that the HostKeyAlgorithms option restricts which
of the keys are actually used by sshd(8).
It is possible to have multiple host key files. It is also
possible to specify public host key files instead. In this case
operations on the private key will be delegated to an
ssh-agent(1).
HostKeyAgent
Identifies the UNIX-domain socket used to communicate with an
agent that has access to the private host keys. If the string
"SSH_AUTH_SOCK" is specified, the location of the socket will be
read from the SSH_AUTH_SOCK environment variable.
HostKeyAlgorithms
Specifies the host key signature algorithms that the server
offers. The default for this option is:
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be obtained
using "ssh -Q HostKeyAlgorithms".
IgnoreRhosts
Specifies whether to ignore per-user .rhosts and .shosts files
during HostbasedAuthentication. The system-wide /etc/hosts.equiv
and /etc/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.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during HostbasedAuthentication and use only
the system-wide known hosts file /etc/ssh/ssh_known_hosts. The
default is M-bM-^@M-^\noM-bM-^@M-^].
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.
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 af21 (Low-Latency
Data) for interactive sessions and cs1 (Lower Effort) for non-
interactive sessions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication.
All authentication styles from login.conf(5) are supported. The
default is yes. The argument to this keyword must be yes or no.
ChallengeResponseAuthentication is a deprecated alias for this.
KerberosAuthentication
Specifies whether the password provided by the user for
PasswordAuthentication will be validated through the Kerberos
KDC. To use this option, the server needs a Kerberos servtab
which allows the verification of the KDC's identity. The default
is no.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to
acquire an AFS token 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 permitted KEX (Key Exchange) algorithms that the
server will offer to clients. The ordering of this list is not
important, as the client specifies the preference order.
Multiple algorithms must be comma-separated.
If the specified list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
specified algorithms will be appended to the default set instead
of replacing them. If the specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y
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 M-bM-^@M-^X^M-bM-^@M-^Y character, then the
specified algorithms will be placed at the head of the default
set.
The supported algorithms are:
curve25519-sha256
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
The default is:
curve25519-sha256,[email protected],
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 supported key exchange algorithms may also be
obtained using "ssh -Q KexAlgorithms".
ListenAddress
Specifies the local addresses sshd(8) should listen on. The
following forms may be used:
ListenAddress hostname|address [rdomain domain]
ListenAddress hostname:port [rdomain domain]
ListenAddress IPv4_address:port [rdomain domain]
ListenAddress [hostname|address]:port [rdomain domain]
The optional rdomain qualifier requests sshd(8) listen in an
explicit routing domain. 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 on the current
default routing domain. Multiple ListenAddress options are
permitted. For more information on routing domains, see
rdomain(4).
LoginGraceTime
The server disconnects after this time if the user has not
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.
LogVerbose
Specify one or more overrides to LogLevel. An override consists
of a pattern lists that matches the source file, function and
line number to force detailed logging for. For example, an
override pattern of:
kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
would enable detailed logging for line 1000 of kex.c, everything
in the kex_exchange_identification() function, and all code in
the packet.c file. This option is intended for debugging and no
overrides are enabled by default.
MACs Specifies the available MAC (message authentication code)
algorithms. The MAC algorithm is used for data integrity
protection. Multiple algorithms must be comma-separated. If the
specified list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified
algorithms will be appended to the default set instead of
replacing them. If the specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y
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 M-bM-^@M-^X^M-bM-^@M-^Y 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
The default is:
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, RDomain,
and Address (with RDomain representing the rdomain(4) on which
the connection was received).
The match patterns may consist of single entries or comma-
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, ChannelTimeout, ChrootDirectory,
ClientAliveCountMax, ClientAliveInterval, DenyGroups, DenyUsers,
DisableForwarding, ExposeAuthInfo, ForceCommand, GatewayPorts,
GSSAPIAuthentication, HostbasedAcceptedAlgorithms,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
IgnoreRhosts, Include, IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, LogLevel, MaxAuthTries, MaxSessions,
PAMServiceName, PasswordAuthentication, PermitEmptyPasswords,
PermitListen, PermitOpen, PermitRootLogin, PermitTTY,
PermitTunnel, PermitUserRC, PubkeyAcceptedAlgorithms,
PubkeyAuthentication, PubkeyAuthOptions, RekeyLimit, RevokedKeys,
RDomain, SetEnv, StreamLocalBindMask, StreamLocalBindUnlink,
TrustedUserCAKeys, UnusedConnectionTimeout, 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).
ModuliFile
Specifies the moduli(5) file that contains the Diffie-Hellman
groups used for the M-bM-^@M-^\diffie-hellman-group-exchange-sha1M-bM-^@M-^] and
M-bM-^@M-^\diffie-hellman-group-exchange-sha256M-bM-^@M-^] key exchange methods. The
default is /etc/moduli.
PAMServiceName
Specifies the service name used for Pluggable Authentication
Modules (PAM) authentication, authorisation and session controls
when UsePAM is enabled. The default is sshd.
PasswordAuthentication
Specifies whether password authentication is allowed. The
default is sshd.
PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings. The
default is no.
PermitListen
Specifies the addresses/ports on which a remote TCP port
forwarding may listen. The listen specification must be one of
the following forms:
PermitListen port
PermitListen host:port
Multiple permissions may be specified by separating them with
whitespace. An argument of any can be used to remove all
restrictions and permit any listen requests. An 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 M-bM-^@M-^X*M-bM-^@M-^Y 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 M-bM-^@M-^\localhostM-bM-^@M-^] if no
listen host was specifically requested, and this name is treated
differently to explicit localhost addresses of M-bM-^@M-^\127.0.0.1M-bM-^@M-^] and
M-bM-^@M-^\::1M-bM-^@M-^].
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 M-bM-^@M-^X*M-bM-^@M-^Y can be used for host or port to allow all hosts or
ports respectively. Otherwise, no pattern matching or address
lookups are performed on supplied names. By default all port
forwarding requests are permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The argument
must be yes, prohibit-password, forced-commands-only, or no. The
default is prohibit-password.
If this option is set to prohibit-password (or its deprecated
alias, without-password), password and keyboard-interactive
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). Valid options
are yes, no or a pattern-list specifying which environment
variable names to accept (for example "LANG,LC_*"). The default
is no. Enabling environment processing may enable users to
bypass access restrictions in some configurations using
mechanisms such as LD_PRELOAD.
PermitUserRC
Specifies whether any ~/.ssh/rc file is executed. The default is
yes.
PerSourceMaxStartups
Specifies the number of unauthenticated connections allowed from
a given source address, or M-bM-^@M-^\noneM-bM-^@M-^] if there is no limit. This
limit is applied in addition to MaxStartups, whichever is lower.
The default is none.
PerSourceNetBlockSize
Specifies the number of bits of source address that are grouped
together for the purposes of applying PerSourceMaxStartups
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.
PerSourcePenalties
Controls penalties for various conditions that may represent
attacks on sshd(8). If a penalty is enforced against a client
then its source address and any others in the same network, as
defined by PerSourceNetBlockSize, will be refused connection for
a period.
A penalty doesn't affect concurrent connections in progress, but
multiple penalties from the same source from concurrent
connections will accumulate up to a maximum. Conversely,
penalties are not applied until a minimum threshold time has been
accumulated.
Penalties are enabled by default with the default settings listed
below but may disabled using the off keyword. The defaults may
be overridden by specifying one or more of the keywords below,
separated by whitespace. All keywords accept arguments, e.g.
"crash:2m".
crash:duration
Specifies how long to refuse clients that cause a crash
of sshd(8) (default: 90s).
authfail:duration
Specifies how long to refuse clients that disconnect
after making one or more unsuccessful authentication
attempts (default: 5s).
noauth:duration
Specifies how long to refuse clients that disconnect
without attempting authentication (default: 1s). This
timeout should be used cautiously otherwise it may
penalise legitimate scanning tools such as
ssh-keyscan(1).
grace-exceeded:duration
Specifies how long to refuse clients that fail to
authenticate after LoginGraceTime (default: 20s).
max:duration
Specifies the maximum time a particular source address
range will be refused access for (default: 10m).
Repeated penalties will accumulate up to this maximum.
min:duration
Specifies the minimum penalty that must accrue before
enforcement begins (default: 15s).
max-sources4:number, max-sources6:number
Specifies the maximum number of client IPv4 and IPv6
address ranges to track for penalties (default: 65536 for
both).
overflow:mode
Controls how the server behaves when max-sources4 or
max-sources6 is exceeded. There are two operating modes:
deny-all, which denies all incoming connections other
than those exempted via PerSourcePenaltyExemptList until
a penalty expires, and permissive, which allows new
connections by removing existing penalties early
(default: permissive). Note that client penalties below
the min threshold count against the total number of