sshd_config —
OpenSSH SSH daemon configuration file
sshd(8) reads configuration data
from
/etc/ssh/sshd_config (or the file specified
with
-f on the command line). The file contains
keyword-argument pairs, one per line. For each keyword, the first obtained
value will be used. Lines starting with
‘
#
’ and empty lines are interpreted as
comments. Arguments may optionally be enclosed in double quotes (") in
order to represent arguments containing spaces.
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
‘*
’ 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 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)). 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.
- CASignatureAlgorithms
- Specifies which algorithms are allowed for signing of
certificates by certificate authorities (CAs). The default is:
ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
Certificates signed using other algorithms will not be accepted for public
key or host-based authentication.
- ChallengeResponseAuthentication
- Specifies whether challenge-response authentication is
allowed (e.g. via PAM or through authentication styles supported in
login.conf(5)) 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.
- 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.
- 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.
- HostbasedAcceptedKeyTypes
- Specifies the key types that will be accepted for hostbased
authentication as a list of comma-separated patterns. 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,
rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,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_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 algorithms that the server offers.
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,
rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
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/shosts.equiv are still used. The default
is yes.
- 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.
- 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 af21 (Low-Latency Data) for
interactive sessions and cs1 (Lower Effort)
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-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,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,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:
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.
- 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,
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,
ChrootDirectory,
ClientAliveCountMax,
ClientAliveInterval,
DenyGroups,
DenyUsers,
ForceCommand,
GatewayPorts,
GSSAPIAuthentication,
HostbasedAcceptedKeyTypes,
HostbasedAuthentication,
HostbasedUsesNameFromPacketOnly,
IPQoS,
KbdInteractiveAuthentication,
KerberosAuthentication,
LogLevel,
MaxAuthTries,
MaxSessions,
PasswordAuthentication,
PermitEmptyPasswords,
PermitListen,
PermitOpen,
PermitRootLogin,
PermitTTY,
PermitTunnel,
PermitUserRC,
PubkeyAcceptedKeyTypes,
PubkeyAuthentication,
RekeyLimit,
RevokedKeys,
RDomain, SetEnv,
StreamLocalBindMask,
StreamLocalBindUnlink,
TrustedUserCAKeys,
X11DisplayOffset,
X11Forwarding and
X11UseLocalHost.
- MaxAuthTries
- Specifies the maximum number of authentication attempts
permitted per connection. Once the number of failures reaches half this
value, additional failures are logged. The default is 6.
- MaxSessions
- Specifies the maximum number of open shell, login or
subsystem (e.g. sftp) sessions permitted per network connection. Multiple
sessions may be established by clients that support connection
multiplexing. Setting MaxSessions to 1 will
effectively disable session multiplexing, whereas setting it to 0 will
prevent all shell, login and subsystem sessions while still permitting
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.
- PermitListen
- Specifies the addresses/ports on which a remote TCP port
forwarding may listen. The listen specification must be one of the
following forms:
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 this name is treated differently to
explicit localhost addresses of “127.0.0.1” and
“::1”.
- PermitOpen
- Specifies the destinations to which TCP port forwarding is
permitted. The forwarding specification must be one of the following
forms:
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,
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.
- PidFile
- Specifies the file that contains the process ID of the SSH
daemon, or none to not write one. The default
is /var/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 list of comma-separated patterns. 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,
rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
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).
- RDomain
- Specifies an explicit routing domain that is applied after
authentication has completed. The user session, as well and any forwarded
or listening IP sockets, will be bound to this
rdomain(4). If the
routing domain is set to %D, then the domain
in which the incoming connection was received will be applied.
- SetEnv
- Specifies one or more environment variables to set in child
sessions started by
sshd(8) as
“NAME=VALUE”. The environment value may be quoted (e.g. if
it contains whitespace characters). Environment variables set by
SetEnv override the default environment and
any variables specified by the user via
AcceptEnv or
PermitUserEnvironment.
- StreamLocalBindMask
- Sets the octal file creation mode mask (umask) used when
creating a Unix-domain socket file for local or remote port forwarding.
This option is only used for port forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain socket file that is
readable and writable only by the owner. Note that not all operating
systems honor the file mode on Unix-domain socket files.
- StreamLocalBindUnlink
- Specifies whether to remove an existing Unix-domain socket
file for local or remote port forwarding before creating a new one. If the
socket file already exists and
StreamLocalBindUnlink is not enabled,
sshd will be unable to forward the port to
the Unix-domain socket file. This option is only used for port forwarding
to a Unix-domain socket file.
The argument must be yes or
no. The default is
no.
- StrictModes
- Specifies whether
sshd(8) should check file
modes and ownership of the user's files and home directory before
accepting login. This is normally desirable because novices sometimes
accidentally leave their directory or files world-writable. The default is
yes. Note that this does not apply to
ChrootDirectory, whose permissions and
ownership are checked unconditionally.
- Subsystem
- Configures an external subsystem (e.g. file transfer
daemon). Arguments should be a subsystem name and a command (with optional
arguments) to execute upon subsystem 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.
- 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/X11R6/bin/xauth.
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)
Arguments to some keywords can make use of tokens, which are expanded at
runtime:
- %%
- A literal ‘%’.
- %D
- The routing domain in which the incoming connection was
received.
- %F
- The fingerprint of the CA key.
- %f
- The fingerprint of the key or certificate.
- %h
- The home directory of the user.
- %i
- The key ID in the certificate.
- %K
- The base64-encoded CA key.
- %k
- The base64-encoded key or certificate for
authentication.
- %s
- The serial number of the certificate.
- %T
- The type of the CA key.
- %t
- The key or certificate type.
- %U
- The numeric user ID of the target user.
- %u
- The username.
AuthorizedKeysCommand accepts the tokens %%, %f,
%h, %k, %t, %U, and %u.
AuthorizedKeysFile accepts the tokens %%, %h, %U,
and %u.
AuthorizedPrincipalsCommand accepts the tokens %%,
%F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
AuthorizedPrincipalsFile accepts the tokens %%, %h,
%U, and %u.
ChrootDirectory accepts the tokens %%, %h, %U, and
%u.
RoutingDomain accepts the token %D.
- /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.
sftp-server(8),
sshd(8)
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.