File Formats sshdconfig(4)
NAME
sshdconfig - sshd configuration file
SYNOPSIS
/etc/ssh/sshdconfig
DESCRIPTION
The sshd(1M) daemon reads configuration data from
/etc/ssh/sshdconfig (or the file specified with sshd -f on
the command line). The file contains keyword-value pairs,
one per line. A line starting with a hash mark (#) and empty
lines are interpreted as comments.
The sshdconfig file supports the following keywords. Unless
otherwise noted, keywords and their arguments are case-
insensitive.
AllowGroups
This keyword can be followed by a number of group names,
separated by spaces. If specified, login is allowed only
for users whose primary group matches one of the pat-
terns. Asterisk (*) and question mark (?) can be used as
wildcards in the patterns. Only group names are valid; a
numerical group ID is not recognized. By default, login
is allowed regardless of the primary group.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The
default is yes. 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 number of user names,
separated by spaces. If specified, login is allowed only
for user names that match one of the patterns. Asterisk
(*) and question mark (?) can be used as wildcards in
the patterns. Only user names are valid; a numerical
user ID is not recognized. By default login is allowed
regardless of the user name.
If a specified pattern takes the form user@host then
user and host are checked separately, restricting logins
to particular users from particular hosts.
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AuthorizedKeysFile
Specifies the file that contains the public keys that
can be used for user authentication. AuthorizedKeysFile
can contain tokens of the form %T, which are substituted
during connection set-up. The following tokens are
defined: %% is replaced by a literal %, %h is replaced
by the home directory of the user being authenticated
and %u is replaced by the username of that user. After
expansion, AuthorizedKeysFile is taken to be an absolute
path or one relative to the user's home directory. The
default is .ssh/authorizedkeys.
Banner
In some jurisdictions, sending a warning message before
authentication can be relevant for getting legal protec-
tion. The contents of the specified file are sent to the
remote user before authentication is allowed. This
option is only available for protocol version 2. By
default, no banner is displayed.
Ciphers
Specifies the ciphers allowed for protocol version 2.
Cipher ordering on the server side is not relevant. Mul-
tiple ciphers must be comma separated.
Valid ciphers are: aes128-ctr, aes192-ctr, aes256-ctr,
aes128-cbc, aes192-cbc, aes256-cbc, arcfour, arcfour128,
arcfour256, 3des-cbc, and blowfish-cbc.
The default cipher list is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour128,
arcfour256,arcfour
Using CBC modes on the server side is not recommended
due to potential security issues in connection with the
SH protocol version 2.
ClientAliveCountMax
Sets the number of client alive messages, (see Clien-
tAliveInterval), that can be sent without sshd receiving
any messages back from the client. If this threshold is
reached while client alive messages are being sent, sshd
disconnects the client, terminating the session. The use
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of client alive messages is very different from
KeepAlive. The client alive messages are sent through
the encrypted channel and therefore are not spoofable.
The TCP keepalive option enabled by KeepAlive is spoofa-
ble. The client alive mechanism is valuable when a
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 are disconnected after approxi-
mately 45 seconds.
ClientAliveInterval
Sets a timeout interval in seconds after which, if no
data has been received from the client, sshd sends a
message through the encrypted channel to request a
response from the client. The default is 0, indicating
that these messages are not sent to the client. This
option applies only to protocol version 2.
Compression
Controls whether the server allows the client to nego-
tiate the use of compression. The default is yes.
DenyGroups
Can be followed by a number of group names, separated by
spaces. Users whose primary group matches one of the
patterns are not allowed to log in. Asterisk (*) and
question mark (?) can be used as wildcards in the pat-
terns. Only group names are valid; a numerical group ID
is not recognized. By default, login is allowed regard-
less of the primary group.
DenyUsers
Can be followed by a number of user names, separated by
spaces. Login is disallowed for user names that match
one of the patterns. Asterisk (*) and question mark (?)
can be used as wildcards in the patterns. Only user
names are valid; a numerical user ID is not recognized.
By default, login is allowed regardless of the user
name.
If a specified pattern takes the form user@host then
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user and host are checked separately, disallowing logins
to particular users from particular hosts.
GatewayPorts
Specifies whether remote hosts are allowed to connect to
ports forwarded for the client. By default, sshd 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 bind remote port forwardings to the wildcard
address, thus allowing remote hosts to connect to for-
warded ports.
The argument can 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. See also RemoteForward in sshconfig(4).
GSAPIAuthentication
Enables/disables GS-API user authentication. The
default is yes.
Currently sshd authorizes client user principals to user
accounts as follows: if the principal name matches the
requested user account, then the principal is author-
ized. Otherwise, GS-API authentication fails.
GSAPIKeyExchange
Enables/disables GS-API-authenticated key exchanges.
The default is yes.
This option also enables the use of the GS-API to
authenticate the user to server after the key exchange.
GS-API key exchange can succeed but the subsequent
authentication using the GS-API fail if the server does
not authorize the user's GS principal name to the tar-
get user account.
Currently sshd authorizes client user principals to user
accounts as follows: if the principal name matches the
requested user account, then the principal is author-
ized. Otherwise, GS-API authentication fails.
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GSAPIStoreDelegatedCredentials
Enables/disables the use of delegated GS-API creden-
tials on the server-side. The default is yes.
Specifically, this option, when enabled, causes the
server to store delegated GS-API credentials in the
user's default GS-API credential store (which for the
Kerberos V mechanism means /tmp/krb5cc).
Note -
sshd does not take any steps to explicitly destroy
stored delegated GS-API credentials upon logout. It
is the responsibility of PAM modules to destroy
credentials associated with a session.
HostbasedAuthentication
Specifies whether to try rhosts-based authentication
with public key authentication. The argument must be yes
or no. The default is no. This option applies to proto-
col version 2 only and is similar to RhostsRSAuthenti-
cation. See sshd(1M) for guidelines on setting up host-
based authentication.
HostbasedUsesNameFromPacketOnly
Controls which hostname is searched for in the files
~/.shosts, /etc/shosts.equiv, and /etc/hosts.equiv. If
this parameter is set to yes, the server uses the name
the client claimed for itself and signed with that
host's key. If set to no, the default, the server uses
the name to which the client's IP address resolves.
Setting this parameter to no disables host-based authen-
tication when using NAT or when the client gets to the
server indirectly through a port-forwarding firewall.
HostKey
Specifies the file containing the private host key used
by SH. The default is /etc/ssh/sshhostkey for proto-
col version 1, and /etc/ssh/sshhostrsakey and
/etc/ssh/sshhostdsakey for protocol version 2. sshd
refuses to use a file if it is group/world-accessible.
It is possible to have multiple host key files. rsa1
keys are used for version 1 and dsa or rsa are used for
version 2 of the SH protocol.
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IgnoreRhosts
Specifies that .rhosts and .shosts files are not used in
authentication. /etc/hosts.equiv and /etc/shosts.equiv
are still used. The default is yes. This parameter
applies to both protocol versions 1 and 2.
IgnoreUserKnownHosts
Specifies whether sshd should ignore the user's
$HOME/.ssh/knownhosts during RhostsRSAuthentication.
The default is no. This parameter applies to both proto-
col versions 1 and 2.
KbdInteractiveAuthentication
Specifies whether authentication by means of the
"keyboard-interactive" authentication method (and PAM)
is allowed. Defaults to yes. (Deprecated: this parameter
can only be set to yes.)
KeepAlive
Specifies whether the system should send keepalive mes-
sages to the other side. If they are sent, death of the
connection or crash of one of the machines is properly
noticed. However, this means that connections die if the
route is down temporarily, which can be an annoyance. On
the other hand, if keepalives are not sent, sessions can
hang indefinitely on the server, leaving ghost users and
consuming server resources.
The default is yes (to send keepalives), and the server
notices if the network goes down or the client host
reboots. This avoids infinitely hanging sessions.
To disable keepalives, the value should be set to no in
both the server and the client configuration files.
KeyRegenerationInterval
In protocol version 1, the ephemeral server key is
automatically regenerated after this many seconds (if it
has been used). The purpose of regeneration is to
prevent decrypting captured sessions by later breaking
into the machine and stealing the keys. The key is never
stored anywhere. If the value is 0, the key is never
regenerated. The default is 3600 (seconds).
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ListenAddress
Specifies what local address sshd should listen on. The
following forms can be used:
ListenAddress hostIPv4addrIPv6addr
ListenAddress hostIPv4addr:port
ListenAddress [hostIPv6addr]:port
If port is not specified, sshd listens on the address
and all prior Port options specified. The default is to
listen on all local addresses. Multiple ListenAddress
options are permitted. Additionally, any Port options
must precede this option for non-port qualified
addresses.
The default is to listen on all local addresses. Multi-
ple options of this type are permitted. Additionally,
the Ports options must precede this option.
LoginGraceTime
The server disconnects after this time (in seconds) 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 mes-
sages from sshd. The possible values are: QUIET, FATAL,
EROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
The default is INFO. DEBUG2 and DEBUG3 each specify
higher levels of debugging output. Logging with level
DEBUG violates the privacy of users and is not recom-
mended.
LookupClientHostnames
Specifies whether or not to lookup the names of client's
addresses. Defaults to yes.
MACs
Specifies the available MAC (message authentication
code) algorithms. The MAC algorithm is used in protocol
version 2 for data integrity protection. Multiple algo-
rithms must be comma-separated. The default is hmac-
md5,hmac-sha1,hmac-sha1-96,hmac-md5-96.
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MaxStartups
Specifies the maximum number of concurrent unauthenti-
cated connections to the sshd daemon. Additional connec-
tions are dropped until authentication succeeds or the
LoginGraceTime expires for a connection. The default is
10.
Alternatively, random early drop can be enabled by
specifying the three colon-separated values
start:rate:full (for example, 10:30:60). Referring to
this example, sshd refuse connection attempts with a
probability of rate/100 (30% in our example) if there
are currently 10 (from the start field) unauthenticated
connections. The probability increases linearly and all
connection attempts are refused if the number of unau-
thenticated connections reaches full (60 in our exam-
ple).
PasswordAuthentication
Specifies whether password authentication is allowed.
The default is yes. This option applies to both protocol
versions 1 and 2.
PermitEmptyPasswords
When password or keyboard-interactive authentication is
allowed, it specifies whether the server allows login to
accounts with empty password strings.
If not set then the /etc/default/login PASREQ value is
used instead.
PASREQ=no is equivalent to PermitEmptyPasswords yes.
PASREQ=yes is equivalent to PermitEmptyPasswords no. If
neither PermitEmptyPasswords or PASREQ are set the
default is no.
PermitRootLogin
Specifies whether the root can log in using ssh(1). The
argument must be yes, without-password, forced-
commands-only, or no. without-password means that root
cannot be authenticated using the "password" or
"keyboard-interactive" methods (see description of
KbdInteractiveAuthentication). forced-commands-only
means that authentication is allowed only for publickey
(for SHv2, or RSA, for SHv1) and only if the matching
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File Formats sshdconfig(4)
authorizedkeys entry for root has a command=
option.
In Solaris, the default /etc/ssh/sshdconfig file is
shipped with PermitRootLogin set to no. If unset by the
administrator, then CONSOLE parameter from
/etc/default/login supplies the default value as fol-
lows: if the CONSOLE parameter is not commented out (it
can even be empty, that is, "CONSOLE="), then without-
password is used as default value. If CONSOLE is com-
mented out, then the default for PermitRootLogin is yes.
The without-password and forced-commands-only settings
are useful for, for example, performing remote adminis-
tration and backups using trusted public keys for
authentication of the remote client, without allowing
access to the root account using passwords.
PermitUserEnvironment
Specifies whether a user's ~/.ssh/environment on the
server side and environment options in the Author-
izedKeysFile file are processed by sshd. The default is
no. Enabling environment processing can enable users to
bypass access restrictions in some configurations using
mechanisms such as LDPRELOAD.
Environment setting from a relevant entry in Author-
izedKeysFile file is processed only if the user was
authenticated using the public key authentication
method. Of the two files used, values of variables set
in ~/.ssh/environment are of higher priority.
PidFile
Allows you to specify an alternative to
/var/run/sshd.pid, the default file for storing the PID
of the sshd listening for connections. See sshd(1M).
Port
Specifies the port number that sshd listens on. The
default is 22. Multiple options of this type are permit-
ted. See also ListenAddress.
PrintLastLog
Specifies whether sshd should display the date and time
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File Formats sshdconfig(4)
when the user last logged in. The default is yes.
PrintMotd
Specifies whether sshd should display the contents of
/etc/motd when a user logs in interactively. (On some
systems it is also displayed by the shell or a shell
startup file, such as /etc/profile.) The default is yes.
Protocol
Specifies the protocol versions sshd should support in
order of preference. The possible values are 1 and 2.
Multiple versions must be comma-separated. The default
is 2,1. This means that ssh tries version 2 and falls
back to version 1 if version 2 is not available.
PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is yes. This option applies to protocol ver-
sion 2 only.
RhostsAuthentication
Specifies whether authentication using rhosts or
/etc/hosts.equiv files is sufficient. Normally, this
method should not be permitted because it is insecure.
RhostsRSAuthentication should be used instead, because
it performs RSA-based host authentication in addition to
normal rhosts or /etc/hosts.equiv authentication. The
default is no. This parameter applies only to protocol
version 1.
RhostsRSAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentica-
tion together with successful RSA host authentication is
allowed. The default is no. This parameter applies only
to protocol version 1.
RSAuthentication
Specifies whether pure RSA authentication is allowed.
The default is yes. This option applies to protocol ver-
sion 1 only.
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ServerKeyBits
Defines the number of bits in the ephemeral protocol
version 1 server key. The minimum value is 512, and the
default is 768.
StrictModes
Specifies whether sshd should check file modes and own-
ership 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.
Subsystem
Configures an external subsystem (for example, a file
transfer daemon). Arguments should be a subsystem name
and a command to execute upon subsystem request. The
command sftp-server(1M) implements the sftp file
transfer subsystem. By default, no subsystems are
defined. This option applies to protocol version 2 only.
SyslogFacility
Gives the facility code that is used when logging mes-
sages from sshd. The possible values are: DAEMON, USER,
AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5,
LOCAL6, and LOCAL7. The default is AUTH.
UseOpenSLEngine
Specifies whether ssh should use the OpenSL PKCS#11
engine for off loading cryptographic operations to the
Cryptographic Framework. Cryptographic operations are
accelerated according to the available installed plug-
ins. When no suitable plug-ins are present this option
does not have an effect. The default is yes.
VerifyReverseMapping
Specifies whether sshd should try to verify the remote
host name and check that the resolved host name for the
remote IP address maps back to the very same IP address.
(A yes setting means "verify".) Setting this parameter
to no can be useful where DNS servers might be down and
thus cause sshd to spend much time trying to resolve the
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client's IP address to a name. This feature is useful
for Internet-facing servers. The default is no.
X11DisplayOffset
Specifies the first display number available for sshd's
X11 forwarding. This prevents sshd from interfering with
real X11 servers. The default is 10.
X11Forwarding
Specifies whether X11 forwarding is permitted. The
default is yes. Disabling X11 forwarding does not
improve security in any way, as users can always install
their own forwarders.
When X11 forwarding is enabled, there can be additional
exposure to the server and to client displays if the
sshd proxy display is configured to listen on the wild-
card address (see X11UseLocalhost). However, 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 can
be exposed to attack when the ssh client requests for-
warding (see the warnings for ForwardX11 in
sshconfig(4)). A system administrator who wants to pro-
tect clients that expose themselves to attack by unwit-
tingly requesting X11 forwarding, should specify a no
setting.
Disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install
their own forwarders.
X11UseLocalhost
Specifies whether sshd 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. How-
ever, some older X11 clients might not function with
this configuration. X11UseLocalhost can 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.
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XAuthLocation
Specifies the location of the xauth(1) program. The
default is /usr/openwin/bin/xauth and sshd attempts to
open it when X11 forwarding is enabled.
Time Formats
sshd command-line arguments and configuration file options
that specify time can be expressed using a sequence of the
form: time[qualifier,] where time is a positive integer
value and qualifier is one of the following:
seconds
s S seconds
m M minutes
h H hours
d D days
w weeks
Each element of the sequence is added together to calculate
the total time value. For example:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour, 30 minutes (90 minutes)
FILES
/etc/ssh/sshdconfig Contains configuration data for
sshd. This file should be writable
by root only, but it is recommended
(though not necessary) that it be
world-readable.
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ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWsshu
Interface Stability Uncommitted
SEE ALSO
login(1), sshd(1M), sshconfig(4), attributes(5), ker-
beros(5)
AUTHORS
OpenSH 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 recent features, and created OpenSH.
Markus Friedl contributed the support for SH protocol ver-
sions 1.5 and 2.0. Niels Provos and Markus Friedl contri-
buted support for privilege separation.
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