.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.\" $OpenBSD: sshd_config.5,v 1.52 2006/02/24 23:43:57 jmc Exp $
-.Dd September 25, 1999
+.\" $OpenBSD: sshd_config.5,v 1.115 2009/12/29 18:03:32 jmc Exp $
+.Dd $Mdocdate$
.Dt SSHD_CONFIG 5
.Os
.Sh NAME
.Nm sshd_config
.Nd OpenSSH SSH daemon configuration file
.Sh SYNOPSIS
-.Bl -tag -width Ds -compact
-.It Pa /etc/ssh/sshd_config
-.El
+.Nm /etc/ssh/sshd_config
.Sh DESCRIPTION
-.Nm sshd
+.Xr sshd 8
reads configuration data from
.Pa /etc/ssh/sshd_config
(or the file specified with
Lines starting with
.Ql #
and empty lines are interpreted as comments.
+Arguments may optionally be enclosed in double quotes
+.Pq \&"
+in order to represent arguments containing spaces.
.Pp
The possible
keywords and their meanings are as follows (note that
The default is not to accept any environment variables.
.It Cm AddressFamily
Specifies which address family should be used by
-.Nm sshd .
+.Xr sshd 8 .
Valid arguments are
.Dq any ,
.Dq inet
(use IPv6 only).
The default is
.Dq any .
+.It Cm AllowAgentForwarding
+Specifies whether
+.Xr ssh-agent 1
+forwarding is permitted.
+The default is
+.Dq 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.
.It Cm AllowGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
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:
+.Cm DenyUsers ,
+.Cm AllowUsers ,
+.Cm DenyGroups ,
+and finally
+.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
+The allow/deny directives are processed in the following order:
+.Cm DenyUsers ,
+.Cm AllowUsers ,
+.Cm DenyGroups ,
+and finally
+.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
The default is
.Dq .ssh/authorized_keys .
.It Cm Banner
-In some jurisdictions, sending a warning message before authentication
-may be relevant for getting legal protection.
The contents of the specified file are sent to the remote user before
authentication is allowed.
+If the argument is
+.Dq none
+then no banner is displayed.
This option is only available for protocol version 2.
By default, no banner is displayed.
.It Cm ChallengeResponseAuthentication
-Specifies whether challenge-response authentication is allowed.
-All authentication styles from
-.Xr login.conf 5
-are supported.
+Specifies whether challenge-response authentication is allowed (e.g. via
+PAM or though authentication styles supported in
+.Xr login.conf 5 )
The default is
.Dq yes .
+.It Cm ChrootDirectory
+Specifies the pathname of a directory to
+.Xr chroot 2
+to after authentication.
+All components of the pathname must be root-owned directories that are
+not writable by any other user or group.
+After the chroot,
+.Xr sshd 8
+changes the working directory to the user's home directory.
+.Pp
+The pathname may contain the following tokens that are expanded at runtime once
+the connecting user has been authenticated: %% 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.
+.Pp
+The
+.Cm 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
+.Xr sh 1 ,
+and basic
+.Pa /dev
+nodes such as
+.Xr null 4 ,
+.Xr zero 4 ,
+.Xr stdin 4 ,
+.Xr stdout 4 ,
+.Xr stderr 4 ,
+.Xr arandom 4
+and
+.Xr tty 4
+devices.
+For file transfer sessions using
+.Dq sftp ,
+no additional configuration of the environment is necessary if the
+in-process sftp server is used,
+though sessions which use logging do require
+.Pa /dev/log
+inside the chroot directory (see
+.Xr sftp-server 8
+for details).
+.Pp
+The default is not to
+.Xr chroot 2 .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
.Dq cast128-cbc .
The default is:
.Bd -literal -offset 3n
-aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
-arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
-aes192-ctr,aes256-ctr
+aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
+aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
+aes256-cbc,arcfour
.Ed
.It Cm ClientAliveCountMax
Sets the number of client alive messages (see below) which may be
.Cm ClientAliveCountMax
is left at the default, unresponsive SSH clients
will be disconnected after approximately 45 seconds.
+This option applies to protocol version 2 only.
.It Cm ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the client,
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:
+.Cm DenyUsers ,
+.Cm AllowUsers ,
+.Cm DenyGroups ,
+and finally
+.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
+The allow/deny directives are processed in the following order:
+.Cm DenyUsers ,
+.Cm AllowUsers ,
+.Cm DenyGroups ,
+and finally
+.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
+.It Cm ForceCommand
+Forces the execution of the command specified by
+.Cm ForceCommand ,
+ignoring any command supplied by the client and
+.Pa ~/.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
+.Cm Match
+block.
+The command originally supplied by the client is available in the
+.Ev SSH_ORIGINAL_COMMAND
+environment variable.
+Specifying a command of
+.Dq internal-sftp
+will force the use of an in-process sftp server that requires no support
+files when used with
+.Cm ChrootDirectory .
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client.
and applies to protocol version 2 only.
The default is
.Dq no .
+.It Cm HostbasedUsesNameFromPacketOnly
+Specifies whether or not the server will attempt to perform a reverse
+name lookup when matching the name in the
+.Pa ~/.shosts ,
+.Pa ~/.rhosts ,
+and
+.Pa /etc/hosts.equiv
+files during
+.Cm HostbasedAuthentication .
+A setting of
+.Dq yes
+means that
+.Xr sshd 8
+uses the name supplied by the client rather than
+attempting to resolve the name from the TCP connection itself.
+The default is
+.Dq no .
.It Cm HostKey
Specifies a file containing a private host key
used by SSH.
The default is 120 seconds.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
-.Nm sshd .
+.Xr sshd 8 .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
The default is INFO.
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is:
-.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
+.Bd -literal -offset indent
+hmac-md5,hmac-sha1,umac-64@openssh.com,
+hmac-ripemd160,hmac-sha1-96,hmac-md5-96
+.Ed
+.It Cm Match
+Introduces a conditional block.
+If all of the criteria on the
+.Cm Match
+line are satisfied, the keywords on the following lines override those
+set in the global section of the config file, until either another
+.Cm Match
+line or the end of the file.
+.Pp
+The arguments to
+.Cm Match
+are one or more criteria-pattern pairs.
+The available criteria are
+.Cm User ,
+.Cm Group ,
+.Cm Host ,
+and
+.Cm Address .
+The match patterns may consist of single entries or comma-separated
+lists and may use the wildcard and negation operators described in the
+.Sx PATTERNS
+section of
+.Xr ssh_config 5 .
+.Pp
+The patterns in an
+.Cm Address
+criteria may additionally contain addresses to match in CIDR
+address/masklen format, e.g.\&
+.Dq 192.0.2.0/24
+or
+.Dq 3ffe:ffff::/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,
+.Dq 192.0.2.0/33
+and
+.Dq 192.0.2.0/8
+respectively.
+.Pp
+Only a subset of keywords may be used on the lines following a
+.Cm Match
+keyword.
+Available keywords are
+.Cm AllowAgentForwarding ,
+.Cm AllowTcpForwarding ,
+.Cm Banner ,
+.Cm ChrootDirectory ,
+.Cm ForceCommand ,
+.Cm GatewayPorts ,
+.Cm GSSAPIAuthentication ,
+.Cm HostbasedAuthentication ,
+.Cm KbdInteractiveAuthentication ,
+.Cm KerberosAuthentication ,
+.Cm MaxAuthTries ,
+.Cm MaxSessions ,
+.Cm PasswordAuthentication ,
+.Cm PermitEmptyPasswords ,
+.Cm PermitOpen ,
+.Cm PermitRootLogin ,
+.Cm PubkeyAuthentication ,
+.Cm RhostsRSAAuthentication ,
+.Cm RSAAuthentication ,
+.Cm X11DisplayOffset ,
+.Cm X11Forwarding
+and
+.Cm X11UseLocalHost .
.It Cm 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.
+.It Cm MaxSessions
+Specifies the maximum number of open sessions permitted per network connection.
+The default is 10.
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
SSH daemon.
the three colon separated values
.Dq start:rate:full
(e.g. "10:30:60").
-.Nm sshd
+.Xr sshd 8
will refuse connection attempts with a probability of
.Dq rate/100
(30%)
server allows login to accounts with empty password strings.
The default is
.Dq no .
+.It Cm PermitOpen
+Specifies the destinations to which TCP port forwarding is permitted.
+The forwarding specification must be one of the following forms:
+.Pp
+.Bl -item -offset indent -compact
+.It
+.Cm PermitOpen
+.Sm off
+.Ar host : port
+.Sm on
+.It
+.Cm PermitOpen
+.Sm off
+.Ar IPv4_addr : port
+.Sm on
+.It
+.Cm PermitOpen
+.Sm off
+.Ar \&[ IPv6_addr \&] : port
+.Sm on
+.El
+.Pp
+Multiple forwards may be specified by separating them with whitespace.
+An argument of
+.Dq any
+can be used to remove all restrictions and permit any forwarding requests.
+By default all port forwarding requests are permitted.
.It Cm PermitRootLogin
Specifies whether root can log in using
.Xr ssh 1 .
device forwarding is allowed.
The argument must be
.Dq yes ,
-.Dq point-to-point ,
-.Dq ethernet ,
-or
+.Dq point-to-point
+(layer 3),
+.Dq ethernet
+(layer 2), or
.Dq no .
+Specifying
+.Dq yes
+permits both
+.Dq point-to-point
+and
+.Dq ethernet .
The default is
.Dq no .
.It Cm PermitUserEnvironment
.Ev LD_PRELOAD .
.It Cm PidFile
Specifies the file that contains the process ID of the
-.Nm sshd
-daemon.
+SSH daemon.
The default is
.Pa /var/run/sshd.pid .
.It Cm Port
.Sq 2 .
Multiple versions must be comma-separated.
The default is
-.Dq 2,1 .
+.Sq 2 .
Note that the order of the protocol list does not indicate preference,
because the client selects among multiple protocol versions offered
by the server.
The default is
.Dq no .
This option applies to protocol version 1 only.
+.It Cm RoutingDomain
+Set the routing domain number.
+The default routing domain is set by the system.
.It Cm RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is
This option applies to protocol version 1 only.
.It Cm ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1 server key.
-The minimum value is 512, and the default is 768.
+The minimum value is 512, and the default is 1024.
.It Cm StrictModes
Specifies whether
.Xr sshd 8
directory or files world-writable.
The default is
.Dq yes .
+Note that this does not apply to
+.Cm ChrootDirectory ,
+whose permissions and ownership are checked unconditionally.
.It Cm Subsystem
Configures an external subsystem (e.g. file transfer daemon).
-Arguments should be a subsystem name and a command to execute upon subsystem
-request.
+Arguments should be a subsystem name and a command (with optional arguments)
+to execute upon subsystem request.
+.Pp
The command
.Xr sftp-server 8
implements the
.Dq sftp
file transfer subsystem.
+.Pp
+Alternately the name
+.Dq internal-sftp
+implements an in-process
+.Dq sftp
+server.
+This may simplify configurations using
+.Cm ChrootDirectory
+to force a different filesystem root on clients.
+.Pp
By default no subsystems are defined.
Note that this option applies to protocol version 2 only.
.It Cm SyslogFacility
Gives the facility code that is used when logging messages from
-.Nm sshd .
+.Xr sshd 8 .
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
The default is AUTH.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
-.Ss Time Formats
-.Nm sshd
+.Sh TIME FORMATS
+.Xr sshd 8
command-line arguments and configuration file options that specify time
may be expressed using a sequence of the form:
.Sm off
is one of the following:
.Pp
.Bl -tag -width Ds -compact -offset indent
-.It Cm <none>
+.It Aq Cm none
seconds
.It Cm s | Cm S
seconds
.Bl -tag -width Ds
.It Pa /etc/ssh/sshd_config
Contains configuration data for
-.Nm sshd .
+.Xr sshd 8 .
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
.El
andersk / openssh.git / blobdiff