.\" (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 1.15 2003/03/28 10:11:43 jmc Exp $
-.Dd September 25, 1999
+.\" $OpenBSD: sshd_config.5,v 1.106 2009/04/21 15:13:17 stevesk Exp $
+.Dd $Mdocdate: April 21 2009 $
.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
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
-.It Cm AFSTokenPassing
-Specifies whether an AFS token may be forwarded to the server.
-Default is
-.Dq no .
+.It Cm AcceptEnv
+Specifies what environment variables sent by the client will be copied into
+the session's
+.Xr environ 7 .
+See
+.Cm SendEnv
+in
+.Xr ssh_config 5
+for how to configure the client.
+Note that environment passing is only supported for protocol 2.
+Variables are specified by name, which may contain the wildcard characters
+.Ql *
+and
+.Ql \&? .
+Multiple environment variables may be separated by whitespace or spread
+across multiple
+.Cm 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.
+.It Cm AddressFamily
+Specifies which address family should be used by
+.Xr sshd 8 .
+Valid arguments are
+.Dq any ,
+.Dq inet
+(use IPv4 only), or
+.Dq inet6
+(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.
If specified, login is allowed only for users whose primary
group or supplementary group list matches one of the patterns.
-.Ql \&*
-and
-.Ql ?
-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 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
+in
+.Xr ssh_config 5
+for more information on patterns.
.It Cm AllowTcpForwarding
Specifies whether TCP forwarding is permitted.
The default is
Note that disabling TCP forwarding does not improve security unless
users are also denied shell access, as they can always install their
own forwarders.
-.Pp
.It Cm 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.
-.Ql \&*
-and
-.Ql ?
-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 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.
+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 AuthorizedKeysFile
Specifies the file that contains the public keys that can be used
for user authentication.
.Cm AuthorizedKeysFile
may 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
+setup.
+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,
.Cm AuthorizedKeysFile
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.
-.Pp
.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 a path to
+.Xr chroot 2
+to after authentication.
+This path, and all its components, 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 path 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.
-The default is
-.Pp
-.Bd -literal
- ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
- aes192-cbc,aes256-cbc''
+The supported ciphers are
+.Dq 3des-cbc ,
+.Dq aes128-cbc ,
+.Dq aes192-cbc ,
+.Dq aes256-cbc ,
+.Dq aes128-ctr ,
+.Dq aes192-ctr ,
+.Dq aes256-ctr ,
+.Dq arcfour128 ,
+.Dq arcfour256 ,
+.Dq arcfour ,
+.Dq blowfish-cbc ,
+and
+.Dq cast128-cbc .
+The default is:
+.Bd -literal -offset 3n
+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
+sent without
+.Xr 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
+.Cm TCPKeepAlive
+(below).
+The client alive messages are sent through the encrypted channel
+and therefore will not be spoofable.
+The TCP keepalive option enabled by
+.Cm TCPKeepAlive
+is spoofable.
+The client alive mechanism is valuable when the client or
+server depend on knowing when a connection has become inactive.
+.Pp
+The default value is 3.
+If
+.Cm ClientAliveInterval
+(see below) is set to 15, and
+.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,
-.Nm sshd
+.Xr 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.
This option applies to protocol version 2 only.
-.It Cm ClientAliveCountMax
-Sets the number of client alive messages (see above) which may be
-sent without
-.Nm sshd
-receiving any messages back from the client. If this threshold is
-reached while client alive messages are being sent,
-.Nm sshd
-will disconnect the client, terminating the session. It is important
-to note that the use of client alive messages is very different from
-.Cm KeepAlive
-(below). The client alive messages are sent through the
-encrypted channel and therefore will not be spoofable. The TCP keepalive
-option enabled by
-.Cm KeepAlive
-is spoofable. The client alive mechanism is valuable when the client or
-server depend on knowing when a connection has become inactive.
-.Pp
-The default value is 3. If
-.Cm ClientAliveInterval
-(above) is set to 15, and
-.Cm ClientAliveCountMax
-is left at the default, unresponsive ssh clients
-will be disconnected after approximately 45 seconds.
.It Cm Compression
-Specifies whether compression is allowed.
+Specifies whether compression is allowed, or delayed until
+the user has authenticated successfully.
The argument must be
-.Dq yes
+.Dq yes ,
+.Dq delayed ,
or
.Dq no .
The default is
-.Dq yes .
+.Dq delayed .
.It Cm 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.
-.Ql \&*
-and
-.Ql ?
-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 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
+in
+.Xr ssh_config 5
+for more information on patterns.
.It Cm 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.
-.Ql \&*
-and
-.Ql ?
-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 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.
+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 DisableUsageStats
+This keyword can be followed by one of the keywords "true", "enabled", "yes",
+"on" or "1" to disable reporting of usage metrics. Or it can be set to "false",
+"disabled", "no", "off", "0" to enable reporting of usage metrics, which is the
+default. Setting the
+.Cm GLOBUS_USAGE_OPTOUT
+environment variable to "1" will also disable the reporting of usage metrics.
+Disabling reporting of usage metrics will cause the
+.Cm UsageStatsTargets
+setting to be ignored.
+.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.
By default,
-.Nm sshd
+.Xr sshd 8
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
-can be used to specify that
-.Nm sshd
-should bind remote port forwardings to the wildcard address,
-thus allowing remote hosts to connect to forwarded ports.
-The argument must be
+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
+.Dq no
+to force remote port forwardings to be available to the local host only,
.Dq yes
-or
+to force remote port forwardings to bind to the wildcard address, or
+.Dq clientspecified
+to allow the client to select the address to which the forwarding is bound.
+The default is
+.Dq no .
+.It Cm GSSAPIAuthentication
+Specifies whether user authentication based on GSSAPI is allowed.
+The default is
+.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPIDelegateCredentials
+Specifies whether delegated credentials are stored in the user's environment.
+The default is
+.Dq yes .
+.It Cm GSSAPIKeyExchange
+Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
+doesn't rely on ssh keys to verify host identity.
+The default is
+.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPICleanupCredentials
+Specifies whether to automatically destroy the user's credentials cache
+on logout.
+The default is
+.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPICredentialsPath
+If specified, the delegated GSSAPI credential is stored in the
+given path, overwriting any existing credentials.
+Paths can be specified with syntax similar to the AuthorizedKeysFile
+option (i.e., accepting %h and %u tokens).
+When using this option,
+setting 'GssapiCleanupCredentials no' is recommended,
+so logging out of one session
+doesn't remove the credentials in use by another session of
+the same user.
+Currently only implemented for the GSI mechanism.
+.It Cm GSIAllowLimitedProxy
+Specifies whether to accept limited proxy credentials for
+authentication.
+The default is
.Dq no .
+.It Cm GSSAPIStrictAcceptorCheck
+Determines whether to be strict about the identity of the GSSAPI acceptor
+a client authenticates against. If
+.Dq yes
+then the client must authenticate against the
+.Pa host
+service on the current hostname. If
+.Dq 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
+.Dq yes .
+Note that this option applies only to protocol version 2 GSSAPI connections,
+and setting it to
+.Dq no
+may only work with recent Kerberos GSSAPI libraries.
+.It Cm GSSAPIStoreCredentialsOnRekey
+Controls whether the user's GSSAPI credentials should be updated following a
+successful connection rekeying. This option can be used to accepted renewed
+or updated credentials from a compatible client. The default is
.Dq no .
.It Cm HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful public key client host authentication is allowed
-(hostbased authentication).
+(host-based authentication).
This option is similar to
.Cm RhostsRSAAuthentication
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.
.Pa /etc/ssh/ssh_host_dsa_key
for protocol version 2.
Note that
-.Nm sshd
+.Xr sshd 8
will refuse to use a file if it is group/world-accessible.
It is possible to have multiple host key files.
.Dq rsa1
and
.Pa .shosts
files will not be used in
-.Cm RhostsAuthentication ,
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
.Dq yes .
.It Cm IgnoreUserKnownHosts
Specifies whether
-.Nm sshd
+.Xr sshd 8
should ignore the user's
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.ssh/known_hosts
during
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
The default is
.Dq no .
-.It Cm KeepAlive
-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 keepalives are not sent,
-sessions may hang indefinitely on the server, leaving
-.Dq ghost
-users and consuming server resources.
-.Pp
-The default is
-.Dq yes
-(to send keepalives), and the server will notice
-if the network goes down or the client host crashes.
-This avoids infinitely hanging sessions.
-.Pp
-To disable keepalives, the value should be set to
-.Dq no .
.It Cm KerberosAuthentication
-Specifies whether Kerberos authentication is allowed.
-This can be in the form of a Kerberos ticket, or if
+Specifies whether the password provided by the user for
.Cm PasswordAuthentication
-is yes, the password provided by the user will be validated through
-the Kerberos KDC.
+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.
-Default is
+The default is
+.Dq no .
+.It Cm 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
.Dq no .
.It Cm KerberosOrLocalPasswd
-If set then if password authentication through Kerberos fails then
+If password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
such as
.Pa /etc/passwd .
-Default is
+The default is
.Dq yes .
-.It Cm KerberosTgtPassing
-Specifies whether a Kerberos TGT may be forwarded to the server.
-Default is
-.Dq no ,
-as this only works when the Kerberos KDC is actually an AFS kaserver.
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
-Default is
+The default is
.Dq yes .
.It Cm KeyRegenerationInterval
In protocol version 1, the ephemeral server key is automatically regenerated
The default is 3600 (seconds).
.It Cm ListenAddress
Specifies the local addresses
-.Nm sshd
+.Xr sshd 8
should listen on.
The following forms may be used:
.Pp
If
.Ar port
is not specified,
-.Nm sshd
-will listen on the address and all prior
+sshd will listen on the address and all prior
.Cm Port
-options specified. The default is to listen on all local
-addresses.
+options specified.
+The default is to listen on all local addresses.
Multiple
.Cm ListenAddress
-options are permitted. Additionally, any
+options are permitted.
+Additionally, any
.Cm Port
-options must precede this option for non port qualified addresses.
+options must precede this option for non-port qualified addresses.
.It Cm LoginGraceTime
The server disconnects after this time if the user has not
successfully logged in.
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.
+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.
The MAC algorithm is used in protocol version 2
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 .
+The default is:
+.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 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
-.Nm sshd
-daemon.
+SSH daemon.
Additional connections will be dropped until authentication succeeds or the
.Cm LoginGraceTime
expires for a connection.
Alternatively, random early drop can be enabled by specifying
the three colon separated values
.Dq start:rate:full
-(e.g., "10:30:60").
-.Nm sshd
+(e.g. "10:30:60").
+.Xr sshd 8
will refuse connection attempts with a probability of
.Dq rate/100
(30%)
are refused if the number of unauthenticated connections reaches
.Dq full
(60).
-.It Cm PAMAuthenticationViaKbdInt
-Specifies whether PAM challenge response authentication is allowed. This
-allows the use of most PAM challenge response authentication modules, but
-it will allow password authentication regardless of whether
-.Cm PasswordAuthentication
-is enabled.
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
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 login using
+Specifies whether root can log in using
.Xr ssh 1 .
The argument must be
.Dq yes ,
.Dq without-password ,
-.Dq forced-commands-only
+.Dq forced-commands-only ,
or
.Dq no .
The default is
.Dq yes .
.Pp
If this option is set to
-.Dq without-password
+.Dq without-password ,
password authentication is disabled for root.
.Pp
If this option is set to
-.Dq forced-commands-only
+.Dq forced-commands-only ,
root login with public key authentication will be allowed,
but only if the
.Ar 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.
+normally not allowed).
+All other authentication methods are disabled for root.
.Pp
If this option is set to
-.Dq no
-root is not allowed to login.
+.Dq no ,
+root is not allowed to log in.
+.It Cm PermitTunnel
+Specifies whether
+.Xr tun 4
+device forwarding is allowed.
+The argument must be
+.Dq yes ,
+.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
Specifies whether
.Pa ~/.ssh/environment
options in
.Pa ~/.ssh/authorized_keys
are processed by
-.Nm sshd .
+.Xr sshd 8 .
The default is
.Dq no .
Enabling environment processing may enable users to bypass access
.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
Specifies the port number that
-.Nm sshd
+.Xr sshd 8
listens on.
The default is 22.
Multiple options of this type are permitted.
.Cm ListenAddress .
.It Cm PrintLastLog
Specifies whether
-.Nm sshd
-should print the date and time when the user last logged in.
+.Xr sshd 8
+should print the date and time of the last user login when a user logs
+in interactively.
The default is
.Dq yes .
.It Cm PrintMotd
Specifies whether
-.Nm sshd
+.Xr sshd 8
should print
.Pa /etc/motd
when a user logs in interactively.
.Dq yes .
.It Cm Protocol
Specifies the protocol versions
-.Nm sshd
+.Xr sshd 8
supports.
The possible values are
-.Dq 1
+.Sq 1
and
-.Dq 2 .
+.Sq 2 .
Multiple versions must be comma-separated.
The default is
.Dq 2,1 .
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
-.It Cm RhostsAuthentication
-Specifies whether authentication using rhosts or /etc/hosts.equiv
-files is sufficient.
-Normally, this method should not be permitted because it is insecure.
-.Cm RhostsRSAAuthentication
-should be used
-instead, because it performs RSA-based host authentication in addition
-to normal rhosts or /etc/hosts.equiv authentication.
-The default is
-.Dq no .
-This option applies to protocol version 1 only.
.It Cm RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful RSA host authentication is allowed.
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
-.Nm sshd
+.Xr 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
The default is
.Dq yes .
.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.
+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.
+.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.
+.It Cm 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
+.Dq ghost
+users and consuming server resources.
+.Pp
+The default is
+.Dq 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.
+.Pp
+To disable TCP keepalive messages, the value should be set to
+.Dq no .
+.It Cm UsageStatsTargets
+This option can be used to specify the target collector hosts to which usage
+metrics should be reported. This setting will be ignored if
+.Cm DisableUsageStats
+is enabled. Multiple targets can be specified separated by comma(s), but no
+space(s). Each target specification is of the format
+.Pa host:port[!tags].
+Tags control what data elements are reported. The following list specifies
+the tags for the corresponding data elements.
+.Pp
+.Bl -item -offset indent -compact
+.It
+.Cm V
+.Sm off
+- OpenSSH version, reported by default.
+.Sm on
+.It
+.Cm v
+.Sm off
+- SSL version, reported by default.
+.Sm on
+.It
+.Cm M
+.Sm off
+- User authentication method used such as "gssapi-keyex", "gssapi-with-mic", etc. Reported by default.
+.Sm on
+.It
+.Cm m
+.Sm off
+- User authentication mechanism used such as "GSI", "Kerberos", etc. Reported by default.
+.Sm on
+.It
+.Cm I
+.Sm off
+- Client IP address. Not reported by default.
+.Sm on
+.It
+.Cm u
+.Sm off
+- User name. Not reported by default.
+.Sm on
+.It
+.Cm U
+.Sm off
+- User DN. Not reported by default.
+.Sm on
+.Pp
+In addition to the above selected information, the following data are
+reported to ALL the specified/default target collectors. There's no way to
+exclude these from being reported other than by disabling the reporting of
+usage metrics altogether:
+.Pp
+.It
+.Cm Component code
+.Sm off
+- 12 for GSI OpenSSH
+.Sm on
+.It
+.Cm Component Data Format version
+.Sm off
+- 0 currently
+.Sm on
+.It
+.Cm IP Address
+.Sm off
+- IP address of reporting server
+.Sm on
+.It
+.Cm Timestamp
+.It
+.Cm Hostname
+.Sm off
+- Host name of reporting server
+.Sm on
+.Pp
+If no tags are specified in a host spec, or the special string "default"
+is specified, the tags VvtrlLB are assumed. A site could choose to allow a
+different set of data to be reported by specifying a different tag set. The
+last 3 tags I, u and U above are more meant for a local collector that a
+site might like to deploy since they could be construed as private information.
+The special string "all" denotes all tags.
+.El
+.Pp
+By default, Usage Metrics reporting is sent to
+.Dq usage-stats.cilogon.org:4810 .
+This can be made explicit by specifying
+.Dq default
+(all by itself) for the
+target specification as in:
+.Pp
+.Bl -item -offset indent -compact
+.It
+.Cm UsageStatsTargets
+.Sm off
+default
+.Sm on
+.El
+.Pp
+If
+.Cm UsageStatsTargets
+is not specified, a comma-separated list of targets
+(without any tags specified) if specified in the environment variable
+.Cm GLOBUS_USAGE_TARGETS
+will be used.
+.Pp
+.It Cm UseDNS
+Specifies whether
+.Xr sshd 8
+should look up the remote host name and check that
+the resolved host name for the remote IP address maps back to the
+very same IP address.
+The default is
+.Dq yes .
.It Cm UseLogin
Specifies whether
.Xr login 1
If
.Cm UsePrivilegeSeparation
is specified, it will be disabled after authentication.
+.It Cm UsePAM
+Enables the Pluggable Authentication Module interface.
+If set to
+.Dq yes
+this will enable PAM authentication using
+.Cm ChallengeResponseAuthentication
+and
+.Cm PasswordAuthentication
+in addition to PAM account and session module processing for all
+authentication types.
+.Pp
+Because PAM challenge-response authentication usually serves an equivalent
+role to password authentication, you should disable either
+.Cm PasswordAuthentication
+or
+.Cm ChallengeResponseAuthentication.
+.Pp
+If
+.Cm UsePAM
+is enabled, you will not be able to run
+.Xr sshd 8
+as a non-root user.
+The default is
+.Dq no .
+.It Cm PermitPAMUserChange
+If set to
+.Dq yes
+this will enable PAM authentication to change the name of the user being
+authenticated. The default is
+.Dq no .
.It Cm UsePrivilegeSeparation
Specifies whether
-.Nm sshd
+.Xr sshd 8
separates privileges by creating an unprivileged child process
to deal with incoming network traffic.
After successful authentication, another process will be created that has
escalation by containing any corruption within the unprivileged processes.
The default is
.Dq yes .
-.It Cm VerifyReverseMapping
-Specifies whether
-.Nm 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.
-The default is
-.Dq no .
.It Cm X11DisplayOffset
Specifies the first display number available for
-.Nm sshd Ns 's
+.Xr sshd 8 Ns 's
X11 forwarding.
-This prevents
-.Nm sshd
-from interfering with real X11 servers.
+This prevents sshd from interfering with real X11 servers.
The default is 10.
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.
.Pp
When X11 forwarding is enabled, there may be additional exposure to
the server and to client displays if the
-.Nm sshd
+.Xr sshd 8
proxy display is configured to listen on the wildcard address (see
.Cm X11UseLocalhost
-below), however this is not the default.
+below), 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
+display server may be exposed to attack when the SSH client requests
forwarding (see the warnings for
.Cm ForwardX11
in
-.Xr ssh_config 5 ).
+.Xr 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
is enabled.
.It Cm X11UseLocalhost
Specifies whether
-.Nm sshd
+.Xr sshd 8
should bind the X11 forwarding server to the loopback address or to
the wildcard address.
By default,
-.Nm sshd
-binds the forwarding server to the loopback address and sets the
+sshd binds the forwarding server to the loopback address and sets the
hostname part of the
.Ev DISPLAY
environment variable to
The default is
.Pa /usr/X11R6/bin/xauth .
.El
-.Ss Time Formats
-.Pp
-.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
+.Sh SEE ALSO
+.Xr sshd 8
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.
-.Sh SEE ALSO
-.Xr sshd 8
andersk / gssapi-openssh.git / blobdiff