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+.\"  -*- nroff -*-
+.\"
+.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
+.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
+.\"                    All rights reserved
+.\"
+.\" As far as I am concerned, the code I have written for this software
+.\" can be used freely for any purpose.  Any derived versions of this
+.\" software must be clearly marked as such, and if the derived work is
+.\" incompatible with the protocol description in the RFC file, it must be
+.\" called by a name other than "ssh" or "Secure Shell".
+.\"
+.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
+.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
+.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (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 1.3 2002/06/20 23:37:12 markus Exp $
+.Dd September 25, 1999
+.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
+.Sh DESCRIPTION
+.Nm sshd
+reads configuration data from
+.Pa /etc/ssh/sshd_config
+(or the file specified with
+.Fl f
+on the command line).
+The file contains keyword-argument pairs, one per line.
+Lines starting with
+.Ql #
+and empty lines are interpreted as comments.
+.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 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.
+.Pp
+.It Cm AllowTcpForwarding
+Specifies whether TCP forwarding is permitted.
+The default is
+.Dq yes .
+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 users 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.
+.Pp
+.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
+%u is replaced by the username of that user.
+After expansion,
+.Cm AuthorizedKeysFile
+is taken to be an absolute path or one relative to the user's home
+directory.
+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.
+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.
+The default is
+.Dq yes .
+.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''
+.Ed
+.It Cm ClientAliveInterval
+Sets a timeout interval in seconds after which if no data has been received
+from the client,
+.Nm sshd
+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.
+The argument must be
+.Dq yes
+or
+.Dq no .
+The default is
+.Dq yes .
+.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.
+.Pp
+.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.
+.It Cm GatewayPorts
+Specifies whether remote hosts are allowed to connect to ports
+forwarded for the client.
+By default,
+.Nm sshd
+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
+.Dq yes
+or
+.Dq no .
+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).
+This option is similar to
+.Cm RhostsRSAAuthentication
+and applies to protocol version 2 only.
+The default is
+.Dq no .
+.It Cm HostKey
+Specifies a file containing a private host key
+used by SSH.
+The default is
+.Pa /etc/ssh/ssh_host_key
+for protocol version 1, and
+.Pa /etc/ssh/ssh_host_rsa_key
+and
+.Pa /etc/ssh/ssh_host_dsa_key
+for protocol version 2.
+Note that
+.Nm sshd
+will refuse to use a file if it is group/world-accessible.
+It is possible to have multiple host key files.
+.Dq rsa1
+keys are used for version 1 and
+.Dq dsa
+or
+.Dq rsa
+are used for version 2 of the SSH protocol.
+.It Cm IgnoreRhosts
+Specifies that
+.Pa .rhosts
+and
+.Pa .shosts
+files will not be used in
+.Cm RhostsAuthentication ,
+.Cm RhostsRSAAuthentication
+or
+.Cm HostbasedAuthentication .
+.Pp
+.Pa /etc/hosts.equiv
+and
+.Pa /etc/shosts.equiv
+are still used.
+The default is
+.Dq yes .
+.It Cm IgnoreUserKnownHosts
+Specifies whether
+.Nm sshd
+should ignore the user's
+.Pa $HOME/.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
+.Cm PasswordAuthentication
+is yes, the password provided by the user 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
+.Dq no .
+.It Cm KerberosOrLocalPasswd
+If set 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
+.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
+.Dq yes .
+.It Cm 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).
+.It Cm ListenAddress
+Specifies the local addresses
+.Nm sshd
+should listen on.
+The following forms may be used:
+.Pp
+.Bl -item -offset indent -compact
+.It
+.Cm ListenAddress
+.Sm off
+.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
+.Sm on
+.It
+.Cm ListenAddress
+.Sm off
+.Ar host No | Ar IPv4_addr No : Ar port
+.Sm on
+.It
+.Cm ListenAddress
+.Sm off
+.Oo
+.Ar host No | Ar IPv6_addr Oc : Ar port
+.Sm on
+.El
+.Pp
+If
+.Ar port
+is not specified,
+.Nm sshd
+will listen on the address and all prior
+.Cm Port
+options specified. The default is to listen on all local
+addresses.  Multiple
+.Cm ListenAddress
+options are permitted. Additionally, any
+.Cm Port
+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.
+If the value is 0, there is no time limit.
+The default is 600 (seconds).
+.It Cm LogLevel
+Gives the verbosity level that is used when logging messages from
+.Nm sshd .
+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.
+.It Cm MACs
+Specifies the available MAC (message authentication code) algorithms.
+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 .
+.It Cm MaxStartups
+Specifies the maximum number of concurrent unauthenticated connections to the
+.Nm sshd
+daemon.
+Additional connections will be dropped until authentication succeeds or the
+.Cm LoginGraceTime
+expires for a connection.
+The default is 10.
+.Pp
+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
+will refuse connection attempts with a probability of
+.Dq rate/100
+(30%)
+if there are currently
+.Dq start
+(10)
+unauthenticated connections.
+The probability increases linearly and all connection attempts
+are refused if the number of unauthenticated connections reaches
+.Dq full
+(60).
+.It Cm PasswordAuthentication
+Specifies whether password authentication is allowed.
+The default is
+.Dq yes .
+.It Cm PermitEmptyPasswords
+When password authentication is allowed, it specifies whether the
+server allows login to accounts with empty password strings.
+The default is
+.Dq no .
+.It Cm PermitRootLogin
+Specifies whether root can login using
+.Xr ssh 1 .
+The argument must be
+.Dq yes ,
+.Dq without-password ,
+.Dq forced-commands-only
+or
+.Dq no .
+The default is
+.Dq yes .
+.Pp
+If this option is set to
+.Dq without-password
+password authentication is disabled for root.
+.Pp
+If this option is set to
+.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.
+.Pp
+If this option is set to
+.Dq no
+root is not allowed to login.
+.It Cm PidFile
+Specifies the file that contains the process identifier of the
+.Nm sshd
+daemon.
+The default is
+.Pa /var/run/sshd.pid .
+.It Cm Port
+Specifies the port number that
+.Nm sshd
+listens on.
+The default is 22.
+Multiple options of this type are permitted.
+See also
+.Cm ListenAddress .
+.It Cm PrintLastLog
+Specifies whether
+.Nm sshd
+should print the date and time when the user last logged in.
+The default is
+.Dq yes .
+.It Cm PrintMotd
+Specifies whether
+.Nm sshd
+should print
+.Pa /etc/motd
+when a user logs in interactively.
+(On some systems it is also printed by the shell,
+.Pa /etc/profile ,
+or equivalent.)
+The default is
+.Dq yes .
+.It Cm Protocol
+Specifies the protocol versions
+.Nm sshd
+should support.
+The possible values are
+.Dq 1
+and
+.Dq 2 .
+Multiple versions must be comma-separated.
+The default is
+.Dq 2,1 .
+.It Cm PubkeyAuthentication
+Specifies whether public key authentication is allowed.
+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.
+The default is
+.Dq no .
+This option applies to protocol version 1 only.
+.It Cm RSAAuthentication
+Specifies whether pure RSA authentication is allowed.
+The default is
+.Dq yes .
+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.
+.It Cm StrictModes
+Specifies whether
+.Nm sshd
+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
+.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.
+The command
+.Xr sftp-server 8
+implements the
+.Dq sftp
+file transfer subsystem.
+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 .
+The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
+LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
+The default is AUTH.
+.It Cm UseLogin
+Specifies whether
+.Xr login 1
+is used for interactive login sessions.
+The default is
+.Dq no .
+Note that
+.Xr login 1
+is never used for remote command execution.
+Note also, that if this is enabled,
+.Cm X11Forwarding
+will be disabled because
+.Xr login 1
+does not know how to handle
+.Xr xauth 1
+cookies.  If
+.Cm UsePrivilegeSeparation
+is specified, it will be disabled after authentication.
+.It Cm UsePrivilegeSeparation
+Specifies whether
+.Nm sshd
+separates privileges by creating an unprivileged child process
+to deal with incoming network traffic.  After successful authentication,
+another process will be created that has the privilege of the authenticated
+user.  The goal of privilege separation is to prevent privilege
+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
+X11 forwarding.
+This prevents
+.Nm sshd
+from interfering with real X11 servers.
+The default is 10.
+.It Cm X11Forwarding
+Specifies whether X11 forwarding is permitted.
+The default is
+.Dq no .
+Note that disabling X11 forwarding does not improve security in any
+way, as users can always install their own forwarders.
+X11 forwarding is automatically disabled if
+.Cm UseLogin
+is enabled.
+.It Cm X11UseLocalhost
+Specifies whether
+.Nm sshd
+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
+hostname part of the
+.Ev DISPLAY
+environment variable to
+.Dq localhost .
+This prevents remote hosts from connecting to the fake display.
+However, some older X11 clients may not function with this
+configuration.
+.Cm X11UseLocalhost
+may be set to
+.Dq no
+to specify that the forwarding server should be bound to the wildcard
+address.
+The argument must be
+.Dq yes
+or
+.Dq no .
+The default is
+.Dq yes .
+.It Cm XAuthLocation
+Specifies the location of the
+.Xr xauth 1
+program.
+The default is
+.Pa /usr/X11R6/bin/xauth .
+.El
+.Ss Time Formats
+.Pp
+.Nm sshd
+command-line arguments and configuration file options that specify time
+may be expressed using a sequence of the form:
+.Sm off
+.Ar time Oo Ar qualifier Oc ,
+.Sm on
+where
+.Ar time
+is a positive integer value and
+.Ar qualifier
+is one of the following:
+.Pp
+.Bl -tag -width Ds -compact -offset indent
+.It Cm <none>
+seconds
+.It Cm s | Cm S
+seconds
+.It Cm m | Cm M
+minutes
+.It Cm h | Cm H
+hours
+.It Cm d | Cm D
+days
+.It Cm w | Cm W
+weeks
+.El
+.Pp
+Each member of the sequence is added together to calculate
+the total time value.
+.Pp
+Time format examples:
+.Pp
+.Bl -tag -width Ds -compact -offset indent
+.It 600
+600 seconds (10 minutes)
+.It 10m
+10 minutes
+.It 1h30m
+1 hour 30 minutes (90 minutes)
+.El
+.Sh FILES
+.Bl -tag -width Ds
+.It Pa /etc/ssh/sshd_config
+Contains configuration data for
+.Nm sshd .
+This file should be writable by root only, but it is recommended
+(though not necessary) that it be world-readable.
+.El
+.Sh AUTHORS
+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.
+.Sh SEE ALSO
+.Xr sshd 8