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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.8,v 1.120 2001/04/22 23:58:36 markus Exp $
+.Dd September 25, 1999
+.Dt SSHD 8
+.Os
+.Sh NAME
+.Nm sshd
+.Nd OpenSSH SSH daemon
+.Sh SYNOPSIS
+.Nm sshd
+.Op Fl deiqD46
+.Op Fl b Ar bits
+.Op Fl f Ar config_file
+.Op Fl g Ar login_grace_time
+.Op Fl h Ar host_key_file
+.Op Fl k Ar key_gen_time
+.Op Fl p Ar port
+.Op Fl u Ar len
+.Op Fl V Ar client_protocol_id
+.Sh DESCRIPTION
+.Nm
+(SSH Daemon) is the daemon program for
+.Xr ssh 1 .
+Together these programs replace rlogin and rsh, and
+provide secure encrypted communications between two untrusted hosts
+over an insecure network.
+The programs are intended to be as easy to
+install and use as possible.
+.Pp
+.Nm
+is the daemon that listens for connections from clients.
+It is normally started at boot from
+.Pa /etc/rc .
+It forks a new
+daemon for each incoming connection.
+The forked daemons handle
+key exchange, encryption, authentication, command execution,
+and data exchange.
+This implementation of
+.Nm
+supports both SSH protocol version 1 and 2 simultaneously.
+.Nm
+works as follows.
+.Pp
+.Ss SSH protocol version 1
+.Pp
+Each host has a host-specific RSA key
+(normally 1024 bits) used to identify the host.
+Additionally, when
+the daemon starts, it generates a server RSA key (normally 768 bits).
+This key is normally regenerated every hour if it has been used, and
+is never stored on disk.
+.Pp
+Whenever a client connects the daemon responds with its public
+host and server keys.
+The client compares the
+RSA host key against its own database to verify that it has not changed.
+The client then generates a 256 bit random number.
+It encrypts this
+random number using both the host key and the server key, and sends
+the encrypted number to the server.
+Both sides then use this
+random number as a session key which is used to encrypt all further
+communications in the session.
+The rest of the session is encrypted
+using a conventional cipher, currently Blowfish or 3DES, with 3DES
+being used by default.
+The client selects the encryption algorithm
+to use from those offered by the server.
+.Pp
+Next, the server and the client enter an authentication dialog.
+The client tries to authenticate itself using
+.Pa .rhosts
+authentication,
+.Pa .rhosts
+authentication combined with RSA host
+authentication, RSA challenge-response authentication, or password
+based authentication.
+.Pp
+Rhosts authentication is normally disabled
+because it is fundamentally insecure, but can be enabled in the server
+configuration file if desired.
+System security is not improved unless
+.Xr rshd 8 ,
+.Xr rlogind 8 ,
+.Xr rexecd 8 ,
+and
+.Xr rexd 8
+are disabled (thus completely disabling
+.Xr rlogin 1
+and
+.Xr rsh 1
+into the machine).
+.Pp
+.Ss SSH protocol version 2
+.Pp
+Version 2 works similarly:
+Each host has a host-specific DSA key used to identify the host.
+However, when the daemon starts, it does not generate a server key.
+Forward security is provided through a Diffie-Hellman key agreement.
+This key agreement results in a shared session key.
+.Pp
+The rest of the session is encrypted using a symmetric cipher, currently
+128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
+The client selects the encryption algorithm
+to use from those offered by the server.
+Additionally, session integrity is provided
+through a cryptographic message authentication code
+(hmac-sha1 or hmac-md5).
+.Pp
+Protocol version 2 provides a public key based
+user (PubkeyAuthentication) or
+client host (HostbasedAuthentication) authentication method,
+conventional password authentication and challenge response based methods.
+.Pp
+.Ss Command execution and data forwarding
+.Pp
+If the client successfully authenticates itself, a dialog for
+preparing the session is entered.
+At this time the client may request
+things like allocating a pseudo-tty, forwarding X11 connections,
+forwarding TCP/IP connections, or forwarding the authentication agent
+connection over the secure channel.
+.Pp
+Finally, the client either requests a shell or execution of a command.
+The sides then enter session mode.
+In this mode, either side may send
+data at any time, and such data is forwarded to/from the shell or
+command on the server side, and the user terminal in the client side.
+.Pp
+When the user program terminates and all forwarded X11 and other
+connections have been closed, the server sends command exit status to
+the client, and both sides exit.
+.Pp
+.Nm
+can be configured using command-line options or a configuration
+file.
+Command-line options override values specified in the
+configuration file.
+.Pp
+.Nm
+rereads its configuration file when it receives a hangup signal,
+.Dv SIGHUP ,
+by executing itself with the name it was started as, ie.
+.Pa /usr/sbin/sshd .
+.Pp
+The options are as follows:
+.Bl -tag -width Ds
+.It Fl b Ar bits
+Specifies the number of bits in the ephemeral protocol version 1
+server key (default 768).
+.Pp
+.It Fl d
+Debug mode.
+The server sends verbose debug output to the system
+log, and does not put itself in the background.
+The server also will not fork and will only process one connection.
+This option is only intended for debugging for the server.
+Multiple -d options increase the debugging level.
+Maximum is 3.
+.It Fl e
+When this option is specified,
+.Nm
+will send the output to the standard error instead of the system log.
+.It Fl f Ar configuration_file
+Specifies the name of the configuration file.
+The default is
+.Pa /etc/sshd_config .
+.Nm
+refuses to start if there is no configuration file.
+.It Fl g Ar login_grace_time
+Gives the grace time for clients to authenticate themselves (default
+600 seconds).
+If the client fails to authenticate the user within
+this many seconds, the server disconnects and exits.
+A value of zero indicates no limit.
+.It Fl h Ar host_key_file
+Specifies the file from which the host key is read (default
+.Pa /etc/ssh_host_key ) .
+This option must be given if
+.Nm
+is not run as root (as the normal
+host file is normally not readable by anyone but root).
+It is possible to have multiple host key files for
+the different protocol versions and host key algorithms.
+.It Fl i
+Specifies that
+.Nm
+is being run from inetd.
+.Nm
+is normally not run
+from inetd because it needs to generate the server key before it can
+respond to the client, and this may take tens of seconds.
+Clients would have to wait too long if the key was regenerated every time.
+However, with small key sizes (e.g., 512) using
+.Nm
+from inetd may
+be feasible.
+.It Fl k Ar key_gen_time
+Specifies how often the ephemeral protocol version 1 server key is
+regenerated (default 3600 seconds, or one hour).
+The motivation for regenerating the key fairly
+often is that the key is not stored anywhere, and after about an hour,
+it becomes impossible to recover the key for decrypting intercepted
+communications even if the machine is cracked into or physically
+seized.
+A value of zero indicates that the key will never be regenerated.
+.It Fl p Ar port
+Specifies the port on which the server listens for connections
+(default 22).
+.It Fl q
+Quiet mode.
+Nothing is sent to the system log.
+Normally the beginning,
+authentication, and termination of each connection is logged.
+.It Fl u Ar len
+This option is used to specify the size of the field
+in the
+.Li utmp
+structure that holds the remote host name.
+If the resolved host name is longer than
+.Ar len ,
+the dotted decimal value will be used instead.
+This allows hosts with very long host names that
+overflow this field to still be uniquely identified.
+Specifying
+.Fl u0
+indicates that only dotted decimal addresses
+should be put into the
+.Pa utmp
+file.
+.It Fl D
+When this option is specified
+.Nm
+will not detach and does not become a daemon.
+This allows easy monitoring of
+.Nm sshd .
+.It Fl 4
+Forces
+.Nm
+to use IPv4 addresses only.
+.It Fl 6
+Forces
+.Nm
+to use IPv6 addresses only.
+.El
+.Sh CONFIGURATION FILE
+.Nm
+reads configuration data from
+.Pa /etc/sshd_config
+(or the file specified with
+.Fl f
+on the command line).
+The file contains keyword-value pairs, one per line.
+Lines starting with
+.Ql #
+and empty lines are interpreted as comments.
+.Pp
+The following keywords are possible.
+.Bl -tag -width Ds
+.It Cm AFSTokenPassing
+Specifies whether an AFS token may be forwarded to the server.
+Default is
+.Dq yes .
+.It Cm AllowGroups
+This keyword can be followed by a list of group names, 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 isn't recognized.
+By default login is allowed regardless of the group list.
+.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 names, 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 isn't recognized.
+By default login is allowed regardless of the user name.
+.Pp
+.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.
+.Pp
+.It Cm ChallengeResponseAuthentication
+Specifies whether
+challenge response
+authentication is allowed.
+Currently there is only support for
+.Xr skey 1
+authentication.
+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
+.Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
+.It Cm CheckMail
+Specifies whether
+.Nm
+should check for new mail for interactive logins.
+The default is
+.Dq no .
+.It Cm ClientAliveInterval
+Sets a timeout interval in seconds after which if no data has been received
+from the client, 
+.Nm
+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
+receiving any messages back from the client. If this threshold is
+reached while client alive messages are being sent, 
+.Nm
+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. You want to use the client
+alive mechanism when you are basing something important on
+clients having an active connection to the server.
+.Pp
+The default value is 3. If you set
+.Cm ClientAliveInterval
+(above) to 15, and leave this value at the default, unresponsive ssh clients
+will be disconnected after approximately 45 seconds. 
+.It Cm DenyGroups
+This keyword can be followed by a number of group names, separated
+by spaces.
+Users whose primary group or supplementary group list matches
+one of the patterns aren't allowed to log in.
+.Ql \&*
+and
+.Ql ?
+can be used as
+wildcards in the patterns.
+Only group names are valid; a numerical group ID isn't recognized.
+By default login is allowed regardless of the group list.
+.Pp
+.It Cm DenyUsers
+This keyword can be followed by a number of user names, 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 isn't recognized.
+By default login is allowed regardless of the user name.
+.It Cm GatewayPorts
+Specifies whether remote hosts are allowed to connect to ports
+forwarded for the client.
+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 the file containing the private host keys (default
+.Pa /etc/ssh_host_key )
+used by SSH protocol versions 1 and 2.
+Note that
+.Nm
+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
+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 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 reboots.
+This avoids infinitely hanging sessions.
+.Pp
+To disable keepalives, the value should be set to
+.Dq no
+in both the server and the client configuration files.
+.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 yes .
+.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
+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
+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 and DEBUG.
+The default is INFO.
+Logging with level DEBUG 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
+.Pp
+.Bd -literal
+  ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
+    hmac-sha1-96,hmac-md5-96''
+.Ed
+.It Cm MaxStartups
+Specifies the maximum number of concurrent unauthenticated connections to the
+.Nm
+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
+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 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 disabled.
+The default is
+.Dq no .
+.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
+daemon.
+The default is
+.Pa /var/run/sshd.pid .
+.It Cm Port
+Specifies the port number that
+.Nm
+listens on.
+The default is 22.
+Multiple options of this type are permitted.
+See also
+.Cm ListenAddress .
+.It Cm PrintLastLog
+Specifies whether
+.Nm
+should print the date and time when the user last logged in.
+The default is
+.Dq yes .
+.It Cm PrintMotd
+Specifies whether
+.Nm
+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
+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 ReverseMappingCheck
+Specifies whether
+.Nm
+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 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
+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.
+Note that
+.Xr login 1
+is never used for remote command execution.
+The default is
+.Dq no .
+.It Cm X11DisplayOffset
+Specifies the first display number available for
+.Nm sshd Ns 's
+X11 forwarding.
+This prevents
+.Nm
+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.
+.It Cm XAuthLocation
+Specifies the location of the
+.Xr xauth 1
+program.
+The default is
+.Pa /usr/X11R6/bin/xauth .
+.El
+.Sh LOGIN PROCESS
+When a user successfully logs in,
+.Nm
+does the following:
+.Bl -enum -offset indent
+.It
+If the login is on a tty, and no command has been specified,
+prints last login time and
+.Pa /etc/motd
+(unless prevented in the configuration file or by
+.Pa $HOME/.hushlogin ;
+see the
+.Sx FILES
+section).
+.It
+If the login is on a tty, records login time.
+.It
+Checks
+.Pa /etc/nologin ;
+if it exists, prints contents and quits
+(unless root).
+.It
+Changes to run with normal user privileges.
+.It
+Sets up basic environment.
+.It
+Reads
+.Pa $HOME/.ssh/environment
+if it exists.
+.It
+Changes to user's home directory.
+.It
+If
+.Pa $HOME/.ssh/rc
+exists, runs it; else if
+.Pa /etc/sshrc
+exists, runs
+it; otherwise runs xauth.
+The
+.Dq rc
+files are given the X11
+authentication protocol and cookie in standard input.
+.It
+Runs user's shell or command.
+.El
+.Sh AUTHORIZED_KEYS FILE FORMAT
+The
+.Pa $HOME/.ssh/authorized_keys
+file lists the RSA keys that are
+permitted for RSA authentication in protocol version 1
+Similarly, the
+.Pa $HOME/.ssh/authorized_keys2
+file lists the DSA and RSA keys that are
+permitted for public key authentication (PubkeyAuthentication)
+in protocol version 2.
+.Pp
+Each line of the file contains one
+key (empty lines and lines starting with a
+.Ql #
+are ignored as
+comments).
+Each RSA public key consists of the following fields, separated by
+spaces: options, bits, exponent, modulus, comment.
+Each protocol version 2 public key consists of:
+options, keytype, base64 encoded key, comment.
+The options fields
+are optional; its presence is determined by whether the line starts
+with a number or not (the option field never starts with a number).
+The bits, exponent, modulus and comment fields give the RSA key for
+protocol version 1; the
+comment field is not used for anything (but may be convenient for the
+user to identify the key).
+For protocol version 2 the keytype is
+.Dq ssh-dss
+or
+.Dq ssh-rsa .
+.Pp
+Note that lines in this file are usually several hundred bytes long
+(because of the size of the RSA key modulus).
+You don't want to type them in; instead, copy the
+.Pa identity.pub ,
+.Pa id_dsa.pub
+or the
+.Pa id_rsa.pub
+file and edit it.
+.Pp
+The options (if present) consist of comma-separated option
+specifications.
+No spaces are permitted, except within double quotes.
+The following option specifications are supported:
+.Bl -tag -width Ds
+.It Cm from="pattern-list"
+Specifies that in addition to RSA authentication, the canonical name
+of the remote host must be present in the comma-separated list of
+patterns
+.Pf ( Ql *
+and
+.Ql ?
+serve as wildcards).
+The list may also contain
+patterns negated by prefixing them with
+.Ql ! ;
+if the canonical host name matches a negated pattern, the key is not accepted.
+The purpose
+of this option is to optionally increase security: RSA authentication
+by itself does not trust the network or name servers or anything (but
+the key); however, if somebody somehow steals the key, the key
+permits an intruder to log in from anywhere in the world.
+This additional option makes using a stolen key more difficult (name
+servers and/or routers would have to be compromised in addition to
+just the key).
+.It Cm command="command"
+Specifies that the command is executed whenever this key is used for
+authentication.
+The command supplied by the user (if any) is ignored.
+The command is run on a pty if the connection requests a pty;
+otherwise it is run without a tty.
+Note that if you want a 8-bit clean channel,
+you must not request a pty or should specify
+.Cm no-pty .
+A quote may be included in the command by quoting it with a backslash.
+This option might be useful
+to restrict certain RSA keys to perform just a specific operation.
+An example might be a key that permits remote backups but nothing else.
+Note that the client may specify TCP/IP and/or X11
+forwarding unless they are explicitly prohibited.
+.It Cm environment="NAME=value"
+Specifies that the string is to be added to the environment when
+logging in using this key.
+Environment variables set this way
+override other default environment values.
+Multiple options of this type are permitted.
+.It Cm no-port-forwarding
+Forbids TCP/IP forwarding when this key is used for authentication.
+Any port forward requests by the client will return an error.
+This might be used, e.g., in connection with the
+.Cm command
+option.
+.It Cm no-X11-forwarding
+Forbids X11 forwarding when this key is used for authentication.
+Any X11 forward requests by the client will return an error.
+.It Cm no-agent-forwarding
+Forbids authentication agent forwarding when this key is used for
+authentication.
+.It Cm no-pty
+Prevents tty allocation (a request to allocate a pty will fail).
+.It Cm permitopen="host:port"
+Limit local 
+.Li ``ssh -L''
+port forwarding such that it may only connect to the specified host and
+port. Multiple 
+.Cm permitopen
+options may be applied separated by commas. No pattern matching is 
+performed on the specified hostnames, they must be literal domains or 
+addresses.
+.El
+.Ss Examples
+1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
+.Pp
+from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
+.Pp
+command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
+.Pp
+permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
+.Sh SSH_KNOWN_HOSTS FILE FORMAT
+The
+.Pa /etc/ssh_known_hosts ,
+.Pa /etc/ssh_known_hosts2 ,
+.Pa $HOME/.ssh/known_hosts ,
+and
+.Pa $HOME/.ssh/known_hosts2
+files contain host public keys for all known hosts.
+The global file should
+be prepared by the administrator (optional), and the per-user file is
+maintained automatically: whenever the user connects from an unknown host
+its key is added to the per-user file.
+.Pp
+Each line in these files contains the following fields: hostnames,
+bits, exponent, modulus, comment.
+The fields are separated by spaces.
+.Pp
+Hostnames is a comma-separated list of patterns ('*' and '?' act as
+wildcards); each pattern in turn is matched against the canonical host
+name (when authenticating a client) or against the user-supplied
+name (when authenticating a server).
+A pattern may also be preceded by
+.Ql !
+to indicate negation: if the host name matches a negated
+pattern, it is not accepted (by that line) even if it matched another
+pattern on the line.
+.Pp
+Bits, exponent, and modulus are taken directly from the RSA host key; they
+can be obtained, e.g., from
+.Pa /etc/ssh_host_key.pub .
+The optional comment field continues to the end of the line, and is not used.
+.Pp
+Lines starting with
+.Ql #
+and empty lines are ignored as comments.
+.Pp
+When performing host authentication, authentication is accepted if any
+matching line has the proper key.
+It is thus permissible (but not
+recommended) to have several lines or different host keys for the same
+names.
+This will inevitably happen when short forms of host names
+from different domains are put in the file.
+It is possible
+that the files contain conflicting information; authentication is
+accepted if valid information can be found from either file.
+.Pp
+Note that the lines in these files are typically hundreds of characters
+long, and you definitely don't want to type in the host keys by hand.
+Rather, generate them by a script
+or by taking
+.Pa /etc/ssh_host_key.pub
+and adding the host names at the front.
+.Ss Examples
+.Bd -literal
+closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
+cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
+.Ed
+.Sh FILES
+.Bl -tag -width Ds
+.It Pa /etc/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.
+.It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
+These three files contain the private parts of the host keys.
+These files should only be owned by root, readable only by root, and not
+accessible to others.
+Note that
+.Nm
+does not start if this file is group/world-accessible.
+.It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
+These three files contain the public parts of the host keys.
+These files should be world-readable but writable only by
+root.
+Their contents should match the respective private parts.
+These files are not
+really used for anything; they are provided for the convenience of
+the user so their contents can be copied to known hosts files.
+These files are created using
+.Xr ssh-keygen 1 .
+.It Pa /etc/primes
+Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
+.It Pa /var/run/sshd.pid
+Contains the process ID of the
+.Nm
+listening for connections (if there are several daemons running
+concurrently for different ports, this contains the pid of the one
+started last).
+The content of this file is not sensitive; it can be world-readable.
+.It Pa $HOME/.ssh/authorized_keys
+Lists the RSA keys that can be used to log into the user's account.
+This file must be readable by root (which may on some machines imply
+it being world-readable if the user's home directory resides on an NFS
+volume).
+It is recommended that it not be accessible by others.
+The format of this file is described above.
+Users will place the contents of their
+.Pa identity.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
+.It Pa $HOME/.ssh/authorized_keys2
+Lists the public keys (RSA or DSA) that can be used to log into the user's account.
+This file must be readable by root (which may on some machines imply
+it being world-readable if the user's home directory resides on an NFS
+volume).
+It is recommended that it not be accessible by others.
+The format of this file is described above.
+Users will place the contents of their
+.Pa id_dsa.pub
+and/or
+.Pa id_rsa.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
+.It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
+These files are consulted when using rhosts with RSA host
+authentication to check the public key of the host.
+The key must be listed in one of these files to be accepted.
+The client uses the same files
+to verify that it is connecting to the correct remote host.
+These files should be writable only by root/the owner.
+.Pa /etc/ssh_known_hosts
+should be world-readable, and
+.Pa $HOME/.ssh/known_hosts
+can but need not be world-readable.
+.It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
+These files are consulted when using protocol version 2 hostbased
+authentication to check the public key of the host.
+The key must be listed in one of these files to be accepted.
+The client uses the same files
+to verify that it is connecting to the correct remote host.
+These files should be writable only by root/the owner.
+.Pa /etc/ssh_known_hosts2
+should be world-readable, and
+.Pa $HOME/.ssh/known_hosts2
+can but need not be world-readable.
+.It Pa /etc/nologin
+If this file exists,
+.Nm
+refuses to let anyone except root log in.
+The contents of the file
+are displayed to anyone trying to log in, and non-root connections are
+refused.
+The file should be world-readable.
+.It Pa /etc/hosts.allow, /etc/hosts.deny
+If compiled with
+.Sy LIBWRAP
+support, tcp-wrappers access controls may be defined here as described in
+.Xr hosts_access 5 .
+.It Pa $HOME/.rhosts
+This file contains host-username pairs, separated by a space, one per
+line.
+The given user on the corresponding host is permitted to log in
+without password.
+The same file is used by rlogind and rshd.
+The file must
+be writable only by the user; it is recommended that it not be
+accessible by others.
+.Pp
+If is also possible to use netgroups in the file.
+Either host or user
+name may be of the form +@groupname to specify all hosts or all users
+in the group.
+.It Pa $HOME/.shosts
+For ssh,
+this file is exactly the same as for
+.Pa .rhosts .
+However, this file is
+not used by rlogin and rshd, so using this permits access using SSH only.
+.It Pa /etc/hosts.equiv
+This file is used during
+.Pa .rhosts
+authentication.
+In the simplest form, this file contains host names, one per line.
+Users on
+those hosts are permitted to log in without a password, provided they
+have the same user name on both machines.
+The host name may also be
+followed by a user name; such users are permitted to log in as
+.Em any
+user on this machine (except root).
+Additionally, the syntax
+.Dq +@group
+can be used to specify netgroups.
+Negated entries start with
+.Ql \&- .
+.Pp
+If the client host/user is successfully matched in this file, login is
+automatically permitted provided the client and server user names are the
+same.
+Additionally, successful RSA host authentication is normally required.
+This file must be writable only by root; it is recommended
+that it be world-readable.
+.Pp
+.Sy "Warning: It is almost never a good idea to use user names in"
+.Pa hosts.equiv .
+Beware that it really means that the named user(s) can log in as
+.Em anybody ,
+which includes bin, daemon, adm, and other accounts that own critical
+binaries and directories.
+Using a user name practically grants the user root access.
+The only valid use for user names that I can think
+of is in negative entries.
+.Pp
+Note that this warning also applies to rsh/rlogin.
+.It Pa /etc/shosts.equiv
+This is processed exactly as
+.Pa /etc/hosts.equiv .
+However, this file may be useful in environments that want to run both
+rsh/rlogin and ssh.
+.It Pa $HOME/.ssh/environment
+This file is read into the environment at login (if it exists).
+It can only contain empty lines, comment lines (that start with
+.Ql # ) ,
+and assignment lines of the form name=value.
+The file should be writable
+only by the user; it need not be readable by anyone else.
+.It Pa $HOME/.ssh/rc
+If this file exists, it is run with /bin/sh after reading the
+environment files but before starting the user's shell or command.
+If X11 spoofing is in use, this will receive the "proto cookie" pair in
+standard input (and
+.Ev DISPLAY
+in environment).
+This must call
+.Xr xauth 1
+in that case.
+.Pp
+The primary purpose of this file is to run any initialization routines
+which may be needed before the user's home directory becomes
+accessible; AFS is a particular example of such an environment.
+.Pp
+This file will probably contain some initialization code followed by
+something similar to:
+.Bd -literal
+        if read proto cookie; then
+                echo add $DISPLAY $proto $cookie | xauth -q -
+        fi
+.Ed
+.Pp
+If this file does not exist,
+.Pa /etc/sshrc
+is run, and if that
+does not exist either, xauth is used to store the cookie.
+.Pp
+This file should be writable only by the user, and need not be
+readable by anyone else.
+.It Pa /etc/sshrc
+Like
+.Pa $HOME/.ssh/rc .
+This can be used to specify
+machine-specific login-time initializations globally.
+This file should be writable only by root, and should 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.
+.Sh SEE ALSO
+.Xr scp 1 ,
+.Xr sftp 1 ,
+.Xr sftp-server 8 ,
+.Xr ssh 1 ,
+.Xr ssh-add 1 ,
+.Xr ssh-agent 1 ,
+.Xr ssh-keygen 1 ,
+.Xr rlogin 1 ,
+.Xr rsh 1
+.Rs
+.%A T. Ylonen
+.%A T. Kivinen
+.%A M. Saarinen
+.%A T. Rinne
+.%A S. Lehtinen
+.%T "SSH Protocol Architecture"
+.%N draft-ietf-secsh-architecture-07.txt
+.%D January 2001
+.%O work in progress material
+.Re
+.Rs
+.%A M. Friedl
+.%A N. Provos
+.%A W. A. Simpson
+.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
+.%N draft-ietf-secsh-dh-group-exchange-00.txt
+.%D January 2001
+.%O work in progress material
+.Re