Put ssh back into the repository
Change-Id: I23324372188fa6ed3f93a32b84365f5df6367590
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+.\"
+.\" 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 1.135 2011/08/02 01:22:11 djm Exp $
+.Dd $Mdocdate: August 2 2011 $
+.Dt SSHD_CONFIG 5
+.Os
+.Sh NAME
+.Nm sshd_config
+.Nd OpenSSH SSH daemon configuration file
+.Sh SYNOPSIS
+.Nm /etc/ssh/sshd_config
+.Sh DESCRIPTION
+.Xr sshd 8
+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.
+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 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.
+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
+.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.
+.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.
+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.
+The format is described in the
+.Sx AUTHORIZED_KEYS FILE FORMAT
+section of
+.Xr sshd 8 .
+.Cm AuthorizedKeysFile
+may contain tokens of the form %T which are substituted during connection
+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
+is taken to be an absolute path or one relative to the user's home
+directory.
+Multiple files may be listed, separated by whitespace.
+The default is
+.Dq .ssh/authorized_keys .ssh/authorized_keys2 .
+.It Cm AuthorizedPrincipalsFile
+Specifies a file that lists principal names that are accepted for
+certificate authentication.
+When using certificates signed by a key listed in
+.Cm TrustedUserCAKeys ,
+this file lists names, one of which must appear in the certificate for it
+to be accepted for authentication.
+Names are listed one per line preceded by key options (as described
+in
+.Sx AUTHORIZED_KEYS FILE FORMAT
+in
+.Xr sshd 8 ) .
+Empty lines and comments starting with
+.Ql #
+are ignored.
+.Pp
+.Cm AuthorizedPrincipalsFile
+may contain tokens of the form %T which are substituted during connection
+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 AuthorizedPrincipalsFile
+is taken to be an absolute path or one relative to the user's home
+directory.
+.Pp
+The default is not to use a principals file \(en in this case, the username
+of the user must appear in a certificate's principals list for it to be
+accepted.
+Note that
+.Cm AuthorizedPrincipalsFile
+is only used when authentication proceeds using a CA listed in
+.Cm TrustedUserCAKeys
+and is not consulted for certification authorities trusted via
+.Pa ~/.ssh/authorized_keys ,
+though the
+.Cm principals=
+key option offers a similar facility (see
+.Xr sshd 8
+for details).
+.It Cm Banner
+The contents of the specified file are sent to the remote user before
+authentication is allowed.
+If the argument is
+.Dq none
+then no banner is displayed.
+This option is only available for protocol version 2.
+By default, no banner is displayed.
+.It Cm ChallengeResponseAuthentication
+Specifies whether challenge-response authentication is allowed (e.g. via
+PAM or though authentication styles supported in
+.Xr login.conf 5 )
+The default is
+.Dq yes .
+.It Cm ChrootDirectory
+Specifies the pathname of a directory to
+.Xr chroot 2
+to after authentication.
+All components of the pathname must be root-owned directories that are
+not writable by any other user or group.
+After the chroot,
+.Xr sshd 8
+changes the working directory to the user's home directory.
+.Pp
+The pathname may contain the following tokens that are expanded at runtime once
+the connecting user has been authenticated: %% is replaced by a literal '%',
+%h is replaced by the home directory of the user being authenticated, and
+%u is replaced by the username of that user.
+.Pp
+The
+.Cm ChrootDirectory
+must contain the necessary files and directories to support the
+user's session.
+For an interactive session this requires at least a shell, typically
+.Xr sh 1 ,
+and basic
+.Pa /dev
+nodes such as
+.Xr null 4 ,
+.Xr zero 4 ,
+.Xr stdin 4 ,
+.Xr stdout 4 ,
+.Xr stderr 4 ,
+.Xr arandom 4
+and
+.Xr tty 4
+devices.
+For file transfer sessions using
+.Dq sftp ,
+no additional configuration of the environment is necessary if the
+in-process sftp server is used,
+though sessions which use logging do require
+.Pa /dev/log
+inside the chroot directory (see
+.Xr sftp-server 8
+for details).
+.Pp
+The default is not to
+.Xr chroot 2 .
+.It Cm Ciphers
+Specifies the ciphers allowed for protocol version 2.
+Multiple ciphers must be comma-separated.
+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,
+.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 Compression
+Specifies whether compression is allowed, or delayed until
+the user has authenticated successfully.
+The argument must be
+.Dq yes ,
+.Dq delayed ,
+or
+.Dq no .
+The default is
+.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.
+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.
+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 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,
+.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 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
+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 no .
+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 HostbasedAuthentication
+Specifies whether rhosts or /etc/hosts.equiv authentication together
+with successful public key client host authentication is allowed
+(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 HostCertificate
+Specifies a file containing a public host certificate.
+The certificate's public key must match a private host key already specified
+by
+.Cm HostKey .
+The default behaviour of
+.Xr sshd 8
+is not to load any certificates.
+.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_dsa_key ,
+.Pa /etc/ssh/ssh_host_ecdsa_key
+and
+.Pa /etc/ssh/ssh_host_rsa_key
+for protocol version 2.
+Note that
+.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
+keys are used for version 1 and
+.Dq dsa ,
+.Dq ecdsa
+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 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
+.Xr sshd 8
+should ignore the user's
+.Pa ~/.ssh/known_hosts
+during
+.Cm RhostsRSAAuthentication
+or
+.Cm HostbasedAuthentication .
+The default is
+.Dq no .
+.It Cm IPQoS
+Specifies the IPv4 type-of-service or DSCP class for the connection.
+Accepted values are
+.Dq af11 ,
+.Dq af12 ,
+.Dq af13 ,
+.Dq af14 ,
+.Dq af22 ,
+.Dq af23 ,
+.Dq af31 ,
+.Dq af32 ,
+.Dq af33 ,
+.Dq af41 ,
+.Dq af42 ,
+.Dq af43 ,
+.Dq cs0 ,
+.Dq cs1 ,
+.Dq cs2 ,
+.Dq cs3 ,
+.Dq cs4 ,
+.Dq cs5 ,
+.Dq cs6 ,
+.Dq cs7 ,
+.Dq ef ,
+.Dq lowdelay ,
+.Dq throughput ,
+.Dq reliability ,
+or a numeric value.
+This option may take one or two arguments, separated by whitespace.
+If one argument is specified, it is used as the packet class unconditionally.
+If two values are specified, the first is automatically selected for
+interactive sessions and the second for non-interactive sessions.
+The default is
+.Dq lowdelay
+for interactive sessions and
+.Dq throughput
+for non-interactive sessions.
+.It Cm KerberosAuthentication
+Specifies whether the password provided by the user for
+.Cm PasswordAuthentication
+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.
+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 password authentication through Kerberos fails then
+the password will be validated via any additional local mechanism
+such as
+.Pa /etc/passwd .
+The default is
+.Dq yes .
+.It Cm KerberosTicketCleanup
+Specifies whether to automatically destroy the user's ticket cache
+file on logout.
+The default is
+.Dq yes .
+.It Cm KexAlgorithms
+Specifies the available KEX (Key Exchange) algorithms.
+Multiple algorithms must be comma-separated.
+The default is
+.Dq ecdh-sha2-nistp256 ,
+.Dq ecdh-sha2-nistp384 ,
+.Dq ecdh-sha2-nistp521 ,
+.Dq diffie-hellman-group-exchange-sha256 ,
+.Dq diffie-hellman-group-exchange-sha1 ,
+.Dq diffie-hellman-group14-sha1 ,
+.Dq diffie-hellman-group1-sha1 .
+.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
+.Xr sshd 8
+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,
+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 120 seconds.
+.It Cm LogLevel
+Gives the verbosity level that is used when logging messages from
+.Xr sshd 8 .
+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:
+.Bd -literal -offset indent
+hmac-md5,hmac-sha1,umac-64@openssh.com,
+hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
+hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
+hmac-sha2-512-96
+.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 AuthorizedKeysFile ,
+.Cm AuthorizedPrincipalsFile ,
+.Cm Banner ,
+.Cm ChrootDirectory ,
+.Cm ForceCommand ,
+.Cm GatewayPorts ,
+.Cm GSSAPIAuthentication ,
+.Cm HostbasedAuthentication ,
+.Cm HostbasedUsesNameFromPacketOnly ,
+.Cm KbdInteractiveAuthentication ,
+.Cm KerberosAuthentication ,
+.Cm MaxAuthTries ,
+.Cm MaxSessions ,
+.Cm PasswordAuthentication ,
+.Cm PermitEmptyPasswords ,
+.Cm PermitOpen ,
+.Cm PermitRootLogin ,
+.Cm PermitTunnel ,
+.Cm PubkeyAuthentication ,
+.Cm RhostsRSAAuthentication ,
+.Cm RSAAuthentication ,
+.Cm X11DisplayOffset ,
+.Cm X11Forwarding
+and
+.Cm X11UseLocalHost .
+.It Cm MaxAuthTries
+Specifies the maximum number of authentication attempts permitted per
+connection.
+Once the number of failures reaches half this value,
+additional failures are logged.
+The default is 6.
+.It Cm MaxSessions
+Specifies the maximum number of open sessions permitted per network connection.
+The default is 10.
+.It Cm MaxStartups
+Specifies the maximum number of concurrent unauthenticated connections to the
+SSH daemon.
+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").
+.Xr sshd 8
+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 PermitOpen
+Specifies the destinations to which TCP port forwarding is permitted.
+The forwarding specification must be one of the following forms:
+.Pp
+.Bl -item -offset indent -compact
+.It
+.Cm PermitOpen
+.Sm off
+.Ar host : port
+.Sm on
+.It
+.Cm PermitOpen
+.Sm off
+.Ar IPv4_addr : port
+.Sm on
+.It
+.Cm PermitOpen
+.Sm off
+.Ar \&[ IPv6_addr \&] : port
+.Sm on
+.El
+.Pp
+Multiple forwards may be specified by separating them with whitespace.
+An argument of
+.Dq any
+can be used to remove all restrictions and permit any forwarding requests.
+By default all port forwarding requests are permitted.
+.It Cm PermitRootLogin
+Specifies whether root can log in using
+.Xr ssh 1 .
+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 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
+and
+.Cm environment=
+options in
+.Pa ~/.ssh/authorized_keys
+are processed by
+.Xr sshd 8 .
+The default is
+.Dq no .
+Enabling environment processing may enable users to bypass access
+restrictions in some configurations using mechanisms such as
+.Ev LD_PRELOAD .
+.It Cm PidFile
+Specifies the file that contains the process ID of the
+SSH daemon.
+The default is
+.Pa /var/run/sshd.pid .
+.It Cm Port
+Specifies the port number that
+.Xr sshd 8
+listens on.
+The default is 22.
+Multiple options of this type are permitted.
+See also
+.Cm ListenAddress .
+.It Cm PrintLastLog
+Specifies whether
+.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
+.Xr sshd 8
+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
+.Xr sshd 8
+supports.
+The possible values are
+.Sq 1
+and
+.Sq 2 .
+Multiple versions must be comma-separated.
+The default is
+.Sq 2 .
+Note that the order of the protocol list does not indicate preference,
+because the client selects among multiple protocol versions offered
+by the server.
+Specifying
+.Dq 2,1
+is identical to
+.Dq 1,2 .
+.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 RevokedKeys
+Specifies a list of revoked public keys.
+Keys listed in this file will be refused for public key authentication.
+Note that if this file is not readable, then public key authentication will
+be refused for all users.
+.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 1024.
+.It Cm StrictModes
+Specifies whether
+.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
+directory or files world-writable.
+The default is
+.Dq yes .
+Note that this does not apply to
+.Cm ChrootDirectory ,
+whose permissions and ownership are checked unconditionally.
+.It Cm Subsystem
+Configures an external subsystem (e.g. file transfer daemon).
+Arguments should be a subsystem name and a command (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
+.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 TrustedUserCAKeys
+Specifies a file containing public keys of certificate authorities that are
+trusted to sign user certificates for authentication.
+Keys are listed one per line; empty lines and comments starting with
+.Ql #
+are allowed.
+If a certificate is presented for authentication and has its signing CA key
+listed in this file, then it may be used for authentication for any user
+listed in the certificate's principals list.
+Note that certificates that lack a list of principals will not be permitted
+for authentication using
+.Cm TrustedUserCAKeys .
+For more details on certificates, see the
+.Sx CERTIFICATES
+section in
+.Xr ssh-keygen 1 .
+.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
+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 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 UsePrivilegeSeparation
+Specifies whether
+.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
+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 .
+If
+.Cm UsePrivilegeSeparation
+is set to
+.Dq sandbox
+then the pre-authentication unprivileged process is subject to additional
+restrictions.
+.It Cm X11DisplayOffset
+Specifies the first display number available for
+.Xr sshd 8 Ns 's
+X11 forwarding.
+This prevents sshd from interfering with real X11 servers.
+The default is 10.
+.It Cm X11Forwarding
+Specifies whether X11 forwarding is permitted.
+The argument must be
+.Dq yes
+or
+.Dq no .
+The default is
+.Dq no .
+.Pp
+When X11 forwarding is enabled, there may be additional exposure to
+the server and to client displays if the
+.Xr sshd 8
+proxy display is configured to listen on the wildcard address (see
+.Cm X11UseLocalhost
+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
+forwarding (see the warnings for
+.Cm ForwardX11
+in
+.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
+.Dq no
+setting.
+.Pp
+Note that disabling X11 forwarding does not prevent users from
+forwarding X11 traffic, as users can always install their own forwarders.
+X11 forwarding is automatically disabled if
+.Cm UseLogin
+is enabled.
+.It Cm X11UseLocalhost
+Specifies whether
+.Xr sshd 8
+should bind the X11 forwarding server to the loopback address or to
+the wildcard address.
+By default,
+sshd binds the forwarding server to the loopback address and sets the
+hostname part of the
+.Ev DISPLAY
+environment variable to
+.Dq localhost .
+This prevents remote hosts from connecting to the proxy 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 full pathname of the
+.Xr xauth 1
+program.
+The default is
+.Pa /usr/X11R6/bin/xauth .
+.El
+.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
+.Ar time Op Ar qualifier ,
+.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 Aq 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
+.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.
+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.