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+.\" -*- nroff -*-
+.\" Author: Tatu Ylonen <>
+.\" Copyright (c) 1995 Tatu Ylonen <>, Espoo, Finland
+.\" All rights reserved
+.\" Created: Sat Apr 22 21:55:14 1995 ylo
+.\" $Id: opensshd.8,v 1.1 1999/10/28 04:03:14 damien Exp $
+.Dd September 25, 1999
+.Dt SSHD 8
+.Nm sshd
+.Nd secure shell daemon
+.Nm sshd
+.Op Fl diq
+.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
+(Secure Shell Daemon) is the daemon program for
+.Xr ssh 1 .
+Together these programs replace rlogin and rsh programs, 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.
+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.
+works as follows. 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.
+Whenever a client connects the daemon, the daemon sends its host
+and server public keys to the client. The client compares the
+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 start to 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 and 3DES, with 3DES
+being is used by default. The client selects the encryption algorithm
+to use from those offered by the server.
+Next, the server and the client enter an authentication dialog. The
+client tries to authenticate itself using
+.Pa .rhosts
+.Pa .rhosts
+authentication combined with RSA host
+authentication, RSA challenge-response authentication, or password
+based authentication.
+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 ,
+.Xr rexd 8
+are disabled (thus completely disabling
+.Xr rlogin 1
+.Xr rsh 1
+into that machine).
+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.
+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.
+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.
+can be configured using command-line options or a configuration
+file. Command-line options override values specified in the
+configuration file.
+The options are as follows:
+.Bl -tag -width Ds
+.It Fl b Ar bits
+Specifies the number of bits in the server key (default 768).
+.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.
+.It Fl f Ar configuration_file
+Specifies the name of the configuration file. The default is
+.Pa /etc/openssh/sshd_config .
+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
+300 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/openssh/ssh_host_key ) .
+This option must be given if
+is not run as root (as the normal
+host file is normally not readable by anyone but root).
+.It Fl i
+Specifies that
+is being run from inetd.
+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
+from inetd may
+be feasible.
+.It Fl k Ar key_gen_time
+Specifies how often the 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 Q
+Do not print an error message if RSA support is missing.
+reads configuration data from
+.Pa /etc/openssh/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.
+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 number of group names, separated
+by spaces. If specified, login is allowed only for users whose primary
+group matches one of the patterns.
+.Ql \&*
+.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 primary group.
+.It Cm AllowUsers
+This keyword can be followed by a number of user names, separated
+by spaces. If specified, login is allowed only for users names that
+match one of the patterns.
+.Ql \&*
+.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 CheckMail
+Specifies whether
+should check for new mail for interactive logins.
+The default is
+.Dq no .
+.It Cm DenyGroups
+This keyword can be followed by a number of group names, separated
+by spaces. Users whose primary group matches one of the patterns
+aren't allowed to log in.
+.Ql \&*
+.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 primary group.
+.It Cm DenyUsers
+This keyword can be followed by a number of user names, separated
+by spaces. Login is allowed disallowed for user names that match
+one of the patterns.
+.Ql \&*
+.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 FascistLogging
+Specifies whether to use verbose logging. Verbose logging violates
+the privacy of users and is not recommended. The argument must be
+.Dq yes
+.Dq no .
+The default is
+.Dq no .
+.It Cm HostKey
+Specifies the file containing the private host key (default
+.Pa /etc/openssh/ssh_host_key ) .
+Note that
+does not start if this file is group/world-accessible.
+.It Cm IgnoreRhosts
+Specifies that rhosts and shosts files will not be used in
+.Pa /etc/hosts.equiv
+.Pa /etc/openssh/shosts.equiv
+are still used. 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 send,
+sessions may hang indefinitely on the server, leaving
+.Dq ghost
+users and consuming server resources.
+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.
+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. 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
+or SecurID. 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
+The 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
+.It Cm ListenAddress
+Specifies what local address
+should listen on.
+The default is to listen to all local 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 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
+.Dq yes .
+.It Cm PermitRootLogin
+Specifies whether the root can log in using
+.Xr ssh 1 .
+The argument must be
+.Dq yes ,
+.Dq without-password
+.Dq no .
+The default is
+.Dq yes .
+If this options is set to
+.Dq without-password
+only password authentication is disabled for root.
+Root login with RSA authentication when the
+.Ar command
+option has been
+specified will be allowed regardless of the value of this setting
+(which may be useful for taking remote backups even if root login is
+normally not allowed).
+.It Cm Port
+Specifies the port number that
+listens on. The default is 22.
+.It Cm PrintMotd
+Specifies whether
+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 QuietMode
+Specifies whether the system runs in quiet mode. In quiet mode,
+nothing is logged in the system log, except fatal errors. The default
+.Dq no .
+.It Cm RandomSeed
+Obsolete. Random number generation uses other techniques.
+.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 .
+.It Cm RhostsRSAAuthentication
+Specifies whether rhosts or /etc/hosts.equiv authentication together
+with successful RSA host authentication is allowed. The default is
+.Dq yes .
+.It Cm RSAAuthentication
+Specifies whether pure RSA authentication is allowed. The default is
+.Dq yes .
+.It Cm ServerKeyBits
+Defines the number of bits in the server key. The minimum value is
+512, and the default is 768.
+.It Cm SkeyAuthentication
+Specifies whether
+.Xr skey 1
+authentication is allowed. The default is
+.Dq yes .
+Note that s/key authentication is enabled only if
+.Cm PasswordAuthentication
+is allowed, too.
+.It Cm StrictModes
+Specifies whether
+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 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. The default is
+.Dq no .
+.It Cm X11Forwarding
+Specifies whether X11 forwarding is permitted. The default is
+.Dq yes .
+Note that disabling X11 forwarding does not improve security in any
+way, as users can always install their own forwarders.
+.It Cm X11DisplayOffset
+Specifies the first display number available for
+.Nm sshd Ns 's
+X11 forwarding. This prevents
+from interfering with real X11 servers.
+When a user successfully logs in,
+does the following:
+.Bl -enum -offset indent
+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
+If the login is on a tty, records login time.
+.Pa /etc/nologin ;
+if it exists, prints contents and quits
+(unless root).
+Changes to run with normal user privileges.
+Sets up basic environment.
+.Pa $HOME/.ssh/environment
+if it exists.
+Changes to user's home directory.
+.Pa $HOME/.ssh/rc
+exists, runs it; else if
+.Pa /etc/openssh/sshrc
+exists, runs
+it; otherwise runs xauth. The
+.Dq rc
+files are given the X11
+authentication protocol and cookie in standard input.
+Runs user's shell or command.
+.Pa $HOME/.ssh/authorized_keys
+file lists the RSA keys that are
+permitted for RSA authentication. Each line of the file contains one
+key (empty lines and lines starting with a
+.Ql #
+are ignored as
+comments). Each line consists of the following fields, separated by
+spaces: options, bits, exponent, modulus, comment. The options field
+is 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; the
+comment field is not used for anything (but may be convenient for the
+user to identify the key).
+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
+file and edit it.
+The options (if present) consists 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 ('*' and '?' serve as wildcards). The list may also contain
+patterns negated by prefixing them with '!'; 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. 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. Notice that the client may specify TCP/IP and/or X11
+forwardings 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
+.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
+.It Cm no-pty
+Prevents tty allocation (a request to allocate a pty will fail).
+.Ss Examples
+1024 33 12121.\|.\|.\|312314325
+from="*,!" 1024 35 23.\|.\|.\|2334 ylo@niksula
+command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323
+.Pa /etc/openssh/ssh_known_hosts
+.Pa $HOME/.ssh/known_hosts
+files contain host public keys for all known hosts. The global file should
+be prepared by the admistrator (optional), and the per-user file is
+maintained automatically: whenever the user connects an unknown host
+its key is added to the per-user file.
+Each line in these files contains the following fields: hostnames,
+bits, exponent, modulus, comment. The fields are separated by spaces.
+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
+.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.
+Bits, exponent, and modulus are taken directly from the host key; they
+can be obtained, e.g., from
+.Pa /etc/openssh/ .
+The optional comment field continues to the end of the line, and is not used.
+Lines starting with
+.Ql #
+and empty lines are ignored as comments.
+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.
+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/openssh/
+and adding the host names at the front.
+.Ss Examples
+closenet,,.\|.\|.\|, 1024 37 159.\|.\|.93
+.Bl -tag -width Ds
+.It Pa /etc/openssh/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/openssh/ssh_host_key
+Contains the private part of the host key.
+This file should only be owned by root, readable only by root, and not
+accessible to others.
+Note that
+does not start if this file is group/world-accessible.
+.It Pa /etc/openssh/
+Contains the public part of the host key.
+This file should be world-readable but writable only by
+root. Its contents should match the private part. This file is not
+really used for anything; it is only provided for the convenience of
+the user so its contents can be copied to known hosts files.
+These two files are created using
+.Xr ssh-keygen 1 .
+.It Pa /var/run/
+Contains the process ID of the
+listening for connections (if there are several daemons running
+concurrently for different ports, this contains the pid of the one
+started last). The contents of this file are not sensitive; it can be
+.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.
+.It Pa /etc/openssh/ssh_known_hosts
+This file is consulted when using rhosts with RSA host
+authentication to check the public key of the host. The key must be
+listed in this file to be accepted.
+.It Pa $HOME/.ssh/known_hosts
+The client uses this file
+.Pa /etc/openssh/ssh_known_hosts
+to verify that the remote host is the one we intended to
+connect. These files should be writable only by root/the owner.
+.Pa /etc/openssh/ssh_known_hosts
+should be world-readable, and
+.Pa $HOME/.ssh/known_hosts
+can but need not be world-readable.
+.It Pa /etc/nologin
+If this file exists,
+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
+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.
+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.
+.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 \&- .
+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.
+.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.
+Note that this warning also applies to rsh/rlogin.
+.It Pa /etc/openssh/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
+in environment). This must call
+.Xr xauth 1
+in that case.
+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.
+This file will probably contain some initialization code followed by
+something similar to: "if read proto cookie; then echo add $DISPLAY
+$proto $cookie | xauth -q -; fi".
+If this file does not exist,
+.Pa /etc/openssh/sshrc
+is run, and if that
+does not exist either, xauth is used to store the cookie.
+This file should be writable only by the user, and need not be
+readable by anyone else.
+.It Pa /etc/openssh/sshrc
+.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.
+Tatu Ylonen <>
+Information about new releases, mailing lists, and other related
+issues can be found from the SSH WWW home page:
+is a derivative of the original (free) ssh 1.2.12 release, but with bugs
+removed and newer features re-added. Rapidly after the 1.2.12 release,
+newer versions bore successively more restrictive licenses. This version
+of OpenSSH
+.Bl -bullet
+has all components of a restrictive nature (ie. patents, see
+.Xr ssl 8 )
+directly removed from the source code; any licensed or patented components
+are chosen from
+external libraries.
+has been updated to support ssh protocol 1.5.
+contains added support for
+.Xr kerberos 8
+authentication and ticket passing.
+supports one-time password authentication with
+.Xr skey 1 .
+The libraries described in
+.Xr ssl 8
+are required for proper operation.
+.Xr rlogin 1 ,
+.Xr rsh 1 ,
+.Xr scp 1 ,
+.Xr ssh 1 ,
+.Xr ssh-add 1 ,
+.Xr ssh-agent 1 ,
+.Xr ssh-keygen 1 ,
+.Xr ssl 8