.\" -*- nroff -*-
.\"
-.\" sshd.8.in
-.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
-.\"
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\" All rights reserved
.\"
-.\" Created: Sat Apr 22 21:55:14 1995 ylo
+.\" 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.
.\"
-.\" $Id$
+.\" 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 1.99 2001/03/02 18:54:31 deraadt Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
-.Nd secure shell daemon
+.Nd OpenSSH secure shell daemon
.Sh SYNOPSIS
.Nm sshd
-.Op Fl diqQ
+.Op Fl diqD46
.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
+.Sh DESCRIPTION
.Nm
-(Secure Shell Daemon) is the daemon program for
+(Secure Shell Daemon) is the daemon program for
.Xr ssh 1 .
-Together these programs replace rlogin and rsh programs, and
+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
+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
+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
+daemon for each incoming connection.
+The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
-.Pp
+This implementation of
+.Nm
+supports both SSH protocol version 1 and 2 simultaneously.
.Nm
-works as follows. Each host has a host-specific RSA key
-(normally 1024 bits) used to identify the host. Additionally, when
+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, 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
+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 start to use this
+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 and 3DES, with 3DES
-being is used by default. The client selects the encryption algorithm
+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
+Next, the server and the client enter an authentication dialog.
+The client tries to authenticate itself using
.Pa .rhosts
authentication,
.Pa .rhosts
.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
+configuration file if desired.
+System security is not improved unless
.Xr rshd 8 ,
.Xr rlogind 8 ,
.Xr rexecd 8 ,
.Xr rlogin 1
and
.Xr rsh 1
-into that machine).
+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.
+The rest of the session is encrypted
+using a symmetric cipher, currently
+Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
+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 authentication method (PubkeyAuthentication)
+and conventional password authentication.
+.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
+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
+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
.Pp
.Nm
can be configured using command-line options or a configuration
-file. Command-line options override values specified in the
+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 .
+.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
Specifies the number of bits in the 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.
+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 increases the debugging level.
+Maximum is 3.
.It Fl f Ar configuration_file
-Specifies the name of the configuration file. The default is
-.Pa /etc/ssh/sshd_config .
+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
-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.
+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/ssh_host_key ) .
+.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.
.It Fl i
Specifies that
.Nm
-is being run from inetd.
+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
+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 server key is regenerated (default 3600
-seconds, or one hour). The motivation for regenerating the key fairly
+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.
+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,
+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.
+.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 V Ar client_protocol_id
-SSH2 compatibility mode.
-When this options is specified
+SSH-2 compatibility mode.
+When this option is specified
.Nm
-assumes the client has sent the given version string
+assumes the client has sent the supplied version string
and skips the
Protocol Version Identification Exchange.
+This option is not intended to be called directly.
+.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/ssh/sshd_config
+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
+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
+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.
+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 primary group.
+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 number of user names, separated
-by spaces. If specified, login is allowed only for users names that
+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.
+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 Ciphers
+Specifies the ciphers allowed for protocol version 2.
+Multiple ciphers must be comma-separated.
+The default is
+.Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
.It Cm CheckMail
Specifies whether
.Nm
.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.
+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 primary group.
+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 allowed disallowed for user names that match
-one of the patterns.
+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.
+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 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 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 HostKey
-Specifies the file containing the private host key (default
-.Pa /etc/ssh/ssh_host_key ) .
+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
-does not start if this file is group/world-accessible.
+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 rhosts and shosts files will not be used in
-authentication.
+Specifies that
+.Pa .rhosts
+and
+.Pa .shosts
+files will not be used in authentication.
.Pa /etc/hosts.equiv
and
-.Pa /etc/ssh/shosts.equiv
-are still used. The default is
-.Dq no .
+.Pa /etc/shosts.equiv
+are still used.
+The default is
+.Dq yes .
.It Cm IgnoreUserKnownHosts
Specifies whether
.Nm
.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
+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,
+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.
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.
+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
+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
+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
-or SecurID. Default is
+.Pa /etc/passwd .
+Default is
.Dq yes .
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT may be forwarded to the server.
-Default is
+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
+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
+(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).
+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 what local address
.Nm
should listen on.
The default is to listen to all local addresses.
+Multiple options of this type are permitted.
+Additionally, the
+.Cm Ports
+options must precede this option.
.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.
+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
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-sha1,hmac-md5,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 PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Dq yes .
+Note that this option applies to both protocol versions 1 and 2.
.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 yes .
+server allows login to accounts with empty password strings.
+The default is
+.Dq no .
.It Cm PermitRootLogin
Specifies whether the root can log in using
.Xr ssh 1 .
The argument must be
.Dq yes ,
-.Dq without-password
+.Dq without-password ,
+.Dq forced-commands-only
or
.Dq no .
The default is
.Dq yes .
-If this options is set to
+.Pp
+If this option is set to
.Dq without-password
-only password authentication is disabled for root.
+password authentication is disabled for root.
.Pp
-Root login with RSA authentication when the
+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 will be allowed regardless of the value of this setting
+option has been specified
(which may be useful for taking remote backups even if root login is
-normally not allowed).
+normally not allowed). All other authentication methods are disabled
+for root.
+.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.
+listens on.
+The default is 22.
+Multiple options of this type are permitted.
.It Cm PrintMotd
Specifies whether
.Nm
-should print
+should print
.Pa /etc/motd
-when a user logs in interactively. (On some systems it is also
-printed by the shell,
+when a user logs in interactively.
+(On some systems it is also printed by the shell,
.Pa /etc/profile ,
-or equivalent.) The default is
+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 1 .
.It Cm RandomSeed
-Obsolete. Random number generation uses other techniques.
+Obsolete.
+Random number generation uses other techniques.
+.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.
+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
.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 .
+with successful RSA host authentication is allowed.
+The default is
+.Dq no .
.It Cm RSAAuthentication
-Specifies whether pure RSA authentication is allowed. The default is
+Specifies whether pure RSA authentication is allowed.
+The default is
.Dq yes .
+Note that this option applies to protocol version 1 only.
.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
+Defines the number of bits in the server key.
+The minimum value is 512, and the default is 768.
+.It Cm ChallengeResponseAuthentication
Specifies whether
-.Xr skey 1
-authentication is allowed. The default is
+challenge reponse
+authentication is allowed.
+Currently there is support for
+.Xr skey 1
+and PAM authentication.
+The default is
.Dq yes .
-Note that s/key authentication is enabled only if
-.Cm PasswordAuthentication
-is allowed, too.
+Note that enabling ChallengeResponseAuthentication for PAM bypasses
+OpenSSH's password checking code, thus rendering options such as
+.Cm PasswordAuthentication
+and
+.Cm PermitEmptyPasswords
+ineffective.
.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
+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.
+LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
+The default is AUTH.
.It Cm UseLogin
Specifies whether
.Xr login 1
-is used. The default is
+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 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
+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,
.Bl -enum -offset indent
.It
If the login is on a tty, and no command has been specified,
-prints last login time and
+prints last login time and
.Pa /etc/motd
(unless prevented in the configuration file or by
.Pa $HOME/.hushlogin ;
see the
-.Sx FILES
+.Sx FILES
section).
.It
If the login is on a tty, records login time.
If
.Pa $HOME/.ssh/rc
exists, runs it; else if
-.Pa /etc/ssh/sshrc
+.Pa /etc/sshrc
exists, runs
-it; otherwise runs xauth. The
+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.
.El
.Sh AUTHORIZED_KEYS FILE FORMAT
-The
+The
.Pa $HOME/.ssh/authorized_keys
file lists the RSA keys that are
-permitted for RSA authentication. Each line of the file contains one
+permitted for RSA authentication in SSH protocols 1.3 and 1.5
+Similarly, the
+.Pa $HOME/.ssh/authorized_keys2
+file lists the DSA and RSA keys that are
+permitted for public key authentication (PubkeyAuthentication)
+in SSH protocol 2.0.
+.Pp
+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
+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; the
+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
+(because of the size of the RSA key modulus).
+You don't want to type them in; instead, copy the
.Pa identity.pub
+or the
+.Pa id_dsa.pub
file and edit it.
.Pp
-The options (if present) consists of comma-separated option
-specifications. No spaces are permitted, except within double quotes.
+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 ('*' 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
+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
+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.
+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.
+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.
+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
+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
.Pp
command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
.Sh SSH_KNOWN_HOSTS FILE FORMAT
-The
-.Pa /etc/ssh/ssh_known_hosts
-and
-.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.
+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.
+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
+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 host key; they
+Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
-.Pa /etc/ssh/ssh_host_key.pub .
+.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
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
+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
+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/ssh_host_key.pub
+or by taking
+.Pa /etc/ssh_host_key.pub
and adding the host names at the front.
.Ss Examples
closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
.Sh FILES
.Bl -tag -width Ds
-.It Pa /etc/ssh/sshd_config
+.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/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
+.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
+(SSH1, SSH2 DSA, and SSH2 RSA) 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/ssh_host_key.pub
-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
+.It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
+There three files contain the public parts of the
+(SSH1, SSH2 DSA, and SSH2 RSA) 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 contents of this file are not sensitive; it can be
-world-readable.
+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.
+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 $HOME/.ssh/authorized_keys2
+Lists the DSA keys that can be used to log into the user's account.
+This file must be readable by root (which may on some machines imply
+it being world-readable if the user's home directory resides on an NFS
+volume).
+It is recommended that it not be accessible by others.
+The format of this file is described above.
+Users will place the contents of their
+.Pa id_dsa.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
.It Pa "/etc/ssh_known_hosts" and "$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.
+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 the remote host is the one we intended to
-connect. These files should be writable only by root/the owner.
-.Pa /etc/ssh/ssh_known_hosts
+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/nologin
-If this file exists,
+If this file exists,
.Nm
-refuses to let anyone except root log in. The contents of the file
+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.
+refused.
+The file should be world-readable.
.It Pa /etc/hosts.allow, /etc/hosts.deny
If compiled with
.Sy LIBWRAP
.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.
+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
+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
.Pa .rhosts .
However, this file is
not used by rlogin and rshd, so using this permits access using SSH only.
-.Pa /etc/hosts.equiv
+.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
+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
+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
+user on this machine (except root).
+Additionally, the syntax
.Dq +@group
-can be used to specify netgroups. Negated entries start with
+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
+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"
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
+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/ssh/shosts.equiv
+.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
+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
+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
+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
+in environment).
+This must call
.Xr xauth 1
in that case.
.Pp
$proto $cookie | xauth -q -; fi".
.Pp
If this file does not exist,
-.Pa /etc/ssh/sshrc
+.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/ssh/sshrc
+.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.
-.Sh AUTHOR
-Tatu Ylonen <ylo@cs.hut.fi>
-.Pp
-Information about new releases, mailing lists, and other related
-issues can be found from the SSH WWW home page:
-.Pp
-.Dl http://www.cs.hut.fi/ssh.
-.Pp
-OpenSSH
-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
-.It
-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.
-.It
-has been updated to support ssh protocol 1.5.
-.It
-contains added support for
-.Xr kerberos 8
-authentication and ticket passing.
-.It
-supports one-time password authentication with
-.Xr skey 1 .
+machine-specific login-time initializations globally.
+This file should be writable only by root, and should be world-readable.
.El
-.Pp
-The libraries described in
-.Xr ssl 8
-are required for proper operation.
+.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 rlogin 1 ,
-.Xr rsh 1 ,
.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 ssl 8
+.Xr rlogin 1 ,
+.Xr rsh 1
andersk / openssh.git / blobdiff