.\" (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 1.142 2001/08/21 21:47:42 stevesk Exp $
+.\" $OpenBSD: sshd.8,v 1.204 2005/02/25 10:55:13 jmc Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Nd OpenSSH SSH daemon
.Sh SYNOPSIS
.Nm sshd
-.Op Fl deiqD46
+.Bk -words
+.Op Fl 46Ddeiqt
.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 o Ar option
.Op Fl p Ar port
.Op Fl u Ar len
-.Op Fl V Ar client_protocol_id
+.Ek
.Sh DESCRIPTION
.Nm
(SSH Daemon) is the daemon program for
.Nm
supports both SSH protocol version 1 and 2 simultaneously.
.Nm
-works as follows.
-.Pp
+works as follows:
.Ss SSH protocol version 1
-.Pp
Each host has a host-specific RSA key
(normally 1024 bits) used to identify the host.
Additionally, when
This key is normally regenerated every hour if it has been used, and
is never stored on disk.
.Pp
-Whenever a client connects the daemon responds with its public
+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.
+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.
.Pp
Next, the server and the client enter an authentication dialog.
The client tries to authenticate itself using
-.Pa .rhosts
-authentication,
-.Pa .rhosts
+.Em .rhosts
authentication combined with RSA host
authentication, RSA challenge-response authentication, or password
based authentication.
.Pp
-Rhosts authentication is normally disabled
-because it is fundamentally insecure, but can be enabled in the server
-configuration file if desired.
-System security is not improved unless
-.Xr rshd 8 ,
-.Xr rlogind 8 ,
-.Xr rexecd 8 ,
+Regardless of the authentication type, the account is checked to
+ensure that it is accessible. An account is not accessible if it is
+locked, listed in
+.Cm DenyUsers
+or its group is listed in
+.Cm DenyGroups
+\&. The definition of a locked account is system dependant. Some platforms
+have their own account database (eg AIX) and some modify the passwd field (
+.Ql \&*LK\&*
+on Solaris,
+.Ql \&*
+on HP-UX, containing
+.Ql Nologin
+on Tru64 and a leading
+.Ql \&!!
+on Linux). If there is a requirement to disable password authentication
+for the account while allowing still public-key, then the passwd field
+should be set to something other than these values (eg
+.Ql NP
+or
+.Ql \&*NP\&*
+).
+.Pp
+.Nm rshd ,
+.Nm rlogind ,
and
-.Xr rexd 8
+.Nm rexecd
are disabled (thus completely disabling
-.Xr rlogin 1
+.Xr rlogin
and
-.Xr rsh 1
+.Xr rsh
into the machine).
-.Pp
.Ss SSH protocol version 2
-.Pp
Version 2 works similarly:
Each host has a host-specific key (RSA or DSA) used to identify the host.
However, when the daemon starts, it does not generate a server key.
This key agreement results in a shared session key.
.Pp
The rest of the session is encrypted using a symmetric cipher, currently
-128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
+128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
The client selects the encryption algorithm
to use from those offered by the server.
Additionally, session integrity is provided
user (PubkeyAuthentication) or
client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response based methods.
-.Pp
.Ss Command execution and data forwarding
-.Pp
If the client successfully authenticates itself, a dialog for
preparing the session is entered.
At this time the client may request
the client, and both sides exit.
.Pp
.Nm
-can be configured using command-line options or a configuration
-file.
+can be configured using command-line options or a configuration file
+(by default
+.Xr sshd_config 5 ) .
Command-line options override values specified in the
configuration file.
.Pp
.Nm
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP ,
-by executing itself with the name it was started as, i.e.,
+by executing itself with the name and options it was started with, e.g.,
.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
.Bl -tag -width Ds
+.It Fl 4
+Forces
+.Nm
+to use IPv4 addresses only.
+.It Fl 6
+Forces
+.Nm
+to use IPv6 addresses only.
.It Fl b Ar bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
+.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 d
Debug mode.
The server sends verbose debug output to the system
log, and does not put itself in the background.
The server also will not fork and will only process one connection.
This option is only intended for debugging for the server.
-Multiple -d options increase the debugging level.
+Multiple
+.Fl d
+options increase the debugging level.
Maximum is 3.
.It Fl e
When this option is specified,
.It Fl f Ar configuration_file
Specifies the name of the configuration file.
The default is
-.Pa /etc/sshd_config .
+.Pa /etc/ssh/sshd_config .
.Nm
refuses to start if there is no configuration file.
.It Fl g Ar login_grace_time
Gives the grace time for clients to authenticate themselves (default
-600 seconds).
+120 seconds).
If the client fails to authenticate the user within
this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
.It Fl h Ar host_key_file
-Specifies the file from which the host key is read (default
-.Pa /etc/ssh_host_key ) .
+Specifies a file from which a host key is read.
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).
+host key files are normally not readable by anyone but root).
+The default is
+.Pa /etc/ssh/ssh_host_key
+for protocol version 1, and
+.Pa /etc/ssh/ssh_host_rsa_key
+and
+.Pa /etc/ssh/ssh_host_dsa_key
+for protocol version 2.
It is possible to have multiple host key files for
the different protocol versions and host key algorithms.
.It Fl i
Specifies that
.Nm
-is being run from inetd.
+is being run from
+.Xr inetd 8 .
.Nm
is normally not run
from inetd because it needs to generate the server key before it can
Specifies how often the ephemeral protocol version 1 server key is
regenerated (default 3600 seconds, or one hour).
The motivation for regenerating the key fairly
-often is that the key is not stored anywhere, and after about an hour,
+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 o Ar option
+Can be used to give options in the format used in the configuration file.
+This is useful for specifying options for which there is no separate
+command-line flag.
+For full details of the options, and their values, see
+.Xr sshd_config 5 .
.It Fl p Ar port
Specifies the port on which the server listens for connections
(default 22).
+Multiple port options are permitted.
+Ports specified in the configuration file are ignored when a
+command-line port is specified.
.It Fl q
Quiet mode.
Nothing is sent to the system log.
.It Fl t
Test mode.
Only check the validity of the configuration file and sanity of the keys.
-This is useful for updating
+This is useful for updating
.Nm
reliably as configuration options may change.
.It Fl u Ar len
should be put into the
.Pa utmp
file.
-.It Fl D
-When this option is specified
-.Nm
-will not detach and does not become a daemon.
-This allows easy monitoring of
-.Nm sshd .
-.It Fl 4
-Forces
-.Nm
-to use IPv4 addresses only.
-.It Fl 6
-Forces
-.Nm
-to use IPv6 addresses only.
+.Fl u0
+may also be used to prevent
+.Nm
+from making DNS requests unless the authentication
+mechanism or configuration requires it.
+Authentication mechanisms that may require DNS include
+.Cm RhostsRSAAuthentication ,
+.Cm HostbasedAuthentication
+and using a
+.Cm from="pattern-list"
+option in a key file.
+Configuration options that require DNS include using a
+USER@HOST pattern in
+.Cm AllowUsers
+or
+.Cm DenyUsers .
.El
.Sh CONFIGURATION FILE
.Nm
reads configuration data from
-.Pa /etc/sshd_config
+.Pa /etc/ssh/sshd_config
(or the file specified with
.Fl f
on the command line).
-The file contains keyword-argument pairs, one per line.
-Lines starting with
-.Ql #
-and empty lines are interpreted as comments.
-.Pp
-The possible
-keywords and their meanings are as follows (note that
-keywords are case-insensitive and arguments are case-sensitive):
-.Bl -tag -width Ds
-.It Cm AFSTokenPassing
-Specifies whether an AFS token may be forwarded to the server.
-Default is
-.Dq yes .
-.It Cm AllowGroups
-This keyword can be followed by a list of group names, separated
-by spaces.
-If specified, login is allowed only for users whose primary
-group or supplementary group list matches one of the patterns.
-.Ql \&*
-and
-.Ql ?
-can be used as
-wildcards in the patterns.
-Only group names are valid; a numerical group ID isn't recognized.
-By default login is allowed regardless of the group list.
-.Pp
-.It Cm AllowTcpForwarding
-Specifies whether TCP forwarding is permitted.
-The default is
-.Dq yes .
-Note that disabling TCP forwarding does not improve security unless
-users are also denied shell access, as they can always install their
-own forwarders.
-.Pp
-.It Cm AllowUsers
-This keyword can be followed by a list of user names, separated
-by spaces.
-If specified, login is allowed only for users names that
-match one of the patterns.
-.Ql \&*
-and
-.Ql ?
-can be used as
-wildcards in the patterns.
-Only user names are valid; a numerical user ID isn't recognized.
-By default login is allowed regardless of the user name.
-If the pattern takes the form USER@HOST then USER and HOST
-are separately checked, allowing you to restrict logins to particular
-users from particular hosts.
-.Pp
-.It Cm AuthorizedKeysFile
-Specifies the file that contains the public keys that can be used
-for user authentication.
-.Cm AuthorizedKeysFile
-may contain tokens of the form %T which are substituted during connection
-set-up. The following tokens are defined: %% is replaced by a literal '%',
-%h is replaced by the home directory of the user being authenticated and
-%u is replaced by the username of that user.
-After expansion,
-.Cm AuthorizedKeysFile
-is taken to be an absolute path or one relative to the user's home
-directory.
-The default is
-.Dq .ssh/authorized_keys
-.It Cm Banner
-In some jurisdictions, sending a warning message before authentication
-may be relevant for getting legal protection.
-The contents of the specified file are sent to the remote user before
-authentication is allowed.
-This option is only available for protocol version 2.
-.Pp
-.It Cm ChallengeResponseAuthentication
-Specifies whether challenge response authentication is allowed.
-All authentication styles from
-.Xr login.conf 5
-are supported.
-The default is
-.Dq yes .
-.It Cm Ciphers
-Specifies the ciphers allowed for protocol version 2.
-Multiple ciphers must be comma-separated.
-The default is
-.Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
-.It Cm ClientAliveInterval
-Sets a timeout interval in seconds after which if no data has been received
-from the client,
-.Nm
-will send a message through the encrypted
-channel to request a response from the client.
-The default
-is 0, indicating that these messages will not be sent to the client.
-This option applies to protocol version 2 only.
-.It Cm ClientAliveCountMax
-Sets the number of client alive messages (see above) which may be
-sent without
-.Nm
-receiving any messages back from the client. If this threshold is
-reached while client alive messages are being sent,
-.Nm
-will disconnect the client, terminating the session. It is important
-to note that the use of client alive messages is very different from
-.Cm Keepalive
-(below). The client alive messages are sent through the
-encrypted channel and therefore will not be spoofable. The TCP keepalive
-option enabled by
-.Cm Keepalive
-is spoofable. You want to use the client
-alive mechanism when you are basing something important on
-clients having an active connection to the server.
-.Pp
-The default value is 3. If you set
-.Cm ClientAliveInterval
-(above) to 15, and leave this value at the default, unresponsive ssh clients
-will be disconnected after approximately 45 seconds.
-.It Cm DenyGroups
-This keyword can be followed by a number of group names, separated
-by spaces.
-Users whose primary group or supplementary group list matches
-one of the patterns aren't allowed to log in.
-.Ql \&*
-and
-.Ql ?
-can be used as
-wildcards in the patterns.
-Only group names are valid; a numerical group ID isn't recognized.
-By default login is allowed regardless of the group list.
-.Pp
-.It Cm DenyUsers
-This keyword can be followed by a number of user names, separated
-by spaces.
-Login is disallowed for user names that match one of the patterns.
-.Ql \&*
-and
-.Ql ?
-can be used as wildcards in the patterns.
-Only user names are valid; a numerical user ID isn't recognized.
-By default login is allowed regardless of the user name.
-.It Cm GatewayPorts
-Specifies whether remote hosts are allowed to connect to ports
-forwarded for the client.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm HostbasedAuthentication
-Specifies whether rhosts or /etc/hosts.equiv authentication together
-with successful public key client host authentication is allowed
-(hostbased authentication).
-This option is similar to
-.Cm RhostsRSAAuthentication
-and applies to protocol version 2 only.
-The default is
-.Dq no .
-.It Cm HostKey
-Specifies the file containing the private host keys (default
-.Pa /etc/ssh_host_key )
-used by SSH protocol versions 1 and 2.
-Note that
-.Nm
-will refuse to use a file if it is group/world-accessible.
-It is possible to have multiple host key files.
-.Dq rsa1
-keys are used for version 1 and
-.Dq dsa
-or
-.Dq rsa
-are used for version 2 of the SSH protocol.
-.It Cm IgnoreRhosts
-Specifies that
-.Pa .rhosts
-and
-.Pa .shosts
-files will not be used in
-.Cm RhostsAuthentication ,
-.Cm RhostsRSAAuthentication
-or
-.Cm HostbasedAuthentication .
-.Pp
-.Pa /etc/hosts.equiv
-and
-.Pa /etc/shosts.equiv
-are still used.
-The default is
-.Dq yes .
-.It Cm IgnoreUserKnownHosts
-Specifies whether
-.Nm
-should ignore the user's
-.Pa $HOME/.ssh/known_hosts
-during
-.Cm RhostsRSAAuthentication
-or
-.Cm HostbasedAuthentication .
-The default is
-.Dq no .
-.It Cm KeepAlive
-Specifies whether the system should send keepalive messages to the
-other side.
-If they are sent, death of the connection or crash of one
-of the machines will be properly noticed.
-However, this means that
-connections will die if the route is down temporarily, and some people
-find it annoying.
-On the other hand, if keepalives are not sent,
-sessions may hang indefinitely on the server, leaving
-.Dq ghost
-users and consuming server resources.
-.Pp
-The default is
-.Dq yes
-(to send keepalives), and the server will notice
-if the network goes down or the client host reboots.
-This avoids infinitely hanging sessions.
-.Pp
-To disable keepalives, the value should be set to
-.Dq no
-in both the server and the client configuration files.
-.It Cm KerberosAuthentication
-Specifies whether Kerberos authentication is allowed.
-This can be in the form of a Kerberos ticket, or if
-.Cm PasswordAuthentication
-is yes, the password provided by the user will be validated through
-the Kerberos KDC.
-To use this option, the server needs a
-Kerberos servtab which allows the verification of the KDC's identity.
-Default is
-.Dq yes .
-.It Cm KerberosOrLocalPasswd
-If set then if password authentication through Kerberos fails then
-the password will be validated via any additional local mechanism
-such as
-.Pa /etc/passwd .
-Default is
-.Dq yes .
-.It Cm KerberosTgtPassing
-Specifies whether a Kerberos TGT may be forwarded to the server.
-Default is
-.Dq no ,
-as this only works when the Kerberos KDC is actually an AFS kaserver.
-.It Cm KerberosTicketCleanup
-Specifies whether to automatically destroy the user's ticket cache
-file on logout.
-Default is
-.Dq yes .
-.It Cm KeyRegenerationInterval
-In protocol version 1, the ephemeral server key is automatically regenerated
-after this many seconds (if it has been used).
-The purpose of regeneration is to prevent
-decrypting captured sessions by later breaking into the machine and
-stealing the keys.
-The key is never stored anywhere.
-If the value is 0, the key is never regenerated.
-The default is 3600 (seconds).
-.It Cm ListenAddress
-Specifies the local addresses
-.Nm
-should listen on.
-The following forms may be used:
-.Pp
-.Bl -item -offset indent -compact
-.It
-.Cm ListenAddress
-.Sm off
-.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
-.Sm on
-.It
-.Cm ListenAddress
-.Sm off
-.Ar host No | Ar IPv4_addr No : Ar port
-.Sm on
-.It
-.Cm ListenAddress
-.Sm off
-.Oo
-.Ar host No | Ar IPv6_addr Oc : Ar port
-.Sm on
-.El
-.Pp
-If
-.Ar port
-is not specified,
-.Nm
-will listen on the address and all prior
-.Cm Port
-options specified. The default is to listen on all local
-addresses. Multiple
-.Cm ListenAddress
-options are permitted. Additionally, any
-.Cm Port
-options must precede this option for non port qualified addresses.
-.It Cm LoginGraceTime
-The server disconnects after this time if the user has not
-successfully logged in.
-If the value is 0, there is no time limit.
-The default is 600 (seconds).
-.It Cm LogLevel
-Gives the verbosity level that is used when logging messages from
-.Nm sshd .
-The possible values are:
-QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
-The default is INFO.
-Logging with level DEBUG violates the privacy of users
-and is not recommended.
-.It Cm MACs
-Specifies the available MAC (message authentication code) algorithms.
-The MAC algorithm is used in protocol version 2
-for data integrity protection.
-Multiple algorithms must be comma-separated.
-The default is
-.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
-.It Cm MaxStartups
-Specifies the maximum number of concurrent unauthenticated connections to the
-.Nm
-daemon.
-Additional connections will be dropped until authentication succeeds or the
-.Cm LoginGraceTime
-expires for a connection.
-The default is 10.
-.Pp
-Alternatively, random early drop can be enabled by specifying
-the three colon separated values
-.Dq start:rate:full
-(e.g., "10:30:60").
-.Nm
-will refuse connection attempts with a probability of
-.Dq rate/100
-(30%)
-if there are currently
-.Dq start
-(10)
-unauthenticated connections.
-The probability increases linearly and all connection attempts
-are refused if the number of unauthenticated connections reaches
-.Dq full
-(60).
-.It Cm PAMAuthenticationViaKbdInt
-Specifies whether PAM challenge response authentication is allowed. This
-allows the use of most PAM challenge response authentication modules, but
-it will allow password authentication regardless of whether
-.Cm PasswordAuthentication
-is disabled.
-The default is
-.Dq no .
-.It Cm PasswordAuthentication
-Specifies whether password authentication is allowed.
-The default is
-.Dq yes .
-.It Cm PermitEmptyPasswords
-When password authentication is allowed, it specifies whether the
-server allows login to accounts with empty password strings.
-The default is
-.Dq no .
-.It Cm PermitRootLogin
-Specifies whether root can login using
-.Xr ssh 1 .
-The argument must be
-.Dq yes ,
-.Dq without-password ,
-.Dq forced-commands-only
-or
-.Dq no .
-The default is
-.Dq yes .
-.Pp
-If this option is set to
-.Dq without-password
-password authentication is disabled for root.
-.Pp
-If this option is set to
-.Dq forced-commands-only
-root login with public key authentication will be allowed,
-but only if the
-.Ar command
-option has been specified
-(which may be useful for taking remote backups even if root login is
-normally not allowed). All other authentication methods are disabled
-for root.
-.Pp
-If this option is set to
-.Dq no
-root is not allowed to login.
-.It Cm PidFile
-Specifies the file that contains the process identifier of the
-.Nm
-daemon.
-The default is
-.Pa /var/run/sshd.pid .
-.It Cm Port
-Specifies the port number that
-.Nm
-listens on.
-The default is 22.
-Multiple options of this type are permitted.
-See also
-.Cm ListenAddress .
-.It Cm PrintLastLog
-Specifies whether
-.Nm
-should print the date and time when the user last logged in.
-The default is
-.Dq yes .
-.It Cm PrintMotd
-Specifies whether
-.Nm
-should print
-.Pa /etc/motd
-when a user logs in interactively.
-(On some systems it is also printed by the shell,
-.Pa /etc/profile ,
-or equivalent.)
-The default is
-.Dq yes .
-.It Cm Protocol
-Specifies the protocol versions
-.Nm
-should support.
-The possible values are
-.Dq 1
-and
-.Dq 2 .
-Multiple versions must be comma-separated.
-The default is
-.Dq 2,1 .
-.It Cm PubkeyAuthentication
-Specifies whether public key authentication is allowed.
-The default is
-.Dq yes .
-Note that this option applies to protocol version 2 only.
-.It Cm ReverseMappingCheck
-Specifies whether
-.Nm
-should try to verify the remote host name and check that
-the resolved host name for the remote IP address maps back to the
-very same IP address.
-The default is
-.Dq no .
-.It Cm RhostsAuthentication
-Specifies whether authentication using rhosts or /etc/hosts.equiv
-files is sufficient.
-Normally, this method should not be permitted because it is insecure.
-.Cm RhostsRSAAuthentication
-should be used
-instead, because it performs RSA-based host authentication in addition
-to normal rhosts or /etc/hosts.equiv authentication.
-The default is
-.Dq no .
-This option applies to protocol version 1 only.
-.It Cm RhostsRSAAuthentication
-Specifies whether rhosts or /etc/hosts.equiv authentication together
-with successful RSA host authentication is allowed.
-The default is
-.Dq no .
-This option applies to protocol version 1 only.
-.It Cm RSAAuthentication
-Specifies whether pure RSA authentication is allowed.
-The default is
-.Dq yes .
-This option applies to protocol version 1 only.
-.It Cm ServerKeyBits
-Defines the number of bits in the ephemeral protocol version 1 server key.
-The minimum value is 512, and the default is 768.
-.It Cm StrictModes
-Specifies whether
-.Nm
-should check file modes and ownership of the
-user's files and home directory before accepting login.
-This is normally desirable because novices sometimes accidentally leave their
-directory or files world-writable.
-The default is
-.Dq yes .
-.It Cm Subsystem
-Configures an external subsystem (e.g., file transfer daemon).
-Arguments should be a subsystem name and a command to execute upon subsystem
-request.
-The command
-.Xr sftp-server 8
-implements the
-.Dq sftp
-file transfer subsystem.
-By default no subsystems are defined.
-Note that this option applies to protocol version 2 only.
-.It Cm SyslogFacility
-Gives the facility code that is used when logging messages from
-.Nm sshd .
-The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
-LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
-The default is AUTH.
-.It Cm UseLogin
-Specifies whether
-.Xr login 1
-is used for interactive login sessions.
-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.
-.It Cm X11DisplayOffset
-Specifies the first display number available for
-.Nm sshd Ns 's
-X11 forwarding.
-This prevents
-.Nm
-from interfering with real X11 servers.
-The default is 10.
-.It Cm X11Forwarding
-Specifies whether X11 forwarding is permitted.
-The default is
-.Dq no .
-Note that disabling X11 forwarding does not improve security in any
-way, as users can always install their own forwarders.
-X11 forwarding is automatically disabled if
-.Cm UseLogin
-is enabled.
-.It Cm XAuthLocation
-Specifies the location of the
-.Xr xauth 1
-program.
-The default is
-.Pa /usr/X11R6/bin/xauth .
-.El
-.Ss Time Formats
-.Pp
-.Nm
-command-line arguments and configuration file options that specify time
-may be expressed using a sequence of the form:
-.Sm off
-.Ar time Oo Ar qualifier Oc ,
-.Sm on
-where
-.Ar time
-is a positive integer value and
-.Ar qualifier
-is one of the following:
-.Pp
-.Bl -tag -width Ds -compact -offset indent
-.It Cm <none>
-seconds
-.It Cm s | Cm S
-seconds
-.It Cm m | Cm M
-minutes
-.It Cm h | Cm H
-hours
-.It Cm d | Cm D
-days
-.It Cm w | Cm W
-weeks
-.El
-.Pp
-Each member of the sequence is added together to calculate
-the total time value.
-.Pp
-Time format examples:
-.Pp
-.Bl -tag -width Ds -compact -offset indent
-.It 600
-600 seconds (10 minutes)
-.It 10m
-10 minutes
-.It 1h30m
-1 hour 30 minutes (90 minutes)
-.El
+The file format and configuration options are described in
+.Xr sshd_config 5 .
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
.It
Sets up basic environment.
.It
-Reads
-.Pa $HOME/.ssh/environment
-if it exists.
+Reads the file
+.Pa $HOME/.ssh/environment ,
+if it exists, and users are allowed to change their environment.
+See the
+.Cm PermitUserEnvironment
+option in
+.Xr sshd_config 5 .
.It
Changes to user's home directory.
.It
If
.Pa $HOME/.ssh/rc
exists, runs it; else if
-.Pa /etc/sshrc
+.Pa /etc/ssh/sshrc
exists, runs
it; otherwise runs xauth.
The
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 options field
+is optional; its presence is determined by whether the line starts
+with a number or not (the options field never starts with a number).
The bits, exponent, modulus and comment fields give the RSA key for
protocol version 1; the
comment field is not used for anything (but may be convenient for the
.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).
+(because of the size of the public key encoding) up to a limit of
+8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
+keys up to 16 kilobits.
You don't want to type them in; instead, copy the
.Pa identity.pub ,
.Pa id_dsa.pub
.Pa id_rsa.pub
file and edit it.
.Pp
+.Nm
+enforces a minimum RSA key modulus size for protocol 1
+and protocol 2 keys of 768 bits.
+.Pp
The options (if present) consist of comma-separated option
specifications.
No spaces are permitted, except within double quotes.
that option keywords are case-insensitive):
.Bl -tag -width Ds
.It Cm from="pattern-list"
-Specifies that in addition to RSA authentication, the canonical name
+Specifies that in addition to public key authentication, the canonical name
of the remote host must be present in the comma-separated list of
patterns
-.Pf ( Ql *
+.Pf ( Ql \&*
and
-.Ql ?
+.Ql \&?
serve as wildcards).
The list may also contain
patterns negated by prefixing them with
-.Ql ! ;
+.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
+of this option is to optionally increase security: public key 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.
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;
+The command is run on a pty if the client requests a pty;
otherwise it is run without a tty.
-Note that if you want a 8-bit clean channel,
-you must not request a pty or should specify
+If an 8-bit clean channel is required,
+one 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.
+to restrict certain public 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.
+Note that this option applies to shell, command or subsystem execution.
.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.
+Environment processing is disabled by default and is
+controlled via the
+.Cm PermitUserEnvironment
+option.
+This option is automatically disabled if
+.Cm UseLogin
+is enabled.
.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.
Limit local
.Li ``ssh -L''
port forwarding such that it may only connect to the specified host and
-port. Multiple
+port.
+IPv6 addresses can be specified with an alternative syntax:
+.Ar host Ns / Ns Ar port .
+Multiple
.Cm permitopen
-options may be applied separated by commas. No pattern matching is
-performed on the specified hostnames, they must be literal domains or
-addresses.
+options may be applied separated by commas.
+No pattern matching is performed on the specified hostnames,
+they must be literal domains or addresses.
.El
.Ss Examples
-1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
+1024 33 12121...312314325 ylo@foo.bar
.Pp
-from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
+from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
.Pp
-command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
+command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
.Pp
-permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
+permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
-.Pa /etc/ssh_known_hosts ,
+.Pa /etc/ssh/ssh_known_hosts
and
.Pa $HOME/.ssh/known_hosts
files contain host public keys for all known hosts.
bits, exponent, modulus, comment.
The fields are separated by spaces.
.Pp
-Hostnames is a comma-separated list of patterns ('*' and '?' act as
+Hostnames is a comma-separated list of patterns
+.Pf ( Ql \&*
+and
+.Ql \&?
+act as
wildcards); each pattern in turn is matched against the canonical host
name (when authenticating a client) or against the user-supplied
name (when authenticating a server).
A pattern may also be preceded by
-.Ql !
+.Ql \&!
to indicate negation: if the host name matches a negated
pattern, it is not accepted (by that line) even if it matched another
pattern on the line.
.Pp
Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
-.Pa /etc/ssh_host_key.pub .
+.Pa /etc/ssh/ssh_host_key.pub .
The optional comment field continues to the end of the line, and is not used.
.Pp
Lines starting with
long, and you definitely don't want to type in the host keys by hand.
Rather, generate them by a script
or by taking
-.Pa /etc/ssh_host_key.pub
+.Pa /etc/ssh/ssh_host_key.pub
and adding the host names at the front.
.Ss Examples
.Bd -literal
-closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
+closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
.Ed
.Sh FILES
.Bl -tag -width Ds
-.It Pa /etc/sshd_config
+.It Pa /etc/ssh/sshd_config
Contains configuration data for
.Nm sshd .
-This file should be writable by root only, but it is recommended
-(though not necessary) that it be world-readable.
-.It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
+The file format and configuration options are described in
+.Xr sshd_config 5 .
+.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
These three files contain the private parts of the host keys.
These files should only be owned by root, readable only by root, and not
accessible to others.
Note that
.Nm
does not start if this file is group/world-accessible.
-.It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
+.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
These three files contain the public parts of the host keys.
These files should be world-readable but writable only by
root.
.Xr ssh-keygen 1 .
.It Pa /etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
+The file format is described in
+.Xr moduli 5 .
+.It Pa /var/empty
+.Xr chroot 2
+directory used by
+.Nm
+during privilege separation in the pre-authentication phase.
+The directory should not contain any files and must be owned by root
+and not group or world-writable.
.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
+concurrently for different ports, this contains the process ID of the one
started last).
The content of this file is not sensitive; it can be world-readable.
.It Pa $HOME/.ssh/authorized_keys
.Pa id_rsa.pub
files into this file, as described in
.Xr ssh-keygen 1 .
-.It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
+.It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
authentication or protocol version 2 hostbased authentication
to check the public key of the host.
The client uses the same files
to verify that it is connecting to the correct remote host.
These files should be writable only by root/the owner.
-.Pa /etc/ssh_known_hosts
+.Pa /etc/ssh/ssh_known_hosts
should be world-readable, and
.Pa $HOME/.ssh/known_hosts
-can but need not be world-readable.
+can, but need not be, world-readable.
+.It Pa /etc/motd
+See
+.Xr motd 5 .
+.It Pa $HOME/.hushlogin
+This file is used to suppress printing the last login time and
+.Pa /etc/motd ,
+if
+.Cm PrintLastLog
+and
+.Cm PrintMotd ,
+respectively,
+are enabled.
+It does not suppress printing of the banner specified by
+.Cm Banner .
.It Pa /etc/nologin
If this file exists,
.Nm
refused.
The file should be world-readable.
.It Pa /etc/hosts.allow, /etc/hosts.deny
-If compiled with
-.Sy LIBWRAP
-support, tcp-wrappers access controls may be defined here as described in
+Access controls that should be enforced by tcp-wrappers are defined here.
+Further details are described in
.Xr hosts_access 5 .
.It Pa $HOME/.rhosts
-This file contains host-username pairs, separated by a space, one per
+This file is used during
+.Cm RhostsRSAAuthentication
+and
+.Cm HostbasedAuthentication
+and 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.
+without a 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.
+It 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.
not used by rlogin and rshd, so using this permits access using SSH only.
.It Pa /etc/hosts.equiv
This file is used during
-.Pa .rhosts
+.Cm RhostsRSAAuthentication
+and
+.Cm HostbasedAuthentication
authentication.
In the simplest form, this file contains host names, one per line.
Users on
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.
+Additionally, successful client host key authentication is required.
This file must be writable only by root; it is recommended
that it be world-readable.
.Pp
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.
+Environment processing is disabled by default and is
+controlled via the
+.Cm PermitUserEnvironment
+option.
.It Pa $HOME/.ssh/rc
-If this file exists, it is run with /bin/sh after reading the
+If this file exists, it is run with
+.Pa /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
+It must not produce any output on stdout; stderr must be used
+instead.
+If X11 forwarding is in use, it will receive the "proto cookie" pair in
+its standard input (and
.Ev DISPLAY
-in environment).
-This must call
+in its environment).
+The script must call
.Xr xauth 1
-in that case.
+because
+.Nm
+will not run xauth automatically to add X11 cookies.
.Pp
The primary purpose of this file is to run any initialization routines
which may be needed before the user's home directory becomes
This file will probably contain some initialization code followed by
something similar to:
.Bd -literal
- if read proto cookie; then
- echo add $DISPLAY $proto $cookie | xauth -q -
- fi
+if read proto cookie && [ -n "$DISPLAY" ]; then
+ if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
+ # X11UseLocalhost=yes
+ echo add unix:`echo $DISPLAY |
+ cut -c11-` $proto $cookie
+ else
+ # X11UseLocalhost=no
+ echo add $DISPLAY $proto $cookie
+ fi | xauth -q -
+fi
.Ed
.Pp
If this file does not exist,
-.Pa /etc/sshrc
+.Pa /etc/ssh/sshrc
is run, and if that
-does not exist either, xauth is used to store the cookie.
+does not exist either, xauth is used to add the cookie.
.Pp
This file should be writable only by the user, and need not be
readable by anyone else.
-.It Pa /etc/sshrc
+.It Pa /etc/ssh/sshrc
Like
.Pa $HOME/.ssh/rc .
This can be used to specify
machine-specific login-time initializations globally.
This file should be writable only by root, and should be world-readable.
.El
-.Sh AUTHORS
-OpenSSH is a derivative of the original and free
-ssh 1.2.12 release by Tatu Ylonen.
-Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
-Theo de Raadt and Dug Song
-removed many bugs, re-added newer features and
-created OpenSSH.
-Markus Friedl contributed the support for SSH
-protocol versions 1.5 and 2.0.
.Sh SEE ALSO
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
+.Xr chroot 2 ,
+.Xr hosts_access 5 ,
.Xr login.conf 5 ,
.Xr moduli 5 ,
+.Xr sshd_config 5 ,
+.Xr inetd 8 ,
.Xr sftp-server 8
.Rs
.%A T. Ylonen
.%A T. Rinne
.%A S. Lehtinen
.%T "SSH Protocol Architecture"
-.%N draft-ietf-secsh-architecture-09.txt
-.%D July 2001
+.%N draft-ietf-secsh-architecture-12.txt
+.%D January 2002
.%O work in progress material
.Re
.Rs
.%A N. Provos
.%A W. A. Simpson
.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
-.%N draft-ietf-secsh-dh-group-exchange-01.txt
-.%D April 2001
+.%N draft-ietf-secsh-dh-group-exchange-02.txt
+.%D January 2002
.%O work in progress material
.Re
+.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.
andersk / openssh.git / blobdiff