.\" (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 1.175 2002/03/29 19:16:22 stevesk Exp $
-.Dd September 25, 1999
+.\" $OpenBSD: sshd.8,v 1.249 2009/10/08 20:42:13 jmc Exp $
+.Dd $Mdocdate$
.Dt SSHD 8
.Os
.Sh NAME
.Nd OpenSSH SSH daemon
.Sh SYNOPSIS
.Nm sshd
-.Op Fl deiqtD46
+.Bk -words
+.Op Fl 46DdeiqTt
.Op Fl b Ar bits
+.Op Fl C Ar connection_spec
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl h Ar host_key_file
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl u Ar len
+.Ek
.Sh DESCRIPTION
.Nm
-(SSH Daemon) is the daemon program for
+(OpenSSH Daemon) is the daemon program for
.Xr ssh 1 .
-Together these programs replace rlogin and rsh, and
-provide secure encrypted communications between two untrusted hosts
+Together these programs replace
+.Xr rlogin 1
+and
+.Xr rsh 1 ,
+and provide secure encrypted communications between two untrusted hosts
over an insecure network.
-The programs are intended to be as easy to
-install and use as possible.
.Pp
.Nm
-is the daemon that listens for connections from clients.
+listens for connections from clients.
It is normally started at boot from
.Pa /etc/rc .
It forks a new
The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
-This implementation of
-.Nm
-supports both SSH protocol version 1 and 2 simultaneously.
-.Nm
-works as follows.
-.Pp
-.Ss SSH protocol version 1
-.Pp
-Each host has a host-specific RSA key
-(normally 1024 bits) used to identify the host.
-Additionally, when
-the daemon starts, it generates a server RSA key (normally 768 bits).
-This key is normally regenerated every hour if it has been used, and
-is never stored on disk.
-.Pp
-Whenever a client connects the daemon responds with its public
-host and server keys.
-The client compares the
-RSA host key against its own database to verify that it has not changed.
-The client then generates a 256 bit random number.
-It encrypts this
-random number using both the host key and the server key, and sends
-the encrypted number to the server.
-Both sides then use this
-random number as a session key which is used to encrypt all further
-communications in the session.
-The rest of the session is encrypted
-using a conventional cipher, currently Blowfish or 3DES, with 3DES
-being used by default.
-The client selects the encryption algorithm
-to use from those offered by the server.
-.Pp
-Next, the server and the client enter an authentication dialog.
-The client tries to authenticate itself using
-.Pa .rhosts
-authentication,
-.Pa .rhosts
-authentication combined with RSA host
-authentication, RSA challenge-response authentication, or password
-based authentication.
-.Pp
-Rhosts authentication is normally disabled
-because it is fundamentally insecure, but can be enabled in the server
-configuration file if desired.
-System security is not improved unless
-.Xr rshd 8 ,
-.Xr rlogind 8 ,
-and
-.Xr rexecd 8
-are disabled (thus completely disabling
-.Xr rlogin 1
-and
-.Xr rsh 1
-into the machine).
-.Pp
-.Ss SSH protocol version 2
-.Pp
-Version 2 works similarly:
-Each host has a host-specific key (RSA or DSA) used to identify the host.
-However, when the daemon starts, it does not generate a server key.
-Forward security is provided through a Diffie-Hellman key agreement.
-This key agreement results in a shared session key.
-.Pp
-The rest of the session is encrypted using a symmetric cipher, currently
-128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
-The client selects the encryption algorithm
-to use from those offered by the server.
-Additionally, session integrity is provided
-through a cryptographic message authentication code
-(hmac-sha1 or hmac-md5).
-.Pp
-Protocol version 2 provides a public key based
-user (PubkeyAuthentication) or
-client host (HostbasedAuthentication) authentication method,
-conventional password authentication and challenge response based methods.
-.Pp
-.Ss Command execution and data forwarding
-.Pp
-If the client successfully authenticates itself, a dialog for
-preparing the session is entered.
-At this time the client may request
-things like allocating a pseudo-tty, forwarding X11 connections,
-forwarding TCP/IP connections, or forwarding the authentication agent
-connection over the secure channel.
-.Pp
-Finally, the client either requests a shell or execution of a command.
-The sides then enter session mode.
-In this mode, either side may send
-data at any time, and such data is forwarded to/from the shell or
-command on the server side, and the user terminal in the client side.
-.Pp
-When the user program terminates and all forwarded X11 and other
-connections have been closed, the server sends command exit status to
-the client, and both sides exit.
.Pp
.Nm
-can be configured using command-line options or a configuration
-file.
-Command-line options override values specified in the
+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).
+server key (default 1024).
+.It Fl C Ar connection_spec
+Specify the connection parameters to use for the
+.Fl T
+extended test mode.
+If provided, any
+.Cm Match
+directives in the configuration file
+that would apply to the specified user, host, and address will be set before
+the configuration is written to standard output.
+The connection parameters are supplied as keyword=value pairs.
+The keywords are
+.Dq user ,
+.Dq host ,
+and
+.Dq addr .
+All are required and may be supplied in any order, either with multiple
+.Fl C
+options or as a comma-separated list.
+.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,
.Nm
will send the output to the standard error instead of the system log.
-.It Fl f Ar configuration_file
+.It Fl f Ar config_file
Specifies the name of the configuration file.
The default is
.Pa /etc/ssh/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
-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 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
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
+However, with small key sizes (e.g. 512) using
.Nm
from inetd may
be feasible.
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.
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.
+Ports specified in the configuration file with the
+.Cm Port
+option are ignored when a command-line port is specified.
+Ports specified using the
+.Cm ListenAddress
+option override command-line ports.
.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 T
+Extended test mode.
+Check the validity of the configuration file, output the effective configuration
+to stdout and then exit.
+Optionally,
+.Cm Match
+rules may be applied by specifying the connection parameters using one or more
+.Fl C
+options.
.It Fl t
Test mode.
Only check the validity of the configuration file and sanity of the keys.
.Pa utmp
file.
.Fl u0
-is also be used to prevent
+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 RhostsAuthentication ,
.Cm RhostsRSAAuthentication ,
-.Cm HostbasedAuthentication
+.Cm HostbasedAuthentication ,
and using a
.Cm from="pattern-list"
option in a key file.
.Cm AllowUsers
or
.Cm DenyUsers .
-.It Fl D
-When this option is specified
-.Nm
-will not detach and does not become a daemon.
-This allows easy monitoring of
-.Nm sshd .
-.It Fl 4
-Forces
-.Nm
-to use IPv4 addresses only.
-.It Fl 6
-Forces
-.Nm
-to use IPv6 addresses only.
.El
-.Sh CONFIGURATION FILE
-.Nm
-reads configuration data from
-.Pa /etc/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 name patterns, separated
-by spaces.
-If specified, login is allowed only for users whose primary
-group or supplementary group list matches one of the patterns.
-.Ql \&*
-and
-.Ql ?
-can be used as
-wildcards in the patterns.
-Only group names are valid; a numerical group ID is not recognized.
-By default, login is allowed for all groups.
-.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 name patterns, 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 is not recognized.
-By default, login is allowed for all users.
-If the pattern takes the form USER@HOST then USER and HOST
-are separately checked, restricting logins to particular
-users from particular hosts.
-.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.
-By default, no banner is displayed.
-.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
-.Pp
-.Bd -literal
- ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
- aes192-cbc,aes256-cbc''
-.Ed
-.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. The client alive mechanism is valuable when the client or
-server depend on knowing when a connection has become inactive.
+.Sh AUTHENTICATION
+The OpenSSH SSH daemon supports SSH protocols 1 and 2.
+The default is to use protocol 2 only,
+though this can be changed via the
+.Cm Protocol
+option in
+.Xr sshd_config 5 .
+Protocol 2 supports both RSA and DSA keys;
+protocol 1 only supports RSA keys.
+For both protocols,
+each host has a host-specific key,
+normally 2048 bits,
+used to identify the host.
+.Pp
+Forward security for protocol 1 is provided through
+an additional server key,
+normally 768 bits,
+generated when the server starts.
+This key is normally regenerated every hour if it has been used, and
+is never stored on disk.
+Whenever a client connects, the daemon responds with its public
+host and server keys.
+The client compares the
+RSA host key against its own database to verify that it has not changed.
+The client then generates a 256-bit random number.
+It encrypts this
+random number using both the host key and the server key, and sends
+the encrypted number to the server.
+Both sides then use this
+random number as a session key which is used to encrypt all further
+communications in the session.
+The rest of the session is encrypted
+using a conventional cipher, currently Blowfish or 3DES, with 3DES
+being used by default.
+The client selects the encryption algorithm
+to use from those offered by the server.
.Pp
-The default value is 3. If
-.Cm ClientAliveInterval
-(above) is set to 15, and
-.Cm ClientAliveCountMax
-is left at the default, unresponsive ssh clients
-will be disconnected after approximately 45 seconds.
-.It Cm DenyGroups
-This keyword can be followed by a list of group name patterns, separated
-by spaces.
-Login is disallowed for users whose primary group or supplementary
-group list matches one of the patterns.
-.Ql \&*
-and
-.Ql ?
-can be used as
-wildcards in the patterns.
-Only group names are valid; a numerical group ID is not recognized.
-By default, login is allowed for all groups.
+For protocol 2,
+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
+128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
+The client selects the encryption algorithm
+to use from those offered by the server.
+Additionally, session integrity is provided
+through a cryptographic message authentication code
+(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
.Pp
-.It Cm DenyUsers
-This keyword can be followed by a list of user name patterns, separated
-by spaces.
-Login is disallowed for user names that match one of the patterns.
+Finally, the server and the client enter an authentication dialog.
+The client tries to authenticate itself using
+host-based authentication,
+public key authentication,
+challenge-response authentication,
+or password authentication.
+.Pp
+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 and UnixWare,
.Ql \&*
-and
-.Ql ?
-can be used as wildcards in the patterns.
-Only user names are valid; a numerical user ID is not recognized.
-By default, login is allowed for all users.
-If the pattern takes the form USER@HOST then USER and HOST
-are separately checked, restricting logins to particular
-users from particular hosts.
-.It Cm GatewayPorts
-Specifies whether remote hosts are allowed to connect to ports
-forwarded for the client.
-By default,
-.Nm
-binds remote port forwardings to the loopback addresss. This
-prevents other remote hosts from connecting to forwarded ports.
-.Cm GatewayPorts
-can be used to specify that
-.Nm
-should bind remote port forwardings to the wildcard address,
-thus allowing remote hosts to connect to forwarded ports.
-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 a file containing a private host key
-used by SSH.
-The default is
-.Pa /etc/ssh/ssh_host_key
-for protocol version 1, and
-.Pa /etc/ssh/ssh_host_rsa_key
-and
-.Pa /etc/ssh/ssh_host_dsa_key
-for protocol version 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
+on HP-UX, containing
+.Ql Nologin
+on Tru64,
+a leading
+.Ql \&*LOCKED\&*
+on FreeBSD and a leading
+.Ql \&!
+on most Linuxes).
+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
-.Cm HostbasedAuthentication .
-The default is
-.Dq no .
-.It Cm KeepAlive
-Specifies whether the system should send TCP keepalive messages to the
-other side.
-If they are sent, death of the connection or crash of one
-of the machines will be properly noticed.
-However, this means that
-connections will die if the route is down temporarily, and some people
-find it annoying.
-On the other hand, if 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 crashes.
-This avoids infinitely hanging sessions.
-.Pp
-To disable keepalives, the value should be set to
-.Dq no .
-.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, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
-The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
-and DEBUG3 each specify higher levels of debugging output.
-Logging with a DEBUG level violates the privacy of users
-and is not recommended.
-.It Cm MACs
-Specifies the available MAC (message authentication code) algorithms.
-The MAC algorithm is used in protocol version 2
-for data integrity protection.
-Multiple algorithms must be comma-separated.
-The default is
-.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.
+.Ql \&*NP\&*
+).
.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 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. If
-.Cm UsePrivilegeSeparation
-is specified, it will be disabled after authentication.
-.It Cm UsePrivilegeSeparation
-Specifies whether
-.Nm
-separated privileges by creating an unprivileged child process
-to deal with incoming network traffic. After successful authentication,
-another process will be created that has the privilege of the authenticated
-user. The goal of privilege separation is to prevent privilege
-escalation by containing any corruption within the unprivileged processes.
-The default is
-.Dq no .
-.It Cm VerifyReverseMapping
-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 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 X11UseLocalhost
-Specifies whether
-.Nm
-should bind the X11 forwarding server to the loopback address or to
-the wildcard address. By default,
-.Nm
-binds the forwarding server to the loopback address and sets the
-hostname part of the
-.Ev DISPLAY
-environment variable to
-.Dq localhost .
-This prevents remote hosts from connecting to the fake display.
-However, some older X11 clients may not function with this
-configuration.
-.Cm X11UseLocalhost
-may be set to
-.Dq no
-to specify that the forwarding server should be bound to the wildcard
-address.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-.It Cm XAuthLocation
-Specifies the 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.
+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 connections, or forwarding the authentication agent
+connection over the secure channel.
.Pp
-Time format examples:
+After this, the client either requests a shell or execution of a command.
+The sides then enter session mode.
+In this mode, either side may send
+data at any time, and such data is forwarded to/from the shell or
+command on the server side, and the user terminal in the client side.
.Pp
-.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
+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.
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
prints last login time and
.Pa /etc/motd
(unless prevented in the configuration file or by
-.Pa $HOME/.hushlogin ;
+.Pa ~/.hushlogin ;
see the
.Sx FILES
section).
.It
Sets up basic environment.
.It
-Reads
-.Pa $HOME/.ssh/environment
-if it exists.
+Reads the file
+.Pa ~/.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 $HOME/.ssh/rc
+.Pa ~/.ssh/rc
exists, runs it; else if
.Pa /etc/ssh/sshrc
exists, runs
.Dq rc
files are given the X11
authentication protocol and cookie in standard input.
+See
+.Sx SSHRC ,
+below.
.It
Runs user's shell or command.
.El
+.Sh SSHRC
+If the file
+.Pa ~/.ssh/rc
+exists,
+.Xr sh 1
+runs it after reading the
+environment files but before starting the user's shell or command.
+It must not produce any output on stdout; stderr must be used
+instead.
+If X11 forwarding is in use, it will receive the "proto cookie" pair in
+its standard input (and
+.Ev DISPLAY
+in its environment).
+The script must call
+.Xr xauth 1
+because
+.Nm
+will not run xauth automatically to add X11 cookies.
+.Pp
+The primary purpose of this file is to run any initialization routines
+which may be needed before the user's home directory becomes
+accessible; AFS is a particular example of such an environment.
+.Pp
+This file will probably contain some initialization code followed by
+something similar to:
+.Bd -literal -offset 3n
+if read proto cookie && [ -n "$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/ssh/sshrc
+is run, and if that
+does not exist either, xauth is used to add the cookie.
.Sh AUTHORIZED_KEYS FILE FORMAT
-.Pa $HOME/.ssh/authorized_keys
-is the default file that lists the public keys that are
-permitted for RSA authentication in protocol version 1
-and for public key authentication (PubkeyAuthentication)
-in protocol version 2.
.Cm AuthorizedKeysFile
-may be used to specify an alternative file.
-.Pp
+specifies the file containing public keys for
+public key authentication;
+if none is specified, the default is
+.Pa ~/.ssh/authorized_keys .
Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
are ignored as
comments).
-Each RSA public key consists of the following fields, separated by
-spaces: options, bits, exponent, modulus, comment.
-Each protocol version 2 public key consists of:
-options, keytype, base64 encoded key, comment.
-The options fields
-are optional; its presence is determined by whether the line starts
-with a number or not (the option field never starts with a number).
-The bits, exponent, modulus and comment fields give the RSA key for
+Protocol 1 public keys consist of the following space-separated fields:
+options, bits, exponent, modulus, comment.
+Protocol 2 public key consist of:
+options, keytype, base64-encoded key, comment.
+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
user to identify the key).
.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_dsa.pub ,
or the
.Pa id_rsa.pub
file and edit it.
The following option specifications are supported (note
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
-of the remote host must be present in the comma-separated list of
-patterns
-.Pf ( Ql *
-and
-.Ql ?
-serve as wildcards).
-The list may also contain
-patterns negated by prefixing them with
-.Ql ! ;
-if the canonical host name matches a negated pattern, the key is not accepted.
-The purpose
-of this option is to optionally increase security: RSA authentication
-by itself does not trust the network or name servers or anything (but
-the key); however, if somebody somehow steals the key, the key
-permits an intruder to log in from anywhere in the world.
-This additional option makes using a stolen key more difficult (name
-servers and/or routers would have to be compromised in addition to
-just the key).
.It Cm command="command"
Specifies that the command is executed whenever this key is used for
authentication.
The command supplied by the user (if any) is ignored.
The command is run on a pty if the client requests a pty;
otherwise it is run without a tty.
-If a 8-bit clean channel is required,
+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
+Note that the client may specify TCP and/or X11
forwarding unless they are explicitly prohibited.
+The command originally supplied by the client is available in the
+.Ev SSH_ORIGINAL_COMMAND
+environment variable.
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
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 from="pattern-list"
+Specifies that in addition to public key authentication, either the canonical
+name of the remote host or its IP address must be present in the
+comma-separated list of patterns.
+See
+.Sx PATTERNS
+in
+.Xr ssh_config 5
+for more information on patterns.
+.Pp
+In addition to the wildcard matching that may be applied to hostnames or
+addresses, a
+.Cm from
+stanza may match IP addresses using CIDR address/masklen notation.
+.Pp
+The purpose 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.
+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 no-agent-forwarding
+Forbids authentication agent forwarding when this key is used for
+authentication.
.It Cm no-port-forwarding
-Forbids TCP/IP forwarding when this key is used for authentication.
+Forbids TCP 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
+This might be used, e.g. in connection with the
.Cm command
option.
+.It Cm no-pty
+Prevents tty allocation (a request to allocate a pty will fail).
+.It Cm no-user-rc
+Disables execution of
+.Pa ~/.ssh/rc .
.It Cm no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
-.It Cm no-agent-forwarding
-Forbids authentication agent forwarding when this key is used for
-authentication.
-.It Cm no-pty
-Prevents tty allocation (a request to allocate a pty will fail).
.It Cm permitopen="host:port"
Limit local
.Li ``ssh -L''
port forwarding such that it may only connect to the specified host and
port.
IPv6 addresses can be specified with an alternative syntax:
-.Ar host/port .
+.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.
+.It Cm tunnel="n"
+Force a
+.Xr tun 4
+device on the server.
+Without this option, the next available device will be used if
+the client requests a tunnel.
.El
-.Ss Examples
-1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
.Pp
-from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
-.Pp
-command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
-.Pp
-permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
+An example authorized_keys file:
+.Bd -literal -offset 3n
+# Comments allowed at start of line
+ssh-rsa AAAAB3Nza...LiPk== user@example.net
+from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
+AAAAB2...19Q== john@example.net
+command="dump /home",no-pty,no-port-forwarding ssh-dss
+AAAAC3...51R== example.net
+permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
+AAAAB5...21S==
+tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
+jane@example.net
+.Ed
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
-.Pa /etc/ssh/ssh_known_hosts ,
+.Pa /etc/ssh/ssh_known_hosts
and
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.ssh/known_hosts
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
+maintained automatically: whenever the user connects from an unknown host,
its key is added to the per-user file.
.Pp
Each line in these files contains the following fields: hostnames,
bits, exponent, modulus, comment.
The fields are separated by spaces.
.Pp
-Hostnames is a comma-separated list of patterns ('*' and '?' act as
+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.
+A hostname or address may optionally be enclosed within
+.Ql \&[
+and
+.Ql \&]
+brackets then followed by
+.Ql \&:
+and a non-standard port number.
+.Pp
+Alternately, hostnames may be stored in a hashed form which hides host names
+and addresses should the file's contents be disclosed.
+Hashed hostnames start with a
+.Ql |
+character.
+Only one hashed hostname may appear on a single line and none of the above
+negation or wildcard operators may be applied.
.Pp
Bits, exponent, and modulus are taken directly from the RSA host key; they
-can be obtained, e.g., from
+can be obtained, for example, from
.Pa /etc/ssh/ssh_host_key.pub .
The optional comment field continues to the end of the line, and is not used.
.Pp
or by taking
.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
-cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
+.Pp
+An example ssh_known_hosts file:
+.Bd -literal -offset 3n
+# Comments allowed at start of line
+closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
+cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
+# A hashed hostname
+|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
+AAAA1234.....=
.Ed
.Sh FILES
-.Bl -tag -width Ds
-.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/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
+.Bl -tag -width Ds -compact
+.It ~/.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 .
+.Pp
+.It ~/.rhosts
+This file is used for host-based authentication (see
+.Xr ssh 1
+for more information).
+On some machines this file may need to be
+world-readable if the user's home directory is on an NFS partition,
+because
+.Nm
+reads it as root.
+Additionally, this file must be owned by the user,
+and must not have write permissions for anyone else.
+The recommended
+permission for most machines is read/write for the user, and not
+accessible by others.
+.Pp
+.It ~/.shosts
+This file is used in exactly the same way as
+.Pa .rhosts ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
+.It ~/.ssh/
+This directory is the default location for all user-specific configuration
+and authentication information.
+There is no general requirement to keep the entire contents of this directory
+secret, but the recommended permissions are read/write/execute for the user,
+and not accessible by others.
+.Pp
+.It ~/.ssh/authorized_keys
+Lists the public keys (RSA/DSA) that can be used for logging in as this user.
+The format of this file is described above.
+The content of the file is not highly sensitive, but the recommended
+permissions are read/write for the user, and not accessible by others.
+.Pp
+If this file, the
+.Pa ~/.ssh
+directory, or the user's home directory are writable
+by other users, then the file could be modified or replaced by unauthorized
+users.
+In this case,
+.Nm
+will not allow it to be used unless the
+.Cm StrictModes
+option has been set to
+.Dq no .
+.Pp
+.It ~/.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.
+Environment processing is disabled by default and is
+controlled via the
+.Cm PermitUserEnvironment
+option.
+.Pp
+.It ~/.ssh/known_hosts
+Contains a list of host keys for all hosts the user has logged into
+that are not already in the systemwide list of known host keys.
+The format of this file is described above.
+This file should be writable only by root/the owner and
+can, but need not be, world-readable.
+.Pp
+.It ~/.ssh/rc
+Contains initialization routines to be run before
+the user's home directory becomes accessible.
+This file should be writable only by the user, and need not be
+readable by anyone else.
+.Pp
+.It /etc/hosts.allow
+.It /etc/hosts.deny
+Access controls that should be enforced by tcp-wrappers are defined here.
+Further details are described in
+.Xr hosts_access 5 .
+.Pp
+.It /etc/hosts.equiv
+This file is for host-based authentication (see
+.Xr ssh 1 ) .
+It should only be writable by root.
+.Pp
+.It /etc/moduli
+Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
+The file format is described in
+.Xr moduli 5 .
+.Pp
+.It /etc/motd
+See
+.Xr motd 5 .
+.Pp
+.It /etc/nologin
+If this file exists,
+.Nm
+refuses to let anyone except root log in.
+The contents of the file
+are displayed to anyone trying to log in, and non-root connections are
+refused.
+The file should be world-readable.
+.Pp
+.It /etc/shosts.equiv
+This file is used in exactly the same way as
+.Pa hosts.equiv ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
+.It /etc/ssh/ssh_host_key
+.It /etc/ssh/ssh_host_dsa_key
+.It /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/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
+does not start if these files are group/world-accessible.
+.Pp
+.It /etc/ssh/ssh_host_key.pub
+.It /etc/ssh/ssh_host_dsa_key.pub
+.It /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.
the user so their contents can be copied to known hosts files.
These files are created using
.Xr ssh-keygen 1 .
-.It Pa /etc/moduli
-Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
-.It Pa /var/run/sshd.pid
-Contains the process ID of the
-.Nm
-listening for connections (if there are several daemons running
-concurrently for different ports, this contains the pid of the one
-started last).
-The content of this file is not sensitive; it can be world-readable.
-.It Pa $HOME/.ssh/authorized_keys
-Lists the public keys (RSA or DSA) that can be used to log into the user's account.
-This file must be readable by root (which may on some machines imply
-it being world-readable if the user's home directory resides on an NFS
-volume).
-It is recommended that it not be accessible by others.
-The format of this file is described above.
-Users will place the contents of their
-.Pa identity.pub ,
-.Pa id_dsa.pub
-and/or
-.Pa id_rsa.pub
-files into this file, as described in
-.Xr ssh-keygen 1 .
-.It Pa "/etc/ssh/ssh_known_hosts" and "$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 key must be listed in one of these files to be accepted.
-The client uses the same files
-to verify that it is connecting to the correct remote host.
-These files should be writable only by root/the owner.
-.Pa /etc/ssh/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,
-.Nm
-refuses to let anyone except root log in.
-The contents of the file
-are displayed to anyone trying to log in, and non-root connections are
-refused.
-The file should be world-readable.
-.It Pa /etc/hosts.allow, /etc/hosts.deny
-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
-line.
-The given user on the corresponding host is permitted to log in
-without password.
-The same file is used by rlogind and rshd.
-The file must
-be writable only by the user; it is recommended that it not be
-accessible by others.
-.Pp
-If is also possible to use netgroups in the file.
-Either host or user
-name may be of the form +@groupname to specify all hosts or all users
-in the group.
-.It Pa $HOME/.shosts
-For ssh,
-this file is exactly the same as for
-.Pa .rhosts .
-However, this file is
-not used by rlogin and rshd, so using this permits access using SSH only.
-.It Pa /etc/hosts.equiv
-This file is used during
-.Pa .rhosts
-authentication.
-In the simplest form, this file contains host names, one per line.
-Users on
-those hosts are permitted to log in without a password, provided they
-have the same user name on both machines.
-The host name may also be
-followed by a user name; such users are permitted to log in as
-.Em any
-user on this machine (except root).
-Additionally, the syntax
-.Dq +@group
-can be used to specify netgroups.
-Negated entries start with
-.Ql \&- .
.Pp
-If the client host/user is successfully matched in this file, login is
-automatically permitted provided the client and server user names are the
-same.
-Additionally, successful RSA host authentication is normally required.
-This file must be writable only by root; it is recommended
-that it be world-readable.
-.Pp
-.Sy "Warning: It is almost never a good idea to use user names in"
-.Pa hosts.equiv .
-Beware that it really means that the named user(s) can log in as
-.Em anybody ,
-which includes bin, daemon, adm, and other accounts that own critical
-binaries and directories.
-Using a user name practically grants the user root access.
-The only valid use for user names that I can think
-of is in negative entries.
-.Pp
-Note that this warning also applies to rsh/rlogin.
-.It Pa /etc/shosts.equiv
-This is processed exactly as
-.Pa /etc/hosts.equiv .
-However, this file may be useful in environments that want to run both
-rsh/rlogin and ssh.
-.It Pa $HOME/.ssh/environment
-This file is read into the environment at login (if it exists).
-It can only contain empty lines, comment lines (that start with
-.Ql # ) ,
-and assignment lines of the form name=value.
-The file should be writable
-only by the user; it need not be readable by anyone else.
-.It Pa $HOME/.ssh/rc
-If this file exists, it is run with /bin/sh after reading the
-environment files but before starting the user's shell or command.
-If X11 spoofing is in use, this will receive the "proto cookie" pair in
-standard input (and
-.Ev DISPLAY
-in environment).
-This must call
-.Xr xauth 1
-in that case.
-.Pp
-The primary purpose of this file is to run any initialization routines
-which may be needed before the user's home directory becomes
-accessible; AFS is a particular example of such an environment.
-.Pp
-This file will probably contain some initialization code followed by
-something similar to:
-.Bd -literal
- if read proto cookie; then
- echo add $DISPLAY $proto $cookie | xauth -q -
- fi
-.Ed
+.It /etc/ssh/ssh_known_hosts
+Systemwide list of known host keys.
+This file should be prepared by the
+system administrator to contain the public host keys of all machines in the
+organization.
+The format of this file is described above.
+This file should be writable only by root/the owner and
+should be world-readable.
.Pp
-If this file does not exist,
-.Pa /etc/ssh/sshrc
-is run, and if that
-does not exist either, xauth is used to store the cookie.
+.It /etc/ssh/sshd_config
+Contains configuration data for
+.Nm sshd .
+The file format and configuration options are described in
+.Xr sshd_config 5 .
.Pp
-This file should be writable only by the user, and need not be
-readable by anyone else.
-.It Pa /etc/ssh/sshrc
-Like
-.Pa $HOME/.ssh/rc .
-This can be used to specify
+.It /etc/ssh/sshrc
+Similar to
+.Pa ~/.ssh/rc ,
+it can be used to specify
machine-specific login-time initializations globally.
This file should be writable only by root, and should be world-readable.
+.Pp
+.It /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.
+.Pp
+.It /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 process ID of the one
+started last).
+The content of this file is not sensitive; it can be world-readable.
.El
+.Sh SEE ALSO
+.Xr scp 1 ,
+.Xr sftp 1 ,
+.Xr ssh 1 ,
+.Xr ssh-add 1 ,
+.Xr ssh-agent 1 ,
+.Xr ssh-keygen 1 ,
+.Xr ssh-keyscan 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
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.
-.Sh SEE ALSO
-.Xr scp 1 ,
-.Xr sftp 1 ,
-.Xr ssh 1 ,
-.Xr ssh-add 1 ,
-.Xr ssh-agent 1 ,
-.Xr ssh-keygen 1 ,
-.Xr login.conf 5 ,
-.Xr moduli 5 ,
-.Xr sftp-server 8
-.Rs
-.%A T. Ylonen
-.%A T. Kivinen
-.%A M. Saarinen
-.%A T. Rinne
-.%A S. Lehtinen
-.%T "SSH Protocol Architecture"
-.%N draft-ietf-secsh-architecture-12.txt
-.%D January 2002
-.%O work in progress material
-.Re
-.Rs
-.%A M. Friedl
-.%A N. Provos
-.%A W. A. Simpson
-.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
-.%N draft-ietf-secsh-dh-group-exchange-02.txt
-.%D January 2002
-.%O work in progress material
-.Re
+.Sh CAVEATS
+System security is not improved unless
+.Nm rshd ,
+.Nm rlogind ,
+and
+.Nm rexecd
+are disabled (thus completely disabling
+.Xr rlogin
+and
+.Xr rsh
+into the machine).
andersk / openssh.git / blobdiff