.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.\" $OpenBSD: sshd_config.5,v 1.3 2002/06/20 23:37:12 markus Exp $
+.\" $OpenBSD: sshd_config.5,v 1.44 2005/07/25 11:59:40 markus Exp $
.Dd September 25, 1999
.Dt SSHD_CONFIG 5
.Os
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 no .
+.It Cm AcceptEnv
+Specifies what environment variables sent by the client will be copied into
+the session's
+.Xr environ 7 .
+See
+.Cm SendEnv
+in
+.Xr ssh_config 5
+for how to configure the client.
+Note that environment passing is only supported for protocol 2.
+Variables are specified by name, which may contain the wildcard characters
+.Ql \&*
+and
+.Ql \&? .
+Multiple environment variables may be separated by whitespace or spread
+across multiple
+.Cm AcceptEnv
+directives.
+Be warned that some environment variables could be used to bypass restricted
+user environments.
+For this reason, care should be taken in the use of this directive.
+The default is not to accept any environment variables.
+.It Cm AddressFamily
+Specifies which address family should be used by
+.Nm sshd .
+Valid arguments are
+.Dq any ,
+.Dq inet
+(use IPv4 only) or
+.Dq inet6
+(use IPv6 only).
+The default is
+.Dq any .
.It Cm AllowGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
group or supplementary group list matches one of the patterns.
.Ql \&*
and
-.Ql ?
+.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
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
+If specified, login is allowed only for user names that
match one of the patterns.
.Ql \&*
and
-.Ql ?
+.Ql \&?
can be used as
wildcards in the patterns.
Only user names are valid; a numerical user ID is not recognized.
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 '%',
+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,
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
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
+The supported ciphers are
+.Dq 3des-cbc ,
+.Dq aes128-cbc ,
+.Dq aes192-cbc ,
+.Dq aes256-cbc ,
+.Dq aes128-ctr ,
+.Dq aes192-ctr ,
+.Dq aes256-ctr ,
+.Dq arcfour128 ,
+.Dq arcfour256 ,
+.Dq arcfour ,
+.Dq blowfish-cbc ,
+and
+.Dq cast128-cbc .
The default is
-.Pp
.Bd -literal
- ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
- aes192-cbc,aes256-cbc''
+ ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
+ arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
+ aes192-ctr,aes256-ctr''
.Ed
-.It Cm ClientAliveInterval
-Sets a timeout interval in seconds after which if no data has been received
-from the client,
-.Nm sshd
-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 sshd
-receiving any messages back from the client. If this threshold is
-reached while client alive messages are being sent,
+receiving any messages back from the client.
+If this threshold is reached while client alive messages are being sent,
.Nm sshd
-will disconnect the client, terminating the session. It is important
-to note that the use of client alive messages is very different from
-.Cm 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
+will disconnect the client, terminating the session.
+It is important to note that the use of client alive messages is very
+different from
+.Cm TCPKeepAlive
+(below).
+The client alive messages are sent through the encrypted channel
+and therefore will not be spoofable.
+The TCP keepalive option enabled by
+.Cm TCPKeepAlive
+is spoofable.
+The client alive mechanism is valuable when the client or
server depend on knowing when a connection has become inactive.
.Pp
-The default value is 3. If
+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 ClientAliveInterval
+Sets a timeout interval in seconds after which if no data has been received
+from the client,
+.Nm sshd
+will send a message through the encrypted
+channel to request a response from the client.
+The default
+is 0, indicating that these messages will not be sent to the client.
+This option applies to protocol version 2 only.
.It Cm Compression
-Specifies whether compression is allowed.
+Specifies whether compression is allowed, or delayed until
+the user has authenticated successfully.
The argument must be
-.Dq yes
+.Dq yes ,
+.Dq delayed ,
or
.Dq no .
The default is
-.Dq yes .
+.Dq delayed .
.It Cm DenyGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
group list matches one of the patterns.
.Ql \&*
and
-.Ql ?
+.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 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.
.Ql \&*
and
-.Ql ?
+.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.
forwarded for the client.
By default,
.Nm sshd
-binds remote port forwardings to the loopback address. This
-prevents other remote hosts from connecting to forwarded ports.
+binds remote port forwardings to the loopback address.
+This prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that
.Nm sshd
-should bind remote port forwardings to the wildcard address,
-thus allowing remote hosts to connect to forwarded ports.
-The argument must be
+should allow remote port forwardings to bind to non-loopback addresses, thus
+allowing other hosts to connect.
+The argument may be
+.Dq no
+to force remote port forwardings to be available to the local host only,
.Dq yes
-or
-.Dq no .
+to force remote port forwardings to bind to the wildcard address, or
+.Dq clientspecified
+to allow the client to select the address to which the forwarding is bound.
The default is
.Dq no .
-.It Cm 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.
+.It Cm GSSAPIAuthentication
+Specifies whether user authentication based on GSSAPI is allowed.
The default is
-.Dq no .
-.It Cm GssapiAuthentication
-Specifies whether authentication based on GSSAPI may be used, either using
-the result of a successful key exchange, or using GSSAPI user
-authentication.
-The default is
.Dq yes .
-.It Cm GssapiKeyExchange
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPICleanupCredentials
+Specifies whether to automatically destroy the user's credentials cache
+on logout.
+The default is
+.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI may be used. When using
GSSAPI key exchange the server need not have a host key.
The default is
.Dq yes .
-.It Cm GssapiUseSessionCredCache
+Note that this option applies to protocol version 2 only.
+.It Cm GSSAPIUseSessionCredCache
Specifies whether a unique credentials cache name should be generated per
session for storing delegated credentials.
The default is
.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm HostbasedAuthentication
+Specifies whether rhosts or /etc/hosts.equiv authentication together
+with successful public key client host authentication is allowed
+(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.
and
.Pa .shosts
files will not be used in
-.Cm RhostsAuthentication ,
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
Specifies whether
.Nm sshd
should ignore the user's
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.ssh/known_hosts
during
.Cm RhostsRSAAuthentication
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
-.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
+Specifies whether the password provided by the user for
.Cm PasswordAuthentication
-is yes, the password provided by the user will be validated through
-the Kerberos KDC.
+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 no .
+.It Cm KerberosGetAFSToken
+If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
+an AFS token before accessing the user's home directory.
+Default is
+.Dq no .
.It Cm KerberosOrLocalPasswd
If set then if password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
.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.
.Nm sshd
will listen on the address and all prior
.Cm Port
-options specified. The default is to listen on all local
-addresses. Multiple
+options specified.
+The default is to listen on all local addresses.
+Multiple
.Cm ListenAddress
-options are permitted. Additionally, any
+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).
+The default is 120 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.
+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
Multiple algorithms must be comma-separated.
The default is
.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
+.It Cm MaxAuthTries
+Specifies the maximum number of authentication attempts permitted per
+connection.
+Once the number of failures reaches half this value,
+additional failures are logged.
+The default is 6.
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
.Nm sshd
The default is
.Dq no .
.It Cm PermitRootLogin
-Specifies whether root can login using
+Specifies whether root can log in using
.Xr ssh 1 .
The argument must be
.Dq yes ,
.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.
+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.
+root is not allowed to log in.
+.It Cm PermitUserEnvironment
+Specifies whether
+.Pa ~/.ssh/environment
+and
+.Cm environment=
+options in
+.Pa ~/.ssh/authorized_keys
+are processed by
+.Nm sshd .
+The default is
+.Dq no .
+Enabling environment processing may enable users to bypass access
+restrictions in some configurations using mechanisms such as
+.Ev LD_PRELOAD .
.It Cm PidFile
-Specifies the file that contains the process identifier of the
+Specifies the file that contains the process ID of the
.Nm sshd
daemon.
The default is
.It Cm PrintLastLog
Specifies whether
.Nm sshd
-should print the date and time when the user last logged in.
+should print the date and time of the last user login when a user logs
+in interactively.
The default is
.Dq yes .
.It Cm PrintMotd
.It Cm Protocol
Specifies the protocol versions
.Nm sshd
-should support.
+supports.
The possible values are
.Dq 1
and
Multiple versions must be comma-separated.
The default is
.Dq 2,1 .
+Note that the order of the protocol list does not indicate preference,
+because the client selects among multiple protocol versions offered
+by the server.
+Specifying
+.Dq 2,1
+is identical to
+.Dq 1,2 .
.It Cm PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
-.It Cm 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 possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
The default is AUTH.
+.It Cm TCPKeepAlive
+Specifies whether the system should send TCP keepalive messages to the
+other side.
+If they are sent, death of the connection or crash of one
+of the machines will be properly noticed.
+However, this means that
+connections will die if the route is down temporarily, and some people
+find it annoying.
+On the other hand, if TCP keepalives are not sent,
+sessions may hang indefinitely on the server, leaving
+.Dq ghost
+users and consuming server resources.
+.Pp
+The default is
+.Dq yes
+(to send TCP keepalive messages), and the server will notice
+if the network goes down or the client host crashes.
+This avoids infinitely hanging sessions.
+.Pp
+To disable TCP keepalive messages, the value should be set to
+.Dq no .
+.It Cm UseDNS
+Specifies whether
+.Nm sshd
+should look up the remote host name and check that
+the resolved host name for the remote IP address maps back to the
+very same IP address.
+The default is
+.Dq yes .
.It Cm UseLogin
Specifies whether
.Xr login 1
.Xr login 1
does not know how to handle
.Xr xauth 1
-cookies. If
+cookies.
+If
.Cm UsePrivilegeSeparation
is specified, it will be disabled after authentication.
+.It Cm UsePAM
+Enables the Pluggable Authentication Module interface.
+If set to
+.Dq yes
+this will enable PAM authentication using
+.Cm ChallengeResponseAuthentication
+and PAM account and session module processing for all authentication types.
+.Pp
+Because PAM challenge-response authentication usually serves an equivalent
+role to password authentication, you should disable either
+.Cm PasswordAuthentication
+or
+.Cm ChallengeResponseAuthentication.
+.Pp
+If
+.Cm UsePAM
+is enabled, you will not be able to run
+.Xr sshd 8
+as a non-root user.
+The default is
+.Dq no .
.It Cm UsePrivilegeSeparation
Specifies whether
.Nm sshd
separates privileges by creating an unprivileged child process
-to deal with incoming network traffic. After successful authentication,
-another process will be created that has the privilege of the authenticated
-user. The goal of privilege separation is to prevent privilege
+to deal with incoming network traffic.
+After successful authentication, another process will be created that has
+the privilege of the authenticated user.
+The goal of privilege separation is to prevent privilege
escalation by containing any corruption within the unprivileged processes.
The default is
.Dq yes .
-.It Cm VerifyReverseMapping
-Specifies whether
-.Nm sshd
-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
The default is 10.
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.
+The argument must be
+.Dq yes
+or
+.Dq no .
The default is
.Dq no .
-Note that disabling X11 forwarding does not improve security in any
-way, as users can always install their own forwarders.
+.Pp
+When X11 forwarding is enabled, there may be additional exposure to
+the server and to client displays if the
+.Nm sshd
+proxy display is configured to listen on the wildcard address (see
+.Cm X11UseLocalhost
+below), however this is not the default.
+Additionally, the authentication spoofing and authentication data
+verification and substitution occur on the client side.
+The security risk of using X11 forwarding is that the client's X11
+display server may be exposed to attack when the ssh client requests
+forwarding (see the warnings for
+.Cm ForwardX11
+in
+.Xr ssh_config 5 ) .
+A system administrator may have a stance in which they want to
+protect clients that may expose themselves to attack by unwittingly
+requesting X11 forwarding, which can warrant a
+.Dq no
+setting.
+.Pp
+Note that disabling X11 forwarding does not prevent users from
+forwarding X11 traffic, as users can always install their own forwarders.
X11 forwarding is automatically disabled if
.Cm UseLogin
is enabled.
Specifies whether
.Nm sshd
should bind the X11 forwarding server to the loopback address or to
-the wildcard address. By default,
+the wildcard address.
+By default,
.Nm sshd
binds the forwarding server to the loopback address and sets the
hostname part of the
.Ev DISPLAY
environment variable to
.Dq localhost .
-This prevents remote hosts from connecting to the fake display.
+This prevents remote hosts from connecting to the proxy display.
However, some older X11 clients may not function with this
configuration.
.Cm X11UseLocalhost
The default is
.Dq yes .
.It Cm XAuthLocation
-Specifies the location of the
+Specifies the full pathname of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Ss Time Formats
-.Pp
.Nm sshd
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 ,
+.Ar time Op Ar qualifier ,
.Sm on
where
.Ar time
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
.El
+.Sh SEE ALSO
+.Xr sshd 8
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.
-.Sh SEE ALSO
-.Xr sshd 8
andersk / gssapi-openssh.git / blobdiff