.\" (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 1.107 2001/03/19 12:10:17 djm Exp $
+.\" $OpenBSD: sshd.8,v 1.139 2001/08/01 22:16:45 markus Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
-.Nd OpenSSH ssh daemon
+.Nd OpenSSH SSH daemon
.Sh SYNOPSIS
.Nm sshd
-.Op Fl diqD46
+.Op Fl deiqD46
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Ss SSH protocol version 2
.Pp
Version 2 works similarly:
-Each host has a host-specific DSA key used to identify the host.
+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
-Blowfish, 3DES, CAST128, Arcfour, 128 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
(hmac-sha1 or hmac-md5).
.Pp
Protocol version 2 provides a public key based
-user authentication method (PubkeyAuthentication)
-and conventional password authentication.
+user (PubkeyAuthentication) or
+client host (HostbasedAuthentication) authentication method,
+conventional password authentication and challenge response based methods.
.Pp
.Ss Command execution and data forwarding
.Pp
.Nm
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP ,
-by executing itself with the name it was started as, ie.
+by executing itself with the name it was started as, i.e.,
.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b Ar bits
-Specifies the number of bits in the server key (default 768).
-.Pp
+Specifies the number of bits in the ephemeral protocol version 1
+server key (default 768).
.It Fl d
Debug mode.
The server sends verbose debug output to the system
log, and does not put itself in the background.
The server also will not fork and will only process one connection.
This option is only intended for debugging for the server.
-Multiple -d options increases the debugging level.
+Multiple -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
Specifies the name of the configuration file.
The default is
is not run as root (as the normal
host file is normally not readable by anyone but root).
It is possible to have multiple host key files for
-the different protocol versions.
+the different protocol versions and host key algorithms.
.It Fl i
Specifies that
.Nm
from inetd may
be feasible.
.It Fl k Ar key_gen_time
-Specifies how often the server key is regenerated (default 3600
-seconds, or one hour).
+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,
it becomes impossible to recover the key for decrypting intercepted
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
+.It Fl t
+Test mode.
+Only check the validity of the configuration file and sanity of the keys.
+This is useful for updating
+.Nm
+reliably as configuration options may change.
.It Fl u Ar len
This option is used to specify the size of the field
in the
will not detach and does not become a daemon.
This allows easy monitoring of
.Nm sshd .
-.It Fl V Ar client_protocol_id
-SSH-2 compatibility mode.
-When this option is specified
-.Nm
-assumes the client has sent the supplied version string
-and skips the
-Protocol Version Identification Exchange.
-This option is not intended to be called directly.
.It Fl 4
Forces
.Nm
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 replaces 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.
This option is only available for protocol version 2.
.Pp
.It Cm ChallengeResponseAuthentication
-Specifies whether
-challenge response
-authentication is allowed.
-Currently there is only support for
-.Xr skey 1
-authentication.
+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 CheckMail
Specifies whether
.Nm
should check for new mail for interactive logins.
The default is
.Dq no .
+.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.
.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 )
.Pa .rhosts
and
.Pa .shosts
-files will not be used in authentication.
+files will not be used in
+.Cm RhostsAuthentication ,
+.Cm RhostsRSAAuthentication
+or
+.Cm HostbasedAuthentication .
+.Pp
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv
should ignore the user's
.Pa $HOME/.ssh/known_hosts
during
-.Cm RhostsRSAAuthentication .
+.Cm RhostsRSAAuthentication
+or
+.Cm HostbasedAuthentication .
The default is
.Dq no .
.It Cm KeepAlive
Default is
.Dq yes .
.It Cm KeyRegenerationInterval
-The server key is automatically regenerated after this many seconds
-(if it has been used).
+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.
If the value is 0, the key is never regenerated.
The default is 3600 (seconds).
.It Cm ListenAddress
-Specifies what local address
+Specifies the local addresses
.Nm
should listen on.
-The default is to listen to all local addresses.
-Multiple options of this type are permitted.
-Additionally, the
-.Cm Ports
-options must precede this option.
+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.
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is
-.Pp
-.Bd -literal
- ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
- hmac-sha1-96,hmac-md5-96''
-.Ed
+.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
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 .
-Note that this option applies to both protocol versions 1 and 2.
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
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
.Dq 2 .
Multiple versions must be comma-separated.
The default is
-.Dq 1 .
+.Dq 2,1 .
.It Cm PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is
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 .
-Note that this option applies to protocol version 1 only.
+This option applies to protocol version 1 only.
.It Cm ServerKeyBits
-Defines the number of bits in the server key.
+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
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.
-The default is
-.Dq no .
+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
.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
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
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
Runs user's shell or command.
.El
.Sh AUTHORIZED_KEYS FILE FORMAT
-The
.Pa $HOME/.ssh/authorized_keys
-file lists the RSA keys that are
-permitted for RSA authentication in SSH protocols 1.3 and 1.5
-Similarly, the
-.Pa $HOME/.ssh/authorized_keys2
-file lists the DSA and RSA keys that are
-permitted for public key authentication (PubkeyAuthentication)
-in SSH protocol 2.0.
+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
Each line of the file contains one
key (empty lines and lines starting with a
Note that lines in this file are usually several hundred bytes long
(because of the size of the RSA key modulus).
You don't want to type them in; instead, copy the
-.Pa identity.pub
-or the
+.Pa identity.pub ,
.Pa id_dsa.pub
+or the
+.Pa id_rsa.pub
file and edit it.
.Pp
The options (if present) consist of comma-separated option
.It Cm no-pty
Prevents tty allocation (a request to allocate a pty will fail).
.It Cm permitopen="host:port"
-Limit local
+Limit local
.Li ``ssh -L''
-port-forwading such that it may only connect to the specified host and
-port. Multiple
+port forwarding such that it may only connect to the specified host and
+port. Multiple
.Cm permitopen
-options may be applied seperated by commas. No pattern matching is
-performed on the specified hostnames, they must be literal domains or
+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
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
.Pa /etc/ssh_known_hosts ,
-.Pa /etc/ssh_known_hosts2 ,
-.Pa $HOME/.ssh/known_hosts ,
and
-.Pa $HOME/.ssh/known_hosts2
+.Pa $HOME/.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
.Pa /etc/ssh_host_key.pub
and adding the host names at the front.
.Ss Examples
-closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
+.Bd -literal
+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
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
-These three files contain the private parts of the
-(SSH1, SSH2 DSA, and SSH2 RSA) host keys.
+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
-There three files contain the public parts of the
-(SSH1, SSH2 DSA, and SSH2 RSA) host keys.
+These three files contain the public parts of the host keys.
These files should be world-readable but writable only by
root.
Their contents should match the respective private parts.
the user so their contents can be copied to known hosts files.
These files are created using
.Xr ssh-keygen 1 .
-.It Pa /etc/primes
+.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
started last).
The content of this file is not sensitive; it can be world-readable.
.It Pa $HOME/.ssh/authorized_keys
-Lists the RSA keys that can be used to log into the user's account.
-This file must be readable by root (which may on some machines imply
-it being world-readable if the user's home directory resides on an NFS
-volume).
-It is recommended that it not be accessible by others.
-The format of this file is described above.
-Users will place the contents of their
-.Pa identity.pub
-files into this file, as described in
-.Xr ssh-keygen 1 .
-.It Pa $HOME/.ssh/authorized_keys2
-Lists the DSA keys that can be used to log into the user's account.
+Lists the public keys (RSA or DSA) that can be used to log into the user's account.
This file must be readable by root (which may on some machines imply
it being world-readable if the user's home directory resides on an NFS
volume).
It is recommended that it not be accessible by others.
The format of this file is described above.
Users will place the contents of their
+.Pa identity.pub ,
.Pa id_dsa.pub
+and/or
+.Pa id_rsa.pub
files into this file, as described in
.Xr ssh-keygen 1 .
.It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
-authentication to check the public key of the host.
+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.
accessible; AFS is a particular example of such an environment.
.Pp
This file will probably contain some initialization code followed by
-something similar to: "if read proto cookie; then echo add $DISPLAY
-$proto $cookie | xauth -q -; fi".
+something similar to:
+.Bd -literal
+ if read proto cookie; then
+ echo add $DISPLAY $proto $cookie | xauth -q -
+ fi
+.Ed
.Pp
If this file does not exist,
.Pa /etc/sshrc
.Sh SEE ALSO
.Xr scp 1 ,
.Xr sftp 1 ,
-.Xr sftp-server 8 ,
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
-.Xr rlogin 1 ,
-.Xr rsh 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-09.txt
+.%D July 2001
+.%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-01.txt
+.%D April 2001
+.%O work in progress material
+.Re
andersk / openssh.git / blobdiff