.\" -*- nroff -*-
.\"
-.\" sshd.8.in
-.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
-.\"
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\" All rights reserved
.\"
-.\" Created: Sat Apr 22 21:55:14 1995 ylo
+.\" As far as I am concerned, the code I have written for this software
+.\" can be used freely for any purpose. Any derived versions of this
+.\" software must be clearly marked as such, and if the derived work is
+.\" incompatible with the protocol description in the RFC file, it must be
+.\" called by a name other than "ssh" or "Secure Shell".
+.\"
+.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
+.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
+.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\" notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\" notice, this list of conditions and the following disclaimer in the
+.\" documentation and/or other materials provided with the distribution.
.\"
-.\" $Id$
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
+.\" $OpenBSD: sshd.8,v 1.73 2000/11/22 15:38:30 provos Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Op Fl h Ar host_key_file
.Op Fl k Ar key_gen_time
.Op Fl p Ar port
+.Op Fl u Ar len
.Op Fl V Ar client_protocol_id
.Sh DESCRIPTION
.Nm
The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
-.Pp
+This implementation of
+.Nm
+supports both SSH protocol version 1 and 2 simultaneously.
.Nm
works as follows.
+.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
Whenever a client connects the daemon responds with its public
host and server keys.
The client compares the
-host key against its own database to verify that it has not changed.
+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
.Xr rsh 1
into the machine).
.Pp
+.Ss SSH protocol version 2
+.Pp
+Version 2 works similarly:
+Each host has a host-specific DSA key used to identify the host.
+However, when the daemon starts, it does not generate a server key.
+Forward security is provided through a Diffie-Hellman key agreement.
+This key agreement results in a shared session key.
+The rest of the session is encrypted
+using a symmetric cipher, currently
+Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
+The client selects the encryption algorithm
+to use from those offered by the server.
+Additionally, session integrity is provided
+through a cryptographic message authentication code
+(hmac-sha1 or hmac-md5).
+.Pp
+Protocol version 2 provides a public key based
+user authentication method (PubkeyAuthentication)
+and conventional password authentication.
+.Pp
+.Ss Command execution and data forwarding
+.Pp
If the client successfully authenticates itself, a dialog for
preparing the session is entered.
At this time the client may request
log, and does not put itself in the background.
The server also will not fork and will only process one connection.
This option is only intended for debugging for the server.
+Multiple -d options increases the debugging level.
+Maximum is 3.
.It Fl f Ar configuration_file
Specifies the name of the configuration file.
The default is
this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
.It Fl h Ar host_key_file
-Specifies the file from which the host key is read (default
+Specifies the file from which the RSA host key is read (default
.Pa /etc/ssh_host_key ) .
This option must be given if
.Nm
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
+.It Fl u Ar len
+This option is used to specify the size of the field
+in the
+.Li utmp
+structure that holds the remote host name.
+If the resolved host name is longer than
+.Ar len ,
+the dotted decimal value will be used instead.
+This allows hosts with very long host names that
+overflow this field to still be uniquely identified.
+Specifying
+.Fl u0
+indicates that only dotted decimal addresses
+should be put into the
+.Pa utmp
+file.
.It Fl Q
Do not print an error message if RSA support is missing.
.It Fl V Ar client_protocol_id
-SSH2 compatibility mode.
+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
.Ql ?
can be used as
wildcards in the patterns.
-Only group names are valid, a numerical group ID isn't recognized.
+Only group names are valid; a numerical group ID isn't recognized.
By default login is allowed regardless of the primary group.
.Pp
+.It Cm AllowTcpForwarding
+Specifies whether TCP forwarding is permitted.
+The default is
+.Dq yes .
+Note that disabling TCP forwarding does not improve security unless
+users are also denied shell access, as they can always install their
+own forwarders.
+.Pp
.It Cm AllowUsers
This keyword can be followed by a number of user names, separated
by spaces.
.Ql ?
can be used as
wildcards in the patterns.
-Only user names are valid, a numerical user ID isn't recognized.
+Only user names are valid; a numerical user ID isn't recognized.
By default login is allowed regardless of the user name.
.Pp
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
The default is
-.Dq blowfish-cbc,3des-cbc,arcfour,cast128-cbc .
+.Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
.It Cm CheckMail
Specifies whether
.Nm
.Ql ?
can be used as
wildcards in the patterns.
-Only group names are valid, a numerical group ID isn't recognized.
+Only group names are valid; a numerical group ID isn't recognized.
By default login is allowed regardless of the primary group.
.Pp
.It Cm DenyUsers
and
.Ql ?
can be used as wildcards in the patterns.
-Only user names are valid, a numerical user ID isn't recognized.
+Only user names are valid; a numerical user ID isn't recognized.
By default login is allowed regardless of the user name.
+.It Cm PubkeyAuthentication
+Specifies whether public key authentication is allowed.
+The default is
+.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm GatewayPorts
+Specifies whether remote hosts are allowed to connect to ports
+forwarded for the client.
+The argument must be
+.Dq yes
+or
+.Dq no .
+The default is
+.Dq no .
.It Cm HostKey
-Specifies the file containing the private host key (default
-.Pa /etc/ssh_host_key ) .
+Specifies the file containing the private host keys (default
+.Pa /etc/ssh_host_key )
+used by SSH protocol versions 1 and 2.
Note that
.Nm
-does not start if this file is group/world-accessible.
+if this file 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
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.
-On the other hand, if keepalives are not send,
+On the other hand, if keepalives are not sent,
sessions may hang indefinitely on the server, leaving
.Dq ghost
users and consuming server resources.
.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
-or SecurID.
+.Pa /etc/passwd .
Default is
.Dq yes .
.It Cm KerberosTgtPassing
The default is INFO.
Logging with level DEBUG violates the privacy of users
and is not recommended.
+.It Cm MaxStartups
+Specifies the maximum number of concurrent unauthenticated connections to the
+.Nm
+daemon.
+Additional connections will be dropped until authentication succeeds or the
+.Cm LoginGraceTime
+expires for a connection.
+The default is 10.
+.Pp
+Alternatively, random early drop can be enabled by specifying
+the three colon separated values
+.Dq start:rate:full
+(e.g., "10:30:60").
+.Nm
+will refuse connection attempts with a probabillity of
+.Dq rate/100
+(30%)
+if there are currently
+.Dq start
+(10)
+unauthenticated connections.
+The probabillity increases linearly and all connection attempts
+are refused if the number of unauthenticated connections reaches
+.Dq full
+(60).
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Dq yes .
+Note that this option applies to both protocol versions 1 and 2.
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
specified will be allowed regardless of the value of this setting
(which may be useful for taking remote backups even if root login is
normally not allowed).
+.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
Specifies whether pure RSA authentication is allowed.
The default is
.Dq yes .
+Note that this option applies to protocol version 1 only.
.It Cm ServerKeyBits
Defines the number of bits in the server key.
The minimum value is 512, and the default is 768.
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 .
.It Cm UseLogin
Specifies whether
.Xr login 1
-is used.
+is used for interactive login sessions.
+Note that
+.Xr login 1
+is never used for remote command execution.
The default is
.Dq no .
.It Cm X11DisplayOffset
.Dq no .
Note that disabling X11 forwarding does not improve security in any
way, as users can always install their own forwarders.
+.It Cm XAuthLocation
+Specifies the location of the
+.Xr xauth 1
+program.
+The default is
+.Pa /usr/X11R6/bin/xauth .
.El
.Sh LOGIN PROCESS
When a user successfully logs in,
The
.Pa $HOME/.ssh/authorized_keys
file lists the RSA keys that are
-permitted for RSA authentication.
+permitted for RSA authentication in SSH protocols 1.3 and 1.5
+Similarly, the
+.Pa $HOME/.ssh/authorized_keys2
+file lists the DSA keys that are
+permitted for DSA authentication in SSH protocol 2.0.
Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
.Pa identity.pub
file and edit it.
.Pp
-The options (if present) consists of comma-separated option
+The options (if present) consist of comma-separated option
specifications.
No spaces are permitted, except within double quotes.
The following option specifications are supported:
This option might be useful
to restrict certain RSA keys to perform just a specific operation.
An example might be a key that permits remote backups but nothing else.
-Notice that the client may specify TCP/IP and/or X11
-forwardings unless they are explicitly prohibited.
+Note that the client may specify TCP/IP and/or X11
+forwarding unless they are explicitly prohibited.
.It Cm environment="NAME=value"
Specifies that the string is to be added to the environment when
logging in using this key.
command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
-.Pa /etc/ssh_known_hosts
+.Pa /etc/ssh_known_hosts ,
+.Pa /etc/ssh_known_hosts2 ,
+.Pa $HOME/.ssh/known_hosts ,
and
-.Pa $HOME/.ssh/known_hosts
+.Pa $HOME/.ssh/known_hosts2
files contain host public keys for all known hosts.
The global file should
be prepared by the administrator (optional), and the per-user file is
-maintained automatically: whenever the user connects 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,
pattern, it is not accepted (by that line) even if it matched another
pattern on the line.
.Pp
-Bits, exponent, and modulus are taken directly from the host key; they
+Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
.Pa /etc/ssh_host_key.pub .
The optional comment field continues to the end of the line, and is not used.
the user so its contents can be copied to known hosts files.
These two files are created using
.Xr ssh-keygen 1 .
+.It Pa /etc/primes
+Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
.It Pa /var/run/sshd.pid
Contains the process ID of the
.Nm
listening for connections (if there are several daemons running
concurrently for different ports, this contains the pid of the one
started last).
-The contents of this file are not sensitive; it can be world-readable.
+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
volume).
It is recommended that it not be accessible by others.
The format of this file is described above.
+Users will place the contents of their
+.Pa identity.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
+.It Pa $HOME/.ssh/authorized_keys2
+Lists the DSA keys that can be used to log into the user's account.
+This file must be readable by root (which may on some machines imply
+it being world-readable if the user's home directory resides on an NFS
+volume).
+It is recommended that it not be accessible by others.
+The format of this file is described above.
+Users will place the contents of their
+.Pa id_dsa.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
.It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
authentication to check the public key of the host.
The key must be listed in one of these files to be accepted.
The client uses the same files
-to verify that the remote host is the one we intended to connect.
+to verify that the remote host is the one it intended to connect.
These files should be writable only by root/the owner.
.Pa /etc/ssh_known_hosts
should be world-readable, and
.Pa .rhosts .
However, this file is
not used by rlogin and rshd, so using this permits access using SSH only.
-.Pa /etc/hosts.equiv
+.It Pa /etc/hosts.equiv
This file is used during
.Pa .rhosts
authentication.
This can be used to specify
machine-specific login-time initializations globally.
This file should be writable only by root, and should be world-readable.
-.Sh AUTHOR
+.El
+.Sh AUTHORS
OpenSSH
is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
but with bugs removed and newer features re-added.
Rapidly after the
1.2.12 release, newer versions of the original ssh bore successively
more restrictive licenses, and thus demand for a free version was born.
+.Pp
This version of OpenSSH
.Bl -bullet
.It
-has all components of a restrictive nature (i.e., patents)
+has all components of a restrictive nature (i.e., patents, see
+.Xr crypto 3 )
directly removed from the source code; any licensed or patented components
are chosen from
external libraries.
.It
-has been updated to support ssh protocol 1.5, making it compatible with
-all other ssh protocol 1 clients and servers.
+has been updated to support SSH protocol 1.5 and 2, making it compatible with
+all other SSH clients and servers.
.It
contains added support for
.Xr kerberos 8
supports one-time password authentication with
.Xr skey 1 .
.El
+.Pp
+OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
+Niels Provos, Theo de Raadt, and Dug Song.
+.Pp
+The support for SSH protocol 2 was written by Markus Friedl.
.Sh SEE ALSO
.Xr scp 1 ,
+.Xr sftp-server 8 ,
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
+.Xr crypto 3 ,
.Xr rlogin 1 ,
.Xr rsh 1
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