.\" -*- 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 1.99 2001/03/02 18:54:31 deraadt Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
-.Nd secure shell daemon
+.Nd OpenSSH secure shell daemon
.Sh SYNOPSIS
.Nm sshd
-.Op Fl diqQ46
+.Op Fl diqD46
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl h Ar host_key_file
.Op Fl k Ar key_gen_time
.Op Fl p Ar port
+.Op Fl u Ar len
.Op Fl V Ar client_protocol_id
.Sh DESCRIPTION
.Nm
.Pp
.Ss SSH protocol version 2
.Pp
-Version 2 works similar:
+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.
(hmac-sha1 or hmac-md5).
.Pp
Protocol version 2 provides a public key based
-user authentication method (DSAAuthentication)
+user authentication method (PubkeyAuthentication)
and conventional password authentication.
.Pp
.Ss Command execution and data forwarding
.Pp
.Nm
rereads its configuration file when it receives a hangup signal,
-.Dv SIGHUP .
+.Dv SIGHUP ,
+by executing itself with the name it was started as, ie.
+.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
.Bl -tag -width Ds
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
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
-300 seconds).
+600 seconds).
If the client fails to authenticate the user within
this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
.It Fl h Ar host_key_file
-Specifies the file from which the RSA host key is read (default
+Specifies the file from which the host key is read (default
.Pa /etc/ssh_host_key ) .
This option must be given if
.Nm
is not run as root (as the normal
host file is normally not readable by anyone but root).
+It is possible to have multiple host key files for
+the different protocol versions.
.It Fl i
Specifies that
.Nm
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
-.It Fl Q
-Do not print an error message if RSA support is missing.
+.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 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 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
Default is
.Dq yes .
.It Cm AllowGroups
-This keyword can be followed by a number of group names, separated
+This keyword can be followed by a list of group names, separated
by spaces.
If specified, login is allowed only for users whose primary
-group matches one of the patterns.
+group or supplementary group list matches one of the patterns.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.
-Only group names are valid, a numerical group ID isn't recognized.
-By default login is allowed regardless of the primary group.
+Only group names are valid; a numerical group ID isn't recognized.
+By default login is allowed regardless of the group list.
+.Pp
+.It Cm AllowTcpForwarding
+Specifies whether TCP forwarding is permitted.
+The default is
+.Dq yes .
+Note that disabling TCP forwarding does not improve security unless
+users are also denied shell access, as they can always install their
+own forwarders.
.Pp
.It Cm AllowUsers
-This keyword can be followed by a number of user names, separated
+This keyword can be followed by a list of user names, separated
by spaces.
If specified, login is allowed only for users names that
match one of the patterns.
.Ql ?
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 Banner
+In some jurisdictions, sending a warning message before authentication
+may be relevant for getting legal protection.
+The contents of the specified file are sent to the remote user before
+authentication is allowed.
+This option is only available for protocol version 2.
+.Pp
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
The default is
-.Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
+.Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
.It Cm CheckMail
Specifies whether
.Nm
.It Cm DenyGroups
This keyword can be followed by a number of group names, separated
by spaces.
-Users whose primary group matches one of the patterns
-aren't allowed to log in.
+Users whose primary group or supplementary group list matches
+one of the patterns aren't allowed to log in.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.
-Only group names are valid, a numerical group ID isn't recognized.
-By default login is allowed regardless of the primary group.
+Only group names are valid; a numerical group ID isn't recognized.
+By default login is allowed regardless of the group list.
.Pp
.It Cm DenyUsers
This keyword can be followed by a number of user names, separated
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 DSAAuthentication
-Specifies whether DSA authentication is allowed.
+.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.
.Dq no .
The default is
.Dq no .
-.It Cm HostDsaKey
-Specifies the file containing the private DSA host key (default
-.Pa /etc/ssh_host_dsa_key )
-used by SSH protocol 2.0.
-Note that
-.Nm
-disables protocol 2.0 if this file is group/world-accessible.
.It Cm HostKey
-Specifies the file containing the private RSA host key (default
+Specifies the file containing the private host keys (default
.Pa /etc/ssh_host_key )
-used by SSH protocols 1.3 and 1.5.
+used by SSH protocol versions 1 and 2.
Note that
.Nm
-disables protocols 1.3 and 1.5 if this file is group/world-accessible.
+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
.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 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
+.Pp
+.Bd -literal
+ ``hmac-sha1,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,
+ hmac-sha1-96,hmac-md5-96''
+.Ed
+.It Cm MaxStartups
+Specifies the maximum number of concurrent unauthenticated connections to the
+.Nm
+daemon.
+Additional connections will be dropped until authentication succeeds or the
+.Cm LoginGraceTime
+expires for a connection.
+The default is 10.
+.Pp
+Alternatively, random early drop can be enabled by specifying
+the three colon separated values
+.Dq start:rate:full
+(e.g., "10:30:60").
+.Nm
+will refuse connection attempts with a probability of
+.Dq rate/100
+(30%)
+if there are currently
+.Dq start
+(10)
+unauthenticated connections.
+The probability increases linearly and all connection attempts
+are refused if the number of unauthenticated connections reaches
+.Dq full
+(60).
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Dq yes .
-Note that this option applies to both protocol version 1 and 2.
+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.
.Xr ssh 1 .
The argument must be
.Dq yes ,
-.Dq without-password
+.Dq without-password ,
+.Dq forced-commands-only
or
.Dq no .
The default is
.Dq yes .
-If this options is set to
+.Pp
+If this option is set to
.Dq without-password
-only password authentication is disabled for root.
+password authentication is disabled for root.
.Pp
-Root login with RSA authentication when the
+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 will be allowed regardless of the value of this setting
+option has been specified
(which may be useful for taking remote backups even if root login is
-normally not allowed).
+normally not allowed). All other authentication methods are disabled
+for root.
.It Cm PidFile
Specifies the file that contains the process identifier of the
.Nm
.It Cm RandomSeed
Obsolete.
Random number generation uses other techniques.
+.It Cm ReverseMappingCheck
+Specifies whether
+.Nm
+should try to verify the remote host name and check that
+the resolved host name for the remote IP address maps back to the
+very same IP address.
+The default is
+.Dq no .
.It Cm RhostsAuthentication
Specifies whether authentication using rhosts or /etc/hosts.equiv
files is sufficient.
.It Cm ServerKeyBits
Defines the number of bits in the server key.
The minimum value is 512, and the default is 768.
-.It Cm SkeyAuthentication
+.It Cm ChallengeResponseAuthentication
Specifies whether
-.Xr skey 1
+challenge reponse
authentication is allowed.
+Currently there is support for
+.Xr skey 1
+and PAM authentication.
The default is
.Dq yes .
-Note that s/key authentication is enabled only if
-.Cm PasswordAuthentication
-is allowed, too.
+Note that enabling ChallengeResponseAuthentication for PAM bypasses
+OpenSSH's password checking code, thus rendering options such as
+.Cm PasswordAuthentication
+and
+.Cm PermitEmptyPasswords
+ineffective.
.It Cm StrictModes
Specifies whether
.Nm
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,
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.
+file lists the DSA and RSA keys that are
+permitted for public key authentication (PubkeyAuthentication)
+in SSH protocol 2.0.
+.Pp
Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
are ignored as
comments).
-Each line consists of the following fields, separated by
+Each RSA public key consists of the following fields, separated by
spaces: options, bits, exponent, modulus, comment.
-The options field
-is optional; its presence is determined by whether the line starts
+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; the
+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).
+For protocol version 2 the keytype is
+.Dq ssh-dss
+or
+.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).
You don't want to type them in; instead, copy the
.Pa identity.pub
+or the
+.Pa id_dsa.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:
The command supplied by the user (if any) is ignored.
The command is run on a pty if the connection requests a pty;
otherwise it is run without a tty.
+Note that if you want a 8-bit clean channel,
+you must not request a pty or should specify
+.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.
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,
.Nm sshd .
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
-.It Pa /etc/ssh_host_key
-Contains the private part of the host key.
-This file should only be owned by root, readable only by root, and not
+.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 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
-Contains the public part of the host key.
-This file should be world-readable but writable only by
+.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 files should be world-readable but writable only by
root.
-Its contents should match the private part.
-This file is not
-really used for anything; it is only provided for the convenience of
-the user so its contents can be copied to known hosts files.
-These two files are created using
+Their contents should match the respective private parts.
+These files are not
+really used for anything; they are provided for the convenience of
+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
+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
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 it is connecting to the correct remote host.
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
-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)
-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 and 2, making it compatible with
-all other SSH clients and servers.
-.It
-contains added support for
-.Xr kerberos 8
-authentication and ticket passing.
-.It
-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 AUTHORS
+OpenSSH is a derivative of the original and free
+ssh 1.2.12 release by Tatu Ylonen.
+Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
+Theo de Raadt and Dug Song
+removed many bugs, re-added newer features and
+created OpenSSH.
+Markus Friedl contributed the support for SSH
+protocol versions 1.5 and 2.0.
.Sh SEE ALSO
.Xr scp 1 ,
+.Xr sftp 1 ,
+.Xr sftp-server 8 ,
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
andersk / openssh.git / blobdiff