src/usr.bin/ssh/sshd.8 - diff

Return to sshd.8 CVS log

Up to [local] / src / usr.bin / ssh

Diff for /src/usr.bin/ssh/sshd.8 between version 1.51 and 1.51.2.4

version 1.51, 2000/05/08 17:42:31

version 1.51.2.4, 2001/03/12 15:44:17

Line 1

.\" -*- nroff -*-

.\"

.\" sshd.8.in

.\"

.\" Author: Tatu Ylonen <ylo@cs.hut.fi>

.\"

.\" 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".

.\"

.\" $Id$

.\"

.\" 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.

.\"

.\" 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.

.\"

.\" $OpenBSD$

.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

Line 104

Line 129

.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.

Line 119

Line 144

(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

Line 162

Line 187

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

Line 170

Line 197

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

Line 211

Line 240

Nothing is sent to the system log.

Normally the beginning,

authentication, and termination of each connection is logged.

.It Fl Q

.It Fl u Ar len

Do not print an error message if RSA support is missing.

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

Line 248

Line 298

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.

Only group names are valid; a numerical group ID isn't recognized.

By default login is allowed regardless of the primary group.

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.

Line 270

Line 328

.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

Line 287

Line 352

.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

Users whose primary group or supplementary group list matches

aren't allowed to log in.

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.

Only group names are valid; a numerical group ID isn't recognized.

By default login is allowed regardless of the primary group.

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

Line 305

Line 370

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

.It Cm PubkeyAuthentication

Specifies whether DSA authentication is allowed.

Specifies whether public key authentication is allowed.

The default is

.Dq yes .

Note that this option applies to protocol version 2 only.

Line 321

Line 386

.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

.Dq rsa

are used for version 2 of the SSH protocol.

.It Cm IgnoreRhosts

Specifies that

.Pa .rhosts

Line 384

Line 449

.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

.Pa /etc/passwd .

or SecurID.

Default is

.Dq yes .

.It Cm KerberosTgtPassing

Line 435

Line 501

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.

Line 450

Line 552

.Xr ssh 1 .

The argument must be

.Dq yes ,

.Dq without-password

.Dq without-password ,

.Dq forced-commands-only

.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

option has been specified

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).

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

Line 502

Line 609

.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.

Line 525

Line 640

.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 only support for

.Xr skey 1

authentication.

The default is

.Dq yes .

Note that s/key authentication is enabled only if

.Cm PasswordAuthentication

is allowed, too.

.It Cm StrictModes

Specifies whether

.Nm

Line 543

Line 658

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 .

Line 552

Line 678

.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

Line 569

Line 698

.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,

Line 622

Line 757

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

file lists the DSA and RSA keys that are

permitted for DSA authentication in SSH protocol 2.0.

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

Each protocol version 2 public key consists of:

is optional; its presence is determined by whether the line starts

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

.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:

Line 675

Line 821

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.

Line 718

Line 867

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,

Line 787

Line 936

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

Line 821

Line 972

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

Line 860

Line 1011

.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.

Line 940

Line 1091

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, see

.Xr ssl 8 )

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

.Sh AUTHORS

The libraries described in

OpenSSH is a derivative of the original and free

.Xr ssl 8

ssh 1.2.12 release by Tatu Ylonen.

are required for proper operation.

Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,

.Pp

Theo de Raadt and Dug Song

OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,

removed many bugs, re-added newer features and

Niels Provos, Theo de Raadt, and Dug Song.

created OpenSSH.

.Pp

Markus Friedl contributed the support for SSH

The support for SSH protocol 2 was written by Markus Friedl.

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 ,

.Xr ssh-keygen 1 ,

.Xr ssl 8 ,

.Xr rlogin 1 ,

.Xr rsh 1