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.\" -*- nroff -*-

.\"

.\" sshd.8.in

.\"

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

.\"

.\" As far as I am concerned, the code I have written for this software

.\" Created: Sat Apr 22 21:55:14 1995 ylo

.\" 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 OpenSSH ssh daemon

.Nd secure shell daemon

.Sh SYNOPSIS

.Nm sshd

.Op Fl diqD46

.Op Fl diqQ46

.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

(SSH Daemon) is the daemon program for

(Secure Shell Daemon) is the daemon program for

.Xr ssh 1 .

Together these programs replace rlogin and rsh, and

provide secure encrypted communications between two untrusted hosts

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.Pp

.Ss SSH protocol version 2

.Pp

Version 2 works similarly:

Version 2 works similar:

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

The rest of the session is encrypted

Blowfish, 3DES, CAST128, Arcfour, 128 bit AES, or 256 bit AES.

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

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(hmac-sha1 or hmac-md5).

.Pp

Protocol version 2 provides a public key based

user authentication method (PubkeyAuthentication)

user authentication method (DSAAuthentication)

and conventional password authentication.

.Pp

.Ss Command execution and data forwarding

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.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

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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

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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

600 seconds).

300 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 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

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

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Nothing is sent to the system log.

Normally the beginning,

authentication, and termination of each connection is logged.

.It Fl u Ar len

.It Fl Q

This option is used to specify the size of the field

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

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

SSH-2 compatibility mode.

SSH2 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

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Default is

.Dq yes .

.It Cm AllowGroups

This keyword can be followed by a list of group names, separated

This keyword can be followed by a number of group names, separated

by spaces.

If specified, login is allowed only for users whose primary

group or supplementary group list matches one of the patterns.

group 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 group list.

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 list of user names, separated

This keyword can be followed by a number of user names, separated

by spaces.

If specified, login is allowed only for users names that

match one of the patterns.

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.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 ChallengeResponseAuthentication

Specifies whether

challenge response

authentication is allowed.

Currently there is only support for

.Xr skey 1

authentication.

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.

.Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .

.It Cm CheckMail

Specifies whether

.Nm

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.It Cm DenyGroups

This keyword can be followed by a number of group names, separated

by spaces.

Users whose primary group or supplementary group list matches

Users whose primary group matches one of the patterns

one of the patterns aren't allowed to log in.

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 group list.

By default login is allowed regardless of the primary group.

.Pp

.It Cm DenyUsers

This keyword can be followed by a number of user names, separated

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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.

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.

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.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 host keys (default

Specifies the file containing the private RSA host key (default

.Pa /etc/ssh_host_key )

used by SSH protocol versions 1 and 2.

used by SSH protocols 1.3 and 1.5.

Note that

.Nm

will refuse to use a file if it is group/world-accessible.

disables protocols 1.3 and 1.5 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

.Dq rsa

are used for version 2 of the SSH protocol.

.It Cm IgnoreRhosts

Specifies that

.Pa .rhosts

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.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

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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-md5,hmac-sha1,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 versions 1 and 2.

Note that this option applies to both protocol version 1 and 2.

.It Cm PermitEmptyPasswords

When password authentication is allowed, it specifies whether the

server allows login to accounts with empty password strings.

The default is

.Dq no .

.It Cm PermitRootLogin

Specifies whether root can login using

Specifies whether the root can log in using

.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 .

.Pp

If this options is set to

If this option is set to

.Dq without-password

password authentication is disabled for root.

only password authentication is disabled for root.

.Pp

If this option is set to

Root login with RSA authentication when the

.Dq forced-commands-only

root login with public key authentication will be allowed,

but only if the

.Ar command

option has been specified

option has been

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). All other authentication methods are disabled

normally not allowed).

for root.

.Pp

If this option is set to

.Dq no

root is not allowed to login.

.It Cm PidFile

Specifies the file that contains the process identifier of the

.Nm

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Multiple versions must be comma-separated.

The default is

.Dq 1 .

.It Cm PubkeyAuthentication

.It Cm RandomSeed

Specifies whether public key authentication is allowed.

Obsolete.

The default is

Random number generation uses other techniques.

.Dq yes .

Note that this option applies to protocol version 2 only.

.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.

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.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

Specifies whether

.Xr skey 1

authentication is allowed.

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

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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 .

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.It Cm UseLogin

Specifies whether

.Xr login 1

is used for interactive login sessions.

is used.

Note that

.Xr login 1

is never used for remote command execution.

The default is

.Dq no .

.It Cm X11DisplayOffset

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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

file lists the DSA keys that are

permitted for public key authentication (PubkeyAuthentication)

permitted for DSA authentication in SSH protocol 2.0.

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 RSA public key consists of the following fields, separated by

Each line consists of the following fields, separated by

spaces: options, bits, exponent, modulus, comment.

Each protocol version 2 public key consists of:

The options field

options, keytype, base64 encoded key, comment.

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

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 for

The bits, exponent, modulus and comment fields give the RSA key; the

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) consist of comma-separated option

The options (if present) consists of comma-separated option

specifications.

No spaces are permitted, except within double quotes.

The following option specifications are supported:

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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.

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authentication.

.It Cm no-pty

Prevents tty allocation (a request to allocate a pty will fail).

.It Cm permitopen="host:port"

Limit local

.Li ``ssh -L''

port-forwading 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

addresses.

.El

.Ss Examples

1024 33 12121.\|.\|.\|312314325 ylo@foo.bar

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from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula

.Pp

command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi

.Pp

permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323

.Sh SSH_KNOWN_HOSTS FILE FORMAT

The

.Pa /etc/ssh_known_hosts ,

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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 from an unknown host

maintained automatically: whenever the user connects an unknown host

its key is added to the per-user file.

.Pp

Each line in these files contains the following fields: hostnames,

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.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, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key

.It Pa /etc/ssh_host_key

These three files contain the private parts of the

Contains the private part of the host key.

(SSH1, SSH2 DSA, and SSH2 RSA) host keys.

This file should only be owned by root, readable only by root, and not

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

.It Pa /etc/ssh_host_key.pub

There three files contain the public parts of the

Contains the public part of the host key.

(SSH1, SSH2 DSA, and SSH2 RSA) host keys.

This file should be world-readable but writable only by

These files should be world-readable but writable only by

root.

Their contents should match the respective private parts.

Its contents should match the private part.

These files are not

This file is not

really used for anything; they are provided for the convenience of

really used for anything; it is only provided for the convenience of

the user so their contents can be copied to known hosts files.

the user so its contents can be copied to known hosts files.

These files are created using

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 content of this file is not sensitive; it can be world-readable.

The contents of this file are 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

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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.

to verify that the remote host is the one we intended to connect.

These files should be writable only by root/the owner.

.Pa /etc/ssh_known_hosts

should be world-readable, and

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.Pa .rhosts .

However, this file is

not used by rlogin and rshd, so using this permits access using SSH only.

.It Pa /etc/hosts.equiv

.Pa /etc/hosts.equiv

This file is used during

.Pa .rhosts

authentication.

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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

.Sh AUTHORS

.Pp

OpenSSH is a derivative of the original and free

The libraries described in

ssh 1.2.12 release by Tatu Ylonen.

.Xr ssl 8

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

are required for proper operation.

Theo de Raadt and Dug Song

.Pp

removed many bugs, re-added newer features and

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

created OpenSSH.

Niels Provos, Theo de Raadt, and Dug Song.

Markus Friedl contributed the support for SSH

.Pp

protocol versions 1.5 and 2.0.

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

.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