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version 1.70, 2000/10/16 09:38:44

version 1.70.2.3, 2001/03/21 19:46:31

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.\" incompatible with the protocol description in the RFC file, it must be

.\" called by a name other than "ssh" or "Secure Shell".

.\"

.\" Redistribution and use in source and binary forms, with or without

.\" modification, are permitted provided that the following conditions

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.Os

.Sh NAME

.Nm sshd

.Nd secure shell daemon

.Nd OpenSSH ssh 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

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.Op Fl V Ar client_protocol_id

.Sh DESCRIPTION

.Nm

(Secure Shell Daemon) is the daemon program for

(SSH 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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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

The rest of the session is encrypted using a symmetric cipher, currently

using a symmetric cipher, currently

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

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 (DSAAuthentication)

user authentication method (PubkeyAuthentication)

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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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

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should be put into the

.Pa utmp

file.

.It Fl Q

.It Fl D

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

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.

When this option is specified

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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.

By default login is allowed regardless of the group list.

.Pp

.It Cm AllowTcpForwarding

Specifies whether TCP forwarding is permitted.

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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.

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

.Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.

.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 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.

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

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can be used as 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.

.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 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

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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

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.Dq start:rate:full

(e.g., "10:30:60").

.Nm

will refuse connection attempts with a probabillity of

will refuse connection attempts with a probability of

.Dq rate/100

(30%)

if there are currently

.Dq start

(10)

unauthenticated connections.

The probabillity increases linearly and all connection attempts

The probability increases linearly and all connection attempts

are refused if the number of unauthenticated connections reaches

.Dq full

(60).

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The default is

.Dq no .

.It Cm PermitRootLogin

Specifies whether the root can log in using

Specifies whether root can login 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 .

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.

.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 RandomSeed

.It Cm PubkeyAuthentication

Obsolete.

Specifies whether public key authentication is allowed.

Random number generation uses other techniques.

The default is

.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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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) consist of comma-separated option

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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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.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

.It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key

Contains the private part of the host key.

These three files contain the private parts of the

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

(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

.It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub

Contains the public part of the host key.

There three files contain the public parts of the

This file should be world-readable but writable only by

(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.

Their contents should match the respective private parts.

This file is not

These files are not

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

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

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

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

These two files are created using

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

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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 the remote host is the one it 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

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machine-specific login-time initializations globally.

This file should be writable only by root, and should be world-readable.

.El

.Sh AUTHOR

.Sh AUTHORS

OpenSSH

OpenSSH is a derivative of the original and free

is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,

ssh 1.2.12 release by Tatu Ylonen.

but with bugs removed and newer features re-added.

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

Rapidly after the

Theo de Raadt and Dug Song

1.2.12 release, newer versions of the original ssh bore successively

removed many bugs, re-added newer features and

more restrictive licenses, and thus demand for a free version was born.

created OpenSSH.

.Pp

Markus Friedl contributed the support for SSH

This version of OpenSSH

protocol versions 1.5 and 2.0.

.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

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 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