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version 1.39, 2000/03/04 07:07:05

version 1.40, 2000/03/11 21:40:03

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.Sh DESCRIPTION

.Nm

(Secure Shell) is a program for logging into a remote machine and for

executing commands on a remote machine. It is intended to replace

executing commands on a remote machine.

It is intended to replace

rlogin and rsh, and provide secure encrypted communications between

two untrusted hosts over an insecure network. X11 connections and

two untrusted hosts over an insecure network.

X11 connections and

arbitrary TCP/IP ports can also be forwarded over the secure channel.

.Pp

.Nm

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exists in the user's home directory on the

remote machine and contains a line containing the name of the client

machine and the name of the user on that machine, the user is

permitted to log in. This form of authentication alone is normally not

permitted to log in.

This form of authentication alone is normally not

allowed by the server because it is not secure.

.Pp

The second (and primary) authentication method is the

.Pa rhosts

.Pa hosts.equiv

method combined with RSA-based host authentication. It

method combined with RSA-based host authentication.

means that if the login would be permitted by

It means that if the login would be permitted by

.Pa \&.rhosts ,

.Pa \&.shosts ,

.Pa /etc/hosts.equiv ,

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.Pa $HOME/.ssh/known_hosts

in the

.Sx FILES

section), only then login is

section), only then login is permitted.

permitted. This authentication method closes security holes due to IP

This authentication method closes security holes due to IP

spoofing, DNS spoofing and routing spoofing. [Note to the

spoofing, DNS spoofing and routing spoofing.

administrator:

[Note to the administrator:

.Pa /etc/hosts.equiv ,

.Pa \&.rhosts ,

and the rlogin/rsh protocol in general, are inherently insecure and should be

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The scheme is based on public-key cryptography: there are cryptosystems

where encryption and decryption are done using separate keys, and it

is not possible to derive the decryption key from the encryption key.

RSA is one such system. The idea is that each user creates a public/private

RSA is one such system.

key pair for authentication purposes. The

The idea is that each user creates a public/private

server knows the public key, and only the user knows the private key.

key pair for authentication purposes.

The server knows the public key, and only the user knows the private key.

The file

.Pa $HOME/.ssh/authorized_keys

lists the public keys that are permitted for logging

in. When the user logs in, the

in.

When the user logs in, the

.Nm

program tells the server which key pair it would like to use for

authentication. The server checks if this key is permitted, and if

authentication.

The server checks if this key is permitted, and if

so, sends the user (actually the

.Nm

program running on behalf of the user) a challenge, a random number,

encrypted by the user's public key. The challenge can only be

encrypted by the user's public key.

decrypted using the proper private key. The user's client then decrypts the

The challenge can only be

decrypted using the proper private key.

The user's client then decrypts the

challenge using the private key, proving that he/she knows the private

key but without disclosing it to the server.

.Pp

.Nm

implements the RSA authentication protocol automatically. The user

implements the RSA authentication protocol automatically.

creates his/her RSA key pair by running

The user creates his/her RSA key pair by running

.Xr ssh-keygen 1 .

This stores the private key in

.Pa \&.ssh/identity

and the public key in

.Pa \&.ssh/identity.pub

in the user's home directory. The user should then

in the user's home directory.

copy the

The user should then copy the

.Pa identity.pub

.Pa \&.ssh/authorized_keys

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file corresponds to the conventional

.Pa \&.rhosts

file, and has one key

per line, though the lines can be very long). After this, the user

per line, though the lines can be very long).

can log in without giving the password. RSA authentication is much

After this, the user can log in without giving the password.

RSA authentication is much

more secure than rhosts authentication.

.Pp

The most convenient way to use RSA authentication may be with an

authentication agent. See

authentication agent.

See

.Xr ssh-agent 1

for more information.

.Pp

If other authentication methods fail,

.Nm

prompts the user for a password. The password is sent to the remote

prompts the user for a password.

The password is sent to the remote

host for checking; however, since all communications are encrypted,

the password cannot be seen by someone listening on the network.

.Pp

When the user's identity has been accepted by the server, the server

either executes the given command, or logs into the machine and gives

the user a normal shell on the remote machine. All communication with

the user a normal shell on the remote machine.

All communication with

the remote command or shell will be automatically encrypted.

.Pp

If a pseudo-terminal has been allocated (normal login session), the

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connections to terminate, it can be backgrounded with

.Ic ~&

(this should not be used while the user shell is active, as it can cause the

shell to hang). All available escapes can be listed with

shell to hang).

All available escapes can be listed with

.Ic ~? .

.Pp

A single tilde character can be sent as

.Ic ~~

(or by following the tilde by a character other than those described above).

The escape character must always follow a newline to be interpreted as

special. The escape character can be changed in configuration files

special.

or on the command line.

The escape character can be changed in configuration files

or on the command line.

.Pp

If no pseudo tty has been allocated, the

session is transparent and can be used to reliably transfer binary

data. On most systems, setting the escape character to

data.

On most systems, setting the escape character to

.Dq none

will also make the session transparent even if a tty is used.

.Pp

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automatically forwarded to the remote side in such a way that any X11

programs started from the shell (or command) will go through the

encrypted channel, and the connection to the real X server will be made

from the local machine. The user should not manually set

from the local machine.

The user should not manually set

.Ev DISPLAY .

Forwarding of X11 connections can be

configured on the command line or in configuration files.

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value set by

.Nm

will point to the server machine, but with a display number greater

than zero. This is normal, and happens because

than zero.

This is normal, and happens because

.Nm

creates a

.Dq proxy

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For this purpose, it will generate a random authorization cookie,

store it in Xauthority on the server, and verify that any forwarded

connections carry this cookie and replace it by the real cookie when

the connection is opened. The real authentication cookie is never

the connection is opened.

The real authentication cookie is never

sent to the server machine (and no cookies are sent in the plain).

.Pp

If the user is using an authentication agent, the connection to the agent

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command line or in a configuration file.

.Pp

Forwarding of arbitrary TCP/IP connections over the secure channel can

be specified either on command line or in a configuration file. One

be specified either on command line or in a configuration file.

possible application of TCP/IP forwarding is a secure connection to an

One possible application of TCP/IP forwarding is a secure connection to an

electronic purse; another is going trough firewalls.

.Pp

.Nm

automatically maintains and checks a database containing RSA-based

identifications for all hosts it has ever been used with. The

identifications for all hosts it has ever been used with.

database is stored in

The database is stored in

.Pa \&.ssh/known_hosts

in the user's home directory. Additionally, the file

in the user's home directory.

Additionally, the file

.Pa /etc/ssh_known_hosts

is automatically checked for known hosts. Any new hosts are

is automatically checked for known hosts.

automatically added to the user's file. If a host's identification

Any new hosts are automatically added to the user's file.

If a host's identification

ever changes,

.Nm

warns about this and disables password authentication to prevent a

trojan horse from getting the user's password. Another purpose of

trojan horse from getting the user's password.

Another purpose of

this mechanism is to prevent man-in-the-middle attacks which could

otherwise be used to circumvent the encryption. The

otherwise be used to circumvent the encryption.

The

.Cm StrictHostKeyChecking

option (see below) can be used to prevent logins to machines whose

host key is not known or has changed.

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.It Fl c Ar blowfish|3des

Selects the cipher to use for encrypting the session.

.Ar 3des

is used by default. It is believed to be secure.

is used by default.

It is believed to be secure.

.Ar 3des

(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.

It is presumably more secure than the

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cipher which is no longer supported in ssh.

.Ar blowfish

is a fast block cipher, it appears very secure and is much faster than

.Ar 3des .

.It Fl e Ar ch|^ch|none

Sets the escape character for sessions with a pty (default:

.Ql ~ ) .

The escape character is only recognized at the beginning of a line. The

The escape character is only recognized at the beginning of a line.

escape character followed by a dot

The escape character followed by a dot

.Pq Ql \&.

closes the connection, followed

by control-Z suspends the connection, and followed by itself sends the

escape character once. Setting the character to

escape character once.

Setting the character to

.Dq none

disables any escapes and makes the session fully transparent.

.It Fl f

Requests

.Nm

to go to background just before command execution. This is useful

to go to background just before command execution.

This is useful if

.Nm

is going to ask for passwords or passphrases, but the user

wants it in the background. This implies

wants it in the background.

This implies

.Fl n .

The recommended way to start X11 programs at a remote site is with

something like

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Allows remote hosts to connect to local forwarded ports.

.It Fl i Ar identity_file

Selects the file from which the identity (private key) for

RSA authentication is read. Default is

RSA authentication is read.

Default is

.Pa \&.ssh/identity

in the user's home directory. Identity files may also be specified on

in the user's home directory.

a per-host basis in the configuration file. It is possible to have

Identity files may also be specified on

multiple

a per-host basis in the configuration file.

It is possible to have multiple

.Fl i

options (and multiple identities specified in

configuration files).

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Disables forwarding of Kerberos tickets and AFS tokens. This may

also be specified on a per-host basis in the configuration file.

.It Fl l Ar login_name

Specifies the user to log in as on the remote machine. This may also

Specifies the user to log in as on the remote machine.

be specified on a per-host basis in the configuration file.

This also may be specified on a per-host basis in the configuration file.

.It Fl n

Redirects stdin from

.Pa /dev/null

(actually, prevents reading from stdin).

This must be used when

.Nm

is run in the background. A common trick is to use this to run X11

is run in the background.

programs in a remote machine. For example,

A common trick is to use this to run X11 programs on a remote machine.

For example,

.Ic ssh -n shadows.cs.hut.fi emacs &

will start an emacs on shadows.cs.hut.fi, and the X11

connection will be automatically forwarded over an encrypted channel.

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.It Fl o Ar option

Can be used to give options in the format used in the config file.

This is useful for specifying options for which there is no separate

command-line flag. The option has the same format as a line in the

command-line flag.

configuration file.

The option has the same format as a line in the configuration file.

.It Fl p Ar port

Port to connect to on the remote host. This can be specified on a

Port to connect to on the remote host.

This can be specified on a

per-host basis in the configuration file.

.It Fl P

Use a non-privileged port for outgoing connections.

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and

.Cm RhostsRSAAuthentication .

.It Fl q

Quiet mode. Causes all warning and diagnostic messages to be

Quiet mode.

suppressed. Only fatal errors are displayed.

Causes all warning and diagnostic messages to be suppressed.

Only fatal errors are displayed.

.It Fl t

Force pseudo-tty allocation. This can be used to execute arbitary

Force pseudo-tty allocation.

screen-based programs on a remote machine, which can be very useful

This can be used to execute arbitary

e.g. when implementing menu services.

screen-based programs on a remote machine, which can be very useful,

e.g., when implementing menu services.

.It Fl v

Verbose mode. Causes

Verbose mode.

Causes

.Nm

to print debugging messages about its progress. This is helpful in

to print debugging messages about its progress.

This is helpful in

debugging connection, authentication, and configuration problems.

The verbose mode is also used to display

.Xr skey 1

challenges, if the user entered "s/key" as password.

.It Fl x

Disables X11 forwarding. This can also be specified on a per-host

Disables X11 forwarding.

basis in a configuration file.

This can also be specified on a per-host basis in a configuration file.

.It Fl X

Enables X11 forwarding.

.It Fl C

Requests compression of all data (including stdin, stdout, stderr, and

data for forwarded X11 and TCP/IP connections). The compression

data for forwarded X11 and TCP/IP connections).

algorithm is the same used by

The compression algorithm is the same used by

.Xr gzip 1 ,

and the

.Dq level

can be controlled by the

.Cm CompressionLevel

option (see below). Compression is desirable on modem lines and other

option (see below).

Compression is desirable on modem lines and other

slow connections, but will only slow down things on fast networks.

The default value can be set on a host-by-host basis in the

configuration files; see the

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option below.

.It Fl L Ar port:host:hostport

Specifies that the given port on the local (client) host is to be

forwarded to the given host and port on the remote side. This works

forwarded to the given host and port on the remote side.

by allocating a socket to listen to

This works by allocating a socket to listen to

.Ar port

on the local side, and whenever a connection is made to this port, the

connection is forwarded over the secure channel, and a connection is

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.Ar host

port

.Ar hostport

from the remote machine. Port forwardings can also be specified in the

from the remote machine.

configuration file. Only root can forward privileged ports.

Port forwardings can also be specified in the configuration file.

Only root can forward privileged ports.

IPv6 addresses can be specified with an alternative syntax:

.Ar port/host/hostport

.It Fl R Ar port:host:hostport

Specifies that the given port on the remote (server) host is to be

forwarded to the given host and port on the local side. This works

forwarded to the given host and port on the local side.

by allocating a socket to listen to

This works by allocating a socket to listen to

.Ar port

on the remote side, and whenever a connection is made to this port, the

connection is forwarded over the secure channel, and a connection is

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.Ar host

port

.Ar hostport

from the local machine. Port forwardings can also be specified in the

from the local machine.

configuration file. Privileged ports can be forwarded only when

Port forwardings can also be specified in the configuration file.

Privileged ports can be forwarded only when

logging in as root on the remote machine.

.It Fl 4

Forces

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and system-wide configuration file

.Pq Pa /etc/ssh_config .

For each parameter, the first obtained value

will be used. The configuration files contain sections bracketed by

will be used.

"Host" specifications, and that section is only applied for hosts that

The configuration files contain sections bracketed by

match one of the patterns given in the specification. The matched

.Dq Host

host name is the one given on the command line.

specifications, and that section is only applied for hosts that

match one of the patterns given in the specification.

The matched host name is the one given on the command line.

.Pp

Since the first obtained value for each parameter is used, more

host-specific declarations should be given near the beginning of the

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and

.Ql ?

can be used as wildcards in the

patterns. A single

patterns.

A single

.Ql \&*

as a pattern can be used to provide global

defaults for all hosts. The host is the

defaults for all hosts.

The host is the

.Ar hostname

argument given on the command line (i.e., the name is not converted to

a canonicalized host name before matching).

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.It Cm BatchMode

If set to

.Dq yes ,

passphrase/password querying will be disabled. This

passphrase/password querying will be disabled.

option is useful in scripts and other batch jobs where you have no

This option is useful in scripts and other batch jobs where you have no

user to supply the password. The argument must be

user to supply the password.

The argument must be

.Dq yes

.Dq no .

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.Dq no ,

the check will not be executed.

.It Cm Cipher

Specifies the cipher to use for encrypting the session. Currently,

Specifies the cipher to use for encrypting the session.

Currently,

.Dq blowfish ,

and

.Dq 3des

are supported. The default is

are supported.

The default is

.Dq 3des .

.It Cm Compression

Specifies whether to use compression. The argument must be

Specifies whether to use compression.

The argument must be

.Dq yes

.Dq no .

.It Cm CompressionLevel

Specifies the compression level to use if compression is enable. The

Specifies the compression level to use if compression is enable.

argument must be an integer from 1 (fast) to 9 (slow, best). The

The argument must be an integer from 1 (fast) to 9 (slow, best).

default level is 6, which is good for most applications. The meaning

The default level is 6, which is good for most applications.

of the values is the same as in

The meaning of the values is the same as in

.Xr gzip 1 .

.It Cm ConnectionAttempts

Specifies the number of tries (one per second) to make before falling

back to rsh or exiting. The argument must be an integer. This may be

back to rsh or exiting.

useful in scripts if the connection sometimes fails.

The argument must be an integer.

This may be useful in scripts if the connection sometimes fails.

.It Cm EscapeChar

Sets the escape character (default:

.Ql ~ ) .

The escape character can also

be set on the command line. The argument should be a single

be set on the command line.

character,

The argument should be a single character,

.Ql ^

followed by a letter, or

.Dq none

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listening on the remote host),

.Xr rsh 1

should automatically be used instead (after a suitable warning about

the session being unencrypted). The argument must be

the session being unencrypted).

The argument must be

.Dq yes

.Dq no .

.It Cm ForwardAgent

Specifies whether the connection to the authentication agent (if any)

will be forwarded to the remote machine. The argument must be

will be forwarded to the remote machine.

The argument must be

.Dq yes

.Dq no .

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Specifies whether X11 connections will be automatically redirected

over the secure channel and

.Ev DISPLAY

set. The argument must be

set.

The argument must be

.Dq yes

.Dq no .

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Specifies a file to use instead of

.Pa /etc/ssh_known_hosts .

.It Cm HostName

Specifies the real host name to log into. This can be used to specify

Specifies the real host name to log into.

nicnames or abbreviations for hosts. Default is the name given on the

This can be used to specify nicknames or abbreviations for hosts.

command line. Numeric IP addresses are also permitted (both on the

Default is the name given on the command line.

command line and in

Numeric IP addresses are also permitted (both on the command line and in

.Cm HostName

specifications).

.It Cm IdentityFile

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.Pa .ssh/identity

in the user's home directory).

Additionally, any identities represented by the authentication agent

will be used for authentication. The file name may use the tilde

will be used for authentication.

syntax to refer to a user's home directory. It is possible to have

The file name may use the tilde

syntax to refer to a user's home directory.

It is possible to have

multiple identity files specified in configuration files; all these

identities will be tried in sequence.

.It Cm KeepAlive

Specifies whether the system should send keepalive messages to the

other side. If they are sent, death of the connection or crash of one

other side.

of the machines will be properly noticed. However, this means that

If they are sent, death of the connection or crash of one

of the machines will be properly noticed.

However, this means that

connections will die if the route is down temporarily, and some people

find it annoying.

.Pp

The default is

.Dq yes

(to send keepalives), and the client will notice

if the network goes down or the remote host dies. This is important

if the network goes down or the remote host dies.

in scripts, and many users want it too.

This is important in scripts, and many users want it too.

.Pp

To disable keepalives, the value should be set to

.Dq no

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.Dq no .

.It Cm LocalForward

Specifies that a TCP/IP port on the local machine be forwarded over

the secure channel to given host:port from the remote machine. The

the secure channel to given host:port from the remote machine.

first argument must be a port number, and the second must be

The first argument must be a port number, and the second must be

host:port. Multiple forwardings may be specified, and additional

host:port.

forwardings can be given on the command line. Only the root can

Multiple forwardings may be specified, and additional

forward privileged ports.

forwardings can be given on the command line.

Only the superuser can forward privileged ports.

.It Cm LogLevel

Gives the verbosity level that is used when logging messages from

.Nm ssh .

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Specifies the number of password prompts before giving up. The

argument to this keyword must be an integer. Default is 3.

.It Cm PasswordAuthentication

Specifies whether to use password authentication. The argument to

Specifies whether to use password authentication.

this keyword must be

The argument to this keyword must be

.Dq yes

.Dq no .

.It Cm Port

Specifies the port number to connect on the remote host. Default is

Specifies the port number to connect on the remote host.

22.

Default is 22.

.It Cm ProxyCommand

Specifies the command to use to connect to the server. The command

Specifies the command to use to connect to the server.

string extends to the end of the line, and is executed with /bin/sh.

The command

In the command string, %h will be substituted by the host name to

string extends to the end of the line, and is executed with

connect and %p by the port. The command can be basically anything,

.Pa /bin/sh .

and should read from its stdin and write to its stdout. It should

In the command string,

eventually connect an

.Ql %h

will be substituted by the host name to

connect and

.Ql %p

by the port.

The command can be basically anything,

and should read from its standard input and write to its standard output.

It should eventually connect an

.Xr sshd 8

server running on some machine, or execute

.Ic sshd -i

somewhere. Host key management will be done using the

somewhere.

Host key management will be done using the

HostName of the host being connected (defaulting to the name typed by

the user).

Note that

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.Pp

.It Cm RemoteForward

Specifies that a TCP/IP port on the remote machine be forwarded over

the secure channel to given host:port from the local machine. The

the secure channel to given host:port from the local machine.

first argument must be a port number, and the second must be

The first argument must be a port number, and the second must be

host:port. Multiple forwardings may be specified, and additional

host:port.

forwardings can be given on the command line. Only the root can

Multiple forwardings may be specified, and additional

forward privileged ports.

forwardings can be given on the command line.

Only the superuser can forward privileged ports.

.It Cm RhostsAuthentication

Specifies whether to try rhosts based authentication. Note that this

Specifies whether to try rhosts based authentication.

Note that this

declaration only affects the client side and has no effect whatsoever

on security. Disabling rhosts authentication may reduce

on security.

Disabling rhosts authentication may reduce

authentication time on slow connections when rhosts authentication is

not used. Most servers do not permit RhostsAuthentication because it

not used.

is not secure (see RhostsRSAAuthentication). The argument to this

Most servers do not permit RhostsAuthentication because it

keyword must be

is not secure (see RhostsRSAAuthentication).

The argument to this keyword must be

.Dq yes

.Dq no .

.It Cm RhostsRSAAuthentication

Specifies whether to try rhosts based authentication with RSA host

authentication. This is the primary authentication method for most

authentication.

sites. The argument must be

This is the primary authentication method for most sites.

The argument must be

.Dq yes

.Dq no .

.It Cm RSAAuthentication

Specifies whether to try RSA authentication. The argument to this

Specifies whether to try RSA authentication.

keyword must be

The argument to this keyword must be

.Dq yes

.Dq no .

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.It Cm SkeyAuthentication

Specifies whether to use

.Xr skey 1

authentication. The argument to

authentication.

this keyword must be

The argument to this keyword must be

.Dq yes

.Dq no .

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.Nm

ssh will never automatically add host keys to the

.Pa $HOME/.ssh/known_hosts

file, and refuses to connect hosts whose host key has changed. This

file, and refuses to connect hosts whose host key has changed.

provides maximum protection against trojan horse attacks. However, it

This provides maximum protection against trojan horse attacks.

can be somewhat annoying if you don't have good

However, it can be somewhat annoying if you don't have good

.Pa /etc/ssh_known_hosts

files installed and frequently

connect new hosts. Basically this option forces the user to manually

connect new hosts.

add any new hosts. Normally this option is disabled, and new hosts

Basically this option forces the user to manually

will automatically be added to the known host files. The host keys of

add any new hosts.

known hosts will be verified automatically in either case. The

Normally this option is disabled, and new hosts

argument must be

will automatically be added to the known host files.

The host keys of

known hosts will be verified automatically in either case.

The argument must be

.Dq yes

.Dq no .

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and

.Cm RhostsRSAAuthentication .

.It Cm User

Specifies the user to log in as. This can be useful if you have a

Specifies the user to log in as.

different user name in different machines. This saves the trouble of

This can be useful if you have a different user name on different machines.

This saves the trouble of

having to remember to give the user name on the command line.

.It Cm UserKnownHostsFile

Specifies a file to use instead of

.Pa $HOME/.ssh/known_hosts .

.It Cm UseRsh

Specifies that rlogin/rsh should be used for this host. It is

Specifies that rlogin/rsh should be used for this host.

possible that the host does not at all support the

It is possible that the host does not at all support the

.Nm

protocol. This causes

protocol.

This causes

.Nm

to immediately exec

to immediately execute

.Xr rsh 1 .

All other options (except

.Cm HostName )

are ignored if this has been specified. The argument must be

are ignored if this has been specified.

The argument must be

.Dq yes

.Dq no .

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.It Ev DISPLAY

The

.Ev DISPLAY

variable indicates the location of the X11 server. It is

variable indicates the location of the X11 server.

automatically set by

It is automatically set by

.Nm

to point to a value of the form

.Dq hostname:n

where hostname indicates

the host where the shell runs, and n is an integer >= 1. Ssh uses

the host where the shell runs, and n is an integer >= 1.

this special value to forward X11 connections over the secure

.Nm

channel. The user should normally not set DISPLAY explicitly, as that

uses this special value to forward X11 connections over the secure

channel.

The user should normally not set DISPLAY explicitly, as that

will render the X11 connection insecure (and will require the user to

manually copy any required authorization cookies).

.It Ev HOME

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Line 857

set for compatibility with systems that use this variable.

.It Ev MAIL

Set to point the user's mailbox.

.It Ev PATH

Set to the default

.Ev PATH ,

as specified when compiling

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indicates the path of a unix-domain socket used to communicate with the

agent.

.It Ev SSH_CLIENT

Identifies the client end of the connection. The variable contains

Identifies the client end of the connection.

The variable contains

three space-separated values: client ip-address, client port number,

and server port number.

.It Ev SSH_TTY

This is set to the name of the tty (path to the device) associated

with the current shell or command. If the current session has no tty,

with the current shell or command.

If the current session has no tty,

this variable is not set.

.It Ev TZ

The timezone variable is set to indicate the present timezone if it

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Line 899

See

.Xr sshd 8 .

.It Pa $HOME/.ssh/identity

Contains the RSA authentication identity of the user. This file

Contains the RSA authentication identity of the user.

This file

contains sensitive data and should be readable by the user but not

accessible by others (read/write/execute).

Note that

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Line 911

sensitive part of this file using 3DES.

.It Pa $HOME/.ssh/identity.pub

Contains the public key for authentication (public part of the

identity file in human-readable form). The contents of this file

identity file in human-readable form).

should be added to

The contents of this file should be added to

.Pa $HOME/.ssh/authorized_keys

on all machines

where you wish to log in using RSA authentication. This file is not

where you wish to log in using RSA authentication.

sensitive and can (but need not) be readable by anyone. This file is

This file is not

sensitive and can (but need not) be readable by anyone.

This file is

never used automatically and is not necessary; it is only provided for

the convenience of the user.

.It Pa $HOME/.ssh/config

This is the per-user configuration file. The format of this file is

This is the per-user configuration file.

described above. This file is used by the

The format of this file is described above.

This file is used by the

.Nm

client. This file does not usually contain any sensitive information,

client.

This file does not usually contain any sensitive information,

but the recommended permissions are read/write for the user, and not

accessible by others.

.It Pa $HOME/.ssh/authorized_keys

Lists the RSA keys that can be used for logging in as this user. The

Lists the RSA keys that can be used for logging in as this user.

format of this file is described in the

The format of this file is described in the

.Xr sshd 8

manual page. In the simplest form the format is the same as the .pub

manual page.

In the simplest form the format is the same as the .pub

identity files (that is, each line contains the number of bits in

modulus, public exponent, modulus, and comment fields, separated by

spaces). This file is not highly sensitive, but the recommended

spaces).

This file is not highly sensitive, but the recommended

permissions are read/write for the user, and not accessible by others.

.It Pa /etc/ssh_known_hosts

Systemwide list of known host keys. This file should be prepared by the

Systemwide list of known host keys.

This file should be prepared by the

system administrator to contain the public host keys of all machines in the

organization. This file should be world-readable. This file contains

organization.

This file should be world-readable.

This file contains

public keys, one per line, in the following format (fields separated

by spaces): system name, number of bits in modulus, public exponent,

modulus, and optional comment field. When different names are used

modulus, and optional comment field.

When different names are used

for the same machine, all such names should be listed, separated by

commas. The format is described on the

commas.

The format is described on the

.Xr sshd 8

manual page.

.Pp

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checking the key, because someone with access to the name servers

would then be able to fool host authentication.

.It Pa /etc/ssh_config

Systemwide configuration file. This file provides defaults for those

Systemwide configuration file.

This file provides defaults for those

values that are not specified in the user's configuration file, and

for those users who do not have a configuration file. This file must

for those users who do not have a configuration file.

be world-readable.

This file must be world-readable.

.It Pa $HOME/.rhosts

This file is used in

.Pa \&.rhosts

authentication to list the

host/user pairs that are permitted to log in. (Note that this file is

host/user pairs that are permitted to log in.

(Note that this file is

also used by rlogin and rsh, which makes using this file insecure.)

Each line of the file contains a host name (in the canonical form

returned by name servers), and then a user name on that host,

separated by a space. One some machines this file may need to be

separated by a space.

One some machines this file may need to be

world-readable if the user's home directory is on a NFS partition,

because

.Xr sshd 8

reads it as root. Additionally, this file must be owned by the user,

reads it as root.

and must not have write permissions for anyone else. The recommended

Additionally, this file must be owned by the user,

and must not have write permissions for anyone else.

The recommended

permission for most machines is read/write for the user, and not

accessible by others.

.Pp

Note that by default

.Xr sshd 8

will be installed so that it requires successful RSA host

authentication before permitting \s+2.\s0rhosts authentication. If your

authentication before permitting \s+2.\s0rhosts authentication.

server machine does not have the client's host key in

If your server machine does not have the client's host key in

.Pa /etc/ssh_known_hosts ,

you can store it in

.Pa $HOME/.ssh/known_hosts .

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Line 1016

.Xr rsh 1 .

.It Pa /etc/hosts.equiv

This file is used during

.Pa \&.rhosts authentication. It contains

.Pa \&.rhosts authentication.

It contains

canonical hosts names, one per line (the full format is described on

the

.Xr sshd 8

manual page). If the client host is found in this file, login is

manual page).

If the client host is found in this file, login is

automatically permitted provided client and server user names are the

same. Additionally, successful RSA host authentication is normally

same.

required. This file should only be writable by root.

Additionally, successful RSA host authentication is normally

required.

This file should only be writable by root.

.It Pa /etc/shosts.equiv

This file is processed exactly as

.Pa /etc/hosts.equiv .

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Line 1059

.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

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.

This version of OpenSSH