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Diff for /src/usr.bin/ssh/sshd.8 between version 1.206 and 1.206.2.2

version 1.206, 2005/03/01 14:59:49

version 1.206.2.2, 2006/02/03 02:53:45

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.Ek

.Sh DESCRIPTION

.Nm

(SSH Daemon) is the daemon program for

(OpenSSH 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

over an insecure network.

The programs are intended to be as easy to

install and use as possible.

.Pp

.Nm

is the daemon that listens for connections from clients.

listens for connections from clients.

It is normally started at boot from

.Pa /etc/rc .

It forks a new

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The forked daemons handle

key exchange, encryption, authentication, command execution,

and data exchange.

This implementation of

.Nm

supports both SSH protocol version 1 and 2 simultaneously.

.Nm

works as follows:

.Ss SSH protocol version 1

Each host has a host-specific RSA key

(normally 1024 bits) used to identify the host.

Additionally, when

the daemon starts, it generates a server RSA key (normally 768 bits).

This key is normally regenerated every hour if it has been used, and

is never stored on disk.

.Pp

Whenever a client connects, the daemon responds with its public

host and server keys.

The client compares the

RSA host key against its own database to verify that it has not changed.

The client then generates a 256-bit random number.

It encrypts this

random number using both the host key and the server key, and sends

the encrypted number to the server.

Both sides then use this

random number as a session key which is used to encrypt all further

communications in the session.

The rest of the session is encrypted

using a conventional cipher, currently Blowfish or 3DES, with 3DES

being used by default.

The client selects the encryption algorithm

to use from those offered by the server.

.Pp

Next, the server and the client enter an authentication dialog.

The client tries to authenticate itself using

.Em rhosts

authentication combined with RSA host

authentication, RSA challenge-response authentication, or password

based authentication.

.Pp

System security is not improved unless

.Nm rshd ,

.Nm rlogind ,

and

.Nm rexecd

are disabled (thus completely disabling

.Xr rlogin

and

.Xr rsh

into the machine).

.Ss SSH protocol version 2

Version 2 works similarly:

Each host has a host-specific key (RSA or DSA) 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.

.Pp

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

128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.

The client selects the encryption algorithm

to use from those offered by the server.

Additionally, session integrity is provided

through a cryptographic message authentication code

(hmac-sha1 or hmac-md5).

.Pp

Protocol version 2 provides a public key based

user (PubkeyAuthentication) or

client host (HostbasedAuthentication) authentication method,

conventional password authentication and challenge response based methods.

.Ss Command execution and data forwarding

If the client successfully authenticates itself, a dialog for

preparing the session is entered.

At this time the client may request

things like allocating a pseudo-tty, forwarding X11 connections,

forwarding TCP/IP connections, or forwarding the authentication agent

connection over the secure channel.

.Pp

Finally, the client either requests a shell or execution of a command.

The sides then enter session mode.

In this mode, either side may send

data at any time, and such data is forwarded to/from the shell or

command on the server side, and the user terminal in the client side.

.Pp

When the user program terminates and all forwarded X11 and other

connections have been closed, the server sends command exit status to

the client, and both sides exit.

.Pp

.Nm

can be configured using command-line options or a configuration file

(by default

.Xr sshd_config 5 ) .

.Xr sshd_config 5 ) ;

Command-line options override values specified in the

command-line options override values specified in the

configuration file.

.Pp

.Nm

rereads its configuration file when it receives a hangup signal,

.Dv SIGHUP ,

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Specifies the port on which the server listens for connections

(default 22).

Multiple port options are permitted.

Ports specified in the configuration file are ignored when a

Ports specified in the configuration file with the

command-line port is specified.

.Cm Port

option are ignored when a command-line port is specified.

Ports specified using the

.Cm ListenAddress

option override command-line ports.

.It Fl q

Quiet mode.

Nothing is sent to the system log.

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mechanism or configuration requires it.

Authentication mechanisms that may require DNS include

.Cm RhostsRSAAuthentication ,

.Cm HostbasedAuthentication

.Cm HostbasedAuthentication ,

and using a

.Cm from="pattern-list"

option in a key file.

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.Cm DenyUsers .

.El

.Sh CONFIGURATION FILE

.Sh AUTHENTICATION

.Nm

The OpenSSH SSH daemon supports SSH protocols 1 and 2.

reads configuration data from

Both protocols are supported by default,

.Pa /etc/ssh/sshd_config

though this can be changed via the

(or the file specified with

.Cm Protocol

.Fl f

option in

on the command line).

The file format and configuration options are described in

.Xr sshd_config 5 .

Protocol 2 supports both RSA and DSA keys;

protocol 1 only supports RSA keys.

For both protocols,

each host has a host-specific key,

normally 2048 bits,

used to identify the host.

.Pp

Forward security for protocol 1 is provided through

an additional server key,

normally 768 bits,

generated when the server starts.

This key is normally regenerated every hour if it has been used, and

is never stored on disk.

Whenever a client connects, the daemon responds with its public

host and server keys.

The client compares the

RSA host key against its own database to verify that it has not changed.

The client then generates a 256-bit random number.

It encrypts this

random number using both the host key and the server key, and sends

the encrypted number to the server.

Both sides then use this

random number as a session key which is used to encrypt all further

communications in the session.

The rest of the session is encrypted

using a conventional cipher, currently Blowfish or 3DES, with 3DES

being used by default.

The client selects the encryption algorithm

to use from those offered by the server.

.Pp

For protocol 2,

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

128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.

The client selects the encryption algorithm

to use from those offered by the server.

Additionally, session integrity is provided

through a cryptographic message authentication code

(hmac-sha1 or hmac-md5).

.Pp

Finally, the server and the client enter an authentication dialog.

The client tries to authenticate itself using

host-based authentication,

public key authentication,

challenge-response authentication,

or password authentication.

.Pp

System security is not improved unless

.Nm rshd ,

.Nm rlogind ,

and

.Nm rexecd

are disabled (thus completely disabling

.Xr rlogin

and

.Xr rsh

into the machine).

.Sh COMMAND EXECUTION AND DATA FORWARDING

If the client successfully authenticates itself, a dialog for

preparing the session is entered.

At this time the client may request

things like allocating a pseudo-tty, forwarding X11 connections,

forwarding TCP connections, or forwarding the authentication agent

connection over the secure channel.

.Pp

Finally, the client either requests a shell or execution of a command.

The sides then enter session mode.

In this mode, either side may send

data at any time, and such data is forwarded to/from the shell or

command on the server side, and the user terminal in the client side.

.Pp

When the user program terminates and all forwarded X11 and other

connections have been closed, the server sends command exit status to

the client, and both sides exit.

.Sh LOGIN PROCESS

When a user successfully logs in,

.Nm

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prints last login time and

.Pa /etc/motd

(unless prevented in the configuration file or by

.Pa $HOME/.hushlogin ;

.Pa ~/.hushlogin ;

see the

.Sx FILES

section).

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Sets up basic environment.

.It

Reads the file

.Pa $HOME/.ssh/environment ,

.Pa ~/.ssh/environment ,

if it exists, and users are allowed to change their environment.

See the

.Cm PermitUserEnvironment

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Changes to user's home directory.

.It

.Pa $HOME/.ssh/rc

.Pa ~/.ssh/rc

exists, runs it; else if

.Pa /etc/ssh/sshrc

exists, runs

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Runs user's shell or command.

.El

.Sh AUTHORIZED_KEYS FILE FORMAT

.Pa $HOME/.ssh/authorized_keys

.Pa ~/.ssh/authorized_keys

is the default file that lists the public keys that are

permitted for RSA authentication in protocol version 1

and for public key authentication (PubkeyAuthentication)

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This option might be useful

to restrict certain public keys to perform just a specific operation.

An example might be a key that permits remote backups but nothing else.

Note that the client may specify TCP/IP and/or X11

Note that the client may specify TCP and/or X11

forwarding unless they are explicitly prohibited.

Note that this option applies to shell, command or subsystem execution.

.It Cm environment="NAME=value"

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.Cm UseLogin

is enabled.

.It Cm no-port-forwarding

Forbids TCP/IP forwarding when this key is used for authentication.

Forbids TCP forwarding when this key is used for authentication.

Any port forward requests by the client will return an error.

This might be used, e.g., in connection with the

.Cm command

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options may be applied separated by commas.

No pattern matching is performed on the specified hostnames,

they must be literal domains or addresses.

.It Cm tunnel="n"

Force a

.Xr tun 4

device on the server.

Without this option, the next available device will be used if

the client requests a tunnel.

.El

.Ss Examples

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

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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

.Pp

tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org

.Sh SSH_KNOWN_HOSTS FILE FORMAT

The

.Pa /etc/ssh/ssh_known_hosts

and

.Pa $HOME/.ssh/known_hosts

.Pa ~/.ssh/known_hosts

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

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concurrently for different ports, this contains the process ID of the one

started last).

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

.It Pa $HOME/.ssh/authorized_keys

.It Pa ~/.ssh/authorized_keys

Lists the public keys (RSA or DSA) that can be used to log into the user's account.

This file must be readable by root (which may on some machines imply

it being world-readable if the user's home directory resides on an NFS

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.Pa id_rsa.pub

files into this file, as described in

.Xr ssh-keygen 1 .

.It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"

.It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"

These files are consulted when using rhosts with RSA host

authentication or protocol version 2 hostbased authentication

to check the public key of the host.

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

.Pa /etc/ssh/ssh_known_hosts

should be world-readable, and

.Pa $HOME/.ssh/known_hosts

.Pa ~/.ssh/known_hosts

can, but need not be, world-readable.

.It Pa /etc/motd

See

.Xr motd 5 .

.It Pa $HOME/.hushlogin

.It Pa ~/.hushlogin

This file is used to suppress printing the last login time and

.Pa /etc/motd ,

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Access controls that should be enforced by tcp-wrappers are defined here.

Further details are described in

.Xr hosts_access 5 .

.It Pa $HOME/.rhosts

.It Pa ~/.rhosts

This file is used during

.Cm RhostsRSAAuthentication

and

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Either host or user

name may be of the form +@groupname to specify all hosts or all users

in the group.

.It Pa $HOME/.shosts

.It Pa ~/.shosts

For ssh,

this file is exactly the same as for

.Pa .rhosts .

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.Pa /etc/hosts.equiv .

However, this file may be useful in environments that want to run both

rsh/rlogin and ssh.

.It Pa $HOME/.ssh/environment

.It Pa ~/.ssh/environment

This file is read into the environment at login (if it exists).

It can only contain empty lines, comment lines (that start with

.Ql # ) ,

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controlled via the

.Cm PermitUserEnvironment

option.

.It Pa $HOME/.ssh/rc

.It Pa ~/.ssh/rc

If this file exists, it is run with

.Pa /bin/sh

after reading the

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readable by anyone else.

.It Pa /etc/ssh/sshrc

.Pa $HOME/.ssh/rc .

.Pa ~/.ssh/rc .

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.