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version 1.1, 1999/09/26 20:53:38

version 1.2, 1999/09/26 22:30:06

Line 11

.\"

.\" $Id$

.\"

.TH SSHD 8 "November 8, 1995" "SSH" "SSH"

.Dd September 25, 1999

.Dt SSHD 8

.SH NAME

.Os

sshd \- secure shell daemon

.Sh NAME

.Nm sshd

.SH SYNOPSIS

.Nd secure shell daemon

.na

.Sh SYNOPSIS

.B sshd

.Nm sshd

[\c

.Op Fl diq

.BI \-b \ bits\fR\c

.Op Fl b Ar bits

]

.Op Fl f Ar config_file

[\c

.Op Fl g Ar login_grace_time

.B \-d \c

.Op Fl h Ar host_key_file

]

.Op Fl k Ar key_gen_time

[\c

.Op Fl p Ar port

.BI \-f \ config_file\fR\c

.Sh DESCRIPTION

]

.Nm

[\c

.BI \-g \ login_grace_time\fR\c

]

[\c

.BI \-h \ host_key_file\fR\c

]

[\c

.B \-i \c

]

[\c

.BI \-k \ key_gen_time\fR\c

]

[\c

.BI \-p \ port\fR\c

]

[\c

.B \-q \c

]

.ad

.SH DESCRIPTION

.LP

.B Sshd

(Secure Shell Daemon) is the daemon program for

.BR ssh ".

.Xr ssh 1 .

Together these programs replace rlogin and rsh programs, 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.

.LP

.Pp

.B Sshd

.Nm

is the daemon that listens for connections from clients. It is

normally started at boot from

.I /etc/rc.local

.Pa /etc/rc .

or equivalent. It forks a new

It forks a new

daemon for each incoming connection. The forked daemons handle

key exchange, encryption, authentication, command execution,

and data exchange.

.LP

.Pp

Sshd works as follows. Each host has a host-specific RSA key

.Nm

works as follows. 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.

.LP

.Pp

Whenever a client connects the daemon, the daemon sends its host

and server public keys to the client. The client compares the

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

Line 83

Line 60

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,

.BR \s-1Blowfish\s0 ",

Blowfish, IDEA, DES, and 3DES.

.BR \s-1IDEA\s0 ",

3DES

.BR \s-1DES\s0 ",

.BR \s-1\&3DES\s0 ",

.B \s-13DES\s0

is used by default. The client selects the encryption algorithm to use

from those offered by the server.

.LP

.Pp

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

client tries to authenticate itself using \|\s+2.\s0rhosts

client tries to authenticate itself using

authentication, \|\s+2.\s0rhosts authentication combined with RSA host

.Pa .rhosts

authentication,

.Pa .rhosts

authentication combined with RSA host

authentication, RSA challenge-response authentication, or password

based authentication.

.LP

.Pp

Rhosts authentication is normally disabled

because it is fundamentally insecure, but can be enabled in the server

configuration file if desired. System security is not improved unless

.BR rshd "(8),

.Xr rshd 8 ,

.BR rlogind "(8),

.Xr rlogind 8 ,

.BR rexecd "(8), and

.Xr rexecd 8 ,

.B rexd "(8)

and

.Xr rexd 8

are disabled (thus completely disabling

.BR rlogin (1)

.Xr rlogin 1

and

.BR rsh (1)

.Xr rsh 1

into that machine).

.LP

.Pp

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.

.LP

.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.

.LP

.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.

.LP

.Pp

.B Sshd

.Nm

can be configured using command-line options or a configuration

file. Command-line options override values specified in the

configuration file.

.Sh OPTIONS

.Bl -tag -width Ds

.SH OPTIONS

.It Fl b Ar bits

.TP

.BI \-b \ bits

Specifies the number of bits in the server key (default 768).

.TP

.Pp

.B \-d

.It Fl d

Debug mode. The server sends verbose debug output to the system

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.

.TP

.It Fl f Ar configuration_file

.BI \-f \ configuration_file

Specifies the name of the configuration file. The default is

.IR /etc/sshd_config ".

.Pa /etc/sshd_config .

.TP

.It Fl g Ar login_grace_time

.BI \-g \ login_grace_time

Gives the grace time for clients to authenticate themselves (default

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.

.TP

.It Fl h Ar host_key_file

.BI \-h \ host_key_file

Specifies the file from which the host key is read (default

.IR /etc/ssh_host_key).

.Pa /etc/ssh_host_key ) .

This option must be given if sshd is not run as root (as the normal

host file is normally not readable by anyone but root).

.TP

.It Fl i

.B \-i

Specifies that sshd is being run from inetd. Sshd is normally not run

from inetd because it needs to generate the server key before it can

respond to the client, and this may take tens of seconds. Clients

would have to wait too long if the key was regenerated every time.

However, with small key sizes (e.g. 512) using sshd from inetd may

be feasible.

.TP

.It Fl k Ar key_gen_time

.BI \-k \ key_gen_time

Specifies how often the server key is regenerated (default 3600

seconds, or one hour). The motivation for regenerating the key fairly

often is that the key is not stored anywhere, and after about an hour,

it becomes impossible to recover the key for decrypting intercepted

communications even if the machine is cracked into or physically

seized. A value of zero indicates that the key will never be regenerated.

.TP

.It Fl p Ar port

.BI \-p \ port

Specifies the port on which the server listens for connections

(default 22).

.TP

.It Fl q

.B \-q

Quiet mode. Nothing is sent to the system log. Normally the beginning,

authentication, and termination of each connection is logged.

.El

.SH CONFIGURATION FILE

.Sh CONFIGURATION FILE

.Nm

.B Sshd

reads configuration data from

.I /etc/sshd_config

.Pa /etc/sshd_config

(or the file specified with -f on the command line). The file

(or the file specified with

contains keyword-value pairs, one per line. Lines starting with '#'

.Fl f

on the command line). The file

contains keyword-value pairs, one per line. Lines starting with

.Ql #

and empty lines are interpreted as comments.

.Pp

The following keywords are possible.

.TP

.Bl -tag -width Ds

.B AFSTokenPassing

.It Cm AFSTokenPassing

Specifies whether to accept AFS tokens passed from the client. Default

is "yes".

.TP

.Dq yes .

.B AllowHosts

.It Cm AllowHosts

This keyword can be followed by any number of host name patterns,

separated by spaces. If specified, login is allowed only from hosts

whose name matches one of the patterns. '*' and '?' can be used as

whose name matches one of the patterns.

.Ql \&*

and

.Ql ?

can be used as

wildcards in the patterns. Normal name servers are used to map the

client's host into a canonical host name. If the name cannot be

mapped, its IP-address is used as the host name. By default all hosts

are allowed to connect.

.Pp

Note that

.B sshd

.Nm

can also be configured to use tcp_wrappers using the --with-libwrap

can also be configured to use tcp_wrappers using the

.Fl -with-libwrap

compile-time configuration option.

.TP

.It Cm DenyHosts

.B DenyHosts

This keyword can be followed by any number of host name patterns,

separated by spaces. If specified, login is disallowed from the hosts

whose name matches any of the patterns.

.TP

.It Cm FascistLogging

.B FascistLogging

Specifies whether to use verbose logging. Verbose logging violates

the privacy of users and is not recommended. The argument must be

"yes" or "no" (without the quotes). The default is "no".

.Dq yes

.TP

.B HostKey

.Dq no .

The default is

.Dq no .

.It Cm HostKey

Specifies the file containing the private host key (default

.IR /etc/ssh_host_key ").

.Pa /etc/ssh_host_key ) .

.TP

.It Cm IgnoreRhosts

.B IgnoreRhosts

Specifies that rhosts and shosts files will not be used in

authentication.

.I /etc/hosts.equiv

.Pa /etc/hosts.equiv

and

.I /etc/shosts.equiv

.Pa /etc/shosts.equiv

are still used. The default is "no".

are still used. The default is

.TP

.Dq no .

.B KeepAlive

.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

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. On the other hand, if keepalives are not send,

sessions may hang indefinitely on the server, leaving "ghost" users

sessions may hang indefinitely on the server, leaving

and consuming server resources.

.Dq ghost

users and consuming server resources.

The default is "yes" (to send keepalives), and the server will notice

.Pp

The default is

.Dq yes

(to send keepalives), and the server will notice

if the network goes down or the client host reboots. This avoids

infinitely hanging sessions.

.Pp

To disable keepalives, the value should be set to "no" in both the

To disable keepalives, the value should be set to

server and the client configuration files.

.Dq no

.TP

in both the server and the client configuration files.

.B KerberosAuthentication

.It Cm KerberosAuthentication

Specifies whether Kerberos authentication is allowed. This can

be in the form of a Kerberos ticket, or if PasswordAuthentication

is yes, the password provided by the user will be validated through

the Kerberos KDC / AFS kaserver / DCE Security Server. Default is yes.

.TP

.It Cm KerberosOrLocalPasswd

.B KerberosOrLocalPasswd

If set then if password authentication through Kerberos fails then

the password will be validated via any additional local mechanism

such as /etc/passwd or SecurID. Default is no.

such as

.TP

.Pa /etc/passwd

.B KerberosTgtPassing

or SecurID. Default is

.Dq no .

.It Cm KerberosTgtPassing

Specifies whether a Kerberos TGT may be forwarded to the server.

Default is no, TGT forwarding does only work with the AFS kaserver.

.TP

.It Cm KerberosTicketCleanup

.B KerberosTicketCleanup

Specifies whether to automatically destroy the user's

ticket cache file on logout. Default is yes.

.TP

.It Cm KeyRegenerationInterval

.B KeyRegenerationInterval

The server key is automatically regenerated after this many seconds

(if it has been used). The purpose of regeneration is to prevent

decrypting captured sessions by later breaking into the machine and

stealing the keys. The key is never stored anywhere. If the value is

0, the key is never regenerated. The default is 3600

(seconds).

.TP

.It Cm LoginGraceTime

.B LoginGraceTime

The server disconnects after this time if the user has not

successfully logged in. If the value is 0, there is no time limit.

The default is 600 (seconds).

.TP

.It Cm PasswordAuthentication

.B PasswordAuthentication

Specifies whether password authentication is allowed.

The default is "yes".

The default is

.TP

.Dq yes .

.B PermitEmptyPasswords

.It Cm PermitEmptyPasswords

When password authentication is allowed, it specifies whether the

server allows login to accounts with empty password strings. The default

is "yes".

.TP

.Dq yes .

.B PermitRootLogin

.It Cm PermitRootLogin

Specifies whether the root can log in using

.BR ssh .

.Xr ssh 1 .

The default is "yes".

The default is

.Dq yes .

Root login with RSA authentication when the "command" option has been

.Pp

Root login with RSA authentication when the

.Ar command

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).

.TP

.It Cm Port

.B Port

Specifies the port number that

.B sshd

.Nm

listens on. The default is 22.

.TP

.It Cm PrintMotd

.B PrintMotd

Specifies whether

.B sshd

.Nm

should print

.I /etc/motd

.Pa /etc/motd

when a user logs in interactively. (On some systems it is also

printed by the shell, /etc/profile, or equivalent.) The default is

printed by the shell,

"yes".

.Pa /etc/profile ,

.TP

or equivalent.) The default is

.B QuietMode

.Dq yes .

.It Cm QuietMode

Specifies whether the system runs in quiet mode. In quiet mode,

nothing is logged in the system log, except fatal errors. The default

is "no".

.TP

.Dq no .

.B RandomSeed

.It Cm RandomSeed

Specifies the file containing the random seed for the server; this

file is created automatically and updated regularly. The default is

.IR /etc/ssh_random_seed ".

.Pa /etc/ssh_random_seed .

.TP

.It Cm RhostsAuthentication

.B RhostsAuthentication

Specifies whether authentication using rhosts or /etc/hosts.equiv

files is sufficient. Normally, this method should not be permitted

because it is insecure. RhostsRSAAuthentication should be used

instead, because it performs RSA-based host authentication in addition

to normal rhosts or /etc/hosts.equiv authentication.

The default is "no".

The default is

.TP

.Dq no .

.B RhostsRSAAuthentication

.It Cm RhostsRSAAuthentication

Specifies whether rhosts or /etc/hosts.equiv authentication together

with successful RSA host authentication is allowed. The default is "yes".

with successful RSA host authentication is allowed. The default is

.TP

.Dq yes .

.B RSAAuthentication

.It Cm RSAAuthentication

Specifies whether pure RSA authentication is allowed. The default is "yes".

Specifies whether pure RSA authentication is allowed. The default is

.TP

.Dq yes .

.B ServerKeyBits

.It Cm ServerKeyBits

Defines the number of bits in the server key. The minimum value is

512, and the default is 768.

.TP

.It Cm StrictModes

.B StrictModes

Specifies whether SSH should check file modes and ownership of the

Specifies whether ssh should check file modes and ownership of the

user's home directory and rhosts files before accepting login. This

is normally desirable because novices sometimes accidentally leave their

directory or files world-writable. The default is "yes".

.TP

.It Cm SyslogFacility

.B SyslogFacility

Gives the facility code that is used when logging messages from

.B sshd.

.Nm sshd .

The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,

LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.

.TP

.It Cm X11Forwarding

.B X11Forwarding

Specifies whether X11 forwarding is permitted. The default is

Specifies whether X11 forwarding is permitted. The default is "yes".

.Dq yes .

Note that disabling X11 forwarding does not improve security in any

way, as users can always install their own forwarders.

.El

.SH LOGIN PROCESS

.Sh LOGIN PROCESS

When a user successfully logs in,

.B sshd

.Nm

does the following:

.IP 1.

.Bl -enum -offset indent

.It

If the login is on a tty, and no command has been specified,

prints last login time and

.B /etc/motd

.Pa /etc/motd

(unless prevented in the configuration file or by

.IR $HOME/\s+2.\s0hushlogin ;

.Pa $HOME/.hushlogin ;

see the FILES section).

see the

.IP 2.

.Sx FILES

section).

.It

If the login is on a tty, records login time.

.IP 3.

.It

Checks /etc/nologin; if it exists, prints contents and quits

Checks

.Pa /etc/nologin ;

if it exists, prints contents and quits

(unless root).

.IP 4.

.It

Changes to run with normal user privileges.

.IP 5.

.It

Sets up basic environment.

.IP 6.

.It

Reads /etc/environment if it exists.

Reads

.IP 7.

.Pa /etc/environment

Reads $HOME/.ssh/environment if it exists.

if it exists.

.IP 8.

.It

Reads

.Pa $HOME/.ssh/environment

if it exists.

.It

Changes to user's home directory.

.IP 9.

.It

If $HOME/.ssh/rc exists, runs it; else if /etc/sshrc exists, runs

it; otherwise runs xauth. The "rc" files are given the X11

.Pa $HOME/.ssh/rc

exists, runs it; else if

.Pa /etc/sshrc

exists, runs

it; otherwise runs xauth. The

.Dq rc

files are given the X11

authentication protocol and cookie in standard input.

.IP 10.

.It

Runs user's shell or command.

.RT

.El

.Sh AUTHORIZED_KEYS FILE FORMAT

.SH AUTHORIZED_KEYS FILE FORMAT

.LP

The

.I \&$HOME/\s+2.\s0ssh/authorized_keys

.Pa $HOME/.ssh/authorized_keys

file lists the RSA keys that are

permitted for RSA authentication. Each line of the file contains one

key (empty lines and lines starting with a '#' are ignored as

key (empty lines and lines starting with a

.Ql #

are ignored as

comments). Each line consists of the following fields, separated by

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

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

Line 410

Line 398

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

comment field is not used for anything (but may be convenient for the

user to identify the key).

.LP

.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

.I identity.pub

.Pa identity.pub

file and edit it.

.LP

.Pp

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

specifications. No spaces are permitted, except within double quotes.

The following option specifications are supported:

.IP

.Bl -tag -width Ds

.ti -.5i

.It Cm from="pattern-list"

\fBfrom="pattern-list" \fR

.br

Specifies that in addition to RSA authentication, the canonical name

of the remote host must be present in the comma-separated list of

patterns ('*' and '?' serve as wildcards). The list may also contain

Line 436

Line 422

additional option makes using a stolen key more difficult (name

servers and/or routers would have to be compromised in addition to

just the key).

.IP

.It Cm command="command"

.ti -.5i

\fBcommand="command"\fR

.br

Specifies that the command is executed whenever this key is used for

authentication. The command supplied by the user (if any) is ignored.

The command is run on a pty if the connection requests a pty;

Line 449

Line 432

example might be a key that permits remote backups but nothing

else. Notice that the client may specify TCP/IP and/or X11

forwardings unless they are explicitly prohibited.

.IP

.It Cm environment="NAME=value"

.ti -.5i

\fBenvironment="NAME=value"\fR

.br

Specifies that the string is to be added to the environment when

logging in using this key. Environment variables set this way

override other default environment values. Multiple options of this

type are permitted.

.TP

.It Cm no-port-forwarding

.B no-port-forwarding

Forbids TCP/IP 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

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

.B command

.Cm command

option.

.TP

.It Cm no-X11-forwarding

.B no-X11-forwarding

Forbids X11 forwarding when this key is used for authentication.

Any X11 forward requests by the client will return an error.

.TP

.It Cm no-agent-forwarding

.B no-agent-forwarding

Forbids authentication agent forwarding when this key is used for

authentication.

.TP

.It Cm no-pty

.B no-pty

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

.El

.SS Examples

.Ss Examples

.LP

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

.LP

.Pp

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

.LP

.Pp

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

.Sh SSH_KNOWN_HOSTS FILE FORMAT

.SH SSH_KNOWN_HOSTS FILE FORMAT

.LP

The

.I /etc/ssh_known_hosts

.Pa /etc/ssh_known_hosts

and

.I \&$HOME/\s+2.\s0ssh/known_hosts

.Pa $HOME/.ssh/known_hosts

files contain host public keys for all known hosts. The global file should

be prepared by the admistrator (optional), and the per-user file is

maintained automatically: whenever the user connects an unknown host

its key is added to the per-user file. The recommended way to create

.I /etc/ssh_known_hosts

.Pa /etc/ssh_known_hosts

is to use the

.B make-ssh-known-hosts

.Xr make-ssh-known-hosts 1

command.

.LP

.Pp

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

bits, exponent, modulus, comment. The fields are separated by spaces.

.LP

.Pp

Hostnames is a comma-separated list of patterns ('*' and '?' act as

wildcards); each pattern in turn is matched against the canonical host

name (when authenticating a client) or against the user-supplied

name (when authenticating a server). A pattern may also be preceded

by '!' to indicate negation: if the host name matches a negated

.Ql !

to indicate negation: if the host name matches a negated

pattern, it is not accepted (by that line) even if it matched another

pattern on the line.

.LP

.Pp

Bits, exponent, and modulus are taken directly from the host key; they

can be obtained e.g. from

can be obtained, e.g., from

.IR /etc/ssh_host_key.pub ".

.Pa /etc/ssh_host_key.pub .

The optional comment field continues to the end of the line, and is not used.

.LP

.Pp

Lines starting with '#' and empty lines are ignored as comments.

Lines starting with

.LP

.Ql #

and empty lines are ignored as comments.

.Pp

When performing host authentication, authentication is accepted if any

matching line has the proper key. It is thus permissible (but not

recommended) to have several lines or different host keys for the same

Line 526

Line 501

from different domains are put in the file. It is possible

that the files contain conflicting information; authentication is

accepted if valid information can be found from either file.

.LP

.Pp

Note that the lines in these files are typically hundreds of characters

long, and you definitely don't want to type in the host keys by hand.

Rather, generate them by a script (see

.BR make-ssh-known-hosts (1))

.Xr make-ssh-known-hosts 1 )

or by taking

.I /etc/ssh_host_key.pub

.Pa /etc/ssh_host_key.pub

and adding the host names at the front.

.Ss Examples

.SS Examples

closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi

.Sh FILES

.SH FILES

.Bl -tag -width Ds

.TP

.It Pa /etc/sshd_config

.I /etc/sshd_config

Contains configuration data for

.BR sshd .

.Nm sshd .

This file should be writable by root only, but it is recommended

(though not necessary) that it be world-readable.

.TP

.It Pa /etc/ssh_host_key

.I /etc/ssh_host_key

Contains the private part of the host key. This file is normally

created automatically by "make install", but can also be created

manually using

.BR ssh-keygen (1).

.Xr ssh-keygen 1 .

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

accessible to others.

.TP

.It Pa /etc/ssh_host_key.pub

.I /etc/ssh_host_key.pub

Contains the public part of the host key. This file is normally

created automatically by "make install", but can also be created

manually. This file should be world-readable but writable only by

root. Its contents should match the private part. This file is not

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

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

.TP

.It Pa /etc/ssh_random_seed

.I /etc/ssh_random_seed

This file contains a seed for the random number generator. This file

should only be accessible by root.

.TP

.It Pa /var/run/sshd.pid

.I /var/run/sshd.pid

Contains the process ID of the

Contains the process id of the

.Nm

.B sshd

listening for connections (if there are several daemons running

concurrently for different ports, this contains the pid of the one

started last). The contents of this file are not sensitive; it can be

world-readable.

.TP

.It Pa $HOME/.ssh/authorized_keys

.I \&$HOME/\s+2.\s0ssh/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

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

volume). It is recommended that it not be accessible by others. The

format of this file is described above.

.TP

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

.I "/etc/ssh_known_hosts\fR and \fI$HOME/\s+2.\s0ssh/known_hosts\fR

These files are consulted when using rhosts with RSA host

authentication to check the public key of the host. The key must be

listed in one of these files to be accepted. (The client uses the

same files to verify that the remote host is the one we intended to

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

.I /etc/ssh_known_hosts

.Pa /etc/ssh_known_hosts

should be world-readable, and \fI$HOME/\s+2.\s0ssh/known_hosts\fR can

should be world-readable, and

but need not be world-readable.

.Pa $HOME/.ssh/known_hosts

.TP

can but need not be world-readable.

.I /etc/nologin

.Pa /etc/nologin

If this file exists,

.B sshd

.Nm

refuses to let anyone except root log in. The contents of the file

are displayed to anyone trying to log in, and non-root connections are

refused. The file should be world-readable.

.TP

.Pa $HOME/.rhosts

.I \&$HOME/\s+2.\s0rhosts

This file contains host-username pairs, separated by a space, one per

line. The given user on the corresponding host is permitted to log in

without password. The same file is used by rlogind and rshd.

.B Ssh

Ssh differs from rlogind

differs from rlogind

and rshd in that it requires RSA host authentication in addition to

validating the host name retrieved from domain name servers (unless

compiled with the \-\-with\-rhosts configuration option). The file must

compiled with the

.Fl -with-rhosts

configuration option). The file must

be writable only by the user; it is recommended that it not be

accessible by others.

.Pp

If is also possible to use netgroups in the file. Either host or user

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

in the group.

.TP

.It Pa $HOME/.shosts

.I \&$HOME/\s+2.\s0shosts

For ssh,

For

this file is exactly the same as for

.B ssh,

.Pa .rhosts .

this file is exactly the same as for \s+2.\s0rhosts. However, this file is

However, this file is

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

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

.B ssh

.Pa /etc/hosts.equiv

only.

This file is used during

.TP

.Pa .rhosts

.I /etc/hosts.equiv

authentication. In the

This file is used during \s+2.\s0rhosts authentication. In the

simplest form, this file contains host names, one per line. Users on

those hosts are permitted to log in without a password, provided they

have the same user name on both machines. The host name may also be

followed by a user name; such users are permitted to log in as

.B any

.Em any

user on this machine (except root). Additionally, the syntax +@group

user on this machine (except root). Additionally, the syntax

can be used to specify netgroups. Negated entries start with '-'.

.Dq +@group

can be used to specify netgroups. Negated entries start with

.Ql \&- .

.Pp

If the client host/user is successfully matched in this file, login is

automatically permitted provided the client and server user names are the

same. Additionally, successful RSA host authentication is normally

required. This file must be writable only by root; it is recommended

that it be world-readable.

.Pp

\fBWarning: It is almost never a good idea to use user names in

.Sy Warning: It is almost never a good idea to use user names in

hosts.equiv.\fR

.Pa hosts.equiv .

Beware that it really means that the named user(s) can log in as

\fBanybody\fR,

.Em anybody ,

which includes bin, daemon, adm, and other accounts that own critical

binaries and directories. Using a user name practically grants the

user root access. The only valid use for user names that I can think

of is in negative entries.

\fBNote that this warning also applies to rsh/rlogin.\fR

.Pp

.TP

Note that this warning also applies to rsh/rlogin.

.I /etc/shosts.equiv

.It Pa /etc/shosts.equiv

This is processed exactly as

.I /etc/hosts.equiv.

.Pa /etc/hosts.equiv .

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

rsh/rlogin and

rsh/rlogin and ssh.

.B ssh.

.It Pa /etc/environment

.TP

.I /etc/environment

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

can only contain empty lines, comment lines (that start with '#'), and

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

assignment lines of the form name=value. This file is processed in

.Ql # ) ,

and assignment lines of the form name=value. This file is processed in

all environments (normal rsh/rlogin only process it on AIX and

potentially some other systems). The file should be writable only by

root, and should be world-readable.

.TP

.It Pa $HOME/.ssh/environment

.I \&$HOME/\s+2.\s0ssh/environment

This file is read into the environment after /etc/environment. It has

the same format. The file should be writable only by the user; it

need not be readable by anyone else.

.TP

.It Pa $HOME/.ssh/rc

.I \&$HOME/\s+2.\s0ssh/rc

If this file exists, it is run with /bin/sh after reading the

environment files but before starting the user's shell or command. If

X11 spoofing is in use, this will receive the "proto cookie" pair in

standard input (and DISPLAY in environment). This must call xauth in

standard input (and

that case.

.Ev DISPLAY

in environment). This must call

.Xr xauth 1

in that case.

.Pp

The primary purpose of this file is to run any initialization routines

which may be needed before the user's home directory becomes

accessible; AFS is a particular example of such an environment.

.Pp

This file will probably contain some initialization code followed by

something similar to: "if read proto cookie; then echo add $DISPLAY

$proto $cookie | xauth -q -; fi".

.Pp

If this file does not exist, /etc/sshrc is run, and if that

If this file does not exist,

.Pa /etc/sshrc

is run, and if that

does not exist either, xauth is used to store the cookie.

.Pp

This file should be writable only by the user, and need not be

readable by anyone else.

.TP

.It Pa /etc/sshrc

.I /etc/sshrc

Like $HOME/\s+2.\s0ssh/rc. This can be used to specify

.Pa $HOME/.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.

.Sh INSTALLATION

.Nm

.SH INSTALLATION

.LP

.B Sshd

is normally run as root. If it is not run as root, it can

only log in as the user it is running as, and password authentication

may not work if the system uses shadow passwords. An alternative

host key file must also be used.

.LP

.Pp

.B Sshd

.Nm

is normally started from

.I /etc/rc.local

.Pa /etc/rc

or equivalent at system boot.

.LP

.Pp

Considerable work has been put to making

.B sshd

.Nm

secure. However, if you find a security problem, please report it

immediately to <ssh-bugs@cs.hut.fi>.

.Sh AUTHOR

.SH AUTHOR

.LP

Tatu Ylonen <ylo@cs.hut.fi>

.LP

.Pp

Information about new releases, mailing lists, and other related

issues can be found from the ssh WWW home page at

issues can be found from the SSH WWW home page:

http://www.cs.hut.fi/ssh.

.Pp

.Dl http://www.cs.hut.fi/ssh.

.SH SEE ALSO

.Sh SEE ALSO

.LP

.Xr make-ssh-known-hosts 1 ,

.BR ssh (1),

.Xr rlogin 1 ,

.BR make-ssh-known-hosts (1),

.Xr rsh 1 ,

.BR ssh-keygen (1),

.Xr scp 1 ,

.BR ssh-agent (1),

.Xr ssh 1 ,

.BR ssh-add (1),

.Xr ssh-add x 1 ,

.BR scp (1),

.Xr ssh-agent 1 ,

.BR rlogin (1),

.Xr ssh-keygen 1

.BR rsh (1)