SSH(1) SSH SSH(1)NAME
ssh - secure shell client (remote login program)
SYNOPSIS
ssh [-l login_name] hostname [command]
ssh [-a] [-c idea|blowfish|des|3des|arcfour|none] [-e escape_char]
[-i identity_file] [-l login_name] [-n] [-k] [-V] [-o option] [-p port]
[-q] [-P] [-t] [-v] [-x] [-C] [-g] [-L port:host:hostport]
[-R port:host:hostport] hostname [command]
DESCRIPTION
Ssh (Secure Shell) a program for logging into a remote machine and for
executing commands in 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 arbi‐
trary TCP/IP ports can also be forwarded over the secure channel.
Ssh connects and logs into the specified hostname. The user must prove
his/her identity to the remote machine using one of several methods.
First, if the machine the user logs in from is listed in
/etc/hosts.equiv or /etc/shosts.equiv on the remote machine, and the
user names are the same on both sides, the user is immediately permit‐
ted to log in. Second, if .rhosts or .shosts 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 nor‐
mally not allowed by the server because it is not secure.
The second (and primary) authentication method is the rhosts or
hosts.equiv method combined with RSA-based host authentication. It
means that if the login would be permitted by .rhosts, .shosts,
/etc/hosts.equiv, or /etc/shosts.equiv, and additionally it can verify
the client's host key (see $HOME/.ssh/known_hosts and
/etc/ssh_known_hosts in the FILES section), only then login is permit‐
ted. This authentication method closes security holes due to IP spoof‐
ing, DNS spoofing and routing spoofing. [Note to the administrator:
/etc/hosts.equiv, .rhosts, and the rlogin/rsh protocol in general, are
inherently insecure and should be disabled if security is desired.]
As a third authentication method, ssh supports RSA based authentica‐
tion. The scheme is based on public-key cryptography: there are cryp‐
tosystems 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 key pair for authentication purposes. The server knows
the public key, and only the user knows the private key. The file
$HOME/.ssh/authorized_keys lists the public keys that are permitted for
logging in. When the user logs in, the ssh program tells the server
which key pair it would like to use for authentication. The server
checks if this key is permitted, and if so, sends the user (actually
the ssh program running on behalf of the user) a challenge, a random
number, encrypted by the user's public key. 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.
Ssh implements the RSA authentication protocol automatically. The user
creates his/her RSA key pair by running ssh-keygen(1). This stores the
private key in .ssh/identity and the public key in .ssh/identity.pub in
the user's home directory. The user should then copy the identity.pub
to .ssh/authorized_keys in his/her home directory on the remote machine
(the authorized_keys file corresponds to the conventional .rhosts file,
and has one key per line, though the lines can be very long). After
this, the user can log in without giving the password. RSA authentica‐
tion is much more secure than rhosts authentication.
The most convenient way to use RSA authentication may be with an
authentication agent. See ssh-agent(1) for more information.
As a fourth authentication method, ssh supports authentication through
TIS authentication server. The idea is that ssh asks TIS authsrv(8) to
authenticate the user. Sometime, usernames in the TIS database cannot
be the same as the local users. This can be the case if the user
authenticates itself with a smartcard or a Digipass. In that case, the
username in the database is usually known as the serial number of the
authentification device. The file /etc/sshd_tis.map contains the map‐
ping between local users and their corresponding name in the TIS data‐
base. If the file does not exist or the user is not found, the corre‐
sponding name in the TIS database is supposed to be the same.
If other authentication methods fail, ssh prompts the user for a pass‐
word. 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.
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 remote command or shell will be automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the
user can disconnect with "~.", and suspend ssh with "~^Z". All for‐
warded connections can be listed with "~#", and if the session blocks
waiting for forwarded X11 or TCP/IP connections to terminate, it can be
backgrounded with "~&" (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 "~?".
A single tilde character can be sent as "~~" (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 or on the command line.
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 ``none'' will also make the session transparent
even if a tty is used.
The session terminates when the command or shell in on the remote
machine exists and all X11 and TCP/IP connections have been closed.
The exit status of the remote program is returned as the exit status of
ssh.
If the user is using X11 (the DISPLAY environment variable is set), the
connection to the X11 display is 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 DISPLAY. Forwarding of X11 connections can be config‐
ured on the command line or in configuration files.
The DISPLAY value set by ssh will point to the server machine, but with
a display number greater than zero. This is normal, and happens
because ssh creates a "proxy" X server on the server machine for for‐
warding the connections over the encrypted channel.
Ssh will also automatically set up Xauthority data on the server
machine. For this purpose, it will generate a random authorization
cookie, store it in Xauthority on the server, and verify that any for‐
warded connections carry this cookie and replace it by the real cookie
when the connection is opened. The real authentication cookie is never
sent to the server machine (and no cookies are sent in the plain).
If the user is using an authentication agent, the connection to the
agent is automatically forwarded to the remote side unless disabled on
command line or in a configuration file.
Forwarding of arbitrary TCP/IP connections over the secure channel can
be specified either on command line or in a configuration file. One
possible application of TCP/IP forwarding is a secure connection to an
electronic purse; another is going trough firewalls.
Ssh automatically maintains and checks a database containing RSA-based
identifications for all hosts it has ever been used with. The database
is stored in .ssh/known_hosts in the user's home directory. Addition‐
ally, the file /etc/ssh_known_hosts is automatically checked for known
hosts. Any new hosts are automatically added to the user's file. If a
host's identification ever changes, ssh warns about this and disables
password authentication to prevent a 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 StrictHostKeyChecking option (see below) can be used
to prevent logins to machines whose host key is not known or has
changed.
OPTIONS-a Disables forwarding of the authentication agent connection.
This may also be specified on a per-host basis in the configura‐
tion file.
-c idea|des|3des|blowfish|arcfour|none
Selects the cipher to use for encrypting the session. idea is
used by default. It is believed to be secure. des is the data
encryption standard, but is breakable by governments, large cor‐
porations, and major criminal organizations. 3des (triple-des)
is encrypt-decrypt-encrypt triple with three different keys. It
is presumably more secure than DES. It is used as default if
both sites do not support IDEA. blowfish is an encryption algo‐
rithm invented by Bruce Schneier. It uses 128 bit keys. arc‐
four is an algorithm published in the Usenet News in 1995. This
algorithm is believed to be equivalent with the RC4 cipher from
RSA Data Security (RC4 is a trademark of RSA Data Security).
This is the fastest algorithm currently supported. none dis‐
ables encryption entirely; it is only intended for debugging,
and it renders the connection insecure.
-e ch|^ch|none
Sets the escape character for sessions with a pty (default: ~).
The escape character is only recognized at the beginning of a
line. The escape character followed by a dot (.) closes the
connection, followed by control-Z suspends the connection, and
followed by itself sends the escape character once. Setting the
character to ´none´ disables any escapes and makes the session
fully transparent.
-f Requests ssh to go to background after authentication is done
and forwardings have been established. This is useful if ssh is
going to ask for passwords or passphrases, but the user wants it
in the background. This may also be useful in scripts. This
implies -n. The recommended way to start X11 programs at a
remote site is with something like "ssh -f host xterm".
-i identity_file
Selects the file from which the identity (private key) for RSA
authentication is read. Default is .ssh/identity in the user's
home directory. Identity files may also be specified on a per-
host basis in the configuration file. It is possible to have
multiple -i options (and multiple identities specified in con‐
figuration files).
-k Disables forwarding of the kerberos tickets. This may also be
specified on a per-host basis in the configuration file.
-l login_name
Specifies the user to log in as on the remote machine. This may
also be specified on a per-host basis in the configuration file.
-n Redirects stdin from /dev/null (actually, prevents reading from
stdin). This must be used when ssh is run in the background. A
common trick is to use this to run X11 programs in a remote
machine. For example, "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. The ssh
program will be put in the background. (This does not work if
ssh needs to ask for a password or passphrase; see also the -f
option.)
-o '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 configuration file.
-p port
Port to connect to on the remote host. This can be specified on
a per-host basis in the configuration file.
-q Quiet mode. Causes all warning and diagnostic messages to be
suppressed. Only fatal errors are displayed.
-P Use non privileged port. With this you cannot use rhosts or
rsarhosts authentications, but it can be used to bypass some
firewalls that dont allow privileged source ports to pass.
-t Force pseudo-tty allocation. This can be used to execute
arbitary screen-based programs on a remote machine, which can be
very useful e.g. when implementing menu services.
-v Verbose mode. Causes ssh to print debugging messages about its
progress. This is helpful in debugging connection, authentica‐
tion, and configuration problems.
-V Print only version number and exit.
-g Allows remote hosts to connect local port forwarding ports. The
default is that only localhost may connect to locally binded
ports.
-x Disables X11 forwarding. This can also be specified on a per-
host basis in a configuration file.
-C Requests compression of all data (including stdin, stdout,
stderr, and data for forwarded X11 and TCP/IP connections). The
compression algorithm is the same used by gzip, and the "level"
can be controlled by the CompressionLevel option (see below).
Compression is desirable on modem lines and other slow connec‐
tions, but will only slow down things on fast networks. The
default value can be set on a host-by-host basis in the configu‐
ration files; see the Compress option below.
-L 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 by allocating a socket to listen to port on the local
side, and whenever a connection is made to this port, the con‐
nection is forwarded over the secure channel, and a connection
is made to host:hostport from the remote machine. Port forward‐
ings can also be specified in the configuration file. Only root
can forward privileged ports.
-R 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 by allocating a socket to listen to port on the remote
side, and whenever a connection is made to this port, the con‐
nection is forwarded over the secure channel, and a connection
is made to host:hostport from the local machine. Port forward‐
ings can also be specified in the configuration file. Privi‐
leged ports can be forwarded only when logging in as root on the
remote machine.
CONFIGURATION FILES
Ssh obtains configuration data from the following sources (in this
order): command line options, user's configuration file
($HOME/.ssh/config), and system-wide configuration file (/etc/ssh_con‐
fig). For each parameter, the first obtained value will be used. The
configuration files contain sections bracketed by "Host" specifica‐
tions, 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.
Since the first obtained value for each parameter is used, more host-
specific declarations should be given near the beginning of the file,
and general defaults at the end.
The configuration file has the following format:
Empty lines and lines starting with ´#´ are comments.
Otherwise a line is of the format "keyword arguments" or "key‐
word = arguments". The possible keywords and their meanings are
as follows (note that the configuration files are case-sensi‐
tive, but keywords are case-insensitive):
Host Restricts the following declarations (up to the next Host key‐
word) to be only for those hosts that match one of the patterns
given after the keyword. ´*´ and ´?´ can be as wildcards in the
patterns. A single ´*´ as a pattern can be used to provide
global defaults for all hosts. The host is the hostname argu‐
ment given on the command line (i.e., the name is not converted
to a canonicalized host name before matching).
BatchMode
If set to "yes", passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where you
have no user to supply the password. The argument must be "yes"
or "no".
Cipher Specifies the cipher to use for encrypting the session. Cur‐
rently, idea, des, 3des, blowfish, arcfour, and none are sup‐
ported. The default is "idea" (or "3des" if "idea" is not sup‐
ported by both hosts). Using "none" (no encryption) is intended
only for debugging, and will render the connection insecure.
ClearAllForwardings
Clears all forwardings after reading all config files and pars‐
ing command line. This is usefull to disable forwardings in con‐
fig file when you want to make second connection to host having
forwardings in config file. Scp sets this on by default so it
will not fail even if you have some forwardings set in config
file.
Compression
Specifies whether to use compression. The argument must be
"yes" or "no".
CompressionLevel
Specifies the compression level to use if compression is enable.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications.
The meaning of the values is the same as in GNU GZIP.
ConnectionAttempts
Specifies the number of tries (one per second) to make before
falling back to rsh or exiting. The argument must be an inte‐
ger. This may be useful in scripts if the connection sometimes
fails.
EscapeChar
Sets the escape character (default: ~). The escape character
can also be set on the command line. The argument should be a
single character, ´^´ followed by a letter, or ``none'' to dis‐
able the escape character entirely (making the connection trans‐
parent for binary data).
FallBackToRsh
Specifies that if connecting via ssh fails due to a connection
refused error (there is no sshd listening on the remote host),
rsh should automatically be used instead (after a suitable warn‐
ing about the session being unencrypted). The argument must be
"yes" or "no".
ForwardAgent
Specifies whether the connection to the authentication agent (if
any) will be forwarded to the remote machine. The argument must
be "yes" or "no".
ForwardX11
Specifies whether X11 connections will be automatically redi‐
rected over the secure channel and DISPLAY set. The argument
must be "yes" or "no".
GatewayPorts
Specifies that also remote hosts may connect to locally for‐
warded ports. The argument must be "yes" or "no".
GlobalKnownHostsFile
Specifies a file to use instead of /etc/ssh_known_hosts.
HostName
Specifies the real host name to log into. This can be used to
specify nicnames or abbreviations for hosts. Default is the
name given on the command line. Numeric IP addresses are also
permitted (both on the command line and in HostName specifica‐
tions).
IdentityFile
Specifies the file from which the user's RSA authentication
identity is read (default .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 syntax to refer to a user's home direc‐
tory. It is possible to have multiple identity files specified
in configuration files; all these identities will be tried in
sequence.
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 tempo‐
rarily, and some people find it annoying.
The default is "yes" (to send keepalives), and the client will
notice if the network goes down or the remote host dies. This
is important in scripts, and many users want it too.
To disable keepalives, the value should be set to "no" in both
the server and the client configuration files.
KerberosAuthentication
Specifies whether Kerberos V5 authentication will be used.
KerberosTgtPassing
Specifies whether a Kerberos V5 TGT will be forwarded to the
server.
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 first argument must be a port number, and the sec‐
ond must be host:port. Multiple forwardings may be specified,
and additional forwardings can be given on the command line.
Only the root can forward privileged ports.
NumberOfPasswordPrompts
Specifies number of password prompts before giving up. The argu‐
ment to must be integer. Note that server also limits number of
attempts (currently 5), so setting this larger doesn't have any
effect. Default value is one.
PasswordAuthentication
Specifies whether to use password authentication. The argument
to this keyword must be "yes" or "no".
PasswordPromptHost
Specifies whether to include the remote host name in the pass‐
word prompt. The argument to this keyword must be "yes" or
"no".
PasswordPromptLogin
Specifies whether to include the remote login name in the pass‐
word prompt. The argument to this keyword must be "yes" or
"no".
Port Specifies the port number to connect on the remote host.
Default is 22.
ProxyCommand
Specifies the command to use to connect to the server. The com‐
mand string extends to the end of the line, and is executed with
/bin/sh. In the command string, %h will be substituted by the
host name to connect and %p by the port. The command can be
basically anything, and should read from its stdin and write to
its stdout. It should eventually connect an sshd server running
on some machine, or execute "sshd -i" somewhere. Host key man‐
agement will be done using the HostName of the host being con‐
nected (defaulting to the name typed by the user).
Note that ssh can also be configured to support the SOCKS system
using the --with-socks4 or --with-socks5 compile-time configura‐
tion option.
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 first argument must be a port number, and the sec‐
ond must be host:port. Multiple forwardings may be specified,
and additional forwardings can be given on the command line.
Only the root can forward privileged ports.
RhostsAuthentication
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 authentication time on slow connections when rhosts
authentication is not used. Most servers do not permit Rhost‐
sAuthentication because it is not secure (see RhostsRSAAuthenti‐
cation). The argument to this keyword must be "yes" or "no".
RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA
host authentication. This is the primary authentication method
for most sites. The argument must be "yes" or "no".
RSAAuthentication
Specifies whether to try RSA authentication. The argument to
this keyword must be "yes" or "no". RSA authentication will
only be attempted if the identity file exists, or an authentica‐
tion agent is running.
StrictHostKeyChecking
If this flag is set to "yes", ssh ssh will never automatically
add host keys to the $HOME/.ssh/known_hosts file, and refuses to
connect hosts whose host key has changed. This provides maximum
protection against trojan horse attacks. However, it can be
somewhat annoying if you don't have good /etc/ssh_known_hosts
files installed and frequently connect new hosts. Basically this
option forces the user to manually add any new hosts. Normally
this option is set to "ask", and new hosts will automatically be
added to the known host files after you have confirmed you
really want to do that. If this is set to "no" then new host
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 "yes", "no" or "ask".
TISAuthentication
Specifies whether to try TIS authentication. The argument to
this keyword must be "yes" or "no".
UsePrivilegedPort
Specifies whether to use privileged port when connecting to
other end. The default is yes if rhosts or rsarhosts authentica‐
tions are enabled.
User Specifies the user to log in as. This can be useful if you have
a different user name in different machines. This saves the
trouble of having to remember to give the user name on the com‐
mand line.
UserKnownHostsFile
Specifies a file to use instead of $HOME/.ssh/known_hosts.
UseRsh Specifies that rlogin/rsh should be used for this host. It is
possible that the host does not at all support the ssh protocol.
This causes ssh to immediately exec rsh. All other options
(except HostName) are ignored if this has been specified. The
argument must be "yes" or "no".
XAuthLocation
Specifies the path to xauth program.
ENVIRONMENT
Ssh will normally set the following environment variables:
DISPLAY
The DISPLAY variable indicates the location of the X11 server.
It is automatically set by ssh to point to a value of the form
"hostname:n" where hostname indicates the host where the shell
runs, and n is an integer >= 1. Ssh 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 manu‐
ally copy any required authorization cookies).
HOME Set to the path of the user's home directory.
LOGNAME
Synonym for USER; set for compatibility with systems that use
this variable.
MAIL Set to point the user's mailbox.
PATH Set to the default PATH, as specified when compiling ssh or, on
some systems, /etc/environment or /etc/default/login.
SSH_AUTH_SOCK
if exists, is used to indicate the path of a unix-domain socket
used to communicate with the authentication agent (or its local
representative).
SSH_CLIENT
Identifies the client end of the connection. The variable con‐
tains three space-separated values: client ip-address, client
port number, and server port number.
SSH_ORIGINAL_COMMAND
This will be the original command line of given by protocol if
forced command is run. It can be used to fetch arguments etc
from the other end.
SSH_TTY
This is set to the name of the tty (path to the device) associ‐
ated with the current shell or command. If the current session
has no tty, this variable is not set.
TZ The timezone variable is set to indicate the present timezone if
it was set when the daemon was started (e.i., the daemon passes
the value on to new connections).
USER Set to the name of the user logging in.
Additionally, ssh reads /etc/environment and $HOME/.ssh/environment,
and adds lines of the format VARNAME=value to the environment. Some
systems may have still additional mechanisms for setting up the envi‐
ronment, such as /etc/default/login on Solaris.
FILES
$HOME/.ssh/known_hosts
Records host keys for all hosts the user has logged into (that
are not in /etc/ssh_known_hosts). See sshd manual page.
$HOME/.ssh/random_seed
Used for seeding the random number generator. This file con‐
tains sensitive data and should read/write for the user and not
accessible for others. This file is created the first time the
program is run and updated automatically. The user should never
need to read or modify this file.
$HOME/.ssh/identity
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. It is possible to specify a
passphrase when generating the key; the passphrase will be used
to encrypt the sensitive part of this file using IDEA.
$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 should be added to $HOME/.ssh/authorized_keys on all
machines where you wish to log in using RSA authentication.
This file is not sensitive and can (but need not) be readable by
anyone. This file is never used automatically and is not neces‐
sary; it is only provided for the convenience of the user.
$HOME/.ssh/config
This is the per-user configuration file. The format of this
file is described above. This file is used by the ssh 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.
$HOME/.ssh/authorized_keys
Lists the RSA keys that can be used for logging in as this user.
The format of this file is described in the sshd 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 modu‐
lus, public exponent, modulus, and comment fields, separated by
spaces). This file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by
others.
/etc/ssh_known_hosts
Systemwide list of known host keys. This file should be pre‐
pared 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 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 for the
same machine, all such names should be listed, separated by com‐
mas. The format is described on the sshd manual page.
The canonical system name (as returned by name servers) is used
by sshd to verify the client host when logging in; other names
are needed because ssh does not convert the user-supplied name
to a canonical name before checking the key, because someone
with access to the name servers would then be able to fool host
authentication.
/etc/ssh_config
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 be world-readable.
$HOME/.rhosts
This file is used in .rhosts authentication to list the
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. This file must be owned by
the user, and must not have write permissions for anyone else.
The recommended permission is read/write for the user, and not
accessible by others.
Note that by default sshd will be installed so that it requires
successful RSA host authentication before permitting .rhosts
authentication. If your server machine does not have the
client's host key in /etc/ssh_known_hosts, you can store it in
$HOME/.ssh/known_hosts. The easiest way to do this is to con‐
nect back to the client from the server machine using ssh; this
will automatically add the host key in $HOME/.ssh/known_hosts.
$HOME/.shosts
This file is used exactly the same way as .rhosts. The purpose
for having this file is to be able to use rhosts authentication
with ssh without permitting login with rlogin or rsh.
/etc/hosts.equiv
This file is used during .rhosts authentication. It contains
canonical hosts names, one per line (the full format is
described on the sshd 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 required. This file should
only be writable by root.
/etc/shosts.equiv
This file is processed exactly as /etc/hosts.equiv. This file
may be useful to permit logins using ssh but not using
rsh/rlogin.
/etc/sshrc
Commands in this file are executed by ssh when the user logs in
just before the user's shell (or command) is started. See the
sshd manual page for more information.
$HOME/.ssh/rc
Commands in this file are executed by ssh when the user logs in
just before the user's shell (or command) is started. See the
sshd manual page for more information.
INSTALLATION
Ssh is normally installed as suid root. It needs root privileges only
for rhosts authentication (rhosts authentication requires that the con‐
nection must come from a privileged port, and allocating such a port
requires root privileges). It also needs to be able to read
/etc/ssh_host_key to perform RSA host authentication. It is possible
to use ssh without root privileges, but rhosts authentication will then
be disabled. Ssh drops any extra privileges immediately after the con‐
nection to the remote host has been made.
Considerable work has been put into making ssh secure. However, if you
find a security problem, please report it immediately to <ssh-
bugs@cs.hut.fi>.
AUTHOR
Tatu Ylonen <ylo@ssh.fi>
Information about new releases, mailing lists, and other related issues
can be found from the ssh WWW home page at http://www.cs.hut.fi/ssh.
SEE ALSOsshd(8), ssh-keygen(1), ssh-agent(1), ssh-add(1), scp(1), make-ssh-
known-hosts(1), rlogin(1), rsh(1), telnet(1)SSH November 8, 1995 SSH(1)