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 permitted 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 normally 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 per
mitted 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 permitted. This authentication method closes
security holes due to IP spoofing, DNS spoofing and rout
ing 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
authentication. The scheme is based on public-key cryp
tography: 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 key pair for authentication pur
poses. The server knows the public key, and only the user
knows the private key. The file $HOME/.ssh/autho
rized_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 authen
tication. 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 pri
vate key, proving that he/she knows the private key but
without disclosing it to the server.
Ssh implements the RSA authentication protocol automati
cally. The user creates his/her RSA key pair by running
ssh-keygen(1). This stores the private key in .ssh/iden
tity and the public key in .ssh/identity.pub in the user's
home directory. The user should then copy the iden
tity.pub to .ssh/authorized_keys in his/her home directory
on the remote machine (the authorized_keys file corre
sponds 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
authentication is much more secure than rhosts authentica
tion.
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 authenti
cation through TIS authentication server. The idea is that
ssh asks TIS authsrv(8) to authenticate the user. Some
time, usernames in the TIS database cannot be the same as
the local users. This can be the case if the user authen
ticates 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 mapping between local users
and their corresponding name in the TIS database. 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 password. The password is sent to the remote host
for checking; however, since all communications are
encrypted, the password cannot be seen by someone listen
ing 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 ses
sion), the user can disconnect with "~.", and suspend ssh
with "~^Z". All forwarded 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 fol
lowing 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 trans
parent 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 automati
cally 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 configured 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 forwarding the connec
tions over the encrypted channel.
Ssh will also automatically set up Xauthority data on the
server machine. For this purpose, it will generate a ran
dom 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 sent to the server machine (and no cookies are sent
in the plain).
If the user is using an authentication agent, the connec
tion 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 contain
ing 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 StrictHostKey
Checking 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 configuration file.
-c idea|des|3des|blowfish|arcfour|none
Selects the cipher to use for encrypting the ses
sion. idea is used by default. It is believed to
be secure. des is the data encryption standard,
but is breakable by governments, large corpora
tions, 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. arcfour 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 disables encryption entirely; it is only
intended for debugging, and it renders the connec
tion insecure.
-e ch|^ch|none
Sets the escape character for sessions with a pty
(default: ~). The escape character is only recog
nized at the beginning of a line. The escape char
acter 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 authentica
tion 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 back
ground. This may also be useful in scripts. This
implies -n. The recommended way to start X11 pro
grams 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. Iden
tity 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 configuration 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 mes
sages to be suppressed. Only fatal errors are dis
played.
-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 priv
ileged 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 imple
menting menu services.
-v Verbose mode. Causes ssh to print debugging mes
sages about its progress. This is helpful in
debugging connection, authentication, and configu
ration problems.
-V Print only version number and exit.
-g Allows remote hosts to connect local port forward
ing ports. The default is that only localhost may
connect to locally binded ports.
-x Disables X11 forwarding. This can also be speci
fied 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 con
trolled by the CompressionLevel 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 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
connection is forwarded over the secure channel,
and a connection is made to host:hostport from the
remote machine. Port forwardings can also be spec
ified 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 allocat
ing a socket to listen to 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 made to host:hostport from the
local machine. Port forwardings can also be speci
fied in the configuration file. Privileged 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 configura
tion file ($HOME/.ssh/config), and system-wide configura
tion file (/etc/ssh_config). For each parameter, the
first obtained value will be used. The configuration
files contain sections bracketed by "Host" 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.
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 com
ments.
Otherwise a line is of the format "keyword argu
ments" or "keyword = arguments". The possible key
words and their meanings are as follows (note that
the configuration files are case-sensitive, but
keywords are case-insensitive):
Host Restricts the following declarations (up to the
next Host keyword) 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 argument 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. Currently, idea, des, 3des, blowfish,
arcfour, and none are supported. The default is
"idea" (or "3des" if "idea" is not supported 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 parsing command line. This is usefull to
disable forwardings in config 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 compres
sion 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 integer. 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 disable the escape
character entirely (making the connection transpar
ent for binary data).
FallBackToRsh
Specifies that if connecting via ssh fails due to a
connection refused error (there is no sshd listen
ing on the remote host), rsh should automatically
be used instead (after a suitable warning about the
session being unencrypted). The argument must be
"yes" or "no".
ForwardAgent
Specifies whether the connection to the authentica
tion agent (if any) will be forwarded to the remote
machine. The argument must be "yes" or "no".
ForwardX11
Specifies whether X11 connections will be automati
cally redirected over the secure channel and DIS
PLAY set. The argument must be "yes" or "no".
GatewayPorts
Specifies that also remote hosts may connect to
locally forwarded 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 specifi
cations).
IdentityFile
Specifies the file from which the user's RSA
authentication identity is read (default .ssh/iden
tity 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 directory. 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 temporarily, 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 configura
tion files.
KerberosAuthentication
Specifies whether Kerberos V5 authentication will
be used.
KerberosTgtPassing
Specifies whether a Kerberos V5 TGT will be for
warded 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 argu
ment must be a port number, and the second must be
host:port. Multiple forwardings may be specified,
and additional forwardings can be given on the com
mand line. Only the root can forward privileged
ports.
NumberOfPasswordPrompts
Specifies number of password prompts before giving
up. The argument 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 password prompt. The argument to this key
word must be "yes" or "no".
PasswordPromptLogin
Specifies whether to include the remote login name
in the password prompt. The argument to this key
word 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 command 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 eventu
ally connect an sshd server running on some
machine, or execute "sshd -i" 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 ssh can also be configured to support the
SOCKS system using the --with-socks4 or --with-
socks5 compile-time configuration 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 argu
ment must be a port number, and the second must be
host:port. Multiple forwardings may be specified,
and additional forwardings can be given on the com
mand line. Only the root can forward privileged
ports.
RhostsAuthentication
Specifies whether to try rhosts based authentica
tion. Note that this declaration only affects the
client side and has no effect whatsoever on secu
rity. Disabling rhosts authentication may reduce
authentication time on slow connections when rhosts
authentication is not used. Most servers do not
permit RhostsAuthentication because it is not
secure (see RhostsRSAAuthentication). The argument
to this keyword must be "yes" or "no".
RhostsRSAAuthentication
Specifies whether to try rhosts based authentica
tion 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 authentication 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. Nor
mally 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 automati
cally 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 con
necting to other end. The default is yes if rhosts
or rsarhosts authentications are enabled.
User Specifies the user to log in as. This can be use
ful 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 command 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 imme
diately exec rsh. All other options (except Host
Name) 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 manually
copy any required authorization cookies).
HOME Set to the path of the user's home directory.
LOGNAME
Synonym for USER; set for compatibility with sys
tems that use this variable.
MAIL Set to point the user's mailbox.
PATH Set to the default PATH, as specified when compil
ing 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 authen
tication agent (or its local representative).
SSH_CLIENT
Identifies the client end of the connection. The
variable contains 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) associated 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 pre
sent 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 VAR
NAME=value to the environment. Some systems may have
still additional mechanisms for setting up the environ
ment, 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 contains 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 oth
ers. 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 auto
matically and is not necessary; it is only provided
for the convenience of the user.
$HOME/.ssh/config
This is the per-user configuration file. The for
mat 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 recom
mended 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 oth
ers.
/etc/ssh_known_hosts
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 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 canon
ical 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 recom
mended 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 connect 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 permit
ting 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 com
mand) 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 com
mand) 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 authen
tication requires that the connection 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 connection 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 ALSO
sshd(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)