Table of Contents
ssh - secure shell client (remote login program)
ssh
[-l login_name ] hostname [command]
ssh [-a] [-c idea |blowfish |des |3des |arcfour
|tss |none ] [-e escape_char ] [-i identity_file ] [-l login_name ] [-n] [-k] [-V]
[-o option ] [-p port ] [-q] [-P] [-t] [-v] [-x] [-C] [-L port:host:hostport ] [-R port:host:hostport
] hostname [command]
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 arbitrary 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 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 permitted. This authentication method closes security
holes due to IP spoofing, 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
authentication. The scheme is based on public-key cryptography: there are
cryptosystems where encryption and decryption are done using separate
keys, and it is not possible to derive the decryption key from the encryption
key. RSA is one such system. The idea is that each user creates a public/private
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 authentication 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 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 corresponding 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 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 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 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 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 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 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 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. Additionally,
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.
- -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 |tss |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 corporations, 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 algorithm 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. TSS
is a fast home-grown algorithm based on MD5. none disables 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 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 messages to be suppressed. Only fatal errors are displayed.
- -P
- Use non priviledged port. With this you cannot use rhosts or rsarhosts
authentications, but it can be used to bypass some firewalls that dont
allow priviledged 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, authentication, and configuration
problems.
- -V
- Print only version number and exit.
- -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 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 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
connection is forwarded over the secure channel, and a connection is made
to host:hostport from the local machine. Port forwardings can also be
specified in the configuration file. Privileged ports can be forwarded
only when logging in as root on the remote machine.
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_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 comments.
- Otherwise a line is of the format
"keyword arguments" or "keyword =
- arguments". The possible keywords 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, tss, 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 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 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 transparent 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 warning 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 redirected over
the secure channel and DISPLAY
set. 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 specifications).
- 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 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 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 second 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 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 ".
- 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 eventually 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 argument 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 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 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
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 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. 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
".
- UsePriviledgedPort
- Specifies whether to use priviledged port when
connecting 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 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 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 immediately exec rsh. All other options (except HostName)
are ignored if this has been specified. The argument must be "yes " or
"no ".
- TISAuthentication
- Specifies whether to try TIS authentication.
The argument to this keyword must be "yes " or "no ".
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
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_AUTHENTICATION_SOCKET
- 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
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 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 environment,
such as /etc/default/login on Solaris.
- $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
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 necessary; 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 modulus,
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 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 commas. 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 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 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.
Ssh is normally installed as suid
root. It needs root privileges only for rhosts authentication (rhosts
authentication 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>.
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.
sshd(8)
, ssh-keygen(1)
, ssh-agent(1)
, ssh-add(1)
, scp(1)
, make-ssh-known-hosts(1)
,
rlogin(1)
, rsh(1)
, telnet(1)
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