ssh - Unix, Linux Command

ssh
connects and logs into the specified
 hostname
(with optional
 user
name).
The user must prove
his/her identity to the remote machine using one of several methods
depending on the protocol version used (see below).


If
 command
is specified,
it is executed on the remote host instead of a login shell.


The options are as follows:


Agent forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the agent’s Unix-domain socket)
can access the local agent through the forwarded connection.
An attacker cannot obtain key material from the agent,
however they can perform operations on the keys that enable them to
authenticate using the identities loaded into the agent.


Protocol version 1 allows specification of a single cipher.
The supported values are
"3des",
"blowfish",
and
"des".
 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is believed to be secure.
 blowfish
is a fast block cipher; it appears very secure and is much faster than
 3des.
 des
is only supported in the
ssh
client for interoperability with legacy protocol 1 implementations
that do not support the
 3des
cipher.
Its use is strongly discouraged due to cryptographic weaknesses.
The default is
"3des".


For protocol version 2,
 cipher_spec
is a comma-separated list of ciphers
listed in order of preference.
The supported ciphers are:
3des-cbc,
aes128-cbc,
aes192-cbc,
aes256-cbc,
aes128-ctr,
aes192-ctr,
aes256-ctr,
arcfour128,
arcfour256,
arcfour,
blowfish-cbc,
and
cast128-cbc.
The default is:

aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr


IPv6 addresses can be specified with an alternative syntax:
.Sm off
.Xo 
[bind_address /]
 port




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.
IPv6 addresses can be specified by enclosing the address in square braces or
using an alternative syntax:
.Sm off
.Xo 
[bind_address /]
 host / port /
 hostport


By default, the listening socket on the server will be bound to the loopback
interface only.
This may be overriden by specifying a
 bind_address.
An empty
 bind_address,
or the address
‘*’,
indicates that the remote socket should listen on all interfaces.
Specifying a remote
 bind_address
will only succeed if the server’s
 GatewayPorts
option is enabled (see
sshd_config(5)).


X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the user’s X authorization database)
can access the local X11 display through the forwarded connection.
An attacker may then be able to perform activities such as keystroke monitoring.


For this reason, X11 forwarding is subjected to X11 SECURITY extension
restrictions by default.
Please refer to the
ssh
-Y 
option and the
 ForwardX11Trusted
directive in
ssh_config(5)
for more information.


ssh
may additionally obtain configuration data from
a per-user configuration file and a system-wide configuration file.
The file format and configuration options are described in
ssh_config(5).


ssh
exits with the exit status of the remote command or with 255
if an error occurred.


The methods available for authentication are:
host-based authentication,
public key authentication,
challenge-response authentication,
and password authentication.
Authentication methods are tried in the order specified above,
though protocol 2 has a configuration option to change the default order:
 PreferredAuthentications.


Host-based authentication works as follows:
If the machine the user logs in from is listed in
 /etc/hosts.equiv
or
 /etc/ssh/shosts.equiv
on the remote machine, and the user names are
the same on both sides, or if the files
 ~/.rhosts
or
 ~/.shosts
exist in the user’s home directory on the
remote machine and contain a line containing the name of the client
machine and the name of the user on that machine, the user is
considered for login.
Additionally, the server
 must
be able to verify the client’s
host key (see the description of
 /etc/ssh/ssh_known_hosts
and
 ~/.ssh/known_hosts,
below)
for login to be 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.]


Public key authentication works as follows:
The scheme is based on public-key cryptography,
using cryptosystems
where encryption and decryption are done using separate keys,
and it is unfeasible to derive the decryption key from the encryption key.
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.
ssh
implements public key authentication protocol automatically,
using either the RSA or DSA algorithms.
Protocol 1 is restricted to using only RSA keys,
but protocol 2 may use either.
The
HISTORY
section of
ssl(8)
contains a brief discussion of the two algorithms.


The file
 ~/.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 client proves that it has access to the private key
and the server checks that the corresponding public key
is authorized to accept the account.


The user creates his/her key pair by running
ssh-keygen(1).
This stores the private key in
 ~/.ssh/identity
(protocol 1),
 ~/.ssh/id_dsa
(protocol 2 DSA),
or
 ~/.ssh/id_rsa
(protocol 2 RSA)
and stores the public key in
 ~/.ssh/identity.pub
(protocol 1),
 ~/.ssh/id_dsa.pub
(protocol 2 DSA),
or
 ~/.ssh/id_rsa.pub
(protocol 2 RSA)
in the user’s home directory.
The user should then copy the public key
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.


The most convenient way to use public key authentication may be with an
authentication agent.
See
ssh-agent(1)
for more information.


Challenge-response authentication works as follows:
The server sends an arbitrary
"challenge"
text, and prompts for a response.
Protocol 2 allows multiple challenges and responses;
protocol 1 is restricted to just one challenge/response.
Examples of challenge-response authentication include
BSD Authentication (see
login.conf(5))
and PAM (some non-OpenBSD systems).


Finally, 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.


ssh
automatically maintains and checks a database containing
identification for all hosts it has ever been used with.
Host keys are stored in
 ~/.ssh/known_hosts
in the user’s home directory.
Additionally, the file
 /etc/ssh/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
server spoofing or man-in-the-middle attacks,
which could otherwise be used to circumvent the encryption.
The
 StrictHostKeyChecking
option can be used to control logins to machines whose
host key is not known or has changed.


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 may use the escape characters noted below.


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 on the remote
machine exits and all X11 and TCP connections have been closed.


A single tilde character can be sent as
 ~~
or by following the tilde by a character other than those described below.
The escape character must always follow a newline to be interpreted as
special.
The escape character can be changed in configuration files using the
 EscapeChar
configuration directive or on the command line by the
-e 
option.


The supported escapes (assuming the default
‘~’)
are:


In the example below, we look at encrypting communication between
an IRC client and server, even though the IRC server does not directly
support encrypted communications.
This works as follows:
the user connects to the remote host using
ssh,
specifying a port to be used to forward connections
to the remote server.
After that it is possible to start the service which is to be encrypted
on the client machine,
connecting to the same local port,
and
ssh
will encrypt and forward the connection.


The following example tunnels an IRC session from client machine
"127.0.0.1"
(localhost)
to remote server
"server.example.com":

$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
$ irc -c ’#users’ -p 1234 pinky 127.0.0.1


This tunnels a connection to IRC server
"server.example.com",
joining channel
"#users",
nickname
"pinky",
using port 1234.
It doesn’t matter which port is used,
as long as it’s greater than 1023
(remember, only root can open sockets on privileged ports)
and doesn’t conflict with any ports already in use.
The connection is forwarded to port 6667 on the remote server,
since that’s the standard port for IRC services.


The
-f 
option backgrounds
ssh
and the remote command
"sleep 10"
is specified to allow an amount of time
(10 seconds, in the example)
to start the service which is to be tunnelled.
If no connections are made within the time specified,
ssh
will exit.


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
 ForwardAgent
variable is set to
"yes"
(or see the description of the
-A 
and
-a 
options above) and
the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side.



     $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key



If the fingerprint is already known,
it can be matched and verified,
and the key can be accepted.
If the fingerprint is unknown,
an alternative method of verification is available:
SSH fingerprints verified by DNS.
An additional resource record (RR),
SSHFP,
is added to a zonefile
and the connecting client is able to match the fingerprint
with that of the key presented.


In this example, we are connecting a client to a server,
"host.example.com".
The SSHFP resource records should first be added to the zonefile for
host.example.com:

$ ssh-keygen -f /etc/ssh/ssh_host_rsa_key.pub -r host.example.com.
$ ssh-keygen -f /etc/ssh/ssh_host_dsa_key.pub -r host.example.com.


The output lines will have to be added to the zonefile.
To check that the zone is answering fingerprint queries:



     $ dig -t SSHFP host.example.com



Finally the client connects:

$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?


See the
 VerifyHostKeyDNS
option in
ssh_config(5)
for more information.


The following example would connect client network 10.0.50.0/24
with remote network 10.0.99.0/24, provided that the SSH server
running on the gateway to the remote network,
at 192.168.1.15, allows it:

# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252


Client access may be more finely tuned via the
 /root/.ssh/authorized_keys
file (see below) and the
 PermitRootLogin
server option.
The following entry would permit connections on the first
tun(4)
device from user
"jane"
and on the second device from user
"john",
if
 PermitRootLogin
is set to
"forced-commands-only":

tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun1" ssh-rsa ... john


Since a SSH-based setup entails a fair amount of overhead,
it may be more suited to temporary setups,
such as for wireless VPNs.
More permanent VPNs are better provided by tools such as
ipsecctl(8)
and
isakmpd(8).


Additionally,
ssh
reads
 ~/.ssh/environment,
and adds lines of the format
"VARNAME=value"
to the environment if the file exists and users are allowed to
change their environment.
For more information, see the
 PermitUserEnvironment
option in
sshd_config(5).






























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