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Remote Access
http://en.wikipedia.org/wiki/Telnet
The old, safe world:
telnet, rlogin, rsh, rcp
Telnet
Telecommunications
Network
TELNET

TELecommunication NETwork
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A network protocol used on Internet or LAN connections
Developed in 1969 beginning with RFC 15
Standardized as IETF STD 8

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The term telnet also refers to software which
implements the client part of the protocol

TELNET clients have been available on most Unix
systems for many, many years



One of the first Internet standards
Available for virtually all platforms
Most network equipment and OSs with a TCP/IP stack
support some kind of TELNET service server for their
remote configuration
Secure Shell has begun to dominate remote access
for Unix-based machines.
TELNET

"To telnet" sometimes used as a verb

Establish or use a TELNET or other interactive TCP connection

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"To change your password, telnet to the server and run the passwd
command"
Typically a user will be telneting to a Unix-like server system
or a simple network device such as a switch
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User might "telnet in from home to check his mail at school"
Use a telnet client to connect local computer to a server
Once the connection is established
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Log in with his account information
Execute commands remotely on that computer


E.g. ls or cd
Client may also be used to make interactive raw-TCP
sessions

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When that option is not available, telnet sessions are equivalent
to raw TCP as long as byte 255 never appears in the data
? What is byte 255 ?
Protocol details
Protocol details

TELNET is a client-server protocol



TELNET predates TCP/IP

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Based on a reliable connection-oriented
transport.
Typically TCP port 23
Originally ran on NCP
The protocol has many extensions

Some adopted as Internet standards
 IETF standards STD 27 through STD 32

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Define various extensions
Most are extremely common.
Other extensions are on the IETF standards
track as proposed standards
Security
Security

TELNET initially developed in 1969

Most networked computers at the time:

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Computer departments of academic institutions
Large private and government research facilities
Security originally not as much of a concern
 Changed after the bandwidth explosion of the
1990s
Enencrypted alternatives made necessary

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Rise in the number of people with access to the Internet
Number of people attempting to crack other people's
servers
Security
in computer security1
recommend that the use of
TELNET for remote logins
should be discontinued
under all normal
circumstances for the
following reasons:
 Experts
1SANS
Institute,
members of the comp.os.linux.security newsgroup
Security

TELNET, by default, does not encrypt
any data sent over the connection
(including passwords)

It is easy to eavesdrop on the
communications


Easy to intercept ids and passwords
Anybody with access to a router, switch,
or gateway located on the network
between the two hosts where TELNET is
being used:

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Can intercept the packets
Obtain login and password information

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Any of several common utilities
E.g. tcpdump and Wireshark
Security

Most implementations of TELNET
lack an authentication scheme

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Cannot ensure that communication
is carried out between the two
desired hosts, and not intercepted in
the middle
Commonly used TELNET daemons
have several vulnerabilities
discovered over the years
Security

Security-related shortcomings have seen
the usage of the TELNET protocol drop
rapidly

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Especially on the public Internet,
In favor of a the ssh protocol
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First released in 1995
SSH provides much of the functionality of telnet
Also has:

Strong encryption

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Prevents sensitive data such as passwords from being
intercepted
Public key authentication

Ensures that the remote computer is actually who it
claims to be
Security

As has happened with other early Internet protocols

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Most TELNET implementations do not support these
extensions

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Extensions to the TELNET protocol provide TLS security and
SASL authentication that address many security issues
Relatively little interest in implementing these
SSH is adequate for most purposes.
The main advantage of TLS-TELNET

Ability to use certificate-authority signed server certificates:

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to authenticate a server host to a client that does not yet have
the server key stored
SSH weakness:

User must trust the first session to a host when it has not yet
acquired the server key
Current status
Current status

TELNET clients are still used (as of
the mid-2000s)
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Often when diagnosing problems
Manually "talk" to other services
without specialized client software

Sometimes used in debugging network
services
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an SMTP, IRC or HTTP server
Serves as a simple way to send commands
to the server and examine the responses
Current status

Other software such as nc (netcat)
or socat on Unix (or PuTTY on
Windows) are finding greater favor
with some system administrators
for testing purposes

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They can be called with arguments not
to send any terminal control
handshaking data
netcat does not distort the \377 octet
which allows raw access to TCP socket
 unlike any standard-compliant TELNET
software
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Current status

TELNET is still very popular in enterprise
networks to access host applications
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TELNET is still widely used for
administration of network elements
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IBM Mainframes
Typically in an internal secure environment
Commissioning
Integration
Maintenance
of core network elements in mobile
communication networks
Current status
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TELNET is also heavily used for
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MUD games played over the Internet
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talkers, MUSHes, MUCKs, MOOes
Resurgent BBS community
Windows Vista

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Telnet.exe is no longer installed by
default
Is still included as an installable feature
rsh
http://en.wikipedia.org/wiki/Remote_Shell
Remote SHell
Remote Shell

rsh (remote shell):

A command line computer program

Can execute shell commands
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As another user
On another computer in a computer network
Remote system on which the rsh executes needs
to be running the rshd daemon.
rsh uses well-known port TCP 514.
Note: rsh command shares the same name as
another common UNIX utility, the restricted
shell
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First appeared in PWB/UNIX; in System V
Release 4
Restricted shell is often located at /usr/lib/rsh.
Remote Shell

rsh originated as part of the BSD Unix
operating system, along with rcp, as part of
the rlogin package on 4.2BSD in 1983

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rsh has been ported to other operating systems
rsh protocol is not secure for network use

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Sends unencrypted information over the network
Some implementations also authenticate by
sending unencrypted passwords over the network
rsh has largely been replaced by the very similar
ssh (secure shell) program on untrusted
networks like the internet
Remote Shell

rsh example:

Execute the command mkdir testdir as
user remoteuser on the computer
host.example.com:
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rsh -l remoteuser host.example.com
"mkdir testdir"
After the command has finished rsh
terminates
If no command is specified then rsh will
log in on the remote system using rlogin
Network location of the remote computer
is looked up using the Domain Name
System
rlogin
http://en.wikipedia.org/wiki/Rlogin
Remote Login
rlogin

rlogin is a Unix software utility that allows users
to log in on another host via a network

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Communicates via TCP port 513
First distributed as part of the 4.2BSD release
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rlogin is also the name of the application layer
protocol used by the software
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Authenticated users can act as if physically present
at the computer
RFC 1258 states:
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part of the TCP/IP protocol suite
"The rlogin facility provides a remote-echoed,
locally flow-controlled virtual terminal with proper
flushing of output."
rlogin communicates with a daemon, rlogind, on
the remote host.
rlogin is similar to the Telnet command
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Not customizable
Can connect only to Unix hosts
rlogin
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rlogin most commonly deployed on
corporate or academic networks
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user account information is shared between all
the Unix machines on the network
 often using NIS
Deployments essentially trust most other
machines (and the network infrastructure
itself)
 the rlogin protocol relies on this trust.
rlogind allows logins without password
(where rlogind trusts a remote rlogin client)
 if the remote host appears in the
/etc/hosts.equiv file
 if the user in question has a .rhosts file in
their home directory
rlogin
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rlogin has several serious security problems:

All information is transmitted unencrypted
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Including passwords!
.rlogin (or .rhosts) file is easy to misuse
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Potentially allows anyone to login without a password
Many corporate system administrators prohibit .rlogin
files
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Protocol partly relies on the remote party's rlogin client
providing information honestly (including source port and
source host name)
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actively search their networks for offenders
A corrupt client is able to forge this and gain access
rlogin protocol has no means of authenticating other
machines' identities, or ensuring that the rlogin client on
a trusted machine is the real rlogin client
Common practice of mounting users' home directories
via NFS exposes rlogin to attack by means of fake
.rhosts files
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Any of NFS' security faults automatically plague rlogin
rlogin
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Due to these serious problems rlogin is
rarely used across untrusted networks
(like the public internet)
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Even in closed deployments it has fallen into
relative disuse
 many Unix and Linux distributions no longer
including it by default
Many networks which formerly relied on rlogin
and telnet
 Replaced them with SSH and its rloginequivalent slogin
rlogin
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Original Berkeley package which provides
rlogin also features rcp and rsh
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Share the hosts.equiv and .rhosts accesscontrol scheme
 Suffer from the same security problems
 Do connect to a different daemon, rshd
ssh suite contains suitable replacements for
both:
 scp replaces rcp
 ssh itself replaces both rlogin and rsh
rcp
http://en.wikipedia.org/wiki/Rcp_%28Unix%29
Remote Copy
rcp
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rcp: the Unix ‘Remote CoPy' command

Command on the Unix used to remotely copy
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Copy one or more files from one computer system to
another
Typically uses
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Has been implemented to alternatively support
Kerberos.
rcp is not secure for network use
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TCP/IP protocol
.rhosts file for authentication
Sends unencrypted information over the network
Largely replaced by the ssh-based utility scp
Etymology:
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rcp is a member of the BSD unix family of 'r' (remote)
commands
Name is a contraction of 'r' remote and 'cp' copy.
Summary
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Host of insecure remote commands
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May be okay for “internal” use
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Developed before security was a major
concern
On “secure” networks
Overall:
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Use modern secure alternatives