Telnet - Personal Web Pages

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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



Network protocol used on Internet or LAN connections
Developed in 1969 beginning with RFC 15
Standardized as IETF STD 8


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
Note: 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


"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



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
 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


When that option is not available, telnet sessions are
equivalent to raw TCP as long as byte value 255 never
appears in the data
? What is byte 255 ?
PROTOCOL DETAILS
Protocol details

TELNET is a client-server protocol



TELNET predates TCP/IP


Based on a reliable connection-oriented transport
Typically TCP port 23
Originally ran on NCP
Protocol has many extensions

Some adopted as Internet standards
 IETF standards STD 27 through STD 32



Define various extensions
Most are extremely common
Other extensions are proposed standards
SECURITY
Security

TELNET initially developed in 1969

Most networked computers at the time:



Security originally not as much of a concern



Computer departments of academic institutions
Large private and government research facilities
Most connections in a “secure” environment
Changed after the bandwidth explosion of the 1990s
Encrypted alternatives made necessary with:


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

Easy to eavesdrop on 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:


Can intercept the packets
Obtain login and password information


Any of several common utilities
E.g. tcpdump and Wireshark
Security

Most implementations of TELNET
lack an authentication scheme


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


Especially on the public Internet
In favor of a the ssh protocol



First released in 1995
SSH provides much of the functionality of telnet
Also has:

Strong encryption


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


Most TELNET implementations do not support these
extensions



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
Main advantage of TLS-TELNET

Ability to use certificate-authority signed server certificates:


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 2010ish)
Often when diagnosing problems
Manually "talk" to other services
without specialized client software

Sometimes used in debugging network
services


SMTP, IRC or HTTP servers
Serves as a simple way to send commands
to the server and examine the responses
Current status

Other software is finding greater favor
with some system administrators for
testing purposes

Examples:





nc (netcat)
socat on Unix
PuTTY on Windows
Can be called with arguments to not 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
Current TELNET status

Still very popular in enterprise networks
to access host applications



Still widely used for administration of
network elements




IBM Mainframes
Typically within a secure internal environment
Commissioning
Integration
Maintenance
Core network elements in mobile
communication networks
Current status

TELNET is also heavily used for

MUD games played over the Internet



talkers, MUSHes, MUCKs, MOOes
Resurgent BBS community
Windows Vista and after:


Telnet.exe no longer installed by default
Still available as an installable feature
rsh
http://en.wikipedia.org/wiki/Remote_Shell
Remote SHell
Remote Shell

rsh (remote shell):

Command line computer program

Executes shell commands




Remote system which rsh accesses



As another user
On another computer in a computer network
One command at a time
Must have the rshd daemon running
rsh uses well-known port TCP 514
Note: rsh command shares the same name as
another common UNIX utility, the restricted
shell

First appeared in PWB/UNIX


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


rsh has been ported to other operating systems
rsh protocol is not secure for network use



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:

To execute the command mkdir testdir
as user remoteuser
 on the computer host.example.com:



After the command has finished rsh terminates


rsh -l remoteuser host.example.com "mkdir testdir"
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 via the Domain Name System
rlogin
http://en.wikipedia.org/wiki/Rlogin
Remote Login
rlogin

rlogin: Unix software utility that allows users to log in
on another host via a network


TCP port 513
First distributed as part of the 4.2BSD release

Note: rlogin is also the name of the application layer
protocol used by the software



Authenticated users can act as if physically present at
the computer
From RFC 1258:


"The rlogin facility provides a remote-echoed, locally
flow-controlled virtual terminal with proper flushing of
output."
rlogin communicates with a daemon on the remote host


part of the TCP/IP protocol suite
rlogind
rlogin



Similar to the Telnet command
Not customizable
Can connect only to Unix hosts
rlogin

Most commonly deployed on corporate or
academic networks



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)
 Remote host must be in the /etc/hosts.equiv file

If user has a .rhosts file in their home directory
rlogin

rlogin has several serious security problems:

All information is transmitted unencrypted


Including passwords!
.rlogin (or .rhosts) file is easy to misuse


Potentially allows anyone to login without a password
Many corporate system administrators prohibit .rlogin files


Protocol partly relies on the remote party's rlogin client
providing information honestly (including source port and
source host name)


A corrupt client is able to forge and gain access
rlogin protocol has no means of authenticating other
machines' identities


Actively search their networks for offenders
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

Any of NFS' security faults automatically plague rlogin
rlogin

Due to these serious problems rlogin is
rarely used across untrusted networks
e.g. like the public internet


Even in closed deployments it has fallen into
relative disuse
 many Unix and Linux distributions no longer
include it by default
Many networks which formerly relied on rlogin
and telnet
 Replaced them with SSH and its rloginequivalent slogin
rlogin

Original Berkeley package which provides
rlogin also features rcp and rsh


Share the hosts.equiv and .rhosts accesscontrol scheme
 Suffer from the same security problems
 Connects to a different daemon: rshd
Note: 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

rcp: the Unix ‘Remote CoPy' command

Command on the Unix used to remotely copy


Copy one or more files from one computer system to
another
Typically uses




Has been implemented to alternatively support
Kerberos
rcp is not secure for network use



TCP/IP protocol
.rhosts file for authentication
Sends unencrypted information over the network
Largely replaced by the ssh-based utility scp
Etymology:


rcp is a member of the BSD Unix family of 'r'
(remote) commands
Name is a contraction of 'r' remote and 'cp' copy
FTP
ftp

File Transfer Protocol


FTP runs exclusively over TCP
Listens on port 21 (default)


Incoming connections from FTP clients
Responds on a dynamic port
Sent by client
 Usually port 20


Syntax

ftp [-options] [-s:filename] [-w:buffer] [host]
Typical use

Connect to an FTP server


Login


ftp machineid
Enter a valid user id an password
Interact with the FPT server

At the ftp> prompt
FTP Options

-s:filename


host


-g


-n


-i


-v


-d


Host name or IP address of the remote host.
Disable filename wildcards.
No auto-login.
No interactive prompts during ftp.
Hide remote server responses.
-w:buffer


Run a text file containing FTP commands
-a

Set buffer size to buffer (default=4096)
Debug
Use any local interface when binding data connection.
Commonly Used FTP Commands

?


append local-file [remote-file]


request help or information about the FTP commands
Append a local file to a file on the remote computer.
ascii

set the mode of file transfer to ASCII

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
binary

set the mode of file transfer to binary

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

the default
transmits seven bits per character
bye


binary mode transmits all eight bits per byte
provides less chance of a transmission error
must be used to transmit files other than ASCII files
exit the FTP environment
same as quit
Commonly Used Commands

cd


change directory on the remote machine
close

terminate a connection with another computer

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delete remote-file

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
delete a file in the current remote directory
same as rm in UNIX
dir [remote-directory] [local-file]



close userid closes the current FTP connection with userid
still leaves you within the FTP environment
List a remote directory's files and subdirectories.
(or save the listing to local-file)
disconnect

Disconnect from the remote host, retaining the ftp prompt
Commonly Used FTP Commands

get remote-file [local-file]

copy one file from the remote machine to the local
machine



help [command]


lcd

request a list of all available FTP commands
change directory on your local machine


ls


get ABC DEF copies file ABC in the current remote directory to
(or on top of) a file named DEF in your current local directory
get ABC copies file ABC in the current remote directory to (or
on top of) a file with the same name, ABC, in your current
local directory
same as UNIX cd
list the names of the files in the current remote directory
mdelete remote-files [ ...]

Delete files on remote host
Commonly Used Commands

mdir remote-files [ ...] local-file

Display a list of a remote directory's files and
subdirectories



Mdir allows you to specify multiple files.
mget remote-files [ ...]

copy multiple files from the remote machine to the local
machine



copies all the files in the current remote directory to your
current local directory, using the same filenames
Notice the use of the wild card character, *.
mkdir directory


you are prompted for a y/n answer before transferring each
file
mget *


(or save the listing to local-file)
make a new directory within the current remote directory
mls remote-files [ ...] local-file

List a remote directory's files and folders. (short format)
Commonly Used Commands

mput local-files [ ...]

copy multiple files from the local machine to the remote machine


you are prompted for a y/n answer before transferring each file
open computer [port]

open a connection with another computer

open userid



prompt




put


Toggle prompting.
Ftp prompts during multiple file transfers to allow you to selectively
retrieve or store files; mget and mput transfer all files if prompting is
turned off.
By default, prompting is on.
copy one file from the local machine to the remote machine
pwd

opens a new FTP connection with userid
must enter a username and password for the useid account

unless it is to be an anonymous connection
find out the pathname of the current directory on the remote machine
Commonly Used FTP Commands

pwd


quit


Display help for remote commands.
rename filename newfilename


exit the FTP environment (same as bye)
remotehelp [command]


find out the pathname of the current directory on the remote
machine
Rename remote files.
rmdir

remove a directory in the current remote directory
Commonly Used FTP Commands

send local-file [remote-file]


status


Display the current status of FTP connections and toggles.
trace


Copy a local file to the remote host.
Toggles packet tracing; trace displays the route of each
packet
type [type-name]

Set or display the file transfer type: `binary' or `ASCII' (the
default)





user user-name [password] [account]


If type-name is not specified, the current type is displayed.
ASCII should be used when transferring text files.
In ASCII text mode, character-set and end-of-line characters are
converted as necessary.
Use `Binary' for transferring executable files.
Specifies a user to the remote host.
verbose

Toggle verbose mode. By default, verbose is on.
Telnet should never be use
because it is unsafe:
95%
Fa
l
se
5%
ue
B.
True
False
Tr
A.
ftp allows both the uploading and
downloading of files to a remote computer
97%
Fa
l
se
3%
ue
B.
True
False
Tr
A.
Summary

Host of insecure remote commands


May be okay for “internal” use


On “secure” networks
May be better for speed


Developed before security was a major
concern
Where security is not a concern
Overall:

Use modern secure alternatives

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