<s>
Telnet	B-Protocol
(	O
short	O
for	O
"	O
teletype	O
network	O
"	O
)	O
is	O
a	O
client/server	B-Operating_System
application	B-Application
protocol	I-Application
that	O
provides	O
access	O
to	O
virtual	B-Application
terminals	I-Application
of	O
remote	B-Protocol
systems	I-Protocol
on	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
or	O
the	O
Internet	O
.	O
</s>
<s>
Telnet	B-Protocol
consists	O
of	O
two	O
components	O
:	O
(	O
1	O
)	O
the	O
protocol	O
itself	O
which	O
specifies	O
how	O
two	O
parties	O
to	O
communicate	O
and	O
(	O
2	O
)	O
the	O
software	O
application	O
that	O
provides	O
the	O
service	O
.	O
</s>
<s>
User	O
data	O
is	O
interspersed	O
in-band	O
with	O
Telnet	B-Protocol
control	O
information	O
in	O
an	O
8-bit	O
byte	O
oriented	O
data	O
connection	O
over	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
.	O
</s>
<s>
Telnet	B-Protocol
was	O
developed	O
in	O
1969	O
beginning	O
with	O
,	O
extended	O
in	O
,	O
and	O
standardized	O
as	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
Internet	O
Standard	O
STD	O
8	O
,	O
one	O
of	O
the	O
first	O
Internet	O
standards	O
.	O
</s>
<s>
Telnet	B-Protocol
transmits	O
all	O
information	O
including	O
usernames	O
and	O
passwords	O
in	O
plaintext	O
so	O
it	O
is	O
not	O
recommended	O
for	O
security-sensitive	O
applications	O
such	O
as	O
remote	B-Protocol
management	I-Protocol
of	O
routers	B-Protocol
.	O
</s>
<s>
Telnet	B-Protocol
's	O
use	O
for	O
this	O
purpose	O
has	O
waned	O
significantly	O
in	O
favor	O
of	O
SSH	B-Protocol
.	O
</s>
<s>
Some	O
extensions	O
to	O
Telnet	B-Protocol
which	O
would	O
provide	O
encryption	O
have	O
been	O
proposed	O
.	O
</s>
<s>
Telnet	B-Protocol
consists	O
of	O
two	O
components	O
:	O
(	O
1	O
)	O
the	O
protocol	O
itself	O
and	O
(	O
2	O
)	O
the	O
service	O
component	O
.	O
</s>
<s>
The	O
telnet	B-Protocol
protocol	O
is	O
a	O
client-server	B-Operating_System
protocol	I-Operating_System
,	O
based	O
on	O
a	O
reliable	B-Protocol
connection-oriented	B-Protocol
transport	O
.	O
</s>
<s>
This	O
protocol	O
is	O
used	O
to	O
establish	O
a	O
connection	O
to	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
port	B-Protocol
number	I-Protocol
23	O
or	O
2323	O
,	O
where	O
a	O
Telnet	B-Protocol
server	O
application	O
is	O
listening	O
.	O
</s>
<s>
Telnet	B-Protocol
predated	O
TCP/IP	O
and	O
originally	O
ran	O
over	O
Network	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
NCP	O
)	O
.	O
</s>
<s>
The	O
telnet	B-Protocol
service	O
is	O
best	O
understood	O
in	O
the	O
context	O
of	O
a	O
user	O
with	O
a	O
simple	O
terminal	O
using	O
the	O
local	O
Telnet	B-Protocol
program	O
(	O
known	O
as	O
the	O
client	B-Protocol
program	I-Protocol
)	O
to	O
run	O
a	O
logon	O
session	O
on	O
a	O
remote	O
computer	O
where	O
the	O
user	O
's	O
communications	O
needs	O
are	O
handled	O
by	O
a	O
Telnet	B-Protocol
server	O
program	O
.	O
</s>
<s>
Even	O
though	O
Telnet	B-Protocol
was	O
an	O
ad	O
hoc	O
protocol	O
with	O
no	O
official	O
definition	O
until	O
March	O
5	O
,	O
1973	O
,	O
the	O
name	O
actually	O
referred	O
to	O
Teletype	O
Over	O
Network	O
Protocol	O
as	O
the	O
RFC	O
206	O
(	O
NIC	O
7176	O
)	O
on	O
Telnet	B-Protocol
makes	O
the	O
connection	O
clear	O
:	O
</s>
<s>
Essentially	O
,	O
it	O
used	O
an	O
8-bit	O
channel	O
to	O
exchange	O
7-bit	B-Protocol
ASCII	I-Protocol
data	O
.	O
</s>
<s>
Any	O
byte	O
with	O
the	O
high	O
bit	O
set	O
was	O
a	O
special	O
Telnet	B-Protocol
character	O
.	O
</s>
<s>
On	O
March	O
5	O
,	O
1973	O
,	O
a	O
Telnet	B-Protocol
protocol	O
standard	O
was	O
defined	O
at	O
UCLA	O
with	O
the	O
publication	O
of	O
two	O
NIC	O
documents	O
:	O
Telnet	B-Protocol
Protocol	O
Specification	O
,	O
NIC	O
15372	O
,	O
and	O
Telnet	B-Protocol
Option	O
Specifications	O
,	O
NIC	O
15373	O
.	O
</s>
<s>
Many	O
extensions	O
were	O
made	O
for	O
Telnet	B-Protocol
because	O
of	O
its	O
negotiable	O
options	O
protocol	O
architecture	O
.	O
</s>
<s>
The	O
Telnet	B-Protocol
service	O
is	O
the	O
application	O
providing	O
services	O
over	O
the	O
Telnet	B-Protocol
protocol	O
.	O
</s>
<s>
Most	O
operating	O
systems	O
provide	O
a	O
service	O
that	O
can	O
be	O
installed	O
or	O
enabled	O
to	O
provide	O
Telnet	B-Protocol
services	O
to	O
clients	O
.	O
</s>
<s>
Telnet	B-Protocol
is	O
vulnerable	O
to	O
network-based	O
cyberattacks	O
,	O
such	O
as	O
packet	B-Protocol
sniffing	I-Protocol
sensitive	O
information	O
including	O
passwords	O
and	O
fingerprinting	B-Algorithm
.	O
</s>
<s>
Telnet	B-Protocol
services	O
can	O
also	O
be	O
exploited	O
to	O
leak	O
information	O
about	O
the	O
server	O
(	O
such	O
as	O
hostnames	O
,	O
IP	O
addresses	O
and	O
brand	O
)	O
by	O
packet	B-Protocol
sniffing	I-Protocol
the	O
banner	O
.	O
</s>
<s>
This	O
information	O
can	O
then	O
be	O
searched	O
to	O
determine	O
if	O
a	O
Telnet	B-Protocol
service	O
accepts	O
a	O
connection	O
without	O
authentication	O
.	O
</s>
<s>
Telnet	B-Protocol
is	O
also	O
frequently	O
exploited	O
by	O
malware	O
due	O
to	O
being	O
improperly	O
configured	O
.	O
</s>
<s>
In	O
fact	O
,	O
Telnet	B-Protocol
is	O
targeted	O
by	O
attackers	O
more	O
frequently	O
than	O
other	O
common	O
protocols	O
,	O
especially	O
when	O
compared	O
to	O
UPnP	B-Application
,	O
CoAP	B-Device
,	O
MQTT	B-Application
,	O
AMQP	B-Operating_System
and	O
XMPP	B-Protocol
.	O
</s>
<s>
Common	O
devices	O
targeted	O
are	O
Internet	B-Operating_System
of	I-Operating_System
things	I-Operating_System
devices	I-Operating_System
,	O
routers	B-Protocol
and	O
modems	O
.	O
</s>
<s>
The	O
SANS	O
Institute	O
recommends	O
that	O
the	O
use	O
of	O
Telnet	B-Protocol
for	O
remote	B-Protocol
logins	I-Protocol
should	O
be	O
discontinued	O
under	O
normal	O
circumstances	O
for	O
the	O
following	O
reasons	O
:	O
</s>
<s>
Telnet	B-Protocol
,	O
by	O
default	O
,	O
does	O
not	O
encrypt	O
any	O
data	O
sent	O
over	O
the	O
connection	O
(	O
including	O
passwords	O
)	O
,	O
and	O
so	O
it	O
is	O
often	O
feasible	O
to	O
eavesdrop	O
on	O
the	O
communications	O
and	O
use	O
the	O
password	O
later	O
for	O
malicious	O
purposes	O
;	O
anybody	O
who	O
has	O
access	O
to	O
a	O
router	B-Protocol
,	O
switch	B-Protocol
,	O
hub	O
or	O
gateway	B-Application
located	O
on	O
the	O
network	O
between	O
the	O
two	O
hosts	O
where	O
Telnet	B-Protocol
is	O
being	O
used	O
can	O
intercept	O
the	O
packets	O
passing	O
by	O
and	O
obtain	O
login	O
,	O
password	O
and	O
whatever	O
else	O
is	O
typed	O
with	O
a	O
packet	B-Protocol
analyzer	I-Protocol
.	O
</s>
<s>
Most	O
Telnet	B-Protocol
implementations	O
lack	O
authentication	O
.	O
</s>
<s>
An	O
estimated	O
22,887	O
Telnet-enabled	O
devices	O
found	O
by	O
security	O
researchers	O
not	O
only	O
lacked	O
authentication	O
but	O
also	O
provided	O
unrestricted	B-Application
access	I-Application
to	O
the	O
system	O
.	O
</s>
<s>
Most	O
Telnet	B-Protocol
authentication	O
mechanisms	O
are	O
vulnerable	O
to	O
intercepted	O
in	O
the	O
middle	O
attacks	O
.	O
</s>
<s>
Extensions	O
to	O
Telnet	B-Protocol
provide	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
security	O
and	O
Simple	O
Authentication	O
and	O
Security	O
Layer	O
(	O
SASL	O
)	O
authentication	O
that	O
address	O
the	O
above	O
concerns	O
.	O
</s>
<s>
However	O
,	O
most	O
Telnet	B-Protocol
implementations	O
do	O
not	O
support	O
these	O
extensions	O
;	O
and	O
they	O
do	O
not	O
address	O
other	O
vulnerabilities	O
such	O
as	O
parsing	O
the	O
banner	O
information	O
.	O
</s>
<s>
IBM	B-Device
5250	I-Device
or	O
3270	O
workstation	O
emulation	O
is	O
supported	O
via	O
custom	O
telnet	B-Protocol
clients	O
,	O
TN5250/TN3270	O
,	O
and	O
IBM	B-Application
i	I-Application
systems	O
.	O
</s>
<s>
Clients	O
and	O
servers	O
designed	O
to	O
pass	O
IBM	B-Device
5250	I-Device
data	O
streams	O
over	O
Telnet	B-Protocol
generally	O
do	O
support	O
SSL	B-Protocol
encryption	I-Protocol
,	O
as	O
SSH	B-Protocol
does	O
not	O
include	O
5250	O
emulation	O
.	O
</s>
<s>
Under	O
IBM	B-Application
i	I-Application
(	O
also	O
known	O
as	O
OS/400	B-Application
)	O
,	O
port	O
992	O
is	O
the	O
default	O
port	O
for	O
secured	O
telnet	B-Protocol
.	O
</s>
<s>
Historically	O
,	O
Telnet	B-Protocol
provided	O
access	O
to	O
a	O
command-line	B-Application
interface	I-Application
on	O
a	O
remote	O
host	O
.	O
</s>
<s>
However	O
,	O
because	O
of	O
serious	O
security	O
concerns	O
when	O
using	O
Telnet	B-Protocol
over	O
an	O
open	O
network	O
such	O
as	O
the	O
Internet	O
,	O
its	O
use	O
for	O
this	O
purpose	O
has	O
waned	O
significantly	O
in	O
favor	O
of	O
SSH	B-Protocol
.	O
</s>
<s>
The	O
usage	O
of	O
Telnet	B-Protocol
for	O
remote	B-Protocol
management	I-Protocol
has	O
declined	O
rapidly	O
,	O
especially	O
on	O
the	O
public	O
Internet	O
,	O
in	O
favor	O
of	O
the	O
Secure	B-Protocol
Shell	I-Protocol
(	O
SSH	B-Protocol
)	O
protocol	O
.	O
</s>
<s>
SSH	B-Protocol
provides	O
much	O
of	O
the	O
functionality	O
of	O
telnet	B-Protocol
,	O
with	O
the	O
addition	O
of	O
strong	O
encryption	O
to	O
prevent	O
sensitive	O
data	O
such	O
as	O
passwords	O
from	O
being	O
intercepted	O
,	O
and	O
public	B-Application
key	I-Application
authentication	O
,	O
to	O
ensure	O
that	O
the	O
remote	O
computer	O
is	O
actually	O
who	O
it	O
claims	O
to	O
be	O
.	O
</s>
<s>
Telnet	B-Protocol
may	O
be	O
used	O
in	O
debugging	O
network	O
services	O
such	O
as	O
SMTP	B-Protocol
,	O
IRC	B-Protocol
,	O
HTTP	B-Protocol
,	O
FTP	B-Protocol
or	O
POP3	B-Protocol
,	O
to	O
issue	O
commands	O
to	O
a	O
server	O
and	O
examine	O
the	O
responses	O
.	O
</s>
<s>
For	O
example	O
,	O
Telnet	B-Protocol
client	B-Protocol
applications	I-Protocol
can	O
establish	O
an	O
interactive	O
TCP	O
session	O
to	O
a	O
port	O
other	O
than	O
the	O
Telnet	B-Protocol
server	O
port	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
command	B-Application
line	I-Application
telnet	B-Protocol
client	B-Protocol
could	O
make	O
an	O
HTTP	B-Protocol
request	O
to	O
a	O
web	O
server	O
on	O
TCP	B-Protocol
port	I-Protocol
80	O
as	O
follows	O
:	O
</s>
<s>
For	O
example	O
,	O
a	O
large	O
number	O
of	O
industrial	O
and	O
scientific	O
devices	O
only	O
have	O
Telnet	B-Protocol
available	O
as	O
a	O
communication	O
option	O
.	O
</s>
<s>
Some	O
are	O
built	O
with	O
only	O
a	O
standard	O
RS-232	O
port	O
and	O
use	O
a	O
serial	O
server	O
hardware	O
appliance	O
to	O
provide	O
the	O
translation	O
between	O
the	O
TCP/Telnet	O
data	O
and	O
the	O
RS-232	O
serial	O
data	O
.	O
</s>
<s>
In	O
such	O
cases	O
,	O
SSH	B-Protocol
is	O
not	O
an	O
option	O
unless	O
the	O
interface	O
appliance	O
can	O
be	O
configured	O
for	O
SSH	B-Protocol
(	O
or	O
is	O
replaced	O
with	O
one	O
supporting	O
SSH	B-Protocol
)	O
.	O
</s>
<s>
Telnet	B-Protocol
is	O
commonly	O
used	O
by	O
amateur	O
radio	O
operators	O
for	O
providing	O
public	O
information	O
.	O
</s>
<s>
Although	O
recommended	O
against	O
,	O
security	O
researchers	O
estimated	O
that	O
7,096,465	O
devices	O
connected	O
to	O
the	O
Internet	O
continue	O
to	O
use	O
Telnet	B-Protocol
,	O
however	O
,	O
much	O
less	O
are	O
often	O
estimated	O
because	O
most	O
estimates	O
only	O
scan	O
for	O
TCP	B-Protocol
port	I-Protocol
23	O
.	O
</s>
<s>
The	O
technical	O
details	O
of	O
Telnet	B-Protocol
are	O
defined	O
by	O
a	O
variety	O
of	O
specifications	O
including	O
RFC	O
854	O
.	O
</s>
<s>
Telnet	B-Protocol
commands	O
consist	O
of	O
at	O
least	O
two	O
bytes	O
.	O
</s>
<s>
All	O
data	O
octets	O
except	O
0xff	O
are	O
transmitted	O
over	O
Telnet	B-Protocol
as	O
is	O
.	O
</s>
<s>
(	O
0xff	O
,	O
or	O
255	O
in	O
decimal	O
,	O
is	O
the	O
IAC	O
byte	O
(	O
Interpret	O
As	O
Command	O
)	O
which	O
signals	O
that	O
the	O
next	O
byte	O
is	O
a	O
telnet	B-Protocol
command	O
.	O
</s>
<s>
The	O
command	O
to	O
insert	O
0xff	O
into	O
the	O
stream	O
is	O
0xff	O
,	O
so	O
0xff	O
must	O
be	O
escaped	O
by	O
doubling	O
it	O
when	O
sending	O
data	O
over	O
the	O
telnet	B-Protocol
protocol	O
.	O
)	O
</s>
<s>
Telnet	B-Protocol
also	O
has	O
a	O
variety	O
of	O
options	O
that	O
terminals	B-General_Concept
implementing	O
Telnet	B-Protocol
should	O
support	O
.	O
</s>
<s>
PuTTY	B-Operating_System
and	O
plink	O
command	B-Application
line	I-Application
are	O
a	O
free	O
,	O
open-source	O
SSH	B-Protocol
,	O
Telnet	B-Protocol
,	O
rlogin	B-Application
,	O
and	O
raw	B-Protocol
TCP	I-Protocol
client	B-Protocol
for	O
Windows	B-Application
,	O
Linux	B-Application
,	O
and	O
Unix	B-Application
.	O
</s>
<s>
AbsoluteTelnet	B-Protocol
is	O
a	O
telnet	B-Protocol
client	B-Protocol
for	O
Windows	B-Application
.	O
</s>
<s>
It	O
also	O
supports	O
SSH	B-Protocol
and	O
SFTP	B-Protocol
,	O
</s>
<s>
Rtelnet	B-Protocol
is	O
a	O
SOCKS	B-Application
client	B-Protocol
version	O
of	O
Telnet	B-Protocol
,	O
providing	O
similar	O
functionality	O
of	O
telnet	B-Protocol
to	O
those	O
hosts	O
which	O
are	O
behind	O
firewall	B-Application
and	O
NAT	B-Protocol
.	O
</s>
<s>
Inetutils	B-Application
includes	O
a	O
telnet	B-Protocol
client	B-Protocol
and	O
server	O
and	O
is	O
installed	O
by	O
default	O
on	O
many	O
Linux	B-Application
distributions	O
.	O
</s>
<s>
telnet.exe	O
command	B-Application
line	I-Application
utility	O
included	O
in	O
default	O
installation	O
of	O
many	O
versions	O
of	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
Star	O
Wars	O
:	O
Episode	O
IV	O
–	O
A	O
New	O
Hope	O
from	O
1977	O
has	O
been	O
recreated	O
as	O
a	O
text	O
art	O
movie	O
served	O
through	O
Telnet	B-Protocol
.	O
</s>
