<s>
In	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
,	O
an	O
X	B-Language
display	I-Language
manager	I-Language
is	O
a	O
graphical	O
login	B-Device
manager	I-Device
which	O
starts	O
a	O
login	B-Operating_System
session	I-Operating_System
on	O
an	O
X	B-Operating_System
server	I-Operating_System
from	O
the	O
same	O
or	O
another	O
computer	O
.	O
</s>
<s>
A	O
display	B-Device
manager	I-Device
presents	O
the	O
user	O
with	O
a	O
login	B-General_Concept
screen	I-General_Concept
.	O
</s>
<s>
When	O
the	O
display	B-Device
manager	I-Device
runs	O
on	O
the	O
user	O
's	O
computer	O
,	O
it	O
starts	O
the	O
X	B-Operating_System
server	I-Operating_System
before	O
presenting	O
the	O
user	O
the	O
login	B-General_Concept
screen	I-General_Concept
,	O
optionally	O
repeating	O
when	O
the	O
user	O
logs	O
out	O
.	O
</s>
<s>
In	O
this	O
condition	O
,	O
the	O
DM	O
realizes	O
in	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
the	O
functionality	O
of	O
and	O
on	O
character-mode	B-General_Concept
terminals	I-General_Concept
.	O
</s>
<s>
When	O
the	O
display	B-Device
manager	I-Device
runs	O
on	O
a	O
remote	O
computer	O
,	O
it	O
acts	O
like	O
a	O
telnet	B-Protocol
server	O
,	O
requesting	O
username	O
and	O
password	O
and	O
starting	O
a	O
remote	O
session	O
.	O
</s>
<s>
X11	B-Operating_System
Release	O
3	O
introduced	O
display	B-Device
managers	I-Device
in	O
October	O
1988	O
with	O
the	O
aim	O
of	O
supporting	O
the	O
standalone	O
X	B-Protocol
terminals	I-Protocol
,	O
just	O
coming	O
onto	O
the	O
market	O
.	O
</s>
<s>
Various	O
display	B-Device
managers	I-Device
continue	O
in	O
routine	O
use	O
to	O
provide	O
a	O
graphical	O
login	B-General_Concept
prompt	O
on	O
standalone	O
computer	B-Device
workstations	I-Device
running	O
X	B-Operating_System
.	O
X11R4	B-Operating_System
introduced	O
the	O
X	B-Language
Display	I-Language
Manager	I-Language
Control	O
Protocol	O
(	O
XDMCP	O
)	O
in	O
December	O
1989	O
to	O
fix	O
problems	O
in	O
the	O
X11R3	O
implementation	O
.	O
</s>
<s>
XDM	B-Language
(	O
the	O
X	B-Operating_System
Window	I-Operating_System
Display	B-Device
Manager	I-Device
)	O
originated	O
in	O
X11R3	O
.	O
</s>
<s>
This	O
first	O
version	O
,	O
written	O
by	O
Keith	O
Packard	O
of	O
the	O
MIT	O
X	B-Operating_System
Consortium	O
,	O
had	O
several	O
limitations	O
,	O
the	O
most	O
notable	O
of	O
which	O
was	O
that	O
it	O
could	O
not	O
detect	O
when	O
users	O
switched	O
X	B-Protocol
terminals	I-Protocol
off	O
and	O
on	O
.	O
</s>
<s>
In	O
X11R3	O
,	O
XDM	B-Language
only	O
knew	O
about	O
an	O
X	B-Protocol
terminal	I-Protocol
from	O
its	O
entry	O
in	O
the	O
file	O
,	O
but	O
XDM	B-Language
only	O
consulted	O
this	O
file	O
when	O
it	O
started	O
.	O
</s>
<s>
Thus	O
every	O
time	O
a	O
user	O
switched	O
a	O
terminal	O
off	O
and	O
on	O
,	O
the	O
system	B-Application
administrator	I-Application
had	O
to	O
send	O
a	O
SIGHUP	B-General_Concept
signal	B-Operating_System
to	O
XDM	B-Language
to	O
instruct	O
it	O
to	O
rescan	O
.	O
</s>
<s>
XDMCP	O
arrived	O
with	O
the	O
introduction	O
of	O
X11R4	B-Operating_System
(	O
December	O
1989	O
)	O
.	O
</s>
<s>
With	O
XDMCP	O
,	O
the	O
X	B-Operating_System
server	I-Operating_System
must	O
actively	O
request	O
a	O
display	B-Device
manager	I-Device
connection	O
from	O
the	O
host	O
.	O
</s>
<s>
An	O
X	B-Operating_System
server	I-Operating_System
using	O
XDMCP	O
therefore	O
no	O
longer	O
requires	O
an	O
entry	O
in	O
.	O
</s>
<s>
A	O
display	B-Device
manager	I-Device
can	O
run	O
on	O
the	O
same	O
computer	O
where	O
the	O
user	O
sits	O
—	O
starting	O
one	O
or	O
more	O
X	B-Operating_System
servers	I-Operating_System
,	O
displaying	O
the	O
login	B-General_Concept
screen	I-General_Concept
at	O
the	O
beginning	O
and	O
(	O
optionally	O
)	O
every	O
time	O
the	O
user	O
logs	O
out	O
—	O
or	O
on	O
a	O
remote	O
one	O
,	O
working	O
according	O
to	O
the	O
XDMCP	O
protocol	O
.	O
</s>
<s>
The	O
XDMCP	O
protocol	O
mandates	O
that	O
the	O
X	B-Operating_System
server	I-Operating_System
starts	O
autonomously	O
and	O
connects	O
to	O
the	O
display	B-Device
manager	I-Device
.	O
</s>
<s>
In	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
paradigm	O
,	O
the	O
server	O
runs	O
on	O
the	O
computer	O
providing	O
the	O
display	O
and	O
input	O
devices	O
.	O
</s>
<s>
A	O
server	O
can	O
connect	O
,	O
using	O
the	O
XDMCP	O
protocol	O
,	O
to	O
a	O
display	B-Device
manager	I-Device
running	O
on	O
another	O
computer	O
,	O
requesting	O
it	O
to	O
start	O
the	O
session	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
X	B-Operating_System
server	I-Operating_System
acts	O
as	O
a	O
graphical	O
telnet	B-Protocol
client	O
while	O
the	O
display	B-Device
manager	I-Device
acts	O
like	O
a	O
telnet	B-Protocol
server	O
:	O
users	O
start	O
programs	O
from	O
the	O
computer	O
running	O
the	O
display	B-Device
manager	I-Device
,	O
while	O
their	O
input	O
and	O
output	O
take	O
place	O
on	O
the	O
computer	O
where	O
the	O
server	O
(	O
and	O
the	O
user	O
)	O
sits	O
.	O
</s>
<s>
An	O
administrator	O
can	O
typically	O
configure	O
an	O
XDMCP	O
Chooser	O
program	O
running	O
on	O
the	O
local	O
computer	O
or	O
X	B-Protocol
terminal	I-Protocol
to	O
connect	O
to	O
a	O
specific	O
host	O
's	O
X	B-Language
display	I-Language
manager	I-Language
or	O
to	O
display	O
a	O
list	O
of	O
suitable	O
hosts	O
that	O
the	O
user	O
can	O
choose	O
from	O
.	O
</s>
<s>
a	O
set	O
of	O
hosts	O
(	O
on	O
the	O
local	O
TCP/IP	B-Protocol
subnet	B-Protocol
)	O
that	O
the	O
XDMCP	O
Chooser	O
determines	O
by	O
a	O
network	B-Architecture
broadcast	I-Architecture
to	O
the	O
available	O
display	B-Device
managers	I-Device
.	O
</s>
<s>
When	O
the	O
user	O
selects	O
a	O
host	O
from	O
the	O
list	O
,	O
the	O
XDMCP	O
Chooser	O
running	O
on	O
the	O
local	O
machine	O
will	O
send	O
a	O
message	O
to	O
the	O
selected	O
remote	O
computer	O
's	O
display	B-Device
manager	I-Device
and	O
instruct	O
it	O
to	O
connect	O
the	O
X	B-Operating_System
server	I-Operating_System
on	O
the	O
local	O
computer	O
or	O
terminal	O
.	O
</s>
<s>
The	O
X	B-Language
Display	I-Language
Manager	I-Language
Control	O
Protocol	O
(	O
XDMCP	O
)	O
uses	O
UDP	B-Protocol
port	B-Protocol
177	O
.	O
</s>
<s>
An	O
X	B-Operating_System
server	I-Operating_System
requests	O
that	O
a	O
display	B-Device
manager	I-Device
start	O
a	O
session	O
by	O
sending	O
a	O
Query	O
packet	O
.	O
</s>
<s>
If	O
the	O
display	B-Device
manager	I-Device
allows	O
access	O
for	O
that	O
X	B-Operating_System
server	I-Operating_System
,	O
it	O
responds	O
by	O
sending	O
a	O
Willing	O
packet	O
back	O
to	O
the	O
X	B-Operating_System
server	I-Operating_System
.	O
</s>
<s>
(	O
The	O
X	B-Operating_System
server	I-Operating_System
can	O
also	O
send	O
BroadcastQuery	O
or	O
IndirectQuery	O
packets	O
to	O
start	O
a	O
session	O
-	O
this	O
mechanism	O
for	O
requesting	O
a	O
session	O
resembles	O
using	O
DHCP	B-Protocol
to	O
request	O
an	O
IP	O
address	O
.	O
)	O
</s>
<s>
The	O
display	B-Device
manager	I-Device
must	O
authenticate	O
itself	O
to	O
the	O
server	O
.	O
</s>
<s>
To	O
do	O
this	O
the	O
X	B-Operating_System
server	I-Operating_System
sends	O
a	O
Request	O
packet	O
to	O
the	O
display	B-Device
manager	I-Device
,	O
which	O
returns	O
an	O
Accept	O
packet	O
.	O
</s>
<s>
If	O
the	O
Accept	O
packet	O
contains	O
the	O
response	O
the	O
X	B-Operating_System
server	I-Operating_System
expects	O
,	O
the	O
display	B-Device
manager	I-Device
is	O
authenticated	O
.	O
</s>
<s>
Producing	O
the	O
correct	O
response	O
might	O
require	O
the	O
display	B-Device
manager	I-Device
to	O
have	O
access	O
to	O
a	O
secret	O
key	O
,	O
for	O
example	O
.	O
</s>
<s>
If	O
authentication	O
succeeds	O
,	O
the	O
X	B-Operating_System
server	I-Operating_System
sends	O
a	O
Manage	O
packet	O
to	O
inform	O
the	O
display	B-Device
manager	I-Device
.	O
</s>
<s>
Then	O
the	O
display	B-Device
manager	I-Device
displays	O
its	O
login	B-General_Concept
screen	I-General_Concept
by	O
connecting	O
to	O
the	O
X	B-Operating_System
server	I-Operating_System
as	O
a	O
regular	O
X	B-Operating_System
client	O
.	O
</s>
<s>
During	O
the	O
session	O
,	O
the	O
server	O
can	O
send	O
KeepAlive	O
packets	O
to	O
the	O
display	B-Device
manager	I-Device
at	O
intervals	O
.	O
</s>
<s>
If	O
the	O
display	B-Device
manager	I-Device
fails	O
to	O
respond	O
with	O
an	O
Alive	O
packet	O
within	O
a	O
certain	O
time	O
,	O
the	O
X	B-Operating_System
server	I-Operating_System
presumes	O
that	O
the	O
display	B-Device
manager	I-Device
has	O
ceased	O
running	O
,	O
and	O
can	O
terminate	O
the	O
connection	O
.	O
</s>
<s>
One	O
problem	O
with	O
XDMCP	O
is	O
that	O
,	O
similarly	O
to	O
telnet	B-Protocol
,	O
the	O
authentication	O
takes	O
place	O
unencrypted	O
.	O
</s>
<s>
It	O
is	O
more	O
secure	O
to	O
use	O
an	O
ssh	B-Protocol
tunnel	B-Protocol
for	O
X	B-Operating_System
traffic	O
.	O
</s>
<s>
The	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
supplies	O
XDM	B-Language
as	O
its	O
standard	O
display	B-Device
manager	I-Device
.	O
</s>
<s>
Programmers	O
have	O
developed	O
other	O
X	B-Language
display	I-Language
managers	I-Language
,	O
both	O
commercial	O
and	O
free	O
,	O
offering	O
additional	O
functionality	O
over	O
the	O
basic	O
display	O
management	O
:	O
</s>
<s>
SDDM	B-Language
,	O
recommended	O
display	B-Device
manager	I-Device
for	O
KDE	B-Language
Plasma	I-Language
5	I-Language
and	O
LXQt	B-Language
.	O
</s>
<s>
Successor	O
to	O
KDM	B-Language
.	O
</s>
<s>
LightDM	B-Device
,	O
a	O
lightweight	O
,	O
modular	O
,	O
cross-desktop	O
,	O
fully	O
themeable	O
desktop	O
display	B-Device
manager	I-Device
by	O
Canonical	O
Ltd	O
.	O
</s>
<s>
,	O
a	O
lightweight	O
,	O
secure	O
and	O
login	B-General_Concept
like	O
console	O
display	B-Device
manager	I-Device
for	O
X	B-Operating_System
,	O
written	O
in	O
C	O
.	O
</s>
<s>
for	O
XDM	B-Language
.	O
</s>
<s>
Easy	O
full	O
install	O
,	O
Xhost	O
Phonebook	O
,	O
X	B-Operating_System
Login	B-General_Concept
,	O
X	B-Operating_System
Desktop	O
Chooser	O
,	O
menu-reconfig	O
,	O
repair	O
utils	O
.	O
</s>
<s>
,	O
a	O
graphical	O
display	B-Device
manager	I-Device
developed	O
for	O
Linux	O
Mint	O
.	O
</s>
<s>
SLiM	B-Device
,	O
an	O
independent	O
login	B-Device
manager	I-Device
.	O
</s>
<s>
On	O
some	O
Unix	O
distributions	O
,	O
the	O
default	O
display	B-Device
manager	I-Device
is	O
selected	O
in	O
file	O
$	O
PREFIX/etc/X11/default	O
-display-manager	O
.	O
</s>
