<s>
The	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
(	O
DHCP	B-Protocol
)	O
is	O
a	O
network	O
management	O
protocol	O
used	O
on	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
networks	O
for	O
automatically	O
assigning	O
IP	B-Protocol
addresses	I-Protocol
and	O
other	O
communication	O
parameters	O
to	O
devices	O
connected	O
to	O
the	O
network	O
using	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
The	O
technology	O
eliminates	O
the	O
need	O
for	O
individually	O
configuring	O
network	O
devices	O
manually	O
,	O
and	O
consists	O
of	O
two	O
network	O
components	O
,	O
a	O
centrally	O
installed	O
network	O
DHCP	B-Protocol
server	I-Protocol
and	O
client	O
instances	O
of	O
the	O
protocol	B-Protocol
stack	I-Protocol
on	O
each	O
computer	O
or	O
device	O
.	O
</s>
<s>
When	O
connected	O
to	O
the	O
network	O
,	O
and	O
periodically	O
thereafter	O
,	O
a	O
client	O
requests	B-Protocol
a	O
set	O
of	O
parameters	O
from	O
the	O
server	B-Application
using	O
DHCP	B-Protocol
.	O
</s>
<s>
DHCP	B-Protocol
can	O
be	O
implemented	O
on	O
networks	O
ranging	O
in	O
size	O
from	O
residential	O
networks	O
to	O
large	O
campus	B-General_Concept
networks	I-General_Concept
and	O
regional	O
ISP	O
networks	O
.	O
</s>
<s>
Many	O
routers	B-Protocol
and	O
residential	B-Application
gateways	I-Application
have	O
DHCP	B-Protocol
server	I-Protocol
capability	O
.	O
</s>
<s>
Most	O
residential	O
network	B-Protocol
routers	I-Protocol
receive	O
a	O
unique	O
IP	B-Protocol
address	I-Protocol
within	O
the	O
ISP	O
network	O
.	O
</s>
<s>
Within	O
a	O
local	B-General_Concept
network	I-General_Concept
,	O
a	O
DHCP	B-Protocol
server	I-Protocol
assigns	O
a	O
local	O
IP	B-Protocol
address	I-Protocol
to	O
each	O
device	O
.	O
</s>
<s>
DHCP	B-Protocol
services	O
exist	O
for	O
networks	O
running	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
,	O
as	O
well	O
as	O
version	O
6	O
(	O
IPv6	B-Protocol
)	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
version	O
of	O
the	O
DHCP	B-Protocol
protocol	O
is	O
commonly	O
called	O
DHCPv6	B-Protocol
.	O
</s>
<s>
The	O
Reverse	B-Protocol
Address	I-Protocol
Resolution	I-Protocol
Protocol	I-Protocol
(	O
RARP	B-Protocol
)	O
was	O
defined	O
in	O
1984	O
for	O
the	O
configuration	O
of	O
simple	O
devices	O
,	O
such	O
as	O
diskless	B-Device
workstations	I-Device
,	O
with	O
a	O
suitable	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Acting	O
in	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
it	O
made	O
implementation	O
difficult	O
on	O
many	O
server	B-Application
platforms	O
.	O
</s>
<s>
It	O
required	O
that	O
a	O
server	B-Application
be	O
present	O
on	O
each	O
individual	O
network	O
link	O
.	O
</s>
<s>
RARP	B-Protocol
was	O
superseded	O
by	O
the	O
Bootstrap	B-Device
Protocol	I-Device
(	O
BOOTP	B-Device
)	O
defined	O
in	O
September	O
1985	O
.	O
</s>
<s>
This	O
introduced	O
the	O
concept	O
of	O
a	O
relay	O
agent	O
,	O
which	O
allowed	O
the	O
forwarding	O
of	O
BOOTP	B-Device
packets	O
across	O
networks	O
,	O
allowing	O
one	O
central	O
BOOTP	B-Device
server	B-Application
to	O
serve	O
hosts	O
on	O
many	O
IP	O
subnets	O
.	O
</s>
<s>
DHCP	B-Protocol
was	O
first	O
defined	O
in	O
October	O
1993	O
.	O
</s>
<s>
It	O
is	O
based	O
on	O
BOOTP	B-Device
,	O
but	O
can	O
dynamically	O
allocate	O
IP	B-Protocol
addresses	I-Protocol
from	O
a	O
pool	O
and	O
reclaim	O
them	O
when	O
they	O
are	O
no	O
longer	O
in	O
use	O
.	O
</s>
<s>
Four	O
years	O
later	O
,	O
the	O
DHCPINFORM	O
message	O
type	O
(	O
used	O
for	O
WPAD	B-Protocol
)	O
and	O
other	O
small	O
changes	O
were	O
added	O
.	O
</s>
<s>
This	O
definition	O
,	O
from	O
1997	O
,	O
remains	O
the	O
core	O
of	O
the	O
standard	O
for	O
IPv4	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
DHCPv6	B-Protocol
was	O
initially	O
defined	O
in	O
2003	O
.	O
</s>
<s>
After	O
updates	O
by	O
many	O
subsequent	O
RFCs	O
,	O
its	O
definition	O
was	O
replaced	O
in	O
2018	O
,	O
where	O
prefix	B-Protocol
delegation	I-Protocol
and	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
were	O
now	O
merged	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
defines	O
how	O
devices	O
communicate	O
within	O
and	O
across	O
local	B-General_Concept
networks	I-General_Concept
on	O
the	O
Internet	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
server	I-Protocol
can	O
manage	O
IP	O
settings	O
for	O
devices	O
on	O
its	O
local	B-General_Concept
network	I-General_Concept
,	O
e.g.	O
,	O
by	O
assigning	O
IP	B-Protocol
addresses	I-Protocol
to	O
those	O
devices	O
automatically	O
and	O
dynamically	O
.	O
</s>
<s>
DHCP	B-Protocol
operates	O
based	O
on	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
.	O
</s>
<s>
When	O
a	O
computer	O
or	O
other	O
device	O
connects	O
to	O
a	O
network	O
,	O
the	O
DHCP	B-Protocol
client	O
software	O
sends	O
a	O
DHCP	B-Protocol
broadcast	B-Operating_System
query	O
requesting	O
the	O
necessary	O
information	O
.	O
</s>
<s>
Any	O
DHCP	B-Protocol
server	I-Protocol
on	O
the	O
network	O
may	O
service	O
the	O
request	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
server	I-Protocol
manages	O
a	O
pool	O
of	O
IP	B-Protocol
addresses	I-Protocol
and	O
information	O
about	O
client	O
configuration	O
parameters	O
such	O
as	O
default	B-Protocol
gateway	I-Protocol
,	O
domain	O
name	O
,	O
the	O
name	B-Protocol
servers	I-Protocol
,	O
and	O
time	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
On	O
receiving	O
a	O
DHCP	B-Protocol
request	O
,	O
the	O
DHCP	B-Protocol
server	I-Protocol
may	O
respond	O
with	O
specific	O
information	O
for	O
each	O
client	O
,	O
as	O
previously	O
configured	O
by	O
an	O
administrator	O
,	O
or	O
with	O
a	O
specific	O
address	O
and	O
any	O
other	O
information	O
valid	O
for	O
the	O
entire	O
network	O
and	O
for	O
the	O
time	O
period	O
for	O
which	O
the	O
allocation	O
(	O
lease	O
)	O
is	O
valid	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
client	O
typically	O
queries	O
this	O
information	O
immediately	O
after	O
booting	B-Operating_System
,	O
and	O
periodically	O
thereafter	O
before	O
the	O
expiration	O
of	O
the	O
information	O
.	O
</s>
<s>
When	O
a	O
DHCP	B-Protocol
client	O
refreshes	O
an	O
assignment	O
,	O
it	O
initially	O
requests	B-Protocol
the	O
same	O
parameter	O
values	O
,	O
but	O
the	O
DHCP	B-Protocol
server	I-Protocol
may	O
assign	O
a	O
new	O
address	O
based	O
on	O
the	O
assignment	O
policies	O
set	O
by	O
administrators	O
.	O
</s>
<s>
On	O
large	O
networks	O
that	O
consist	O
of	O
multiple	O
links	O
,	O
a	O
single	O
DHCP	B-Protocol
server	I-Protocol
may	O
service	O
the	O
entire	O
network	O
when	O
aided	O
by	O
DHCP	B-Protocol
relay	O
agents	O
located	O
on	O
the	O
interconnecting	O
routers	B-Protocol
.	O
</s>
<s>
Such	O
agents	O
relay	O
messages	O
between	O
DHCP	B-Protocol
clients	O
and	O
DHCP	B-Protocol
servers	I-Protocol
located	O
on	O
different	O
subnets	O
.	O
</s>
<s>
Depending	O
on	O
implementation	O
,	O
the	O
DHCP	B-Protocol
server	I-Protocol
may	O
have	O
three	O
methods	O
of	O
allocating	O
IP	B-Protocol
addresses	I-Protocol
:	O
</s>
<s>
Dynamic	O
allocation	O
A	O
network	O
administrator	O
reserves	O
a	O
range	O
of	O
IP	B-Protocol
addresses	I-Protocol
for	O
DHCP	B-Protocol
,	O
and	O
each	O
DHCP	B-Protocol
client	O
on	O
the	O
LAN	B-General_Concept
is	O
configured	O
to	O
request	O
an	O
IP	B-Protocol
address	I-Protocol
from	O
the	O
DHCP	B-Protocol
server	I-Protocol
during	O
network	O
initialization	O
.	O
</s>
<s>
The	O
request-and-grant	O
process	O
uses	O
a	O
lease	O
concept	O
with	O
a	O
controllable	O
time	O
period	O
,	O
allowing	O
the	O
DHCP	B-Protocol
server	I-Protocol
to	O
reclaim	O
and	O
then	O
reallocate	O
IP	B-Protocol
addresses	I-Protocol
that	O
are	O
not	O
renewed	O
.	O
</s>
<s>
Automatic	O
allocation	O
The	O
DHCP	B-Protocol
server	I-Protocol
permanently	O
assigns	O
an	O
IP	B-Protocol
address	I-Protocol
to	O
a	O
requesting	O
client	O
from	O
a	O
range	O
defined	O
by	O
an	O
administrator	O
.	O
</s>
<s>
This	O
is	O
like	O
dynamic	O
allocation	O
,	O
but	O
the	O
DHCP	B-Protocol
server	I-Protocol
keeps	O
a	O
table	O
of	O
past	O
IP	B-Protocol
address	I-Protocol
assignments	O
,	O
so	O
that	O
it	O
can	O
preferentially	O
assign	O
to	O
a	O
client	O
the	O
same	O
IP	B-Protocol
address	I-Protocol
that	O
the	O
client	O
previously	O
had	O
.	O
</s>
<s>
Manual	O
allocation	O
This	O
method	O
is	O
also	O
variously	O
called	O
static	B-Protocol
DHCP	I-Protocol
allocation	O
,	O
fixed	O
address	O
allocation	O
,	O
reservation	O
,	O
and	O
MAC/IP	O
address	O
binding	O
.	O
</s>
<s>
An	O
administrator	O
maps	O
a	O
unique	O
identifier	O
(	O
a	O
client	O
id	O
or	O
MAC	O
address	O
)	O
for	O
each	O
client	O
to	O
an	O
IP	B-Protocol
address	I-Protocol
,	O
which	O
is	O
offered	O
to	O
the	O
requesting	O
client	O
.	O
</s>
<s>
DHCP	B-Protocol
servers	I-Protocol
may	O
be	O
configured	O
to	O
fall	O
back	O
to	O
other	O
methods	O
if	O
this	O
fails	O
.	O
</s>
<s>
DHCP	B-Protocol
services	O
are	O
used	O
for	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
and	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
details	O
of	O
the	O
protocol	O
for	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
differ	O
sufficiently	O
that	O
they	O
may	O
be	O
considered	O
separate	O
protocols	O
.	O
</s>
<s>
For	O
the	O
IPv6	B-Protocol
operation	O
,	O
devices	O
may	O
alternatively	O
use	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
hosts	O
may	O
also	O
use	O
link-local	B-Protocol
addressing	I-Protocol
to	O
achieve	O
operations	O
restricted	O
to	O
the	O
local	B-General_Concept
network	I-General_Concept
link	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
employs	O
a	O
connectionless	B-Protocol
service	O
model	O
,	O
using	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
.	O
</s>
<s>
It	O
is	O
implemented	O
with	O
two	O
UDP	B-Protocol
port	O
numbers	O
for	O
its	O
operations	O
which	O
are	O
the	O
same	O
as	O
for	O
the	O
bootstrap	B-Device
protocol	I-Device
(	O
BOOTP	B-Device
)	O
.	O
</s>
<s>
The	O
server	B-Application
listens	O
on	O
UDP	B-Protocol
port	O
number	O
67	O
,	O
and	O
the	O
client	O
listens	O
on	O
UDP	B-Protocol
port	O
number	O
68	O
.	O
</s>
<s>
DHCP	B-Protocol
operations	O
fall	O
into	O
four	O
phases	O
:	O
server	B-Application
discovery	O
,	O
IP	O
lease	O
offer	O
,	O
IP	O
lease	O
request	O
,	O
and	O
IP	O
lease	O
acknowledgement	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
operation	O
begins	O
with	O
clients	O
broadcasting	B-Operating_System
a	O
request	O
.	O
</s>
<s>
If	O
the	O
client	O
and	O
server	B-Application
are	O
in	O
different	O
Broadcast	B-Architecture
Domains	I-Architecture
,	O
a	O
DHCP	B-Protocol
Helper	O
or	O
DHCP	B-Protocol
Relay	O
Agent	O
may	O
be	O
used	O
.	O
</s>
<s>
Clients	O
requesting	O
renewal	O
of	O
an	O
existing	O
lease	O
may	O
communicate	O
directly	O
via	O
UDP	B-Protocol
unicast	B-Protocol
,	O
since	O
the	O
client	O
already	O
has	O
an	O
established	O
IP	B-Protocol
address	I-Protocol
at	O
that	O
point	O
.	O
</s>
<s>
Additionally	O
,	O
there	O
is	O
a	O
BROADCAST	B-Operating_System
flag	O
(	O
1	O
bit	O
in	O
2	O
byte	O
flags	O
field	O
,	O
where	O
all	O
other	O
bits	O
are	O
reserved	O
and	O
so	O
are	O
set	O
to	O
0	O
)	O
the	O
client	O
can	O
use	O
to	O
indicate	O
in	O
which	O
way	O
(	O
broadcast	B-Operating_System
or	O
unicast	B-Protocol
)	O
it	O
can	O
receive	O
the	O
DHCPOFFER	O
:	O
0x8000	O
for	O
broadcast	B-Operating_System
,	O
0x0000	O
for	O
unicast	B-Protocol
.	O
</s>
<s>
Usually	O
,	O
the	O
DHCPOFFER	O
is	O
sent	O
through	O
unicast	B-Protocol
.	O
</s>
<s>
For	O
those	O
hosts	O
which	O
cannot	O
accept	O
unicast	B-Protocol
packets	O
before	O
IP	B-Protocol
addresses	I-Protocol
are	O
configured	O
,	O
this	O
flag	O
can	O
be	O
used	O
to	O
work	O
around	O
this	O
issue	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
client	O
broadcasts	B-Operating_System
a	O
DHCPDISCOVER	O
message	O
on	O
the	O
network	O
subnet	O
using	O
the	O
destination	O
address	O
(	O
limited	O
broadcast	B-Operating_System
)	O
or	O
the	O
specific	O
subnet	O
broadcast	B-Operating_System
address	O
(	O
directed	O
broadcast	B-Operating_System
)	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
client	O
may	O
also	O
request	O
its	O
last	O
known	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
If	O
the	O
client	O
remains	O
connected	O
to	O
the	O
same	O
network	O
,	O
the	O
server	B-Application
may	O
grant	O
the	O
request	O
.	O
</s>
<s>
Otherwise	O
,	O
it	O
depends	O
whether	O
the	O
server	B-Application
is	O
set	O
up	O
as	O
authoritative	O
or	O
not	O
.	O
</s>
<s>
An	O
authoritative	O
server	B-Application
denies	O
the	O
request	O
,	O
causing	O
the	O
client	O
to	O
issue	O
a	O
new	O
request	O
.	O
</s>
<s>
A	O
non-authoritative	O
server	B-Application
simply	O
ignores	O
the	O
request	O
,	O
leading	O
to	O
an	O
implementation-dependent	O
timeout	O
for	O
the	O
client	O
to	O
expire	O
the	O
request	O
and	O
ask	O
for	O
a	O
new	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
UDP	B-Protocol
:	O
source	O
port	O
=	O
68	O
;	O
destination	O
port	O
=	O
67	O
Octet	O
0	O
Octet	O
1	O
Octet	O
2	O
Octet	O
3	O
OP	O
HTYPE	O
HLEN	O
HOPS	O
0x01	O
0x01	O
0x06	O
0x00	O
XID	O
0x3903F326	O
SECS	O
FLAGS	O
0x0000	O
0x0000	O
CIADDR	O
(	O
Client	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
YIADDR	O
(	O
Your	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
SIADDR	O
(	O
Server	B-Application
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
GIADDR	O
(	O
Gateway	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
CHADDR	O
(	O
Client	O
hardware	O
address	O
)	O
0x00053C04	O
0x8D590000	O
0x00000000	O
0x00000000	O
192	O
octets	O
of	O
0s	O
,	O
or	O
overflow	O
space	O
for	O
additional	O
options	O
;	O
BOOTP	B-Device
legacy	O
.	O
</s>
<s>
When	O
a	O
DHCP	B-Protocol
server	I-Protocol
receives	O
a	O
DHCPDISCOVER	O
message	O
from	O
a	O
client	O
,	O
which	O
is	O
an	O
IP	B-Protocol
address	I-Protocol
lease	O
request	O
,	O
the	O
DHCP	B-Protocol
server	I-Protocol
reserves	O
an	O
IP	B-Protocol
address	I-Protocol
for	O
the	O
client	O
and	O
makes	O
a	O
lease	O
offer	O
by	O
sending	O
a	O
DHCPOFFER	O
message	O
to	O
the	O
client	O
.	O
</s>
<s>
This	O
message	O
contains	O
the	O
client	O
's	O
client	O
id	O
(	O
traditionally	O
a	O
MAC	O
address	O
)	O
,	O
the	O
IP	B-Protocol
address	I-Protocol
that	O
the	O
server	B-Application
is	O
offering	O
,	O
the	O
subnet	O
mask	O
,	O
the	O
lease	O
duration	O
,	O
and	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
DHCP	B-Protocol
server	I-Protocol
making	O
the	O
offer	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
server	I-Protocol
may	O
also	O
take	O
notice	O
of	O
the	O
hardware-level	O
MAC	O
address	O
in	O
the	O
underlying	O
transport	O
layer	O
:	O
according	O
to	O
current	O
RFCs	O
the	O
transport	O
layer	O
MAC	O
address	O
may	O
be	O
used	O
if	O
no	O
client	O
ID	O
is	O
provided	O
in	O
the	O
DHCP	B-Protocol
packet	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
server	I-Protocol
determines	O
the	O
configuration	O
based	O
on	O
the	O
client	O
's	O
hardware	O
address	O
as	O
specified	O
in	O
the	O
CHADDR	O
(	O
client	O
hardware	O
address	O
)	O
field	O
.	O
</s>
<s>
In	O
the	O
following	O
example	O
the	O
server	B-Application
(	O
)	O
specifies	O
the	O
client	O
's	O
IP	B-Protocol
address	I-Protocol
in	O
the	O
YIADDR	O
(	O
your	O
IP	B-Protocol
address	I-Protocol
)	O
field	O
.	O
</s>
<s>
UDP	B-Protocol
:	O
source	O
port	O
=	O
67	O
;	O
destination	O
port	O
=	O
68	O
Octet	O
0	O
Octet	O
1	O
Octet	O
2	O
Octet	O
3	O
OP	O
HTYPE	O
HLEN	O
HOPS	O
0x02	O
0x01	O
0x06	O
0x00	O
XID	O
0x3903F326	O
SECS	O
FLAGS	O
0x0000	O
0x0000	O
CIADDR	O
(	O
Client	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
YIADDR	O
(	O
Your	O
IP	B-Protocol
address	I-Protocol
)	O
0xC0A80164	O
(	O
)	O
SIADDR	O
(	O
Server	B-Application
IP	B-Protocol
address	I-Protocol
)	O
0xC0A80101	O
(	O
)	O
GIADDR	O
(	O
Gateway	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
CHADDR	O
(	O
Client	O
hardware	O
address	O
)	O
0x00053C04	O
0x8D590000	O
0x00000000	O
0x00000000	O
192	O
octets	O
of	O
0s	O
;	O
BOOTP	B-Device
legacy	O
.	O
</s>
<s>
Magic	O
cookie	B-Application
0x63825363	O
DHCP	B-Protocol
options	O
53	O
:	O
2	O
(	O
DHCP	B-Protocol
Offer	O
)	O
1	O
(	O
subnet	O
mask	O
)	O
:	O
3	O
(	O
Router	B-Protocol
)	O
:	O
51	O
(	O
IP	B-Protocol
address	I-Protocol
lease	O
time	O
)	O
:	O
86400s	O
(	O
1	O
day	O
)	O
54	O
(	O
DHCP	B-Protocol
server	I-Protocol
)	O
:	O
6	O
(	O
DNS	B-Protocol
servers	I-Protocol
)	O
:	O
</s>
<s>
In	O
response	O
to	O
the	O
DHCP	B-Protocol
offer	O
,	O
the	O
client	O
replies	O
with	O
a	O
DHCPREQUEST	O
message	O
,	O
broadcast	B-Operating_System
to	O
the	O
server	B-Application
,	O
requesting	O
the	O
offered	O
address	O
.	O
</s>
<s>
A	O
client	O
can	O
receive	O
DHCP	B-Protocol
offers	O
from	O
multiple	O
servers	O
,	O
but	O
it	O
will	O
accept	O
only	O
one	O
DHCP	B-Protocol
offer	O
.	O
</s>
<s>
Before	O
claiming	O
an	O
IP	B-Protocol
address	I-Protocol
,	O
the	O
client	O
will	O
broadcast	B-Operating_System
an	O
ARP	O
request	O
,	O
in	O
order	O
to	O
find	O
if	O
there	O
is	O
another	O
host	O
present	O
in	O
the	O
network	O
with	O
the	O
proposed	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
client	O
must	O
send	O
the	O
server	B-Application
identification	O
option	O
in	O
the	O
DHCPREQUEST	O
message	O
,	O
indicating	O
the	O
server	B-Application
whose	O
offer	O
the	O
client	O
has	O
selected	O
.	O
</s>
<s>
When	O
other	O
DHCP	B-Protocol
servers	I-Protocol
receive	O
this	O
message	O
,	O
they	O
withdraw	O
any	O
offers	O
that	O
they	O
have	O
made	O
to	O
the	O
client	O
and	O
return	O
their	O
offered	O
IP	B-Protocol
address	I-Protocol
to	O
the	O
pool	O
of	O
available	O
addresses	O
.	O
</s>
<s>
UDP	B-Protocol
:	O
source	O
port	O
=	O
68	O
;	O
destination	O
port	O
=	O
67	O
Octet	O
0	O
Octet	O
1	O
Octet	O
2	O
Octet	O
3	O
OP	O
HTYPE	O
HLEN	O
HOPS	O
0x01	O
0x01	O
0x06	O
0x00	O
XID	O
0x3903F326	O
SECS	O
FLAGS	O
0x0000	O
0x0000	O
CIADDR	O
(	O
Client	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
YIADDR	O
(	O
Your	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
SIADDR	O
(	O
Server	B-Application
IP	B-Protocol
address	I-Protocol
)	O
0xC0A80101	O
(	O
)	O
GIADDR	O
(	O
Gateway	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
CHADDR	O
(	O
Client	O
hardware	O
address	O
)	O
0x00053C04	O
0x8D590000	O
0x00000000	O
0x00000000	O
192	O
octets	O
of	O
0s	O
;	O
BOOTP	B-Device
legacy	O
.	O
</s>
<s>
Magic	O
cookie	B-Application
0x63825363	O
DHCP	B-Protocol
options	O
53	O
:	O
3	O
(	O
DHCP	B-Protocol
Request	O
)	O
50	O
:	O
requested	O
54	O
(	O
DHCP	B-Protocol
server	I-Protocol
)	O
:	O
</s>
<s>
When	O
the	O
DHCP	B-Protocol
server	I-Protocol
receives	O
the	O
DHCPREQUEST	O
message	O
from	O
the	O
client	O
,	O
the	O
configuration	O
process	O
enters	O
its	O
final	O
phase	O
.	O
</s>
<s>
The	O
protocol	O
expects	O
the	O
DHCP	B-Protocol
client	O
to	O
configure	O
its	O
network	O
interface	O
with	O
the	O
negotiated	O
parameters	O
.	O
</s>
<s>
After	O
the	O
client	O
obtains	O
an	O
IP	B-Protocol
address	I-Protocol
,	O
it	O
should	O
probe	O
the	O
newly	O
received	O
address	O
(	O
e.g.	O
</s>
<s>
with	O
ARP	O
Address	O
Resolution	O
Protocol	O
)	O
to	O
prevent	O
address	O
conflicts	O
caused	O
by	O
overlapping	O
address	O
pools	O
of	O
DHCP	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
If	O
this	O
probe	O
finds	O
another	O
computer	O
using	O
that	O
address	O
,	O
the	O
computer	O
should	O
send	O
DHCPDECLINE	O
,	O
broadcast	B-Operating_System
,	O
to	O
the	O
server	B-Application
.	O
</s>
<s>
UDP	B-Protocol
:	O
source	O
port	O
=	O
67	O
;	O
destination	O
port	O
=	O
68	O
Octet	O
0	O
Octet	O
1	O
Octet	O
2	O
Octet	O
3	O
OP	O
HTYPE	O
HLEN	O
HOPS	O
0x02	O
0x01	O
0x06	O
0x00	O
XID	O
0x3903F326	O
SECS	O
FLAGS	O
0x0000	O
0x0000	O
CIADDR	O
(	O
Client	O
IP	B-Protocol
address	I-Protocol
)	O
0x00000000	O
YIADDR	O
(	O
Your	O
IP	B-Protocol
address	I-Protocol
)	O
0xC0A80164	O
(	O
)	O
SIADDR	O
(	O
Server	B-Application
IP	B-Protocol
address	I-Protocol
)	O
0xC0A80101	O
(	O
)	O
GIADDR	O
(	O
Gateway	O
IP	B-Protocol
address	I-Protocol
switched	O
by	O
relay	O
)	O
0x00000000	O
CHADDR	O
(	O
Client	O
hardware	O
address	O
)	O
0x00053C04	O
0x8D590000	O
0x00000000	O
0x00000000	O
192	O
octets	O
of	O
0s	O
.	O
</s>
<s>
BOOTP	B-Device
legacy	O
Magic	O
cookie	B-Application
0x63825363	O
DHCP	B-Protocol
options	O
53	O
:	O
5	O
(	O
DHCP	B-Protocol
ACK	O
)	O
1	O
(	O
subnet	O
mask	O
)	O
:	O
3	O
(	O
Router	B-Protocol
)	O
:	O
51	O
(	O
IP	B-Protocol
address	I-Protocol
lease	O
time	O
)	O
:	O
86400s	O
(	O
1	O
day	O
)	O
54	O
(	O
DHCP	B-Protocol
server	I-Protocol
)	O
:	O
6	O
(	O
DNS	B-Protocol
servers	I-Protocol
)	O
:	O
</s>
<s>
A	O
DHCP	B-Protocol
client	O
may	O
request	O
more	O
information	O
than	O
the	O
server	B-Application
sent	O
with	O
the	O
original	O
DHCPOFFER	O
.	O
</s>
<s>
For	O
example	O
,	O
browsers	O
use	O
DHCP	B-Protocol
Inform	O
to	O
obtain	O
web	O
proxy	O
settings	O
via	O
WPAD	B-Protocol
.	O
</s>
<s>
The	O
client	O
sends	O
a	O
request	O
to	O
the	O
DHCP	B-Protocol
server	I-Protocol
to	O
release	O
the	O
DHCP	B-Protocol
information	O
and	O
the	O
client	O
deactivates	O
its	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
As	O
client	O
devices	O
usually	O
do	O
not	O
know	O
when	O
users	O
may	O
unplug	O
them	O
from	O
the	O
network	O
,	O
the	O
protocol	O
does	O
not	O
mandate	O
the	O
sending	O
of	O
DHCP	B-Protocol
Release	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
server	I-Protocol
can	O
provide	O
optional	O
configuration	O
parameters	O
to	O
the	O
client	O
.	O
</s>
<s>
RFC	O
2132	O
describes	O
the	O
available	O
DHCP	B-Protocol
options	O
defined	O
by	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
-	O
DHCP	B-Protocol
and	O
BOOTP	B-Device
PARAMETERS	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
client	O
can	O
select	O
,	O
manipulate	O
and	O
overwrite	O
parameters	O
provided	O
by	O
a	O
DHCP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
This	O
is	O
called	O
Type	B-Protocol
–	I-Protocol
length	I-Protocol
–	I-Protocol
value	I-Protocol
encoding	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
DHCP	B-Protocol
message-type	O
option	O
for	O
an	O
offer	O
would	O
appear	O
as	O
0x35	O
,	O
0x01	O
,	O
0x02	O
,	O
where	O
0x35	O
is	O
code	O
53	O
for	O
"	O
DHCP	B-Protocol
message	O
type	O
"	O
,	O
0x01	O
means	O
one	O
octet	O
follows	O
and	O
0x02	O
is	O
the	O
value	O
of	O
"	O
offer	O
"	O
.	O
</s>
<s>
The	O
following	O
tables	O
list	O
the	O
available	O
DHCP	B-Protocol
options	O
.	O
</s>
<s>
+	O
RFC	O
1497	O
(	O
BOOTP	B-Device
Vendor	O
Information	O
Extensions	O
)	O
vendor	O
extensions	O
Code	O
Name	O
Length	O
Notes	O
0	O
Pad	O
0	O
octets	O
Can	O
be	O
used	O
to	O
pad	O
other	O
options	O
so	O
that	O
they	O
are	O
aligned	O
to	O
the	O
word	O
boundary	O
;	O
is	O
not	O
followed	O
by	O
length	O
byte	O
1	O
Subnet	O
mask	O
4	O
octets	O
Client	O
's	O
subnet	O
mask	O
as	O
per	O
RFC	O
950	O
.	O
</s>
<s>
If	O
both	O
the	O
subnet	O
mask	O
and	O
the	O
router	B-Protocol
option	O
(	O
option	O
3	O
)	O
are	O
included	O
,	O
the	O
subnet	O
mask	O
option	O
must	O
be	O
first	O
.	O
</s>
<s>
3	O
Router	B-Protocol
Multiples	O
of	O
4	O
octets	O
Available	O
routers	B-Protocol
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
4	O
Time	B-Protocol
server	I-Protocol
Multiples	O
of	O
4	O
octets	O
Available	O
time	B-Protocol
servers	I-Protocol
to	O
synchronise	O
with	O
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
5	O
Name	B-Protocol
server	I-Protocol
Multiples	O
of	O
4	O
octets	O
Available	O
IEN	B-Protocol
116	I-Protocol
name	B-Protocol
servers	I-Protocol
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
6	O
Domain	B-Protocol
name	I-Protocol
server	I-Protocol
Multiples	O
of	O
4	O
octets	O
Available	O
DNS	B-Protocol
servers	I-Protocol
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
7	O
Log	O
server	B-Application
Multiples	O
of	O
4	O
octets	O
Available	O
log	O
servers	O
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
8	O
Cookie	B-Application
server	B-Application
Multiples	O
of	O
4	O
octets	O
Cookie	B-Application
in	O
this	O
case	O
means	O
"	O
fortune	O
cookie	B-Application
"	O
or	O
"	O
quote	O
of	O
the	O
day	O
"	O
,	O
a	O
pithy	O
or	O
humorous	O
anecdote	O
often	O
sent	O
as	O
part	O
of	O
a	O
logon	O
process	O
on	O
large	O
computers	O
;	O
it	O
has	O
nothing	O
to	O
do	O
with	O
cookies	B-Application
sent	I-Application
by	I-Application
websites	I-Application
.	O
</s>
<s>
9	O
LPR	O
Server	B-Application
Multiples	O
of	O
4	O
octets	O
A	O
list	O
of	O
RFC	O
1179	O
line	O
printer	O
servers	O
available	O
to	O
the	O
client	O
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
10	O
Impress	O
server	B-Application
Multiples	O
of	O
4	O
octets	O
A	O
list	O
of	O
Imagen	O
Impress	O
servers	O
available	O
to	O
the	O
client	O
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
11	O
Resource	O
location	O
server	B-Application
Multiples	O
of	O
4	O
octets	O
A	O
list	O
of	O
RFC	O
887	O
Resource	O
Location	O
servers	O
available	O
to	O
the	O
client	O
,	O
should	O
be	O
listed	O
in	O
order	O
of	O
preference	O
12	O
Host	O
name	O
Minimum	O
of	O
1	O
octet	O
Name	O
of	O
the	O
client	O
.	O
</s>
<s>
This	O
table	O
lists	O
the	O
DHCP	B-Protocol
message	O
types	O
,	O
documented	O
in	O
RFC	O
2132	O
,	O
RFC	O
3203	O
,	O
RFC	O
4388	O
,	O
RFC	O
6926	O
and	O
RFC	O
7724	O
.	O
</s>
<s>
An	O
option	O
exists	O
to	O
identify	O
the	O
vendor	O
and	O
functionality	O
of	O
a	O
DHCP	B-Protocol
client	O
.	O
</s>
<s>
The	O
information	O
is	O
a	O
variable-length	B-Algorithm
string	I-Algorithm
of	O
characters	O
or	O
octets	O
which	O
has	O
a	O
meaning	O
specified	O
by	O
the	O
vendor	O
of	O
the	O
DHCP	B-Protocol
client	O
.	O
</s>
<s>
One	O
method	O
by	O
which	O
a	O
DHCP	B-Protocol
client	O
can	O
communicate	O
to	O
the	O
server	B-Application
that	O
it	O
is	O
using	O
a	O
certain	O
type	O
of	O
hardware	O
or	O
firmware	O
is	O
to	O
set	O
a	O
value	O
in	O
its	O
DHCP	B-Protocol
requests	B-Protocol
called	O
the	O
Vendor	O
Class	O
Identifier	O
(	O
VCI	O
)	O
(	O
Option	O
60	O
)	O
.	O
</s>
<s>
The	O
value	O
to	O
which	O
this	O
option	O
is	O
set	O
gives	O
the	O
DHCP	B-Protocol
server	I-Protocol
a	O
hint	O
about	O
any	O
required	O
extra	O
information	O
that	O
this	O
client	O
needs	O
in	O
a	O
DHCP	B-Protocol
response	O
.	O
</s>
<s>
Some	O
types	O
of	O
set-top	O
boxes	O
set	O
the	O
VCI	O
to	O
inform	O
the	O
DHCP	B-Protocol
server	I-Protocol
about	O
the	O
hardware	O
type	O
and	O
functionality	O
of	O
the	O
device	O
.	O
</s>
<s>
An	O
Aruba	O
campus	O
wireless	B-Device
access	I-Device
point	I-Device
,	O
for	O
example	O
,	O
supplies	O
value	O
'	O
ArubaAP	O
 '	O
as	O
option	O
60	O
in	O
its	O
DHCPDISCOVER	O
message	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
server	I-Protocol
can	O
then	O
augment	O
its	O
DHCPOFFER	O
with	O
an	O
IP	B-Protocol
address	I-Protocol
of	O
an	O
Aruba	O
wireless	B-Device
controller	I-Device
in	O
option	O
43	O
,	O
so	O
the	O
access	O
point	O
knows	O
where	O
to	O
register	O
itself	O
.	O
</s>
<s>
Setting	O
a	O
VCI	O
by	O
the	O
client	O
allows	O
a	O
DHCP	B-Protocol
server	I-Protocol
to	O
differentiate	O
between	O
client	O
machines	O
and	O
process	O
the	O
requests	B-Protocol
from	O
them	O
appropriately	O
.	O
</s>
<s>
The	O
relay	O
agent	O
information	O
option	O
(	O
option	O
82	O
)	O
specifies	O
container	O
for	O
attaching	O
sub-options	O
to	O
DHCP	B-Protocol
requests	B-Protocol
transmitted	O
between	O
a	O
DHCP	B-Protocol
relay	O
and	O
a	O
DHCP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
In	O
small	O
networks	O
,	O
where	O
only	O
one	O
IP	O
subnet	O
is	O
being	O
managed	O
,	O
DHCP	B-Protocol
clients	O
communicate	O
directly	O
with	O
DHCP	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
However	O
,	O
DHCP	B-Protocol
servers	I-Protocol
can	O
also	O
provide	O
IP	B-Protocol
addresses	I-Protocol
for	O
multiple	O
subnets	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
a	O
DHCP	B-Protocol
client	O
that	O
has	O
not	O
yet	O
acquired	O
an	O
IP	B-Protocol
address	I-Protocol
cannot	O
communicate	O
directly	O
with	O
a	O
DHCP	B-Protocol
server	I-Protocol
not	O
on	O
the	O
same	O
subnet	O
,	O
as	O
the	O
client	O
's	O
broadcast	B-Operating_System
can	O
only	O
be	O
received	O
on	O
its	O
own	O
subnet	O
.	O
</s>
<s>
In	O
order	O
to	O
allow	O
DHCP	B-Protocol
clients	O
on	O
subnets	O
not	O
directly	O
served	O
by	O
DHCP	B-Protocol
servers	I-Protocol
to	O
communicate	O
with	O
DHCP	B-Protocol
servers	I-Protocol
,	O
DHCP	B-Protocol
relay	O
agents	O
can	O
be	O
installed	O
on	O
these	O
subnets	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
relay	O
agent	O
runs	O
on	O
a	O
network	O
device	O
,	O
capable	O
of	O
routing	B-Protocol
between	O
the	O
client	O
's	O
subnet	O
and	O
the	O
subnet	O
of	O
the	O
DHCP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
The	O
DHCP	B-Protocol
client	O
broadcasts	B-Operating_System
on	O
the	O
local	O
link	O
;	O
the	O
relay	O
agent	O
receives	O
the	O
broadcast	B-Operating_System
and	O
transmits	O
it	O
to	O
one	O
or	O
more	O
DHCP	B-Protocol
servers	I-Protocol
using	O
unicast	B-Protocol
.	O
</s>
<s>
The	O
IP	B-Protocol
addresses	I-Protocol
of	O
the	O
DHCP	B-Protocol
servers	I-Protocol
are	O
manually	O
configured	O
in	O
the	O
relay	O
agent	O
.	O
</s>
<s>
The	O
relay	O
agent	O
stores	O
its	O
own	O
IP	B-Protocol
address	I-Protocol
,	O
from	O
the	O
interface	O
on	O
which	O
it	O
has	O
received	O
the	O
client	O
's	O
broadcast	B-Operating_System
,	O
in	O
the	O
GIADDR	O
field	O
of	O
the	O
DHCP	B-Protocol
packet	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
server	I-Protocol
uses	O
the	O
GIADDR-value	O
to	O
determine	O
the	O
subnet	O
,	O
and	O
subsequently	O
the	O
corresponding	O
address	O
pool	O
,	O
from	O
which	O
to	O
allocate	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
When	O
the	O
DHCP	B-Protocol
server	I-Protocol
replies	O
to	O
the	O
client	O
,	O
it	O
sends	O
the	O
reply	O
to	O
the	O
GIADDR-address	O
,	O
again	O
using	O
unicast	B-Protocol
.	O
</s>
<s>
The	O
relay	O
agent	O
then	O
retransmits	O
the	O
response	O
on	O
the	O
local	B-General_Concept
network	I-General_Concept
,	O
using	O
unicast	B-Protocol
(	O
in	O
most	O
cases	O
)	O
to	O
the	O
newly	O
reserved	O
IP	B-Protocol
address	I-Protocol
,	O
in	O
an	O
Ethernet	B-Protocol
frame	I-Protocol
directed	O
to	O
the	O
client	O
's	O
MAC	O
address	O
.	O
</s>
<s>
The	O
client	O
should	O
accept	O
the	O
packet	O
as	O
its	O
own	O
,	O
even	O
when	O
that	O
IP	B-Protocol
address	I-Protocol
is	O
not	O
yet	O
set	O
on	O
the	O
interface	O
.	O
</s>
<s>
Directly	O
after	O
processing	O
the	O
packet	O
,	O
the	O
client	O
sets	O
the	O
IP	B-Protocol
address	I-Protocol
on	O
its	O
interface	O
and	O
is	O
ready	O
for	O
regular	O
IP	O
communication	O
,	O
directly	O
thereafter	O
.	O
</s>
<s>
If	O
the	O
client	O
's	O
implementation	O
of	O
the	O
IP	O
stack	O
does	O
not	O
accept	O
unicast	B-Protocol
packets	O
when	O
it	O
has	O
no	O
IP	B-Protocol
address	I-Protocol
yet	O
,	O
the	O
client	O
may	O
set	O
the	O
broadcast	B-Operating_System
bit	O
in	O
the	O
FLAGS	O
field	O
when	O
sending	O
a	O
DHCPDISCOVER	O
packet	O
.	O
</s>
<s>
The	O
relay	O
agent	O
will	O
use	O
the	O
broadcast	B-Operating_System
IP	B-Protocol
address	I-Protocol
(	O
and	O
the	O
clients	O
MAC	O
address	O
)	O
to	O
inform	O
the	O
client	O
of	O
the	O
server	B-Application
's	O
DHCPOFFER	O
.	O
</s>
<s>
The	O
communication	O
between	O
the	O
relay	O
agent	O
and	O
the	O
DHCP	B-Protocol
server	I-Protocol
typically	O
uses	O
both	O
a	O
source	O
and	O
destination	O
UDP	B-Protocol
port	O
of	O
67	O
.	O
</s>
<s>
A	O
DHCP	B-Protocol
client	O
can	O
receive	O
these	O
messages	O
from	O
a	O
server	B-Application
:	O
</s>
<s>
The	O
client	O
moves	O
through	O
DHCP	B-Protocol
states	O
depending	O
on	O
how	O
the	O
server	B-Application
responds	O
to	O
the	O
messages	O
that	O
the	O
client	O
sends	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
ensures	O
reliability	O
in	O
several	O
ways	O
:	O
periodic	O
renewal	O
,	O
rebinding	O
,	O
and	O
failover	O
.	O
</s>
<s>
DHCP	B-Protocol
clients	O
are	O
allocated	O
leases	O
that	O
last	O
for	O
some	O
period	O
of	O
time	O
.	O
</s>
<s>
They	O
do	O
this	O
by	O
sending	O
a	O
unicast	B-Protocol
DHCPREQUEST	O
message	O
to	O
the	O
DHCP	B-Protocol
server	I-Protocol
that	O
granted	O
the	O
original	O
lease	O
.	O
</s>
<s>
If	O
that	O
server	B-Application
is	O
down	O
or	O
unreachable	O
,	O
it	O
will	O
fail	O
to	O
respond	O
to	O
the	O
DHCPREQUEST	O
.	O
</s>
<s>
However	O
,	O
in	O
that	O
case	O
the	O
client	O
repeats	O
the	O
DHCPREQUEST	O
from	O
time	O
to	O
time	O
,	O
so	O
if	O
the	O
DHCP	B-Protocol
server	I-Protocol
comes	O
back	O
up	O
or	O
becomes	O
reachable	O
again	O
,	O
the	O
DHCP	B-Protocol
client	O
will	O
succeed	O
in	O
contacting	O
it	O
and	O
renew	O
the	O
lease	O
.	O
</s>
<s>
If	O
the	O
DHCP	B-Protocol
server	I-Protocol
is	O
unreachable	O
for	O
an	O
extended	O
period	O
of	O
time	O
,	O
the	O
DHCP	B-Protocol
client	O
will	O
attempt	O
to	O
rebind	O
,	O
by	O
broadcasting	B-Operating_System
its	O
DHCPREQUEST	O
rather	O
than	O
unicasting	B-Protocol
it	O
.	O
</s>
<s>
Because	O
it	O
is	O
broadcast	B-Operating_System
,	O
the	O
DHCPREQUEST	O
message	O
will	O
reach	O
all	O
available	O
DHCP	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
If	O
some	O
other	O
DHCP	B-Protocol
server	I-Protocol
is	O
able	O
to	O
renew	O
the	O
lease	O
,	O
it	O
will	O
do	O
so	O
at	O
this	O
time	O
.	O
</s>
<s>
In	O
order	O
for	O
rebinding	O
to	O
work	O
,	O
when	O
the	O
client	O
successfully	O
contacts	O
a	O
backup	O
DHCP	B-Protocol
server	I-Protocol
,	O
that	O
server	B-Application
must	O
have	O
accurate	O
information	O
about	O
the	O
client	O
's	O
binding	O
.	O
</s>
<s>
A	O
proposal	O
for	O
implementing	O
fault-tolerant	B-General_Concept
DHCP	B-Protocol
servers	I-Protocol
was	O
submitted	O
to	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
,	O
but	O
never	O
formalized	O
.	O
</s>
<s>
When	O
the	O
lease	O
expires	O
,	O
the	O
client	O
must	O
stop	O
using	O
the	O
IP	B-Protocol
address	I-Protocol
granted	O
to	O
it	O
in	O
its	O
lease	O
.	O
</s>
<s>
At	O
that	O
time	O
it	O
will	O
restart	O
the	O
DHCP	B-Protocol
process	O
from	O
the	O
beginning	O
by	O
broadcasting	B-Operating_System
a	O
DHCPDISCOVER	O
message	O
.	O
</s>
<s>
Since	O
its	O
lease	O
has	O
expired	O
,	O
it	O
will	O
accept	O
any	O
IP	B-Protocol
address	I-Protocol
offered	O
to	O
it	O
.	O
</s>
<s>
Once	O
it	O
has	O
a	O
new	O
IP	B-Protocol
address	I-Protocol
(	O
presumably	O
from	O
a	O
different	O
DHCP	B-Protocol
server	I-Protocol
)	O
it	O
will	O
once	O
again	O
be	O
able	O
to	O
use	O
the	O
network	O
.	O
</s>
<s>
However	O
,	O
since	O
its	O
IP	B-Protocol
address	I-Protocol
has	O
changed	O
,	O
any	O
ongoing	O
connections	O
will	O
be	O
broken	O
.	O
</s>
<s>
The	O
basic	O
methodology	O
of	O
DHCP	B-Protocol
was	O
developed	O
for	O
networks	O
based	O
on	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
.	O
</s>
<s>
Since	O
the	O
development	O
and	O
deployment	O
of	O
IPv6	B-Protocol
networks	O
,	O
DHCP	B-Protocol
has	O
also	O
been	O
used	O
for	O
assigning	O
parameters	O
in	O
such	O
networks	O
,	O
despite	O
the	O
inherent	O
features	O
of	O
IPv6	B-Protocol
for	O
stateless	B-Protocol
address	I-Protocol
autoconfiguration	I-Protocol
.	O
</s>
<s>
The	O
IPv6	B-Protocol
version	O
of	O
the	O
protocol	O
is	O
designated	O
as	O
DHCPv6	B-Protocol
.	O
</s>
<s>
The	O
base	O
DHCP	B-Protocol
does	O
not	O
include	O
any	O
mechanism	O
for	O
authentication	O
.	O
</s>
<s>
Unauthorized	O
DHCP	B-Protocol
servers	I-Protocol
providing	O
false	O
information	O
to	O
clients	O
.	O
</s>
<s>
Resource	O
exhaustion	O
attacks	O
from	O
malicious	O
DHCP	B-Protocol
clients	O
.	O
</s>
<s>
Because	O
the	O
client	O
has	O
no	O
way	O
to	O
validate	O
the	O
identity	O
of	O
a	O
DHCP	B-Protocol
server	I-Protocol
,	O
unauthorized	O
DHCP	B-Protocol
servers	I-Protocol
(	O
commonly	O
called	O
"	O
rogue	B-Protocol
DHCP	I-Protocol
"	O
)	O
can	O
be	O
operated	O
on	O
networks	O
,	O
providing	O
incorrect	O
information	O
to	O
DHCP	B-Protocol
clients	O
.	O
</s>
<s>
Because	O
the	O
DHCP	B-Protocol
server	I-Protocol
provides	O
the	O
DHCP	B-Protocol
client	O
with	O
server	B-Application
IP	B-Protocol
addresses	I-Protocol
,	O
such	O
as	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
one	O
or	O
more	O
DNS	B-Protocol
servers	I-Protocol
,	O
an	O
attacker	O
can	O
convince	O
a	O
DHCP	B-Protocol
client	O
to	O
do	O
its	O
DNS	B-Protocol
lookups	O
through	O
its	O
own	O
DNS	B-Protocol
server	I-Protocol
,	O
and	O
can	O
therefore	O
provide	O
its	O
own	O
answers	O
to	O
DNS	B-Protocol
queries	I-Protocol
from	O
the	O
client	O
.	O
</s>
<s>
This	O
in	O
turn	O
allows	O
the	O
attacker	O
to	O
redirect	O
network	O
traffic	O
through	O
itself	O
,	O
allowing	O
it	O
to	O
eavesdrop	O
on	O
connections	O
between	O
the	O
client	O
and	O
network	B-Application
servers	I-Application
it	O
contacts	O
,	O
or	O
to	O
simply	O
replace	O
those	O
network	B-Application
servers	I-Application
with	O
its	O
own	O
.	O
</s>
<s>
Because	O
the	O
DHCP	B-Protocol
server	I-Protocol
has	O
no	O
secure	O
mechanism	O
for	O
authenticating	O
the	O
client	O
,	O
clients	O
can	O
gain	O
unauthorized	O
access	O
to	O
IP	B-Protocol
addresses	I-Protocol
by	O
presenting	O
credentials	O
,	O
such	O
as	O
client	O
identifiers	O
,	O
that	O
belong	O
to	O
other	O
DHCP	B-Protocol
clients	O
.	O
</s>
<s>
This	O
also	O
allows	O
DHCP	B-Protocol
clients	O
to	O
exhaust	O
the	O
DHCP	B-Protocol
server	I-Protocol
's	O
store	O
of	O
IP	B-Protocol
addresses	I-Protocol
—	O
by	O
presenting	O
new	O
credentials	O
each	O
time	O
it	O
asks	O
for	O
an	O
address	O
,	O
the	O
client	O
can	O
consume	O
all	O
the	O
available	O
IP	B-Protocol
addresses	I-Protocol
on	O
a	O
particular	O
network	O
link	O
,	O
preventing	O
other	O
DHCP	B-Protocol
clients	O
from	O
getting	O
service	O
.	O
</s>
<s>
DHCP	B-Protocol
does	O
provide	O
some	O
mechanisms	O
for	O
mitigating	O
these	O
problems	O
.	O
</s>
<s>
The	O
Relay	O
Agent	O
Information	O
Option	O
protocol	O
extension	O
(	O
usually	O
referred	O
to	O
in	O
the	O
industry	O
by	O
its	O
actual	O
number	O
as	O
Option	O
82	O
)	O
allows	O
network	O
operators	O
to	O
attach	O
tags	O
to	O
DHCP	B-Protocol
messages	O
as	O
these	O
messages	O
arrive	O
on	O
the	O
network	O
operator	O
's	O
trusted	O
network	O
.	O
</s>
<s>
Because	O
the	O
client	O
has	O
no	O
access	O
to	O
the	O
network	O
upstream	O
of	O
the	O
relay	O
agent	O
,	O
the	O
lack	O
of	O
authentication	O
does	O
not	O
prevent	O
the	O
DHCP	B-Protocol
server	I-Protocol
operator	O
from	O
relying	O
on	O
the	O
authorization	O
token	O
.	O
</s>
<s>
Another	O
extension	O
,	O
Authentication	O
for	O
DHCP	B-Protocol
Messages	O
(	O
RFC	O
3118	O
)	O
,	O
provides	O
a	O
mechanism	O
for	O
authenticating	O
DHCP	B-Protocol
messages	O
.	O
</s>
<s>
As	O
of	O
2002	O
,	O
this	O
extension	O
had	O
not	O
seen	O
widespread	O
adoption	O
because	O
of	O
the	O
problems	O
of	O
managing	O
keys	O
for	O
large	O
numbers	O
of	O
DHCP	B-Protocol
clients	O
.	O
</s>
<s>
This	O
fact	O
,	O
combined	O
with	O
the	O
introduction	O
of	O
802.1x	O
,	O
slowed	O
the	O
deployment	O
and	O
take-rate	O
of	O
authenticated	O
DHCP	B-Protocol
,	O
and	O
it	O
has	O
never	O
been	O
widely	O
deployed	O
.	O
</s>
<s>
A	O
2010	O
book	O
notes	O
that:[T]here	O
have	O
been	O
very	O
few	O
implementations	O
of	O
DHCP	B-Protocol
Authentication	O
.	O
</s>
<s>
Architectural	O
proposals	O
from	O
2008	O
involve	O
authenticating	O
DHCP	B-Protocol
requests	B-Protocol
using	O
802.1x	O
or	O
PANA	B-Protocol
(	O
both	O
of	O
which	O
transport	O
EAP	B-Protocol
)	O
.	O
</s>
<s>
An	O
IETF	O
proposal	O
was	O
made	O
for	O
including	O
EAP	B-Protocol
in	O
DHCP	B-Protocol
itself	O
,	O
the	O
so-called	O
EAPoDHCP	O
;	O
this	O
does	O
not	O
appear	O
to	O
have	O
progressed	O
beyond	O
IETF	O
draft	O
level	O
,	O
the	O
last	O
of	O
which	O
dates	O
to	O
2010	O
.	O
</s>
