<s>
In	O
computer	B-Architecture
networks	I-Architecture
,	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
is	O
an	O
application	O
that	O
sits	O
in	O
front	O
of	O
back-end	O
applications	O
and	O
forwards	O
client	O
(	O
e.g.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
help	O
increase	O
scalability	O
,	O
performance	O
,	O
resilience	O
and	O
security	O
.	O
</s>
<s>
The	O
resources	O
returned	O
to	O
the	O
client	O
appear	O
as	O
if	O
they	O
originated	O
from	O
the	O
web	B-Application
server	I-Application
itself	O
.	O
</s>
<s>
Large	O
websites	O
and	O
content	B-Operating_System
delivery	I-Operating_System
networks	I-Operating_System
use	O
reverse	B-General_Concept
proxies	I-General_Concept
,	O
together	O
with	O
other	O
techniques	O
,	O
to	O
balance	B-Application
the	I-Application
load	I-Application
between	O
internal	O
servers	O
.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
can	O
keep	O
a	O
cache	B-General_Concept
of	O
static	O
content	O
,	O
which	O
further	O
reduces	O
the	O
load	O
on	O
these	O
internal	O
servers	O
and	O
the	O
internal	O
network	O
.	O
</s>
<s>
It	O
is	O
also	O
common	O
for	O
reverse	B-General_Concept
proxies	I-General_Concept
to	O
add	O
features	O
such	O
as	O
compression	B-General_Concept
or	O
TLS	B-Protocol
encryption	I-Protocol
to	O
the	O
communication	O
channel	O
between	O
the	O
client	O
and	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
are	O
typically	O
owned	O
or	O
managed	O
by	O
the	O
web	O
service	O
,	O
and	O
they	O
are	O
accessed	O
by	O
clients	O
from	O
the	O
public	O
Internet	O
.	O
</s>
<s>
Reverse	B-General_Concept
proxy	I-General_Concept
servers	O
are	O
implemented	O
in	O
popular	O
open-source	B-Application
web	B-Application
servers	I-Application
such	O
as	O
Apache	B-Application
,	O
Nginx	B-Operating_System
,	O
and	O
Caddy	B-Language
.	O
</s>
<s>
This	O
software	O
can	O
inspect	O
HTTP	O
headers	O
,	O
which	O
,	O
for	O
example	O
,	O
allows	O
it	O
to	O
present	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
to	O
the	O
Internet	O
while	O
relaying	O
requests	O
to	O
different	O
internal	O
servers	O
based	O
on	O
the	O
domain	O
name	O
of	O
the	O
HTTP	O
request	O
.	O
</s>
<s>
Dedicated	O
reverse	B-General_Concept
proxy	I-General_Concept
servers	O
such	O
as	O
the	O
open	B-Application
source	I-Application
software	I-Application
HAProxy	B-Language
and	O
Squid	B-Protocol
are	O
used	O
by	O
some	O
of	O
the	O
biggest	O
websites	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
can	O
hide	O
the	O
existence	O
and	O
characteristics	O
of	O
origin	O
servers	O
.	O
</s>
<s>
Application	B-Protocol
firewall	I-Protocol
features	O
can	O
protect	O
against	O
common	O
web-based	O
attacks	O
,	O
like	O
a	O
denial-of-service	O
attack	O
(	O
DoS	O
)	O
or	O
distributed	O
denial-of-service	O
attacks	O
(	O
DDoS	O
)	O
.	O
</s>
<s>
Without	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
,	O
removing	O
malware	O
or	O
initiating	O
takedowns	O
,	O
for	O
example	O
,	O
can	O
be	O
difficult	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
secure	B-Protocol
websites	I-Protocol
,	O
a	O
web	B-Application
server	I-Application
may	O
not	O
perform	O
TLS	B-Protocol
encryption	I-Protocol
itself	O
,	O
but	O
instead	O
offload	O
the	O
task	O
to	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
that	O
may	O
be	O
equipped	O
with	O
TLS	B-Protocol
acceleration	I-Protocol
hardware	O
.	O
</s>
<s>
(	O
See	O
TLS	B-Protocol
termination	I-Protocol
proxy	I-Protocol
.	O
)	O
</s>
<s>
A	O
reverse	B-General_Concept
proxy	I-General_Concept
can	O
distribute	B-Application
the	I-Application
load	I-Application
from	O
incoming	O
requests	O
to	O
several	O
servers	O
,	O
with	O
each	O
server	O
supporting	O
its	O
own	O
application	O
area	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
reverse	O
proxying	O
web	B-Application
servers	I-Application
,	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
may	O
have	O
to	O
rewrite	O
the	O
URL	O
in	O
each	O
incoming	O
request	O
in	O
order	O
to	O
match	O
the	O
relevant	O
internal	O
location	O
of	O
the	O
requested	O
resource	O
.	O
</s>
<s>
A	O
reverse	B-General_Concept
proxy	I-General_Concept
can	O
reduce	O
load	O
on	O
its	O
origin	O
servers	O
by	O
caching	B-Protocol
static	O
content	O
and	O
dynamic	O
content	O
,	O
known	O
as	O
web	B-Application
acceleration	I-Application
.	O
</s>
<s>
Proxy	B-Protocol
caches	I-Protocol
of	O
this	O
sort	O
can	O
often	O
satisfy	O
a	O
considerable	O
number	O
of	O
website	O
requests	O
,	O
greatly	O
reducing	O
the	O
load	O
on	O
the	O
origin	O
server(s )	O
.	O
</s>
<s>
A	O
reverse	B-General_Concept
proxy	I-General_Concept
can	O
optimize	O
content	O
by	O
compressing	B-Protocol
it	O
in	O
order	O
to	O
speed	O
up	O
loading	O
times	O
.	O
</s>
<s>
In	O
a	O
technique	O
named	O
"	O
spoon-feeding	O
"	O
,	O
a	O
dynamically	O
generated	O
page	O
can	O
be	O
produced	O
all	O
at	O
once	O
and	O
served	O
to	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
,	O
which	O
can	O
then	O
return	O
it	O
to	O
the	O
client	O
a	O
little	O
bit	O
at	O
a	O
time	O
.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
can	O
operate	O
wherever	O
multiple	O
web-servers	B-Application
must	O
be	O
accessible	O
via	O
a	O
single	O
public	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
web	B-Application
servers	I-Application
listen	O
on	O
different	O
ports	O
in	O
the	O
same	O
machine	O
,	O
with	O
the	O
same	O
local	O
IP	B-Protocol
address	I-Protocol
or	O
,	O
possibly	O
,	O
on	O
different	O
machines	O
with	O
different	O
local	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
reverse	B-General_Concept
proxy	I-General_Concept
analyzes	O
each	O
incoming	O
request	O
and	O
delivers	O
it	O
to	O
the	O
right	O
server	O
within	O
the	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
can	O
perform	O
A/B	O
testing	O
and	O
multivariate	O
testing	O
without	O
placing	O
JavaScript	O
tags	O
or	O
code	O
into	O
pages	O
.	O
</s>
<s>
A	O
reverse	B-General_Concept
proxy	I-General_Concept
can	O
add	O
access	O
authentication	O
to	O
a	O
web	B-Application
server	I-Application
that	O
does	O
not	O
have	O
any	O
authentication	O
.	O
</s>
<s>
A	O
reverse	B-General_Concept
proxy	I-General_Concept
can	O
track	O
all	O
IP	B-Protocol
addresses	I-Protocol
making	O
requests	O
through	O
it	O
and	O
it	O
can	O
also	O
read	O
and	O
modify	O
any	O
non-encrypted	O
traffic	O
.	O
</s>
<s>
When	O
the	O
transit	O
traffic	O
is	O
encrypted	O
and	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
needs	O
to	O
filter/cache/compress	O
or	O
otherwise	O
modify	O
or	O
improve	O
the	O
traffic	O
,	O
the	O
proxy	O
first	O
must	O
decrypt	O
and	O
re-encrypt	O
communications	O
.	O
</s>
<s>
This	O
requires	O
the	O
proxy	O
to	O
possess	O
the	O
TLS	B-Protocol
certificate	O
and	O
its	O
corresponding	O
private	O
key	O
,	O
extending	O
the	O
number	O
of	O
systems	O
that	O
can	O
have	O
access	O
to	O
non-encrypted	O
data	O
and	O
making	O
it	O
a	O
more	O
valuable	O
target	O
for	O
attackers	O
.	O
</s>
<s>
The	O
vast	O
majority	O
of	O
external	O
data	B-Application
breaches	I-Application
happen	O
either	O
when	O
hackers	O
succeed	O
in	O
abusing	O
an	O
existing	O
reverse	B-General_Concept
proxy	I-General_Concept
that	O
was	O
intentionally	O
deployed	O
by	O
an	O
organisation	O
,	O
or	O
when	O
hackers	O
succeed	O
in	O
converting	B-Application
an	O
existing	O
Internet-facing	O
server	O
into	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
server	O
.	O
</s>
<s>
When	O
an	O
organisation	O
allows	O
external	O
access	O
to	O
such	O
internal	O
applications	O
via	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
,	O
they	O
might	O
unintentionally	O
increase	O
their	O
own	O
attack	O
surface	O
and	O
invite	O
hackers	O
.	O
</s>
<s>
If	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
is	O
not	O
configured	O
to	O
filter	O
attacks	O
or	O
it	O
does	O
not	O
receive	O
daily	O
updates	O
to	O
keep	O
its	O
attack	O
signature	O
database	O
up	O
to	O
date	O
,	O
a	O
zero-day	O
vulnerability	O
can	O
pass	O
through	O
unfiltered	O
,	O
enabling	O
attackers	O
to	O
gain	O
control	O
of	O
the	O
system(s )	O
that	O
are	O
behind	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
server	O
.	O
</s>
<s>
Using	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
of	O
a	O
third	O
party	O
(	O
e.g.	O
</s>
<s>
Cloudflare	B-Application
,	O
Imperva	O
)	O
places	O
the	O
entire	O
triad	O
of	O
confidentiality	O
,	O
integrity	O
and	O
availability	O
in	O
the	O
hands	O
of	O
the	O
third	O
party	O
who	O
operates	O
the	O
proxy	O
.	O
</s>
<s>
If	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
is	O
fronting	O
many	O
different	O
domains	O
,	O
its	O
outage	O
(	O
e.g.	O
</s>
<s>
Reverse	B-General_Concept
proxies	I-General_Concept
can	O
also	O
become	O
a	O
single	B-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
if	O
there	O
is	O
no	O
other	O
way	O
to	O
access	O
the	O
back	O
end	O
server	O
.	O
</s>
