<s>
Squid	B-Protocol
is	O
a	O
caching	B-Protocol
and	O
forwarding	O
HTTP	B-Protocol
web	O
proxy	B-Application
.	O
</s>
<s>
It	O
has	O
a	O
wide	O
variety	O
of	O
uses	O
,	O
including	O
speeding	O
up	O
a	O
web	B-Application
server	I-Application
by	O
caching	B-Protocol
repeated	O
requests	O
,	O
caching	B-Protocol
web	O
,	O
DNS	B-Protocol
and	O
other	O
computer	B-Architecture
network	I-Architecture
lookups	O
for	O
a	O
group	O
of	O
people	O
sharing	O
network	O
resources	O
,	O
and	O
aiding	O
security	O
by	O
filtering	O
traffic	O
.	O
</s>
<s>
Although	O
primarily	O
used	O
for	O
HTTP	B-Protocol
and	O
FTP	B-Protocol
,	O
Squid	B-Protocol
includes	O
limited	O
support	O
for	O
several	O
other	O
protocols	O
including	O
Internet	B-Protocol
Gopher	I-Protocol
,	O
SSL	B-Protocol
,	O
TLS	B-Protocol
and	O
HTTPS	B-Protocol
.	O
</s>
<s>
Squid	B-Protocol
does	O
not	O
support	O
the	O
SOCKS	B-Application
protocol	O
,	O
unlike	O
Privoxy	B-Language
,	O
with	O
which	O
Squid	B-Protocol
can	O
be	O
used	O
in	O
order	O
to	O
provide	O
SOCKS	B-Application
support	O
.	O
</s>
<s>
Squid	B-Protocol
was	O
originally	O
designed	O
to	O
run	O
as	O
a	O
daemon	B-Operating_System
on	O
Unix-like	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
A	O
Windows	B-Application
port	O
was	O
maintained	O
up	O
to	O
version	O
2.7	O
.	O
</s>
<s>
New	O
versions	O
available	O
on	O
Windows	B-Application
use	O
the	O
Cygwin	B-Language
environment	O
.	O
</s>
<s>
Squid	B-Protocol
is	O
free	B-Application
software	I-Application
released	O
under	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
.	O
</s>
<s>
Squid	B-Protocol
was	O
originally	O
developed	O
as	O
the	O
Harvest	B-Protocol
object	I-Protocol
cache	I-Protocol
,	O
part	O
of	O
the	O
Harvest	B-Protocol
project	I-Protocol
at	O
the	O
University	O
of	O
Colorado	O
Boulder	O
.	O
</s>
<s>
Duane	O
Wessels	O
forked	O
the	O
"	O
last	O
pre-commercial	O
version	O
of	O
Harvest	O
"	O
and	O
renamed	O
it	O
to	O
Squid	B-Protocol
to	O
avoid	O
confusion	O
with	O
the	O
commercial	O
fork	O
called	O
Cached	O
2.0	O
,	O
which	O
became	O
NetCache	B-Protocol
.	O
</s>
<s>
Squid	B-Protocol
version	O
1.0.0	O
was	O
released	O
in	O
July	O
1996	O
.	O
</s>
<s>
SquidNT	B-Application
,	O
a	O
port	O
of	O
the	O
Squid	B-Protocol
proxy	I-Protocol
server	I-Protocol
was	O
merged	O
into	O
the	O
main	O
Squid	B-Protocol
project	O
in	O
September	O
2006	O
.	O
</s>
<s>
Squid	B-Protocol
is	O
now	O
developed	O
almost	O
exclusively	O
through	O
volunteer	O
efforts	O
.	O
</s>
<s>
After	O
a	O
Squid	B-Protocol
proxy	I-Protocol
server	I-Protocol
is	O
installed	O
,	O
web	B-Application
browsers	I-Application
can	O
be	O
configured	O
to	O
use	O
it	O
as	O
a	O
proxy	B-Application
HTTP	B-Application
server	I-Application
,	O
allowing	O
Squid	B-Protocol
to	O
retain	O
copies	O
of	O
the	O
documents	O
returned	O
,	O
which	O
,	O
on	O
repeated	O
requests	O
for	O
the	O
same	O
documents	O
,	O
can	O
reduce	O
access	O
time	O
as	O
well	O
as	O
bandwidth	O
consumption	O
.	O
</s>
<s>
This	O
is	O
often	O
useful	O
for	O
Internet	O
service	O
providers	O
to	O
increase	O
speed	O
to	O
their	O
customers	O
,	O
and	O
LANs	B-General_Concept
that	O
share	O
an	O
Internet	O
connection	O
.	O
</s>
<s>
Because	O
the	O
caching	B-Protocol
servers	O
are	O
controlled	O
by	O
the	O
web	O
service	O
operator	O
,	O
caching	B-Protocol
proxies	O
do	O
not	O
anonymize	O
the	O
user	O
and	O
should	O
not	O
be	O
confused	O
with	O
anonymizing	O
proxies	O
.	O
</s>
<s>
A	O
client	B-Protocol
program	I-Protocol
(	O
e.g.	O
</s>
<s>
browser	B-Application
)	O
either	O
has	O
to	O
specify	O
explicitly	O
the	O
proxy	B-Application
server	I-Application
it	O
wants	O
to	O
use	O
(	O
typical	O
for	O
ISP	O
customers	O
)	O
,	O
or	O
it	O
could	O
be	O
using	O
a	O
proxy	B-Application
without	O
any	O
extra	O
configuration	O
:	O
"	O
transparent	O
caching	B-Protocol
"	O
,	O
in	O
which	O
case	O
all	O
outgoing	O
HTTP	B-Protocol
requests	I-Protocol
are	O
intercepted	O
by	O
Squid	B-Protocol
and	O
all	O
responses	O
are	O
cached	O
.	O
</s>
<s>
The	O
latter	O
is	O
typically	O
a	O
corporate	O
set-up	O
(	O
all	O
clients	O
are	O
on	O
the	O
same	O
LAN	B-General_Concept
)	O
and	O
often	O
introduces	O
the	O
privacy	O
concerns	O
mentioned	O
above	O
.	O
</s>
<s>
Squid	B-Protocol
has	O
some	O
features	O
that	O
can	O
help	O
anonymize	O
connections	O
,	O
such	O
as	O
disabling	O
or	O
changing	O
specific	O
header	O
fields	O
in	O
a	O
client	B-Protocol
's	I-Protocol
HTTP	B-Protocol
requests	I-Protocol
.	O
</s>
<s>
Whether	O
these	O
are	O
set	O
,	O
and	O
what	O
they	O
are	O
set	O
to	O
do	O
,	O
is	O
up	O
to	O
the	O
person	O
who	O
controls	O
the	O
computer	O
running	O
Squid	B-Protocol
.	O
</s>
<s>
People	O
requesting	O
pages	O
through	O
a	O
network	O
which	O
transparently	O
uses	O
Squid	B-Protocol
may	O
not	O
know	O
whether	O
this	O
information	O
is	O
being	O
logged	O
.	O
</s>
<s>
The	O
above	O
setup	O
—	O
caching	B-Protocol
the	O
contents	O
of	O
an	O
unlimited	O
number	O
of	O
webservers	B-Application
for	O
a	O
limited	O
number	O
of	O
clients	O
—	O
is	O
the	O
classical	O
one	O
.	O
</s>
<s>
Another	O
setup	O
is	O
"	O
reverse	B-General_Concept
proxy	I-General_Concept
"	O
or	O
"	O
webserver	B-Application
acceleration	O
"	O
(	O
using	O
)	O
.	O
</s>
<s>
In	O
this	O
mode	O
,	O
the	O
cache	O
serves	O
an	O
unlimited	O
number	O
of	O
clients	O
for	O
a	O
limited	O
number	O
of	O
—	O
or	O
just	O
one	O
—	O
web	B-Application
servers	I-Application
.	O
</s>
<s>
As	O
an	O
example	O
,	O
if	O
slow.example.com	O
is	O
a	O
"	O
real	O
"	O
web	B-Application
server	I-Application
,	O
and	O
www.example.com	O
is	O
the	O
Squid	B-Protocol
cache	B-Protocol
server	I-Protocol
that	O
"	O
accelerates	O
"	O
it	O
,	O
the	O
first	O
time	O
any	O
page	O
is	O
requested	O
from	O
www.example.com	O
,	O
the	O
cache	B-Protocol
server	I-Protocol
would	O
get	O
the	O
actual	O
page	O
from	O
slow.example.com,	O
but	O
later	O
requests	O
would	O
get	O
the	O
stored	O
copy	O
directly	O
from	O
the	O
accelerator	O
(	O
for	O
a	O
configurable	O
period	O
,	O
after	O
which	O
the	O
stored	O
copy	O
would	O
be	O
discarded	O
)	O
.	O
</s>
<s>
This	O
does	O
,	O
however	O
,	O
mean	O
that	O
the	O
source	O
server	O
cannot	O
accurately	O
report	O
on	O
its	O
traffic	O
numbers	O
without	O
additional	O
configuration	O
,	O
as	O
all	O
requests	O
would	O
seem	O
to	O
have	O
come	O
from	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
.	O
</s>
<s>
A	O
way	O
to	O
adapt	O
the	O
reporting	O
on	O
the	O
source	O
server	O
is	O
to	O
use	O
the	O
X-Forwarded-For	B-Protocol
HTTP	B-Protocol
header	O
reported	O
by	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
,	O
to	O
get	O
the	O
real	O
client	B-Protocol
's	I-Protocol
IP	O
address	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
a	O
single	O
Squid	B-Protocol
server	I-Protocol
to	O
serve	O
both	O
as	O
a	O
normal	O
and	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
simultaneously	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
business	O
might	O
host	O
its	O
own	O
website	O
on	O
a	O
web	B-Application
server	I-Application
,	O
with	O
a	O
Squid	B-Protocol
server	I-Protocol
acting	O
as	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
between	O
clients	O
(	O
customers	O
accessing	O
the	O
website	O
from	O
outside	O
the	O
business	O
)	O
and	O
the	O
web	B-Application
server	I-Application
.	O
</s>
<s>
The	O
same	O
Squid	B-Protocol
server	I-Protocol
could	O
act	O
as	O
a	O
classical	O
web	B-Protocol
cache	I-Protocol
,	O
caching	B-Protocol
HTTP	B-Protocol
requests	I-Protocol
from	O
clients	O
within	O
the	O
business	O
(	O
i.e.	O
,	O
employees	O
accessing	O
the	O
internet	O
from	O
their	O
workstations	O
)	O
,	O
so	O
accelerating	O
web	O
access	O
and	O
reducing	O
bandwidth	O
demands	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
feature	O
of	O
the	O
HTTP	B-Protocol
protocol	I-Protocol
is	O
to	O
limit	O
a	O
request	O
to	O
the	O
range	O
of	O
data	O
in	O
the	O
resource	O
being	O
referenced	O
.	O
</s>
<s>
This	O
feature	O
is	O
used	O
extensively	O
by	O
video	O
streaming	O
websites	O
such	O
as	O
YouTube	B-General_Concept
,	O
so	O
that	O
if	O
a	O
user	O
clicks	O
to	O
the	O
middle	O
of	O
the	O
video	O
progress	O
bar	O
,	O
the	O
server	O
can	O
begin	O
to	O
send	O
data	O
from	O
the	O
middle	O
of	O
the	O
file	O
,	O
rather	O
than	O
sending	O
the	O
entire	O
file	O
from	O
the	O
beginning	O
and	O
the	O
user	O
waiting	O
for	O
the	O
preceding	O
data	O
to	O
finish	O
loading	O
.	O
</s>
<s>
Partial	O
downloads	O
are	O
also	O
extensively	O
used	O
by	O
Microsoft	B-Application
Windows	I-Application
Update	O
so	O
that	O
extremely	O
large	O
update	O
packages	O
can	O
download	O
in	O
the	O
background	O
and	O
pause	O
halfway	O
through	O
the	O
download	O
,	O
if	O
the	O
user	O
turns	O
off	O
their	O
computer	O
or	O
disconnects	O
from	O
the	O
Internet	O
.	O
</s>
<s>
The	O
Metalink	B-Application
download	O
format	O
enables	O
clients	O
to	O
do	O
segmented	O
downloads	O
by	O
issuing	O
partial	O
requests	O
and	O
spreading	O
these	O
over	O
a	O
number	O
of	O
mirrors	O
.	O
</s>
<s>
Squid	B-Protocol
can	O
relay	O
partial	O
requests	O
to	O
the	O
origin	O
web	B-Application
server	I-Application
.	O
</s>
<s>
In	O
order	O
for	O
a	O
partial	O
request	O
to	O
be	O
satisfied	O
at	O
a	O
fast	O
speed	O
from	O
cache	O
,	O
Squid	B-Protocol
requires	O
a	O
full	O
copy	O
of	O
the	O
same	O
object	O
to	O
already	O
exist	O
in	O
its	O
storage	O
.	O
</s>
<s>
If	O
a	O
proxy	B-Application
video	O
user	O
is	O
watching	O
a	O
video	O
stream	O
and	O
browses	O
to	O
a	O
different	O
page	O
before	O
the	O
video	O
completely	O
downloads	O
,	O
Squid	B-Protocol
cannot	O
keep	O
the	O
partial	O
download	O
for	O
reuse	O
and	O
simply	O
discards	O
the	O
data	O
.	O
</s>
<s>
Squid	B-Protocol
can	O
run	O
on	O
the	O
following	O
operating	B-General_Concept
systems	I-General_Concept
:	O
</s>
