<s>
In	O
HTTP	B-Protocol
,	O
""	O
(	O
a	O
misspelling	O
of	O
Referrer	B-Protocol
)	O
is	O
an	O
optional	O
HTTP	B-Protocol
header	I-Protocol
field	I-Protocol
that	O
identifies	O
the	O
address	O
of	O
the	O
web	O
page	O
(	O
i.e.	O
,	O
the	O
URI	B-Protocol
or	O
IRI	B-Protocol
)	O
,	O
from	O
which	O
the	O
resource	O
has	O
been	O
requested	O
.	O
</s>
<s>
By	O
checking	O
the	O
referrer	B-Protocol
,	O
the	O
server	O
providing	O
the	O
new	O
web	O
page	O
can	O
see	O
where	O
the	O
request	O
originated	O
.	O
</s>
<s>
In	O
the	O
most	O
common	O
situation	O
,	O
this	O
means	O
that	O
when	O
a	O
user	O
clicks	O
a	O
hyperlink	O
in	O
a	O
web	B-Application
browser	I-Application
,	O
causing	O
the	O
browser	B-Application
to	O
send	O
a	O
request	O
to	O
the	O
server	O
holding	O
the	O
destination	O
web	O
page	O
,	O
the	O
request	O
may	O
include	O
the	O
field	O
,	O
which	O
indicates	O
the	O
last	O
page	O
the	O
user	O
was	O
on	O
(	O
the	O
one	O
where	O
they	O
clicked	O
the	O
link	O
)	O
.	O
</s>
<s>
Web	O
sites	O
and	O
web	B-Application
servers	I-Application
log	B-Application
the	O
content	O
of	O
the	O
received	O
field	O
to	O
identify	O
the	O
web	O
page	O
from	O
which	O
the	O
user	O
followed	O
a	O
link	O
,	O
for	O
promotional	O
or	O
statistical	O
purposes	O
.	O
</s>
<s>
To	O
mitigate	O
security	O
risks	O
,	O
browsers	B-Application
have	O
been	O
steadily	O
reducing	O
the	O
amount	O
of	O
information	O
sent	O
in	O
Referer	B-Protocol
.	O
</s>
<s>
As	O
of	O
March	O
2021	O
,	O
by	O
default	O
Chrome	B-Application
,	O
Chromium-based	O
Edge	B-Application
,	O
Firefox	B-Application
,	O
Safari	B-Application
default	O
to	O
sending	O
only	O
the	O
origin	O
in	O
cross-origin	O
requests	O
,	O
stripping	O
out	O
everything	O
but	O
the	O
domain	O
name	O
.	O
</s>
<s>
The	O
misspelling	O
of	O
referrer	B-Protocol
was	O
introduced	O
in	O
the	O
original	O
proposal	O
by	O
computer	O
scientist	O
Phillip	O
Hallam-Baker	O
to	O
incorporate	O
the	O
header	O
field	O
into	O
the	O
HTTP	B-Protocol
specification	O
.	O
</s>
<s>
The	O
misspelling	O
was	O
set	O
in	O
stone	O
by	O
the	O
time	O
(	O
May	O
1996	O
)	O
of	O
its	O
incorporation	O
into	O
the	O
Request	O
for	O
Comments	O
standards	O
document	O
RFC	O
1945	O
(	O
which	O
'	O
reflects	O
common	O
usage	O
of	O
the	O
protocol	O
referred	O
to	O
as	O
"	O
HTTP/1.0	B-Protocol
"	O
 '	O
at	O
that	O
time	O
)	O
;	O
document	O
co-author	O
Roy	O
Fielding	O
remarked	O
in	O
March	O
1995	O
that	O
"	O
neither	O
one	O
(	O
or	O
referrer	B-Protocol
)	O
is	O
understood	O
by	O
"	O
the	O
standard	O
Unix	B-Application
spell	I-Application
checker	I-Application
of	O
the	O
period	O
.	O
</s>
<s>
has	O
since	O
become	O
a	O
widely	O
used	O
spelling	O
in	O
the	O
industry	O
when	O
discussing	O
HTTP	B-Protocol
referrers	I-Protocol
;	O
usage	O
of	O
the	O
misspelling	O
is	O
not	O
universal	O
,	O
though	O
,	O
as	O
the	O
correct	O
spelling	O
"	O
referrer	B-Protocol
"	O
is	O
used	O
in	O
some	O
web	O
specifications	O
such	O
as	O
or	O
the	O
Document	B-General_Concept
Object	I-General_Concept
Model	I-General_Concept
.	O
</s>
<s>
When	O
visiting	O
a	O
web	O
page	O
,	O
the	O
referrer	B-Protocol
or	O
referring	B-Protocol
page	I-Protocol
is	O
the	O
URL	O
of	O
the	O
previous	O
web	O
page	O
from	O
which	O
a	O
link	O
was	O
followed	O
.	O
</s>
<s>
More	O
generally	O
,	O
a	O
referrer	B-Protocol
is	O
the	O
URL	O
of	O
a	O
previous	O
item	O
which	O
led	O
to	O
this	O
request	O
.	O
</s>
<s>
The	O
referrer	B-Protocol
for	O
an	O
image	O
,	O
for	O
example	O
,	O
is	O
generally	O
the	O
HTML	B-Language
page	O
on	O
which	O
it	O
is	O
to	O
be	O
displayed	O
.	O
</s>
<s>
The	O
referrer	B-Protocol
field	O
is	O
an	O
optional	O
part	O
of	O
the	O
HTTP	B-Protocol
request	O
sent	O
by	O
the	O
web	B-Application
browser	I-Application
to	O
the	O
web	B-Application
server	I-Application
.	O
</s>
<s>
Many	O
websites	O
log	B-Application
referrers	B-Protocol
as	O
part	O
of	O
their	O
attempt	O
to	O
track	B-Application
their	I-Application
users	I-Application
.	O
</s>
<s>
Most	O
web	B-Application
log	I-Application
analysis	I-Application
software	I-Application
can	O
process	O
this	O
information	O
.	O
</s>
<s>
Because	O
referrer	B-Protocol
information	O
can	O
violate	O
privacy	O
,	O
some	O
web	B-Application
browsers	I-Application
allow	O
the	O
user	O
to	O
disable	O
the	O
sending	O
of	O
referrer	B-Protocol
information	O
.	O
</s>
<s>
Some	O
proxy	B-Application
and	O
firewall	B-Application
software	I-Application
will	O
also	O
filter	O
out	O
referrer	B-Protocol
information	O
,	O
to	O
avoid	O
leaking	O
the	O
location	O
of	O
non-public	O
websites	O
.	O
</s>
<s>
This	O
can	O
,	O
in	O
turn	O
,	O
cause	O
problems	O
:	O
some	O
web	B-Application
servers	I-Application
block	O
parts	O
of	O
their	O
website	O
to	O
web	B-Application
browsers	I-Application
that	O
do	O
not	O
send	O
the	O
right	O
referrer	B-Protocol
information	O
,	O
in	O
an	O
attempt	O
to	O
prevent	O
deep	O
linking	O
or	O
unauthorised	O
use	O
of	O
images	O
(	O
bandwidth	O
theft	O
)	O
.	O
</s>
<s>
Some	O
proxy	B-Application
software	O
has	O
the	O
ability	O
to	O
give	O
the	O
top-level	O
address	O
of	O
the	O
target	O
website	O
as	O
the	O
referrer	B-Protocol
,	O
which	O
reduces	O
these	O
problems	O
but	O
can	O
still	O
in	O
some	O
cases	O
divulge	O
the	O
user	O
's	O
last-visited	O
web	O
page	O
.	O
</s>
<s>
Many	O
blogs	O
publish	O
referrer	B-Protocol
information	O
in	O
order	O
to	O
link	O
back	O
to	O
people	O
who	O
are	O
linking	O
to	O
them	O
,	O
and	O
hence	O
broaden	O
the	O
conversation	O
.	O
</s>
<s>
This	O
has	O
led	O
,	O
in	O
turn	O
,	O
to	O
the	O
rise	O
of	O
referrer	B-Protocol
spam	O
:	O
the	O
sending	O
of	O
fake	O
referrer	B-Protocol
information	O
in	O
order	O
to	O
popularize	O
the	O
spammer	O
's	O
website	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
access	O
the	O
referrer	B-Protocol
information	O
on	O
the	O
client	O
side	O
using	O
document.referrer	O
in	O
JavaScript	B-Language
.	O
</s>
<s>
However	O
,	O
the	O
referrer	B-Protocol
field	O
does	O
not	O
always	O
include	O
search	O
keywords	O
,	O
such	O
as	O
when	O
using	O
Google	B-Application
Search	I-Application
with	O
https	B-Protocol
.	I-Protocol
</s>
<s>
Most	O
web	B-Application
servers	I-Application
maintain	O
logs	O
of	O
all	O
traffic	O
,	O
and	O
record	O
the	O
HTTP	B-Protocol
referrer	I-Protocol
sent	O
by	O
the	O
web	B-Application
browser	I-Application
for	O
each	O
request	O
.	O
</s>
<s>
This	O
raises	O
a	O
number	O
of	O
privacy	O
concerns	O
,	O
and	O
as	O
a	O
result	O
,	O
a	O
number	O
of	O
systems	O
to	O
prevent	O
web	B-Application
servers	I-Application
being	O
sent	O
the	O
real	O
referring	O
URL	O
have	O
been	O
developed	O
.	O
</s>
<s>
These	O
systems	O
work	O
either	O
by	O
blanking	O
the	O
referrer	B-Protocol
field	O
or	O
by	O
replacing	O
it	O
with	O
inaccurate	O
data	O
.	O
</s>
<s>
Generally	O
,	O
Internet-security	B-Application
suites	O
blank	O
the	O
referrer	B-Protocol
data	O
,	O
while	O
web-based	O
servers	O
replace	O
it	O
with	O
a	O
false	O
URL	O
,	O
usually	O
their	O
own	O
.	O
</s>
<s>
This	O
raises	O
the	O
problem	O
of	O
referrer	B-Protocol
spam	O
.	O
</s>
<s>
The	O
technical	O
details	O
of	O
both	O
methods	O
are	O
fairly	O
consistent	O
–	O
software	O
applications	O
act	O
as	O
a	O
proxy	B-Application
server	I-Application
and	O
manipulate	O
the	O
HTTP	B-Protocol
request	O
,	O
while	O
web-based	O
methods	O
load	O
websites	O
within	O
frames	O
,	O
causing	O
the	O
web	B-Application
browser	I-Application
to	O
send	O
a	O
referrer	B-Protocol
URL	O
of	O
their	O
website	O
address	O
.	O
</s>
<s>
Some	O
web	B-Application
browsers	I-Application
give	O
their	O
users	O
the	O
option	O
to	O
turn	O
off	O
referrer	B-Protocol
fields	O
in	O
the	O
request	O
header	O
.	O
</s>
<s>
Most	O
web	B-Application
browsers	I-Application
do	O
not	O
send	O
the	O
referrer	B-Protocol
field	O
when	O
they	O
are	O
instructed	O
to	O
redirect	O
using	O
the	O
"	O
Refresh	O
"	O
field	O
.	O
</s>
<s>
This	O
does	O
not	O
include	O
some	O
versions	O
of	O
Opera	B-Application
and	O
many	O
mobile	O
web	B-Application
browsers	I-Application
.	O
</s>
<s>
If	O
a	O
website	O
is	O
accessed	O
from	O
a	O
HTTP	B-Protocol
Secure	I-Protocol
(	O
HTTPS	B-Protocol
)	O
connection	O
and	O
a	O
link	O
points	O
to	O
anywhere	O
except	O
another	O
secure	O
location	O
,	O
then	O
the	O
referrer	B-Protocol
field	O
is	O
not	O
sent	O
.	O
</s>
<s>
The	O
HTML5	B-Application
standard	O
added	O
support	O
for	O
the	O
attribute/value	O
rel	O
=	O
"	O
noreferrer	O
"	O
,	O
which	O
instructs	O
the	O
user	O
agent	O
to	O
not	O
send	O
a	O
referrer	B-Protocol
.	O
</s>
<s>
Another	O
referrer	B-Protocol
hiding	O
method	O
is	O
to	O
convert	O
the	O
original	O
link	O
URL	O
to	O
a	O
Data	O
URI	O
scheme-based	O
URL	O
containing	O
small	O
HTML	B-Language
page	O
with	O
a	O
meta	B-Protocol
refresh	I-Protocol
to	O
the	O
original	O
URL	O
.	O
</s>
<s>
When	O
the	O
user	O
is	O
redirected	O
from	O
the	O
data	B-Protocol
:	I-Protocol
page	O
,	O
the	O
original	O
referrer	B-Protocol
is	O
hidden	O
.	O
</s>
<s>
Content	B-Protocol
Security	I-Protocol
Policy	I-Protocol
standard	O
version	O
1.1	O
introduced	O
a	O
new	O
referrer	B-Protocol
directive	O
that	O
allows	O
more	O
control	O
over	O
the	O
browser	B-Application
's	O
behaviour	O
in	O
regards	O
to	O
the	O
referrer	B-Protocol
header	I-Protocol
.	O
</s>
<s>
Specifically	O
it	O
allows	O
the	O
webmaster	O
to	O
instruct	O
the	O
browser	B-Application
not	O
to	O
block	O
referrer	B-Protocol
at	O
all	O
,	O
reveal	O
it	O
only	O
when	O
moving	O
with	O
the	O
same	O
origin	O
etc	O
.	O
</s>
