<s>
In	O
computer	O
network	O
communications	O
,	O
the	O
HTTP	B-Protocol
404	I-Protocol
,	O
404	B-Protocol
not	I-Protocol
found	I-Protocol
,	O
404	B-Protocol
,	O
404	B-Protocol
error	I-Protocol
,	O
page	B-Protocol
not	I-Protocol
found	I-Protocol
or	O
file	B-Error_Name
not	I-Error_Name
found	I-Error_Name
error	B-Error_Name
message	I-Error_Name
is	O
a	O
hypertext	B-Protocol
transfer	I-Protocol
protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
standard	B-Protocol
response	I-Protocol
code	I-Protocol
,	O
to	O
indicate	O
that	O
the	O
browser	B-Application
was	O
able	O
to	O
communicate	O
with	O
a	O
given	O
server	B-Application
,	O
but	O
the	O
server	B-Application
could	O
not	O
find	O
what	O
was	O
requested	O
.	O
</s>
<s>
The	O
error	O
may	O
also	O
be	O
used	O
when	O
a	O
server	B-Application
does	O
not	O
wish	O
to	O
disclose	O
whether	O
it	O
has	O
the	O
requested	O
information	O
.	O
</s>
<s>
The	O
website	O
hosting	O
server	B-Application
will	O
typically	O
generate	O
a	O
"	O
404	B-Protocol
Not	I-Protocol
Found	I-Protocol
"	O
web	O
page	O
when	O
a	O
user	O
attempts	O
to	O
follow	O
a	O
broken	O
or	O
dead	O
link	O
;	O
hence	O
the	O
404	B-Protocol
error	I-Protocol
is	O
one	O
of	O
the	O
most	O
recognizable	O
errors	O
encountered	O
on	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
When	O
communicating	O
via	O
HTTP	B-Protocol
,	O
a	O
server	B-Application
is	O
required	O
to	O
respond	O
to	O
a	O
request	O
,	O
such	O
as	O
a	O
web	B-Application
browser	I-Application
request	O
for	O
a	O
web	O
page	O
,	O
with	O
a	O
numeric	O
response	O
code	O
and	O
an	O
optional	O
,	O
mandatory	O
,	O
or	O
disallowed	O
(	O
based	O
upon	O
the	O
status	O
code	O
)	O
message	O
.	O
</s>
<s>
In	O
code	O
404	B-Protocol
,	O
the	O
first	O
digit	O
indicates	O
a	O
client	O
error	O
,	O
such	O
as	O
a	O
mistyped	O
Uniform	O
Resource	O
Locator	O
(	O
URL	O
)	O
.	O
</s>
<s>
HTTP	B-Protocol
's	O
use	O
of	O
three-digit	O
codes	O
is	O
similar	O
to	O
the	O
use	O
of	O
such	O
codes	O
in	O
earlier	O
protocols	O
such	O
as	O
FTP	B-Protocol
and	O
NNTP	B-Protocol
.	O
</s>
<s>
At	O
the	O
HTTP	B-Protocol
level	O
,	O
a	O
404	B-Protocol
response	O
code	O
is	O
followed	O
by	O
a	O
human-readable	O
"	O
reason	O
phrase	O
"	O
.	O
</s>
<s>
The	O
HTTP	B-Protocol
specification	O
suggests	O
the	O
phrase	O
"	O
Not	B-Protocol
Found	I-Protocol
"	O
and	O
many	O
web	B-Application
servers	I-Application
by	O
default	O
issue	O
an	O
HTML	O
page	O
that	O
includes	O
both	O
the	O
404	B-Protocol
code	O
and	O
the	O
"	O
Not	B-Protocol
Found	I-Protocol
"	O
phrase	O
.	O
</s>
<s>
A	O
404	B-Protocol
error	I-Protocol
is	O
often	O
returned	O
when	O
pages	O
have	O
been	O
moved	O
or	O
deleted	O
.	O
</s>
<s>
In	O
the	O
first	O
case	O
,	O
it	O
is	O
better	O
to	O
employ	O
URL	O
mapping	O
or	O
URL	O
redirection	O
by	O
returning	O
a	O
301	O
Moved	O
Permanently	O
response	O
,	O
which	O
can	O
be	O
configured	O
in	O
most	O
server	B-Application
configuration	O
files	O
,	O
or	O
through	O
URL	B-Protocol
rewriting	I-Protocol
;	O
in	O
the	O
second	O
case	O
,	O
a	O
410	O
Gone	O
should	O
be	O
returned	O
.	O
</s>
<s>
Because	O
these	O
two	O
options	O
require	O
special	O
server	B-Application
configuration	O
,	O
most	O
websites	O
do	O
not	O
make	O
use	O
of	O
them	O
.	O
</s>
<s>
404	B-Protocol
errors	I-Protocol
should	O
not	O
be	O
confused	O
with	O
DNS	B-Protocol
errors	I-Protocol
,	O
which	O
appear	O
when	O
the	O
given	O
URL	O
refers	O
to	O
a	O
server	B-Application
name	O
that	O
does	O
not	O
exist	O
.	O
</s>
<s>
A	O
404	B-Protocol
error	I-Protocol
indicates	O
that	O
the	O
server	B-Application
itself	O
was	O
found	O
,	O
but	O
that	O
the	O
server	B-Application
was	O
not	O
able	O
to	O
retrieve	O
the	O
requested	O
page	O
.	O
</s>
<s>
Some	O
websites	O
report	O
a	O
"	O
not	B-Protocol
found	I-Protocol
"	O
error	O
by	O
returning	O
a	O
standard	O
web	O
page	O
with	O
a	O
"	O
200	O
OK	O
"	O
response	O
code	O
,	O
falsely	O
reporting	O
that	O
the	O
page	O
loaded	O
properly	O
;	O
this	O
is	O
known	O
as	O
a	O
soft	O
404	B-Protocol
.	O
</s>
<s>
The	O
term	O
"	O
soft	O
404	B-Protocol
"	O
was	O
introduced	O
in	O
2004	O
by	O
Ziv	O
Bar-Yossef	O
et	O
al	O
.	O
</s>
<s>
Soft	B-Protocol
404s	I-Protocol
are	O
problematic	O
for	O
automated	O
methods	O
of	O
discovering	O
whether	O
a	O
link	O
is	O
broken	O
.	O
</s>
<s>
Some	O
search	O
engines	O
,	O
like	O
Yahoo	B-Application
and	O
Google	B-Application
,	O
use	O
automated	O
processes	O
to	O
detect	O
soft	B-Protocol
404s	I-Protocol
.	O
</s>
<s>
Soft	B-Protocol
404s	I-Protocol
can	O
occur	O
as	O
a	O
result	O
of	O
configuration	O
errors	O
when	O
using	O
certain	O
HTTP	B-Application
server	I-Application
software	O
,	O
for	O
example	O
with	O
the	O
Apache	B-Application
software	O
,	O
when	O
an	O
Error	O
Document	O
404	B-Protocol
(	O
specified	O
in	O
a	O
.htaccess	B-Protocol
file	O
)	O
is	O
specified	O
as	O
an	O
absolute	O
path	O
(	O
e.g.	O
</s>
<s>
This	O
can	O
also	O
be	O
done	O
on	O
purpose	O
to	O
force	O
some	O
browsers	B-Application
(	O
like	O
Internet	B-Application
Explorer	I-Application
)	O
to	O
display	O
a	O
customized	O
404	B-Protocol
error	B-Error_Name
message	I-Error_Name
rather	O
than	O
replacing	O
what	O
is	O
served	O
with	O
a	O
browser-specific	O
"	O
friendly	O
"	O
error	B-Error_Name
message	I-Error_Name
(	O
in	O
Internet	B-Application
Explorer	I-Application
,	O
this	O
behavior	O
is	O
triggered	O
when	O
a	O
404	B-Protocol
is	O
served	O
and	O
the	O
received	O
HTML	O
is	O
shorter	O
than	O
a	O
certain	O
length	O
,	O
and	O
can	O
be	O
manually	O
disabled	O
by	O
the	O
user	O
)	O
.	O
</s>
<s>
There	O
are	O
also	O
"	O
soft	O
3XX	O
"	O
errors	O
where	O
content	B-Operating_System
is	O
returned	O
with	O
a	O
status	O
200	O
but	O
comes	O
from	O
a	O
redirected	O
page	O
,	O
such	O
as	O
when	O
missing	O
pages	O
are	O
redirected	O
to	O
the	O
domain	O
root/home	O
page	O
.	O
</s>
<s>
Some	O
proxy	B-Application
servers	I-Application
generate	O
a	O
404	B-Protocol
error	I-Protocol
when	O
a	O
500-range	O
error	O
code	O
would	O
be	O
more	O
correct	O
.	O
</s>
<s>
If	O
the	O
proxy	B-Application
server	I-Application
is	O
unable	O
to	O
satisfy	O
a	O
request	O
for	O
a	O
page	O
because	O
of	O
a	O
problem	O
with	O
the	O
remote	O
host	O
(	O
such	O
as	O
hostname	O
resolution	O
failures	O
or	O
refused	O
TCP	O
connections	O
)	O
,	O
this	O
should	O
be	O
described	O
as	O
a	O
5xx	O
Internal	O
Server	B-Application
Error	O
,	O
but	O
might	O
deliver	O
a	O
404	B-Protocol
instead	O
.	O
</s>
<s>
This	O
can	O
confuse	O
programs	O
that	O
expect	O
and	O
act	O
on	O
specific	O
responses	O
,	O
as	O
they	O
can	O
no	O
longer	O
easily	O
distinguish	O
between	O
an	O
absent	O
web	B-Application
server	I-Application
and	O
a	O
missing	O
web	O
page	O
on	O
a	O
web	B-Application
server	I-Application
that	O
is	O
present	O
.	O
</s>
<s>
In	O
July	O
2004	O
,	O
the	O
UK	O
telecom	O
provider	O
BT	O
Group	O
deployed	O
the	O
Cleanfeed	O
content	B-Operating_System
blocking	O
system	O
,	O
which	O
returns	O
a	O
404	B-Protocol
error	I-Protocol
to	O
any	O
request	O
for	O
content	B-Operating_System
identified	O
as	O
potentially	O
illegal	O
by	O
the	O
Internet	O
Watch	O
Foundation	O
.	O
</s>
<s>
Other	O
ISPs	O
return	O
a	O
HTTP	B-Protocol
403	I-Protocol
"	O
forbidden	O
"	O
error	O
in	O
the	O
same	O
circumstances	O
.	O
</s>
<s>
The	O
practice	O
of	O
employing	O
fake	O
404	B-Protocol
errors	I-Protocol
as	O
a	O
means	O
to	O
conceal	O
censorship	O
has	O
also	O
been	O
reported	O
in	O
Thailand	O
and	O
Tunisia	O
.	O
</s>
<s>
In	O
Tunisia	O
,	O
where	O
censorship	O
was	O
severe	O
before	O
the	O
2011	O
revolution	O
,	O
people	O
became	O
aware	O
of	O
the	O
nature	O
of	O
the	O
fake	O
404	B-Protocol
errors	I-Protocol
and	O
created	O
an	O
imaginary	O
character	O
named	O
"	O
Ammar	O
404	B-Protocol
"	O
who	O
represents	O
"	O
the	O
invisible	O
censor	O
"	O
.	O
</s>
<s>
The	O
webserver	B-Application
software	O
developed	O
by	O
Microsoft	O
,	O
Microsoft	B-Application
's	I-Application
Internet	I-Application
Information	I-Application
Services	I-Application
(	O
IIS	O
)	O
,	O
returns	O
a	O
set	O
of	O
substatus	O
codes	O
with	O
its	O
404	B-Protocol
responses	O
.	O
</s>
<s>
The	O
substatus	O
codes	O
take	O
the	O
form	O
of	O
decimal	O
numbers	O
appended	O
to	O
the	O
404	B-Protocol
status	O
code	O
.	O
</s>
<s>
Microsoft	O
's	O
IIS	O
7.0	O
,	O
IIS	O
7.5	O
,	O
and	O
IIS	O
8.0	O
servers	O
define	O
the	O
following	O
HTTP	B-Protocol
substatus	O
codes	O
to	O
indicate	O
a	O
more	O
specific	O
cause	O
of	O
a	O
404	B-Protocol
error	I-Protocol
:	O
</s>
<s>
404.0	O
–	O
Not	B-Protocol
found	I-Protocol
.	O
</s>
<s>
404.1	O
–	O
Site	O
Not	B-Protocol
Found	I-Protocol
.	O
</s>
<s>
404.2	O
–	O
ISAPI	B-Library
or	O
CGI	B-Language
restriction	O
.	O
</s>
<s>
404.3	O
–	O
MIME	B-Protocol
type	O
restriction	O
.	O
</s>
<s>
404.4	O
–	O
No	O
handler	O
configured	O
.	O
</s>
<s>
404.5	O
–	O
Denied	O
by	O
request	O
filtering	O
configuration	O
.	O
</s>
<s>
404.6	O
–	O
Verb	O
denied	O
.	O
</s>
<s>
404.7	O
–	O
File	O
extension	O
denied	O
.	O
</s>
<s>
404.8	O
–	O
Hidden	O
namespace	O
.	O
</s>
<s>
404.9	O
–	O
File	O
attribute	O
hidden	O
.	O
</s>
<s>
404.10	O
–	O
Request	O
header	O
too	O
long	O
.	O
</s>
<s>
404.11	O
–	O
Request	O
contains	O
double	O
escape	O
sequence	O
.	O
</s>
<s>
404.12	O
–	O
Request	O
contains	O
high-bit	O
characters	O
.	O
</s>
<s>
404.13	O
–	O
Content	B-Operating_System
length	O
too	O
large	O
.	O
</s>
<s>
404.14	O
–	O
Request	O
URL	O
too	O
long	O
.	O
</s>
<s>
404.15	O
–	O
Query	O
string	O
too	O
long	O
.	O
</s>
<s>
404.16	O
–	O
DAV	O
request	O
sent	O
to	O
the	O
static	O
file	O
handler	O
.	O
</s>
<s>
404.17	O
–	O
Dynamic	O
content	B-Operating_System
mapped	O
to	O
the	O
static	O
file	O
handler	O
via	O
a	O
wildcard	O
MIME	B-Protocol
mapping	O
.	O
</s>
<s>
404.18	O
–	O
Query	O
string	O
sequence	O
denied	O
.	O
</s>
<s>
404.19	O
–	O
Denied	O
by	O
filtering	O
rule	O
.	O
</s>
<s>
404.20	O
–	O
Too	O
Many	O
URL	O
Segments	O
.	O
</s>
<s>
Web	B-Application
servers	I-Application
can	O
typically	O
be	O
configured	O
to	O
display	O
a	O
customised	O
404	B-Protocol
error	I-Protocol
page	O
,	O
including	O
a	O
more	O
natural	O
description	O
,	O
the	O
parent	O
site	O
's	O
branding	O
,	O
and	O
sometimes	O
a	O
site	O
map	O
,	O
a	O
search	O
form	O
or	O
404-page	O
widget	O
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
,	O
however	O
,	O
will	O
not	O
display	O
custom	O
pages	O
unless	O
they	O
are	O
larger	O
than	O
512	O
bytes	O
,	O
opting	O
instead	O
to	O
display	O
a	O
"	O
friendly	O
"	O
error	O
page	O
.	O
</s>
<s>
Google	B-Application
Chrome	I-Application
included	O
similar	O
functionality	O
,	O
where	O
the	O
404	B-Protocol
is	O
replaced	O
with	O
alternative	O
suggestions	O
generated	O
by	O
Google	B-Application
algorithms	O
,	O
if	O
the	O
page	O
is	O
under	O
512	O
bytes	O
in	O
size	O
.	O
</s>
<s>
Another	O
problem	O
is	O
that	O
if	O
the	O
page	O
does	O
not	O
provide	O
a	O
favicon	O
,	O
and	O
a	O
separate	O
custom	O
404-page	O
exists	O
,	O
extra	O
traffic	O
and	O
longer	O
loading	O
times	O
will	O
be	O
generated	O
on	O
every	O
page	O
view	O
.	O
</s>
<s>
Many	O
organizations	O
use	O
404	B-Protocol
error	I-Protocol
pages	O
as	O
an	O
opportunity	O
to	O
inject	O
humor	O
into	O
what	O
may	O
otherwise	O
be	O
a	O
serious	O
website	O
.	O
</s>
<s>
During	O
the	O
2015	O
UK	O
general	O
election	O
campaign	O
the	O
main	O
political	O
parties	O
all	O
used	O
their	O
404	B-Protocol
pages	I-Protocol
to	O
either	O
take	O
aim	O
at	O
political	O
opponents	O
or	O
show	O
relevant	O
policies	O
to	O
potential	O
supporters	O
.	O
</s>
<s>
In	O
Europe	O
,	O
the	O
NotFound	O
project	O
,	O
created	O
by	O
multiple	O
European	O
organizations	O
including	O
Missing	O
Children	O
Europe	O
and	O
Child	O
Focus	O
,	O
encourages	O
site	O
operators	O
to	O
add	O
a	O
snippet	B-Application
of	I-Application
code	I-Application
to	O
serve	O
customized	O
404	B-Protocol
error	I-Protocol
pages	O
which	O
provide	O
data	O
about	O
missing	O
children	O
.	O
</s>
<s>
While	O
many	O
websites	O
send	O
additional	O
information	O
in	O
a	O
404	B-Protocol
error	I-Protocol
message	O
—	O
such	O
as	O
a	O
link	O
to	O
the	O
homepage	O
of	O
a	O
website	O
or	O
a	O
search	O
box	O
—	O
some	O
also	O
endeavor	O
to	O
find	O
the	O
correct	O
web	O
page	O
the	O
user	O
wanted	O
.	O
</s>
<s>
Extensions	O
are	O
available	O
for	O
some	O
content	B-Protocol
management	I-Protocol
systems	I-Protocol
(	O
CMSs	O
)	O
to	O
do	O
this	O
.	O
</s>
<s>
A	O
number	O
of	O
tools	O
exist	O
that	O
crawl	O
through	O
a	O
website	O
to	O
find	O
pages	O
that	O
return	O
404	B-Protocol
status	O
codes	O
.	O
</s>
<s>
The	O
limitation	O
of	O
these	O
tools	O
is	O
that	O
they	O
only	O
find	O
links	O
within	O
one	O
particular	O
website	O
,	O
and	O
ignore	O
404s	B-Protocol
resulting	O
from	O
links	O
on	O
other	O
websites	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
these	O
tools	O
miss	O
out	O
on	O
83%	O
of	O
the	O
404s	B-Protocol
on	O
websites	O
.	O
</s>
<s>
One	O
way	O
around	O
this	O
is	O
to	O
find	O
404	B-Protocol
errors	I-Protocol
by	O
analyzing	O
external	O
links	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
effective	O
ways	O
to	O
discover	O
404	B-Protocol
errors	I-Protocol
is	O
by	O
using	O
Google	B-Application
Search	I-Application
Console	I-Application
,	O
Google	B-Application
Analytics	I-Application
or	O
crawling	O
software	O
.	O
</s>
<s>
Another	O
common	O
method	O
is	O
tracking	O
traffic	O
to	O
404	B-Protocol
pages	I-Protocol
using	O
log	O
file	O
analysis	O
.	O
</s>
<s>
This	O
can	O
be	O
useful	O
to	O
understand	O
more	O
about	O
what	O
404s	B-Protocol
users	O
reached	O
on	O
the	O
site	O
.	O
</s>
<s>
Another	O
method	O
of	O
tracking	O
traffic	O
to	O
404	B-Protocol
pages	I-Protocol
is	O
using	O
JavaScript-based	O
traffic	O
tracking	O
tools	O
.	O
</s>
