<s>
robots.txt	O
is	O
a	O
standard	O
used	O
by	O
websites	O
to	O
indicate	O
to	O
visiting	O
web	B-Application
crawlers	I-Application
and	O
other	O
web	B-Protocol
robots	I-Protocol
which	O
portions	O
of	O
the	O
website	O
they	O
are	O
allowed	O
to	O
visit	O
.	O
</s>
<s>
Not	O
all	O
robots	O
comply	O
with	O
the	O
standard	O
;	O
email	O
harvesters	O
,	O
spambots	B-Protocol
,	O
malware	O
and	O
robots	O
that	O
scan	O
for	O
security	O
vulnerabilities	O
may	O
even	O
start	O
with	O
the	O
portions	O
of	O
the	O
website	O
where	O
they	O
have	O
been	O
told	O
to	O
stay	O
out	O
.	O
</s>
<s>
Charles	O
Stross	O
claims	O
to	O
have	O
provoked	O
Koster	O
to	O
suggest	O
robots.txt	O
,	O
after	O
he	O
wrote	O
a	O
badly-behaved	O
web	B-Application
crawler	I-Application
that	O
inadvertently	O
caused	O
a	O
denial-of-service	O
attack	O
on	O
Koster	O
's	O
server	O
.	O
</s>
<s>
It	O
quickly	O
became	O
a	O
de	O
facto	O
standard	O
that	O
present	O
and	O
future	O
web	B-Application
crawlers	I-Application
were	O
expected	O
to	O
follow	O
;	O
most	O
complied	O
,	O
including	O
those	O
operated	O
by	O
search	O
engines	B-Application
such	O
as	O
WebCrawler	B-Application
,	O
Lycos	B-Application
,	O
and	O
AltaVista	O
.	O
</s>
<s>
On	O
July	O
1	O
,	O
2019	O
,	O
Google	B-Application
announced	O
the	O
proposal	O
of	O
the	O
Robots	O
Exclusion	O
Protocol	O
as	O
an	O
official	O
standard	O
under	O
Internet	O
Engineering	O
Task	O
Force	O
.	O
</s>
<s>
When	O
a	O
site	O
owner	O
wishes	O
to	O
give	O
instructions	O
to	O
web	B-Protocol
robots	I-Protocol
they	O
place	O
a	O
text	O
file	O
called	O
in	O
the	O
root	O
of	O
the	O
web	O
site	O
hierarchy	O
(	O
e.g.	O
</s>
<s>
If	O
this	O
file	O
does	O
not	O
exist	O
,	O
web	B-Protocol
robots	I-Protocol
assume	O
that	O
the	O
website	O
owner	O
does	O
not	O
wish	O
to	O
place	O
any	O
limitations	O
on	O
crawling	O
the	O
entire	O
site	O
.	O
</s>
<s>
A	O
robots.txt	O
file	O
contains	O
instructions	O
for	O
bots	B-Protocol
indicating	O
which	O
web	O
pages	O
they	O
can	O
and	O
cannot	O
access	O
.	O
</s>
<s>
Robots.txt	O
files	O
are	O
particularly	O
important	O
for	O
web	B-Application
crawlers	I-Application
from	O
search	O
engines	B-Application
such	O
as	O
Google	B-Application
.	I-Application
</s>
<s>
A	O
robots.txt	O
file	O
covers	O
one	O
origin	B-Protocol
.	O
</s>
<s>
Some	O
major	O
search	O
engines	B-Application
following	O
this	O
standard	O
include	O
Ask	O
,	O
AOL	O
,	O
Baidu	O
,	O
DuckDuckGo	O
,	O
Google	B-Application
,	O
Yahoo	O
!,	O
and	O
Yandex	B-Application
.	O
</s>
<s>
For	O
some	O
years	O
,	O
the	O
Internet	O
Archive	O
did	O
not	O
crawl	B-Application
sites	I-Application
with	O
robots.txt	O
,	O
but	O
in	O
April	O
2017	O
,	O
it	O
announced	O
that	O
it	O
would	O
no	O
longer	O
honour	O
directives	O
in	O
the	O
robots.txt	O
files	O
.	O
</s>
<s>
Despite	O
the	O
use	O
of	O
the	O
terms	O
"	O
allow	O
"	O
and	O
"	O
disallow	O
"	O
,	O
the	O
protocol	O
is	O
purely	O
advisory	O
and	O
relies	O
on	O
the	O
compliance	O
of	O
the	O
web	B-Protocol
robot	I-Protocol
.	O
</s>
<s>
Malicious	O
web	B-Protocol
robots	I-Protocol
are	O
unlikely	O
to	O
honor	O
robots.txt	O
;	O
some	O
may	O
even	O
use	O
the	O
robots.txt	O
as	O
a	O
guide	O
to	O
find	O
disallowed	O
links	O
and	O
go	O
straight	O
to	O
them	O
.	O
</s>
<s>
Many	O
robots	O
also	O
pass	O
a	O
special	O
user-agent	B-Protocol
to	O
the	O
web	O
server	O
when	O
fetching	O
content	O
.	O
</s>
<s>
A	O
web	O
administrator	O
could	O
also	O
configure	O
the	O
server	O
to	O
automatically	O
return	O
failure	O
(	O
or	O
pass	B-Device
alternative	I-Device
content	I-Device
)	O
when	O
it	O
detects	O
a	O
connection	O
using	O
one	O
of	O
the	O
robots	O
.	O
</s>
<s>
Some	O
sites	O
,	O
such	O
as	O
Google	B-Application
,	O
host	O
a	O
humans.txt	B-General_Concept
file	O
that	O
displays	O
information	O
meant	O
for	O
humans	O
to	O
read	O
.	O
</s>
<s>
Some	O
sites	O
such	O
as	O
GitHub	B-Application
redirect	O
humans.txt	B-General_Concept
to	O
an	O
about	O
page	O
.	O
</s>
<s>
Previously	O
,	O
Google	B-Application
had	O
a	O
joke	O
file	O
hosted	O
at	O
/killer	O
-robots.txt	O
instructing	O
the	B-General_Concept
Terminator	I-General_Concept
not	O
to	O
kill	O
the	O
company	O
founders	O
Larry	O
Page	O
and	O
Sergey	O
Brin	O
.	O
</s>
<s>
A	O
few	O
robot	O
operators	O
,	O
such	O
as	O
Google	B-Application
,	O
support	O
several	O
user-agent	B-Protocol
strings	O
that	O
allow	O
the	O
operator	O
to	O
deny	O
access	O
to	O
a	O
subset	O
of	O
their	O
services	O
by	O
using	O
specific	O
user-agent	B-Protocol
strings	O
.	O
</s>
<s>
Example	O
demonstrating	O
multiple	O
user-agents	B-Protocol
:	O
</s>
<s>
It	O
is	O
used	O
when	O
the	O
multiple	O
burst	O
of	O
visits	O
from	O
bots	B-Protocol
is	O
slowing	O
down	O
the	O
host	O
.	O
</s>
<s>
Yandex	B-Application
interprets	O
the	O
value	O
as	O
the	O
number	O
of	O
seconds	O
to	O
wait	O
between	O
subsequent	O
visits	O
.	O
</s>
<s>
Google	B-Application
provides	O
an	O
interface	O
in	O
its	O
search	B-Application
console	I-Application
for	O
webmasters	O
,	O
to	O
control	O
the	O
Googlebot	B-Application
's	O
subsequent	O
visits	O
.	O
</s>
<s>
Some	O
crawlers	O
(	O
Yandex	B-Application
)	O
support	O
a	O
Host	O
directive	O
,	O
allowing	O
websites	O
with	O
multiple	O
mirrors	O
to	O
specify	O
their	O
preferred	O
domain	O
:	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
the	O
X-Robots-Tag	O
can	O
be	O
added	O
to	O
non-HTML	O
files	O
by	O
using	O
.htaccess	B-Protocol
and	O
httpd.conf	B-Application
files	O
.	O
</s>
<s>
The	O
Robots	O
Exclusion	O
Protocol	O
requires	O
crawlers	O
to	O
parse	O
at	O
least	O
500	O
kibibytes	O
(	O
KiB	O
)	O
of	O
robots.txt	O
files	O
,	O
which	O
Google	B-Application
maintains	O
as	O
a	O
500	O
kibibyte	O
file	O
size	O
restriction	O
for	O
robots.txt	O
files	O
.	O
</s>
