<s>
In	O
computing	O
,	O
a	O
user	B-Protocol
agent	I-Protocol
is	O
any	O
software	O
,	O
acting	O
on	O
behalf	O
of	O
a	O
user	O
,	O
which	O
"	O
retrieves	O
,	O
renders	O
and	O
facilitates	O
end-user	O
interaction	O
with	O
Web	O
content	O
"	O
.	O
</s>
<s>
A	O
user	B-Protocol
agent	I-Protocol
is	O
therefore	O
a	O
special	O
kind	O
of	O
software	B-General_Concept
agent	I-General_Concept
.	O
</s>
<s>
Some	O
prominent	O
examples	O
of	O
user	B-Protocol
agents	I-Protocol
are	O
web	B-Application
browsers	I-Application
and	O
email	B-Protocol
readers	I-Protocol
.	O
</s>
<s>
Often	O
,	O
a	O
user	B-Protocol
agent	I-Protocol
acts	O
as	O
the	O
client	B-Protocol
in	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
In	O
some	O
contexts	O
,	O
such	O
as	O
within	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
,	O
the	O
term	O
user	B-Protocol
agent	I-Protocol
refers	O
to	O
both	O
end	O
points	O
of	O
a	O
communications	O
session	O
.	O
</s>
<s>
When	O
a	O
software	B-General_Concept
agent	I-General_Concept
operates	O
in	O
a	O
network	O
protocol	O
,	O
it	O
often	O
identifies	O
itself	O
,	O
its	O
application	O
type	O
,	O
operating	B-General_Concept
system	I-General_Concept
,	O
device	O
model	O
,	O
software	O
vendor	O
,	O
or	O
software	O
revision	O
,	O
by	O
submitting	O
a	O
characteristic	O
identification	O
string	O
to	O
its	O
operating	O
peer	O
.	O
</s>
<s>
In	O
HTTP	O
,	O
SIP	O
,	O
and	O
NNTP	O
protocols	O
,	O
this	O
identification	O
is	O
transmitted	O
in	O
a	O
header	O
field	O
User-Agent	B-Protocol
.	O
</s>
<s>
Bots	B-Protocol
,	O
such	O
as	O
Web	B-Application
crawlers	I-Application
,	O
often	O
also	O
include	O
a	O
URL	O
and/or	O
e-mail	O
address	O
so	O
that	O
the	O
Webmaster	O
can	O
contact	O
the	O
operator	O
of	O
the	O
bot	B-Protocol
.	O
</s>
<s>
In	O
HTTP	O
,	O
the	O
User-Agent	B-Protocol
string	O
is	O
often	O
used	O
for	O
content	B-Protocol
negotiation	I-Protocol
,	O
where	O
the	O
origin	O
server	O
selects	O
suitable	O
content	O
or	O
operating	O
parameters	O
for	O
the	O
response	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
User-Agent	B-Protocol
string	O
might	O
be	O
used	O
by	O
a	O
web	O
server	O
to	O
choose	O
variants	O
based	O
on	O
the	O
known	O
capabilities	O
of	O
a	O
particular	O
version	O
of	O
client	B-Protocol
software	I-Protocol
.	O
</s>
<s>
The	O
concept	O
of	O
content	O
tailoring	O
is	O
built	O
into	O
the	O
HTTP	O
standard	O
in	O
"	O
for	O
the	O
sake	O
of	O
tailoring	O
responses	O
to	O
avoid	O
particular	O
user	B-Protocol
agent	I-Protocol
limitations	O
"	O
.	O
</s>
<s>
The	O
User-Agent	B-Protocol
string	O
is	O
one	O
of	O
the	O
criteria	O
by	O
which	O
Web	B-Application
crawlers	I-Application
may	O
be	O
excluded	O
from	O
accessing	O
certain	O
parts	O
of	O
a	O
website	O
using	O
the	O
Robots	O
Exclusion	O
Standard	O
(	O
robots.txt	O
file	O
)	O
.	O
</s>
<s>
As	O
with	O
many	O
other	O
HTTP	O
request	O
headers	O
,	O
the	O
information	O
in	O
the	O
"	O
User-Agent	B-Protocol
"	O
string	O
contributes	O
to	O
the	O
information	O
that	O
the	O
client	B-Protocol
sends	O
to	O
the	O
server	O
,	O
since	O
the	O
string	O
can	O
vary	O
considerably	O
from	O
user	O
to	O
user	O
.	O
</s>
<s>
The	O
User-Agent	B-Protocol
string	O
format	O
is	O
currently	O
specified	O
by	O
section	O
10.1.5	O
of	O
.	O
</s>
<s>
The	O
format	O
of	O
the	O
User-Agent	B-Protocol
string	O
in	O
HTTP	O
is	O
a	O
list	O
of	O
product	O
tokens	O
(	O
keywords	O
)	O
with	O
optional	O
comments	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
user	O
's	O
product	O
were	O
called	O
WikiBrowser	O
,	O
their	O
user	B-Protocol
agent	I-Protocol
string	O
might	O
be	O
WikiBrowser/1.0	O
Gecko/1.0	O
.	O
</s>
<s>
During	O
the	O
first	O
browser	B-Application
war	I-Application
,	O
many	O
web	O
servers	O
were	O
configured	O
to	O
send	O
web	O
pages	O
that	O
required	O
advanced	O
features	O
,	O
including	O
frames	B-General_Concept
,	O
to	O
clients	O
that	O
were	O
identified	O
as	O
some	O
version	O
of	O
Mozilla	B-Operating_System
only	O
.	O
</s>
<s>
Other	O
browsers	B-Application
were	O
considered	O
to	O
be	O
older	O
products	O
such	O
as	O
Mosaic	B-Application
,	O
Cello	B-Application
,	O
or	O
Samba	B-Operating_System
,	O
and	O
would	O
be	O
sent	O
a	O
bare	O
bones	O
HTML	O
document	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
most	O
Web	B-Application
browsers	I-Application
use	O
a	O
User-Agent	B-Protocol
string	O
value	O
as	O
follows	O
:	O
</s>
<s>
For	O
example	O
,	O
Safari	B-Application
on	O
the	O
iPad	O
has	O
used	O
the	O
following	O
:	O
</s>
<s>
Mozilla/5.0	O
:	O
Previously	O
used	O
to	O
indicate	O
compatibility	O
with	O
the	O
Mozilla	B-Operating_System
rendering	O
engine	O
.	O
</s>
<s>
(	O
iPad	O
;	O
U	O
;	O
CPU	O
OS	O
3_2_1	O
like	O
Mac	O
OS	O
X	O
;	O
en-us	O
)	O
:	O
Details	O
of	O
the	O
system	O
in	O
which	O
the	O
browser	B-Application
is	O
running	O
.	O
</s>
<s>
AppleWebKit/531.21.10	O
:	O
The	O
platform	O
the	O
browser	B-Application
uses	O
.	O
</s>
<s>
(	O
KHTML	O
,	O
like	O
Gecko	O
)	O
:	O
Browser	B-Application
platform	O
details	O
.	O
</s>
<s>
Mobile/7B405	O
:	O
This	O
is	O
used	O
by	O
the	O
browser	B-Application
to	O
indicate	O
specific	O
enhancements	O
that	O
are	O
available	O
directly	O
in	O
the	O
browser	B-Application
or	O
through	O
third	O
parties	O
.	O
</s>
<s>
Before	O
migrating	O
to	O
the	O
Chromium	B-Language
code	O
base	O
,	O
Opera	B-Application
was	O
the	O
most	O
widely	O
used	O
web	B-Application
browser	I-Application
that	O
did	O
not	O
have	O
the	O
User-Agent	B-Protocol
string	O
with	O
"	O
Mozilla	B-Operating_System
"	O
(	O
instead	O
beginning	O
it	O
with	O
"	O
Opera	B-Application
"	O
)	O
.	O
</s>
<s>
Since	O
July	O
15	O
,	O
2013	O
,	O
Opera	B-Application
's	O
User-Agent	B-Protocol
string	O
begins	O
with	O
"	O
Mozilla/5.0	O
"	O
and	O
,	O
to	O
avoid	O
encountering	O
legacy	O
server	O
rules	O
,	O
no	O
longer	O
includes	O
the	O
word	O
"	O
Opera	B-Application
"	O
(	O
instead	O
using	O
the	O
string	O
"	O
OPR	O
"	O
to	O
denote	O
the	O
Opera	B-Application
version	O
)	O
.	O
</s>
<s>
Automated	O
web	B-Application
crawling	I-Application
tools	O
can	O
use	O
a	O
simplified	O
form	O
,	O
where	O
an	O
important	O
field	O
is	O
contact	O
information	O
in	O
case	O
of	O
problems	O
.	O
</s>
<s>
By	O
convention	O
the	O
word	O
"	O
bot	B-Protocol
"	O
is	O
included	O
in	O
the	O
name	O
of	O
the	O
agent	O
.	O
</s>
<s>
Automated	B-Application
agents	I-Application
are	O
expected	O
to	O
follow	O
rules	O
in	O
a	O
special	O
file	O
called	O
"	O
robots.txt	O
"	O
.	O
</s>
<s>
The	O
popularity	O
of	O
various	O
Web	B-Application
browser	I-Application
products	O
has	O
varied	O
throughout	O
the	O
Web	O
's	O
history	O
,	O
and	O
this	O
has	O
influenced	O
the	O
design	O
of	O
websites	O
in	O
such	O
a	O
way	O
that	O
websites	O
are	O
sometimes	O
designed	O
to	O
work	O
well	O
only	O
with	O
particular	O
browsers	B-Application
,	O
rather	O
than	O
according	O
to	O
uniform	O
standards	O
by	O
the	O
World	O
Wide	O
Web	O
Consortium	O
(	O
W3C	O
)	O
or	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
Websites	O
often	O
include	O
code	O
to	O
detect	O
browser	B-Application
version	O
to	O
adjust	O
the	O
page	O
design	O
sent	O
according	O
to	O
the	O
user	B-Protocol
agent	I-Protocol
string	O
received	O
.	O
</s>
<s>
This	O
may	O
mean	O
that	O
less-popular	O
browsers	B-Application
are	O
not	O
sent	O
complex	O
content	O
(	O
even	O
though	O
they	O
might	O
be	O
able	O
to	O
deal	O
with	O
it	O
correctly	O
)	O
or	O
,	O
in	O
extreme	O
cases	O
,	O
refused	O
all	O
content	O
.	O
</s>
<s>
Thus	O
,	O
various	O
browsers	B-Application
have	O
a	O
feature	O
to	O
cloak	O
or	O
spoof	O
their	O
identification	O
to	O
force	O
certain	O
server-side	O
content	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Android	O
browser	B-Application
identifies	O
itself	O
as	O
Safari	B-Application
(	O
among	O
other	O
things	O
)	O
in	O
order	O
to	O
aid	O
compatibility	O
.	O
</s>
<s>
Other	O
HTTP	B-Application
client	I-Application
programs	O
,	O
like	O
download	B-Protocol
managers	I-Protocol
and	O
offline	B-Protocol
browsers	I-Protocol
,	O
often	O
have	O
the	O
ability	O
to	O
change	O
the	O
user	B-Protocol
agent	I-Protocol
string	O
.	O
</s>
<s>
A	O
result	O
of	O
user	B-Protocol
agent	I-Protocol
spoofing	O
may	O
be	O
that	O
collected	O
statistics	B-Application
of	I-Application
Web	I-Application
browser	I-Application
usage	I-Application
are	O
inaccurate	O
.	O
</s>
<s>
User	B-Protocol
agent	I-Protocol
sniffing	O
is	O
the	O
practice	O
of	O
websites	O
showing	O
different	O
or	O
adjusted	O
content	O
when	O
viewed	O
with	O
certain	O
user	B-Protocol
agents	I-Protocol
.	O
</s>
<s>
An	O
example	O
of	O
this	O
is	O
Microsoft	B-Operating_System
Exchange	I-Operating_System
Server	I-Operating_System
2003	O
's	O
Outlook	O
Web	O
Access	O
feature	O
.	O
</s>
<s>
When	O
viewed	O
with	O
Internet	B-Application
Explorer	I-Application
6	O
or	O
newer	O
,	O
more	O
functionality	O
is	O
displayed	O
compared	O
to	O
the	O
same	O
page	O
in	O
any	O
other	O
browsers	B-Application
.	O
</s>
<s>
User	B-Protocol
agent	I-Protocol
sniffing	O
is	O
considered	O
poor	O
practice	O
,	O
since	O
it	O
encourages	O
browser-specific	O
design	O
and	O
penalizes	O
new	O
browsers	B-Application
with	O
unrecognized	O
user	B-Protocol
agent	I-Protocol
identifications	O
.	O
</s>
<s>
Instead	O
,	O
the	O
W3C	O
recommends	O
creating	O
standard	O
HTML	O
markup	O
,	O
allowing	O
correct	O
rendering	O
in	O
as	O
many	O
browsers	B-Application
as	O
possible	O
,	O
and	O
to	O
test	O
for	O
specific	O
browser	B-Application
features	O
rather	O
than	O
particular	O
browser	B-Application
versions	O
or	O
brands	O
.	O
</s>
<s>
Websites	O
intended	O
for	O
display	O
by	O
mobile	O
phones	O
often	O
rely	O
on	O
user	B-Protocol
agent	I-Protocol
sniffing	O
,	O
since	O
mobile	B-Application
browsers	I-Application
often	O
differ	O
greatly	O
from	O
each	O
other	O
.	O
</s>
<s>
Web	B-Application
browsers	I-Application
created	O
in	O
the	O
United	O
States	O
,	O
such	O
as	O
Netscape	B-Protocol
Navigator	I-Protocol
and	O
Internet	B-Application
Explorer	I-Application
,	O
previously	O
used	O
the	O
letters	O
U	O
,	O
I	O
,	O
and	O
N	O
to	O
specify	O
the	O
encryption	O
strength	O
in	O
the	O
user	B-Protocol
agent	I-Protocol
string	O
.	O
</s>
<s>
Until	O
1996	O
,	O
when	O
the	O
United	O
States	O
government	O
allowed	O
encryption	O
with	O
keys	O
longer	O
than	O
40	O
bits	O
to	O
be	O
exported	O
,	O
vendors	O
shipped	O
various	O
browser	B-Application
versions	O
with	O
different	O
encryption	O
strengths	O
.	O
</s>
<s>
"	O
U	O
"	O
stands	O
for	O
"	O
USA	O
"	O
(	O
for	O
the	O
version	O
with	O
128-bit	O
encryption	O
)	O
,	O
"	O
I	O
"	O
stands	O
for	O
"	O
International	O
"	O
the	O
browser	B-Application
has	O
40-bit	O
encryption	O
and	O
can	O
be	O
used	O
anywhere	O
in	O
the	O
world	O
and	O
"	O
N	O
"	O
stands	O
(	O
de	O
facto	O
)	O
for	O
"	O
None	O
"	O
(	O
no	O
encryption	O
)	O
.	O
</s>
<s>
In	O
2020	O
,	O
Google	B-Application
announced	O
that	O
they	O
would	O
be	O
phasing	O
out	O
support	O
for	O
the	O
User-Agent	B-Protocol
header	O
in	O
their	O
Google	B-Application
Chrome	I-Application
browser	I-Application
.	O
</s>
<s>
They	O
stated	O
that	O
other	O
major	O
web	B-Application
browser	I-Application
vendors	O
were	O
supportive	O
of	O
the	O
move	O
,	O
but	O
that	O
they	O
did	O
not	O
know	O
when	O
other	O
vendors	O
would	O
follow	O
suit	O
.	O
</s>
<s>
Google	B-Application
stated	O
that	O
a	O
new	O
feature	O
called	O
Client	B-Protocol
Hints	I-Protocol
would	O
replace	O
the	O
functionality	O
of	O
the	O
User-Agent	B-Protocol
string	O
.	O
</s>
<s>
Starting	O
with	O
Firefox	B-Application
110	O
released	O
in	O
February	O
2023	O
,	O
Mozilla	B-Operating_System
has	O
had	O
to	O
temporarily	O
freeze	O
portions	O
of	O
the	O
browser	B-Application
's	O
user-agent	B-Protocol
at	O
version	O
109	O
.	O
</s>
<s>
This	O
was	O
done	O
due	O
to	O
several	O
websites	O
incorrectly	O
recognizing	O
a	O
development	O
version	O
of	O
the	O
browser	B-Application
(	O
which	O
identified	O
itself	O
by	O
the	O
string	O
Mozilla/5.0	O
(	O
Windows	O
NT	O
10.0	O
;	O
Win64	O
;	O
rv:110.0	O
)	O
Gecko/20100101	O
Firefox/110.0	O
)	O
as	O
the	O
deprecated	B-General_Concept
Internet	B-Application
Explorer	I-Application
11	I-Application
(	O
which	O
reports	O
Mozilla/5.0	O
(	O
Windows	O
NT	O
10.0	O
;	O
Trident/7.0	O
;	O
rv:11.0	O
)	O
like	O
Gecko	O
)	O
.	O
</s>
<s>
The	O
problem	O
will	O
self-correct	O
after	O
the	O
release	O
of	O
Firefox	B-Application
120	O
,	O
as	O
only	O
versions	O
110	O
through	O
119	O
are	O
affected	O
by	O
it	O
.	O
</s>
