<s>
Server	B-Protocol
Name	I-Protocol
Indication	I-Protocol
(	O
SNI	O
)	O
is	O
an	O
extension	O
to	O
the	O
Transport	O
Layer	O
Security	O
(	O
TLS	O
)	O
computer	O
networking	O
protocol	O
by	O
which	O
a	O
client	B-Protocol
indicates	O
which	O
hostname	B-Protocol
it	O
is	O
attempting	O
to	O
connect	O
to	O
at	O
the	O
start	O
of	O
the	O
handshaking	O
process	O
.	O
</s>
<s>
The	O
extension	O
allows	O
a	O
server	O
to	O
present	O
one	O
of	O
multiple	O
possible	O
certificates	O
on	O
the	O
same	O
IP	B-Protocol
address	I-Protocol
and	O
TCP	B-Protocol
port	I-Protocol
number	O
and	O
hence	O
allows	O
multiple	O
secure	O
(	O
HTTPS	B-Protocol
)	O
websites	O
(	O
or	O
any	O
other	O
service	O
over	O
TLS	O
)	O
to	O
be	O
served	O
by	O
the	O
same	O
IP	B-Protocol
address	I-Protocol
without	O
requiring	O
all	O
those	O
sites	O
to	O
use	O
the	O
same	O
certificate	O
.	O
</s>
<s>
It	O
is	O
the	O
conceptual	O
equivalent	O
to	O
HTTP/1.1	O
name-based	O
virtual	O
hosting	O
,	O
but	O
for	O
HTTPS	B-Protocol
.	I-Protocol
</s>
<s>
This	O
also	O
allows	O
a	O
proxy	O
to	O
forward	O
client	B-Protocol
traffic	O
to	O
the	O
right	O
server	O
during	O
TLS/SSL	O
handshake	O
.	O
</s>
<s>
The	O
desired	O
hostname	B-Protocol
is	O
not	O
encrypted	O
in	O
the	O
original	O
SNI	O
extension	O
,	O
so	O
an	O
eavesdropper	O
can	O
see	O
which	O
site	O
is	O
being	O
requested	O
.	O
</s>
<s>
Prior	O
to	O
SNI	O
,	O
when	O
making	O
a	O
TLS	O
connection	O
,	O
the	O
client	B-Protocol
had	O
no	O
way	O
to	O
specify	O
which	O
site	O
it	O
is	O
trying	O
to	O
connect	O
to	O
.	O
</s>
<s>
In	O
more	O
detail	O
,	O
when	O
making	O
a	O
TLS	O
connection	O
,	O
the	O
client	B-Protocol
requests	O
a	O
digital	O
certificate	O
from	O
the	O
web	O
server	O
.	O
</s>
<s>
Once	O
the	O
server	O
sends	O
the	O
certificate	O
,	O
the	O
client	B-Protocol
examines	O
it	O
and	O
compares	O
the	O
name	O
it	O
was	O
trying	O
to	O
connect	O
to	O
with	O
the	O
name(s )	O
included	O
in	O
the	O
certificate	O
.	O
</s>
<s>
A	O
server	O
that	O
is	O
responsible	O
for	O
multiple	O
hostnames	B-Protocol
is	O
likely	O
to	O
need	O
to	O
present	O
a	O
different	O
certificate	O
for	O
each	O
name	O
(	O
or	O
small	O
group	O
of	O
names	O
)	O
.	O
</s>
<s>
Such	O
"	O
unified	B-Protocol
communications	I-Protocol
certificates	I-Protocol
"	O
must	O
be	O
reissued	O
every	O
time	O
the	O
list	O
of	O
domains	O
changes	O
.	O
</s>
<s>
Name-based	O
virtual	O
hosting	O
allows	O
multiple	O
DNS	O
hostnames	B-Protocol
to	O
be	O
hosted	O
by	O
a	O
single	O
server	O
(	O
usually	O
a	O
web	O
server	O
)	O
on	O
the	O
same	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
To	O
achieve	O
this	O
,	O
the	O
server	O
uses	O
a	O
hostname	B-Protocol
presented	O
by	O
the	O
client	B-Protocol
as	O
part	O
of	O
the	O
protocol	O
(	O
for	O
HTTP	O
the	O
name	O
is	O
presented	O
in	O
the	O
host	O
header	O
)	O
.	O
</s>
<s>
However	O
,	O
when	O
using	O
HTTPS	B-Protocol
,	O
the	O
TLS	O
handshake	O
happens	O
before	O
the	O
server	O
sees	O
any	O
HTTP	O
headers	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
was	O
not	O
possible	O
for	O
the	O
server	O
to	O
use	O
the	O
information	O
in	O
the	O
HTTP	O
host	O
header	O
to	O
decide	O
which	O
certificate	O
to	O
present	O
and	O
as	O
such	O
only	O
names	O
covered	O
by	O
the	O
same	O
certificate	O
could	O
be	O
served	O
from	O
the	O
same	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
In	O
practice	O
,	O
this	O
meant	O
that	O
an	O
HTTPS	B-Protocol
server	O
could	O
only	O
serve	O
one	O
domain	O
(	O
or	O
small	O
group	O
of	O
domains	O
)	O
per	O
IP	B-Protocol
address	I-Protocol
for	O
secured	O
and	O
efficient	O
browsing	O
.	O
</s>
<s>
Assigning	O
a	O
separate	O
IP	B-Protocol
address	I-Protocol
for	O
each	O
site	O
increases	O
the	O
cost	O
of	O
hosting	O
,	O
since	O
requests	O
for	O
IP	B-Protocol
addresses	I-Protocol
must	O
be	O
justified	O
to	O
the	O
regional	O
Internet	O
registry	O
and	O
IPv4	O
addresses	O
are	O
now	O
exhausted	O
.	O
</s>
<s>
SNI	O
addresses	O
this	O
issue	O
by	O
having	O
the	O
client	B-Protocol
send	O
the	O
name	O
of	O
the	O
virtual	O
domain	O
as	O
part	O
of	O
the	O
TLS	O
negotiation	O
's	O
ClientHello	O
message	O
.	O
</s>
<s>
Therefore	O
,	O
with	O
clients	O
and	O
servers	O
that	O
implement	O
SNI	O
,	O
a	O
server	O
with	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
can	O
serve	O
a	O
group	O
of	O
domain	O
names	O
for	O
which	O
it	O
is	O
impractical	O
to	O
get	O
a	O
common	O
certificate	O
.	O
</s>
<s>
Server	B-Protocol
Name	I-Protocol
Indication	I-Protocol
payload	O
is	O
not	O
encrypted	O
,	O
thus	O
the	O
hostname	B-Protocol
of	O
the	O
server	O
the	O
client	B-Protocol
tries	O
to	O
connect	O
to	O
is	O
visible	O
to	O
a	O
passive	O
eavesdropper	O
.	O
</s>
<s>
Presently	O
,	O
there	O
are	O
multiple	O
technologies	O
attempting	O
to	O
hide	O
Server	B-Protocol
Name	I-Protocol
Indication	I-Protocol
.	O
</s>
<s>
Domain	O
fronting	O
is	O
a	O
technique	O
of	O
replacing	O
the	O
desired	O
host	B-Protocol
name	I-Protocol
in	O
SNI	O
with	O
another	O
one	O
hosted	O
by	O
the	O
same	O
server	O
or	O
,	O
more	O
frequently	O
,	O
network	O
of	O
servers	O
known	O
as	O
Content	B-Operating_System
Delivery	I-Operating_System
Network	I-Operating_System
.	O
</s>
<s>
When	O
a	O
client	B-Protocol
uses	O
domain	O
fronting	O
,	O
it	O
replaces	O
the	O
server	O
domain	O
in	O
SNI	O
(	O
unencrypted	O
)	O
,	O
but	O
leaves	O
it	O
in	O
the	O
HTTP	O
host	O
header	O
(	O
which	O
is	O
encrypted	O
by	O
TLS	O
)	O
so	O
that	O
server	O
can	O
serve	O
the	O
right	O
content	O
.	O
</s>
<s>
Encrypted	O
Client	B-Protocol
Hello	O
(	O
ECH	O
)	O
is	O
a	O
TLS	O
1.3	O
protocol	O
extension	O
that	O
enables	O
encryption	O
of	O
the	O
whole	O
Client	B-Protocol
Hello	O
message	O
,	O
which	O
is	O
sent	O
during	O
the	O
early	O
stage	O
of	O
TLS	O
1.3	O
negotiation	O
.	O
</s>
<s>
ECH	O
encrypts	O
the	O
payload	O
with	O
a	O
public	O
key	O
that	O
the	O
relying	O
party	O
(	O
a	O
web	O
browser	O
)	O
needs	O
to	O
know	O
in	O
advance	O
,	O
which	O
means	O
ECH	O
is	O
most	O
effective	O
with	O
large	O
CDNs	B-Operating_System
known	O
to	O
browser	O
vendors	O
in	O
advance	O
.	O
</s>
<s>
Firefox	B-Application
85	O
removed	O
support	O
for	O
ESNI	O
.	O
</s>
<s>
In	O
contrast	O
to	O
ECH	O
,	O
Encrypted	O
SNI	O
encrypted	O
just	O
the	O
SNI	O
rather	O
than	O
the	O
whole	O
Client	B-Protocol
Hello	O
.	O
</s>
<s>
Opt-in	O
support	O
for	O
this	O
version	O
was	O
incorporated	O
into	O
Firefox	B-Application
in	O
October	O
2018	O
and	O
required	O
enabling	O
DNS-over-HTTPS	O
.	O
</s>
<s>
Also	O
,	O
to	O
use	O
ECH	O
,	O
the	O
client	B-Protocol
must	O
not	O
propose	O
TLS	O
versions	O
below	O
1.3	O
.	O
</s>
<s>
In	O
2004	O
,	O
a	O
patch	O
for	O
adding	O
TLS/SNI	O
into	O
OpenSSL	B-Language
was	O
created	O
by	O
the	O
EdelKey	O
project	O
.	O
</s>
<s>
In	O
2006	O
,	O
this	O
patch	O
was	O
then	O
ported	O
to	O
the	O
development	O
branch	O
of	O
OpenSSL	B-Language
,	O
and	O
in	O
2007	O
it	O
was	O
back-ported	O
to	O
OpenSSL	B-Language
0.9.8	O
(	O
first	O
released	O
in	O
0.9.8f	O
)	O
.	O
</s>
<s>
For	O
an	O
application	O
program	O
to	O
implement	O
SNI	O
,	O
the	O
TLS	O
library	O
it	O
uses	O
must	O
implement	O
it	O
and	O
the	O
application	O
must	O
pass	O
the	O
hostname	B-Protocol
to	O
the	O
TLS	O
library	O
.	O
</s>
<s>
+Support	O
for	O
SNI	O
Software	O
Type	O
Supported	O
Notes	O
Supported	O
since	O
Alpine	B-Language
(	O
email	B-Protocol
client	I-Protocol
)	O
IMAP	B-Protocol
email	B-Protocol
client	I-Protocol
Since	O
version	O
2.22	O
2019-02-18	O
BSAFE	B-Protocol
Micro	O
Edition	O
Suite	O
Library	O
Version	O
5.0	O
Internet	B-Application
Explorer	I-Application
Web	O
browser	O
Since	O
version	O
7	O
on	O
Vista	O
(	O
not	O
supported	O
on	O
XP	B-Application
)	O
2006	O
Edge	B-Application
Web	O
browser	O
All	O
versions	O
Mozilla	B-Application
Firefox	I-Application
Web	O
browser	O
Since	O
version	O
2.0	O
2006	O
cURL	B-Language
Command-line	O
tool	O
and	O
library	O
Since	O
version	O
7.18.1	O
2008	O
Safari	B-Application
Web	O
browser	O
Not	O
supported	O
on	O
Windows	B-Application
XP	I-Application
Google	B-Application
Chrome	I-Application
Web	O
browser	O
2010	O
BlackBerry	B-Application
10	I-Application
Web	O
browser	O
Supported	O
in	O
all	O
BB10	O
releases	O
2013	O
BlackBerry	B-Application
OS	I-Application
Barracuda	O
WAF	B-Protocol
Reverse	O
Proxy	O
Supported	O
since	O
version	O
7.8	O
2013	O
Barracuda	O
ADC	B-Protocol
Load	O
balancer	O
Frontend	O
support	O
since	O
version	O
4.0	O
and	O
backend	O
support	O
from	O
v5.2	O
Frontend	O
2013	O
/	O
Backend	O
2015	O
Windows	B-Operating_System
Mobile	I-Operating_System
Web	O
browser	O
Some	O
time	O
after	O
6.5	O
Android	B-Application
default	O
browser	O
Web	O
browser	O
Honeycomb	O
(	O
3.x	O
)	O
for	O
tablets	O
and	O
Ice	O
Cream	O
Sandwich	O
(	O
4.x	O
)	O
for	O
phones	O
2011	O
Firefox	B-Application
for	I-Application
Android	I-Application
Web	O
browser	O
Supported	O
for	O
browsing	O
.	O
</s>
