<s>
OpenVPN	B-Language
is	O
a	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
system	O
that	O
implements	O
techniques	O
to	O
create	O
secure	O
point-to-point	O
or	O
site-to-site	O
connections	O
in	O
routed	O
or	O
bridged	O
configurations	O
and	O
remote	O
access	O
facilities	O
.	O
</s>
<s>
It	O
implements	O
both	O
client	B-Operating_System
and	I-Operating_System
server	I-Operating_System
applications	O
.	O
</s>
<s>
OpenVPN	B-Language
allows	O
peers	O
to	O
authenticate	O
each	O
other	O
using	O
pre-shared	O
secret	O
keys	O
,	O
certificates	O
or	O
username/password	O
.	O
</s>
<s>
When	O
used	O
in	O
a	O
multiclient-server	O
configuration	O
,	O
it	O
allows	O
the	O
server	B-Application
to	O
release	O
an	O
authentication	O
certificate	O
for	O
every	O
client	B-Protocol
,	O
using	O
signatures	O
and	O
certificate	B-Architecture
authority	I-Architecture
.	O
</s>
<s>
It	O
uses	O
the	O
OpenSSL	B-Language
encryption	O
library	B-Library
extensively	O
,	O
as	O
well	O
as	O
the	O
TLS	B-Protocol
protocol	I-Protocol
,	O
and	O
contains	O
many	O
security	O
and	O
control	O
features	O
.	O
</s>
<s>
It	O
uses	O
a	O
custom	O
security	O
protocol	O
that	O
utilizes	O
SSL/TLS	B-Protocol
for	O
key	O
exchange	O
.	O
</s>
<s>
It	O
is	O
capable	O
of	O
traversing	O
network	B-Protocol
address	I-Protocol
translators	I-Protocol
(	O
NATs	O
)	O
and	O
firewalls	B-Application
.	O
</s>
<s>
OpenVPN	B-Language
has	O
been	O
ported	O
and	O
embedded	O
to	O
several	O
systems	O
.	O
</s>
<s>
For	O
example	O
,	O
DD-WRT	B-Protocol
has	O
the	O
OpenVPN	B-Language
server	B-Application
function	O
.	O
</s>
<s>
SoftEther	B-Application
VPN	I-Application
,	O
a	O
multi-protocol	O
VPN	B-Application
server	I-Application
,	O
also	O
has	O
an	O
implementation	O
of	O
OpenVPN	B-Language
protocol	O
.	O
</s>
<s>
It	O
was	O
written	O
by	O
James	O
Yonan	O
and	O
is	O
free	B-Application
software	I-Application
,	O
released	O
under	O
the	O
terms	O
of	O
the	O
GNU	O
General	O
Public	O
License	O
version	O
2	O
(	O
GPLv2	O
)	O
.	O
</s>
<s>
OpenVPN	B-Language
uses	O
the	O
OpenSSL	B-Language
library	B-Library
to	O
provide	O
encryption	O
of	O
both	O
the	O
data	O
and	O
control	O
channels	O
.	O
</s>
<s>
It	O
lets	O
OpenSSL	B-Language
do	O
all	O
the	O
encryption	O
and	O
authentication	O
work	O
,	O
allowing	O
OpenVPN	B-Language
to	O
use	O
all	O
the	O
ciphers	O
available	O
in	O
the	O
OpenSSL	B-Language
package	O
.	O
</s>
<s>
It	O
can	O
also	O
use	O
the	O
HMAC	B-Algorithm
packet	O
authentication	O
feature	O
to	O
add	O
an	O
additional	O
layer	O
of	O
security	O
to	O
the	O
connection	O
(	O
referred	O
to	O
as	O
an	O
"	O
HMAC	B-Algorithm
Firewall	B-Application
"	O
by	O
the	O
creator	O
)	O
.	O
</s>
<s>
Support	O
for	O
mbed	B-Language
TLS	I-Language
is	O
available	O
starting	O
from	O
version	O
2.3	O
.	O
</s>
<s>
OpenVPN	B-Language
has	O
several	O
ways	O
to	O
authenticate	O
peers	O
with	O
each	O
other	O
.	O
</s>
<s>
OpenVPN	B-Language
offers	O
pre-shared	O
keys	O
,	O
certificate-based	O
,	O
and	O
username/password	O
-based	O
authentication	O
.	O
</s>
<s>
Preshared	O
secret	O
key	O
is	O
the	O
easiest	O
,	O
and	O
certificate-based	O
is	O
the	O
most	O
robust	O
and	O
feature-rich	B-Architecture
.	O
</s>
<s>
However	O
,	O
to	O
make	O
use	O
of	O
username/password	O
authentications	O
,	O
OpenVPN	B-Language
depends	O
on	O
third-party	O
modules	O
.	O
</s>
<s>
OpenVPN	B-Language
can	O
run	O
over	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
or	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
transports	O
,	O
multiplexing	O
created	O
SSL	O
tunnels	O
on	O
a	O
single	O
TCP/UDP	O
port	O
(	O
RFC	O
3948	O
for	O
UDP	O
)	O
.	O
</s>
<s>
From	O
2.3.x	O
series	O
on	O
,	O
OpenVPN	B-Language
fully	O
supports	O
IPv6	O
as	O
protocol	O
of	O
the	O
virtual	O
network	O
inside	O
a	O
tunnel	O
and	O
the	O
OpenVPN	B-Language
applications	O
can	O
also	O
establish	O
connections	O
via	O
IPv6	O
.	O
</s>
<s>
It	O
has	O
the	O
ability	O
to	O
work	O
through	O
most	O
proxy	B-Application
servers	I-Application
(	O
including	O
HTTP	B-Protocol
)	O
and	O
is	O
good	O
at	O
working	O
through	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
and	O
getting	O
out	O
through	O
firewalls	B-Application
.	O
</s>
<s>
The	O
server	B-Application
configuration	O
has	O
the	O
ability	O
to	O
"	O
push	O
"	O
certain	O
network	O
configuration	O
options	O
to	O
the	O
clients	O
.	O
</s>
<s>
OpenVPN	B-Language
offers	O
two	O
types	O
of	O
interfaces	O
for	O
networking	O
via	O
the	O
Universal	B-Protocol
TUN/TAP	I-Protocol
driver	I-Protocol
.	O
</s>
<s>
OpenVPN	B-Language
can	O
optionally	O
use	O
the	O
LZO	B-Language
compression	O
library	B-Library
to	O
compress	O
the	O
data	O
stream	O
.	O
</s>
<s>
Port	O
1194	O
is	O
the	O
official	O
IANA	O
assigned	O
port	O
number	O
for	O
OpenVPN	B-Language
.	O
</s>
<s>
OpenVPN	B-Language
's	O
use	O
of	O
common	O
network	O
protocols	O
(	O
TCP	O
and	O
UDP	O
)	O
makes	O
it	O
a	O
desirable	O
alternative	O
to	O
IPsec	B-Protocol
in	O
situations	O
where	O
an	O
ISP	O
may	O
block	O
specific	O
VPN	B-Application
protocols	O
in	O
order	O
to	O
force	O
users	O
to	O
subscribe	O
to	O
a	O
higher-priced	O
,	O
"	O
business	O
grade	O
"	O
service	O
tier	O
.	O
</s>
<s>
Their	O
argument	O
was	O
that	O
conducting	O
remote	O
work	O
via	O
a	O
VPN	B-Application
can	O
adversely	O
affect	O
the	O
network	O
performance	O
of	O
their	O
regular	O
residential	O
subscribers	O
.	O
</s>
<s>
So	O
,	O
anyone	O
wishing	O
to	O
use	O
VPN	B-Application
would	O
have	O
to	O
subscribe	O
to	O
higher-priced	O
,	O
business-grade	O
service	O
tier	O
.	O
</s>
<s>
When	O
OpenVPN	B-Language
uses	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
transports	O
to	O
establish	O
a	O
tunnel	O
,	O
performance	O
will	O
be	O
acceptable	O
only	O
as	O
long	O
as	O
there	O
is	O
sufficient	O
excess	O
bandwidth	O
on	O
the	O
un-tunneled	O
network	O
link	O
to	O
guarantee	O
that	O
the	O
tunneled	O
TCP	O
timers	O
do	O
not	O
expire	O
.	O
</s>
<s>
OpenVPN	B-Language
offers	O
various	O
internal	O
security	O
features	O
.	O
</s>
<s>
It	O
has	O
up	O
to	O
256-bit	O
encryption	O
through	O
the	O
OpenSSL	B-Language
library	B-Library
,	O
although	O
some	O
service	O
providers	O
may	O
offer	O
lower	O
rates	O
,	O
effectively	O
providing	O
some	O
of	O
the	O
fastest	O
VPN	B-Application
available	O
to	O
consumers	O
.	O
</s>
<s>
It	O
runs	O
in	O
userspace	B-Operating_System
instead	O
of	O
requiring	O
IP	O
stack	O
(	O
therefore	O
kernel	O
)	O
operation	O
.	O
</s>
<s>
OpenVPN	B-Language
has	O
the	O
ability	O
to	O
drop	O
root	O
privileges	O
,	O
use	O
to	O
prevent	O
swapping	O
sensitive	O
data	O
to	O
disk	O
,	O
enter	O
a	O
chroot	B-Application
jail	I-Application
after	O
initialization	O
,	O
and	O
apply	O
a	O
SELinux	B-Application
context	O
after	O
initialization	O
.	O
</s>
<s>
OpenVPN	B-Language
runs	O
a	O
custom	O
security	O
protocol	O
based	O
on	O
SSL	O
and	O
TLS	B-Protocol
,	O
rather	O
than	O
supporting	O
IKE	O
,	O
IPsec	B-Protocol
,	O
L2TP	B-Application
or	O
PPTP	B-Protocol
.	O
</s>
<s>
OpenVPN	B-Language
offers	O
support	O
of	O
smart	O
cards	O
via	O
PKCS	O
#11	O
-based	O
cryptographic	O
tokens	O
.	O
</s>
<s>
OpenVPN	B-Language
can	O
be	O
extended	O
with	O
third-party	O
plug-ins	B-Application
or	O
scripts	O
,	O
which	O
can	O
be	O
called	O
at	O
defined	O
entry	O
points	O
.	O
</s>
<s>
The	O
purpose	O
of	O
this	O
is	O
often	O
to	O
extend	O
OpenVPN	B-Language
with	O
more	O
advanced	O
logging	O
,	O
enhanced	O
authentication	O
with	O
username	O
and	O
passwords	O
,	O
dynamic	O
firewall	B-Application
updates	O
,	O
RADIUS	B-Protocol
integration	O
and	O
so	O
on	O
.	O
</s>
<s>
The	O
plug-ins	B-Application
are	O
dynamically	O
loadable	O
modules	O
,	O
usually	O
written	O
in	O
C	B-Language
,	O
while	O
the	O
scripts	O
interface	O
can	O
execute	O
any	O
scripts	O
or	O
binaries	O
available	O
to	O
OpenVPN	B-Language
.	O
</s>
<s>
In	O
the	O
OpenVPN	B-Language
source	O
code	O
there	O
are	O
some	O
examples	O
of	O
such	O
plug-ins	B-Application
,	O
including	O
a	O
PAM	B-Application
authentication	O
plug-in	B-Application
.	O
</s>
<s>
Several	O
third-party	O
plug-ins	B-Application
also	O
exist	O
to	O
authenticate	O
against	O
LDAP	B-Protocol
or	O
SQL	O
databases	O
such	O
as	O
SQLite	B-Language
and	O
MySQL	B-Application
.	O
</s>
<s>
It	O
is	O
available	O
on	O
Solaris	B-Application
,	O
Linux	B-Application
,	O
OpenBSD	B-Operating_System
,	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
QNX	B-Operating_System
,	O
macOS	B-Application
and	O
Windows	O
XP	O
and	O
later	O
.	O
</s>
<s>
OpenVPN	B-Language
is	O
available	O
for	O
mobile	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
including	O
Maemo	B-Operating_System
,	O
Windows	B-Operating_System
Mobile	I-Operating_System
6.5	O
and	O
below	O
,	O
iOS	B-Application
3GS+	O
devices	O
,	O
jailbroken	B-Application
iOS	B-Application
3.1.2	O
+	O
devices	O
,	O
Android	B-Application
4.0	I-Application
+	O
devices	O
,	O
and	O
Android	B-Application
devices	O
that	O
have	O
had	O
the	O
Cyanogenmod	B-Application
aftermarket	O
firmware	O
flashed	O
or	O
have	O
the	O
correct	O
kernel	O
module	O
installed	O
.	O
</s>
<s>
It	O
is	O
not	O
compatible	O
with	O
some	O
mobile	O
phone	O
OSes	O
,	O
including	O
Palm	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
It	O
is	O
not	O
a	O
"	O
web-based	O
"	O
VPN	B-Application
shown	O
as	O
a	O
web	O
page	O
such	O
as	O
Citrix	B-Application
or	O
Terminal	B-Protocol
Services	I-Protocol
Web	I-Protocol
access	I-Protocol
;	O
the	O
program	O
is	O
installed	O
independently	O
and	O
configured	O
by	O
editing	O
text	O
files	O
manually	O
,	O
rather	O
than	O
through	O
a	O
GUI-based	O
wizard	O
.	O
</s>
<s>
OpenVPN	B-Language
is	O
not	O
compatible	O
with	O
VPN	B-Application
clients	I-Application
that	O
use	O
the	O
IPsec	B-Protocol
over	O
L2TP	B-Application
or	O
PPTP	B-Protocol
protocols	O
.	O
</s>
<s>
The	O
entire	O
package	O
consists	O
of	O
one	O
binary	O
for	O
both	O
client	B-Operating_System
and	I-Operating_System
server	I-Operating_System
connections	O
,	O
an	O
optional	O
configuration	B-Protocol
file	I-Protocol
,	O
and	O
one	O
or	O
more	O
key	O
files	O
depending	O
on	O
the	O
authentication	O
method	O
used	O
.	O
</s>
<s>
OpenVPN	B-Language
has	O
been	O
integrated	O
into	O
several	O
router	B-Protocol
firmware	O
packages	O
allowing	O
users	O
to	O
run	O
OpenVPN	B-Language
in	O
client	B-Protocol
or	O
server	B-Application
mode	O
from	O
their	O
network	B-Protocol
routers	I-Protocol
.	O
</s>
<s>
A	O
router	B-Protocol
running	O
OpenVPN	B-Language
in	O
client	B-Protocol
mode	O
,	O
for	O
example	O
,	O
allows	O
any	O
device	O
on	O
a	O
network	O
to	O
access	O
a	O
VPN	B-Application
without	O
needing	O
the	O
capability	O
to	O
install	O
OpenVPN	B-Language
.	O
</s>
<s>
Notable	O
firmware	O
packages	O
with	O
OpenVPN	B-Language
integration	O
include	O
:	O
</s>
<s>
OpenVPN	B-Language
has	O
also	O
been	O
implemented	O
in	O
some	O
manufacturer	O
router	B-Protocol
firmware	O
.	O
</s>
<s>
OpenVPN	B-Language
has	O
been	O
integrated	O
into	O
SoftEther	B-Application
VPN	I-Application
,	O
an	O
open-source	O
multi-protocol	O
VPN	B-Application
server	I-Application
,	O
to	O
allow	O
users	O
to	O
connect	O
to	O
the	O
VPN	B-Application
server	I-Application
from	O
existing	O
OpenVPN	B-Language
clients	O
.	O
</s>
<s>
OpenVPN	B-Language
is	O
also	O
integrated	O
into	O
Vyos	B-Protocol
,	O
an	O
open-source	O
routing	O
operating	B-General_Concept
system	I-General_Concept
forked	O
from	O
the	O
Vyatta	B-Protocol
software	B-Protocol
router	I-Protocol
.	O
</s>
<s>
OpenVPN	B-Language
is	O
available	O
in	O
two	O
versions	O
:	O
</s>
<s>
OpenVPN	B-Language
Access	O
Server	B-Application
(	O
OpenVPN-AS	O
)	O
is	O
based	O
on	O
the	O
Community	O
Edition	O
,	O
but	O
provides	O
additional	O
paid	O
and	O
proprietary	O
features	O
like	O
LDAP	B-Protocol
integration	O
,	O
SMB	O
server	B-Application
,	O
Web	O
UI	O
management	O
and	O
provides	O
a	O
set	O
of	O
installation	O
and	O
configuration	O
tools	O
that	O
are	O
reported	O
to	O
simplify	O
the	O
rapid	O
deployment	O
of	O
a	O
VPN	B-Application
remote-access	O
solution	O
.	O
</s>
<s>
The	O
Access	O
Server	B-Application
edition	O
relies	O
heavily	O
on	O
iptables	B-Application
for	O
load	B-Application
balancing	I-Application
and	O
it	O
has	O
never	O
been	O
available	O
on	O
Windows	O
for	O
this	O
reason	O
.	O
</s>
<s>
This	O
version	O
is	O
also	O
able	O
to	O
dynamically	O
create	O
client	B-Protocol
(	O
"	O
OpenVPN	B-Language
Connect	O
"	O
)	O
installers	O
,	O
which	O
include	O
a	O
client	B-Protocol
profile	O
for	O
connecting	O
to	O
a	O
particular	O
Access	O
Server	B-Application
instance	O
.	O
</s>
<s>
However	O
,	O
the	O
user	O
does	O
not	O
need	O
to	O
have	O
an	O
Access	O
Server	B-Application
client	B-Protocol
in	O
order	O
to	O
connect	O
to	O
the	O
Access	O
Server	B-Application
instance	O
;	O
the	O
client	B-Protocol
from	O
the	O
OpenVPN	B-Language
Community	O
Edition	O
can	O
be	O
used	O
.	O
</s>
