<s>
Slirp	B-Protocol
(	O
sometimes	O
capitalized	O
SLiRP	B-Protocol
)	O
is	O
a	O
software	O
program	O
that	O
emulates	B-Application
a	O
PPP	B-Protocol
,	O
SLIP	B-Protocol
,	O
or	O
CSLIP	B-Protocol
connection	O
to	O
the	O
Internet	O
using	O
a	O
text-based	B-Application
shell	O
account	O
.	O
</s>
<s>
Its	O
original	O
purpose	O
became	O
largely	O
obsolete	O
as	O
dedicated	O
dial-up	O
PPP	B-Protocol
connections	I-Protocol
and	O
broadband	O
Internet	O
access	O
became	O
widely	O
available	O
and	O
inexpensive	O
.	O
</s>
<s>
It	O
then	O
found	O
additional	O
use	O
in	O
connecting	O
mobile	O
devices	O
,	O
such	O
as	O
PDAs	B-Application
,	O
via	O
their	O
serial	B-Protocol
ports	I-Protocol
.	O
</s>
<s>
Another	O
significant	O
use	O
case	O
is	O
firewall	B-Application
piercing/port	O
forwarding	O
.	O
</s>
<s>
One	O
typical	O
use	O
of	O
Slirp	B-Protocol
creates	O
a	O
general	O
purpose	O
network	O
connection	O
over	O
a	O
SSH	B-Protocol
session	O
on	O
which	O
port	B-Protocol
forwarding	I-Protocol
is	O
restricted	O
.	O
</s>
<s>
Shell	O
accounts	O
normally	O
only	O
allow	O
the	O
use	O
of	O
command	B-Application
line	I-Application
or	O
text-based	B-Application
software	O
,	O
but	O
by	O
logging	O
into	O
a	O
shell	O
account	O
and	O
running	O
Slirp	B-Protocol
on	O
the	O
remote	O
server	O
,	O
a	O
user	O
can	O
transform	O
their	O
shell	O
account	O
into	O
a	O
general	O
purpose	O
SLIP/PPP	O
network	O
connection	O
,	O
allowing	O
them	O
to	O
run	O
any	O
TCP/IP	B-Protocol
-based	O
application	O
—	O
including	O
standard	O
GUI	B-Application
software	O
such	O
as	O
the	O
formerly	O
popular	O
Netscape	B-Protocol
Navigator	I-Protocol
—	O
on	O
their	O
computer	O
.	O
</s>
<s>
This	O
was	O
especially	O
useful	O
in	O
the	O
1990s	O
because	O
simple	O
shell	O
accounts	O
were	O
less	O
expensive	O
and/or	O
more	O
widely	O
available	O
than	O
full	O
SLIP/PPP	O
accounts	O
.	O
</s>
<s>
These	O
command	O
line-only	O
connections	O
became	O
more	O
versatile	O
with	O
SLIP/PPP	O
,	O
enabling	O
the	O
use	O
of	O
arbitrary	O
TCP/IP	B-Protocol
-based	O
applications	O
.	O
</s>
<s>
Many	O
guides	O
to	O
using	O
university	O
dial-up	O
connections	O
with	O
Slirp	B-Protocol
were	O
published	O
online	O
(	O
e.g.	O
</s>
<s>
Use	O
of	O
TCP/IP	B-Protocol
emulations	B-Application
software	O
like	O
Slirp	B-Protocol
,	O
and	O
its	O
commercial	O
competitor	O
TIA	B-Protocol
was	O
banned	O
by	O
some	O
shell	O
account	O
providers	O
,	O
who	O
believed	O
its	O
users	O
violated	O
their	O
terms	O
of	O
service	O
or	O
consumed	O
too	O
much	O
bandwidth	O
.	O
</s>
<s>
Slirp	B-Protocol
is	O
also	O
useful	O
for	O
connecting	O
PDAs	B-Application
and	O
other	O
mobile	O
devices	O
to	O
the	O
Internet	O
:	O
by	O
connecting	O
such	O
a	O
device	O
to	O
a	O
computer	O
running	O
Slirp	B-Protocol
,	O
via	O
a	O
serial	O
cable	O
or	O
USB	B-Protocol
,	O
the	O
mobile	O
device	O
can	O
connect	O
to	O
the	O
Internet	O
.	O
</s>
<s>
Unlike	O
a	O
true	O
SLIP/PPP	O
connection	O
,	O
provided	O
by	O
a	O
dedicated	O
server	O
,	O
a	O
Slirp	B-Protocol
connection	O
does	O
not	O
strictly	O
obey	O
the	O
principle	O
of	O
end-to-end	B-Application
connectivity	I-Application
envisioned	O
by	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
The	O
remote	O
end	O
of	O
the	O
connection	O
,	O
running	O
on	O
the	O
shell	O
account	O
,	O
cannot	O
allocate	O
a	O
new	O
IP	B-Protocol
address	I-Protocol
and	O
route	B-Protocol
traffic	O
to	O
it	O
.	O
</s>
<s>
Thus	O
the	O
local	O
computer	O
cannot	O
accept	O
arbitrary	O
incoming	O
connections	O
,	O
although	O
Slirp	B-Protocol
can	O
use	O
port	B-Protocol
forwarding	I-Protocol
to	O
accept	O
incoming	O
traffic	O
for	O
specific	O
ports	B-Protocol
.	O
</s>
<s>
This	O
limitation	O
is	O
similar	O
to	O
that	O
of	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
.	O
</s>
<s>
It	O
does	O
provide	O
enhanced	O
security	O
as	O
a	O
side	O
effect	O
,	O
effectively	O
acting	O
as	O
a	O
firewall	B-Application
between	O
the	O
local	O
computer	O
and	O
the	O
Internet	O
.	O
</s>
<s>
Slirp	B-Protocol
is	O
free	B-Application
software	I-Application
licensed	O
under	O
a	O
by	O
its	O
original	O
author	O
.	O
</s>
<s>
Debian	O
maintainers	O
have	O
taken	O
over	O
some	O
maintenance	O
tasks	O
,	O
such	O
as	O
modifying	O
Slirp	B-Protocol
to	O
work	O
correctly	O
on	O
64-bit	B-Device
computers	I-Device
.	O
</s>
<s>
Despite	O
being	O
largely	O
obsolete	O
,	O
Slirp	B-Protocol
made	O
a	O
great	O
influence	O
on	O
the	O
networking	B-Protocol
stacks	I-Protocol
used	O
in	O
virtual	B-Architecture
machines	I-Architecture
and	O
other	O
virtualized	O
environments	O
.	O
</s>
<s>
The	O
established	O
practice	O
of	O
connecting	O
the	O
virtual	B-Architecture
machines	I-Architecture
to	O
the	O
host	O
's	O
network	B-Protocol
stack	I-Protocol
was	O
to	O
use	O
the	O
various	O
packet	B-Protocol
injection	I-Protocol
mechanisms	O
.	O
</s>
<s>
Raw	B-Protocol
sockets	I-Protocol
,	O
being	O
one	O
of	O
such	O
mechanisms	O
,	O
were	O
originally	O
used	O
for	O
that	O
purpose	O
,	O
and	O
,	O
due	O
to	O
many	O
problems	O
and	O
limitations	O
,	O
were	O
later	O
replaced	O
with	O
the	O
TAP	B-Protocol
device	I-Protocol
.	O
</s>
<s>
Packet	B-Protocol
injection	I-Protocol
is	O
a	O
privileged	O
operation	O
that	O
may	O
introduce	O
a	O
security	O
threat	O
,	O
something	O
that	O
the	O
introduction	O
of	O
TAP	B-Protocol
device	I-Protocol
solved	O
only	O
partially	O
.	O
</s>
<s>
Slirp-derived	O
NAT	B-Protocol
implementation	O
brought	O
a	O
solution	O
to	O
this	O
long-standing	O
problem	O
.	O
</s>
<s>
It	O
was	O
discovered	O
that	O
Slirp	B-Protocol
has	O
the	O
full	O
NAPT	B-Protocol
implementation	O
as	O
a	O
stand-alone	O
user-space	B-Operating_System
code	O
,	O
whereas	O
other	O
NAT	B-Protocol
engines	O
are	O
usually	O
embedded	O
into	O
a	O
network	B-Protocol
protocol	I-Protocol
stack	I-Protocol
and/or	O
do	O
not	O
cooperate	O
with	O
the	O
host	O
OS	O
when	O
doing	O
PAT	O
(	O
use	O
their	O
own	O
port	O
ranges	O
and	O
require	O
packet	B-Protocol
injection	I-Protocol
)	O
.	O
</s>
<s>
QEMU	B-Application
project	O
have	O
adopted	O
the	O
appropriate	O
code	O
portions	O
of	O
the	O
Slirp	B-Protocol
package	O
and	O
got	O
the	O
permission	O
from	O
its	O
original	O
authors	O
to	O
re-license	O
it	O
under	O
3-clause	O
BSD	O
license	O
.	O
</s>
<s>
Such	O
license	O
change	O
allowed	O
many	O
other	O
FOSS	B-License
projects	O
to	O
adopt	O
the	O
QEMU-provided	O
Slirp	B-Protocol
portions	O
,	O
which	O
was	O
(	O
and	O
still	O
is	O
)	O
not	O
possible	O
with	O
the	O
original	O
Slirp	B-Protocol
codebase	O
because	O
of	O
the	O
license	O
compatibility	O
problems	O
.	O
</s>
<s>
Some	O
of	O
the	O
notable	O
adopters	O
are	O
VDE	B-Architecture
and	O
VirtualBox	B-Operating_System
projects	O
.	O
</s>
<s>
Even	O
though	O
the	O
Slirp-derived	O
code	O
was	O
heavily	O
criticized	O
,	O
to	O
date	O
there	O
is	O
no	O
competing	O
implementation	O
available	O
.	O
</s>
