<s>
A	O
pluggable	B-Application
authentication	I-Application
module	I-Application
(	O
PAM	O
)	O
is	O
a	O
mechanism	O
to	O
integrate	O
multiple	O
low-level	O
authentication	O
schemes	O
into	O
a	O
high-level	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
.	O
</s>
<s>
It	O
was	O
first	O
proposed	O
by	O
Sun	O
Microsystems	O
in	O
an	O
Open	B-Application
Software	I-Application
Foundation	I-Application
Request	O
for	O
Comments	O
(	O
RFC	O
)	O
86.0	O
dated	O
October	O
1995	O
.	O
</s>
<s>
It	O
was	O
adopted	O
as	O
the	O
authentication	O
framework	O
of	O
the	O
Common	B-Application
Desktop	I-Application
Environment	I-Application
.	O
</s>
<s>
As	O
a	O
stand-alone	O
open-source	B-License
infrastructure	O
,	O
PAM	O
first	O
appeared	O
in	O
Red	O
Hat	O
Linux	B-Application
3.0.4	O
in	O
August	O
1996	O
in	O
the	O
Linux	B-Application
PAM	I-Application
project	O
.	O
</s>
<s>
PAM	O
is	O
currently	O
supported	O
in	O
the	O
AIX	B-Application
operating	I-Application
system	I-Application
,	O
DragonFly	B-Application
BSD	I-Application
,	O
FreeBSD	B-Operating_System
,	O
HP-UX	B-Application
,	O
Linux	B-Application
,	O
macOS	B-Application
,	O
NetBSD	B-Device
and	O
Solaris	B-Application
.	O
</s>
<s>
Since	O
no	O
central	O
standard	O
of	O
PAM	O
behavior	O
exists	O
,	O
there	O
was	O
a	O
later	O
attempt	O
to	O
standardize	O
PAM	O
as	O
part	O
of	O
the	O
X/Open	O
UNIX	O
standardization	O
process	O
,	O
resulting	O
in	O
the	O
X/Open	B-Application
Single	I-Application
Sign-on	I-Application
(	O
XSSO	B-Application
)	O
standard	O
.	O
</s>
<s>
This	O
standard	O
was	O
not	O
ratified	O
,	O
but	O
the	O
standard	O
draft	O
has	O
served	O
as	O
a	O
reference	O
point	O
for	O
later	O
PAM	O
implementations	O
(	O
for	O
example	O
,	O
OpenPAM	B-Application
)	O
.	O
</s>
<s>
Since	O
most	O
PAM	O
implementations	O
do	O
not	O
interface	O
with	O
remote	O
clients	O
themselves	O
,	O
PAM	O
,	O
on	O
its	O
own	O
,	O
cannot	O
implement	O
Kerberos	B-Protocol
,	O
the	O
most	O
common	O
type	O
of	O
SSO	O
used	O
in	O
Unix	O
environments	O
.	O
</s>
<s>
This	O
led	O
to	O
SSO	O
's	O
incorporation	O
as	O
the	O
"	O
primary	O
authentication	O
"	O
portion	O
of	O
the	O
would-be	O
XSSO	B-Application
standard	O
and	O
the	O
advent	O
of	O
technologies	O
such	O
as	O
SPNEGO	B-Protocol
and	O
SASL	O
.	O
</s>
<s>
This	O
lack	O
of	O
functionality	O
is	O
also	O
the	O
reason	O
SSH	B-Protocol
does	O
its	O
own	O
authentication	O
mechanism	O
negotiation	O
.	O
</s>
<s>
In	O
most	O
PAM	O
implementations	O
,	O
pam_krb5	O
only	O
fetches	O
Ticket	B-Protocol
Granting	I-Protocol
Tickets	I-Protocol
,	O
which	O
involves	O
prompting	O
the	O
user	O
for	O
credentials	O
,	O
and	O
this	O
is	O
only	O
used	O
for	O
the	O
initial	O
login	O
in	O
an	O
SSO	O
environment	O
.	O
</s>
<s>
To	O
fetch	O
a	O
service	O
ticket	O
for	O
a	O
particular	O
application	O
,	O
and	O
not	O
prompt	O
the	O
user	O
to	O
enter	O
credentials	O
again	O
,	O
that	O
application	O
must	O
be	O
specifically	O
coded	O
to	O
support	O
Kerberos	B-Protocol
.	O
</s>
<s>
This	O
is	O
because	O
pam_krb5	O
cannot	O
itself	O
get	O
service	O
tickets	O
,	O
although	O
there	O
are	O
versions	O
of	O
PAM-KRB5	O
that	O
are	O
attempting	O
to	O
work	O
around	O
the	O
issue	O
.	O
</s>
