libptytty/doc/libptytty.3.pod

=head1 NAME

libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog handling

=head1 SYNOPSIS

   cc ... -lptytty

   #include <libptytty.h>


   // C++
   ptytty *pty = ptytty::create ();

   if (!pty->get ())
     // error allocating pty

   if (we want utmp)
     pty->login (process_pid, 0, "remote.host");
   else if (we want utmp AND wtmp/lastlog)
     pty->login (process_pid, 1, "remote.host");

   // we are done with it
   delete pty;


   // C
   PTYTTY pty = ptytty_create ();

   if (!ptytty_get (pty))
     // error allocating pty

   if (we want utmp)
     ptytty_login (pty, process_pid, 0, "remote.host");
   else if (we want utmp AND wtmp/lastlog)
     ptytty_login (pty, process_pid, 1, "remote.host");

   // we are done with it
   ptytty_delete (pty);

See also the F<eg/> directory, which currently contains the F<c-sample.c>
file that spawns a login shell from C using libptytty.

=head1 DESCRIPTION

Libptytty is a small library that offers pseudo-tty management in an
OS-independent way.  It was created out of frustration over the many
differences of pty/tty handling in different operating systems for the use
inside C<rxvt-unicode>.

In addition to offering mere pty/tty management, it also offers session
database support (utmp and optional wtmp/lastlog updates for login
shells).

It also supports fork'ing after startup and dropping privileges in the
calling process, so in case the calling process gets compromised by the
user starting the program there is less to gain, as only the helper
process runs with privileges (e.g. setuid/setgid), which reduces the area
of attack immensely.

Libptytty is written in C++, but it also offers a C-only API.

=head1 SECURITY CONSIDERATIONS

I<< B<It is of paramount importance that you at least read the following
paragraph!> >>

If you write a typical terminal-like program that just wants one or more
ptys, you should call the C<ptytty::init ()> method (C: C<ptytty_init ()>
function) as the very first thing in your program:

   int main (int argc, char *argv[])
   {
      // do nothing here
      ptytty::init ();
      // in C: ptytty_init ();

      // initialise, parse arguments, etc.
   }

This checks whether the program runs setuid or setgid. If yes then it will
fork a helper process and drop privileges.

Some programs need finer control over if and when this helper process
is started, and if and how to drop privileges. For those programs, the
methods C<ptytty::use_helper> and C<ptytty::drop_privileges> (and possibly
C<ptytty::sanitise_stdfd>) are more useful.

=head1 C++ INTERFACE: THE ptytty CLASS

=head2 STATIC METHODS

=over 4

=item ptytty::init ()

The default way to initialise libptytty. Must be called immediately as
the first thing in the C<main> function, or earlier e.g. during static
construction time. The earlier, the better.

This method calls C<sanitise_stdfd> and then checks whether the program runs
with setuid/setgid permissions and, if yes, spawns a helper process for
pty/tty management. It then drops the privileges completely, so the actual
program runs without setuid/setgid privileges.

=item ptytty::use_helper ()

Tries to start a helper process that retains privileges even when the
calling process does not. This is usually called from C<ptytty::init> when
it detects that the program is running setuid or setgid, but can be called
manually if it is inconvenient to drop privileges at startup, or when
you are not running setuid/setgid but want to drop privileges (e.g. when
running as a root-started daemon).

This method will try not to start more than one helper process. The same
helper process can usually be used both from the process starting it and
all its fork'ed (not exec'ed) children.

=item ptytty::drop_privileges ()

Drops privileges completely, i.e. sets real, effective and saved user id
to the real user id. Also aborts if this cannot be achieved. Useful to
make sure that the process doesn't run with special privileges.

=item ptytty::sanitise_stdfd ()

Checks whether file descriptors 0, 1 and 2 (stdin, stdout and stderr) are
valid (open) and, if not, connects them to F</dev/tty> or F</dev/null> if
possible (and aborts otherwise). This is necessary because libptytty might
want to output error messages to those descriptors, which at the time of
outputting the error message, might be connected to something unsuitable
opened by the unsuspecting program itself (this can be a security issue).

=item bool success = ptytty::send_fd (int socket, int fd)

Utility method to send a file descriptor over a unix domain
socket. Returns true if successful, false otherwise. This method is only
exposed for your convenience and is not required for normal operation.

=item int fd = ptytty::recv_fd (int socket)

Utility method to receive a file descriptor over a unix domain
socket. Returns the fd if successful and C<-1> otherwise. This method
is only exposed for your convenience and is not required for normal
operation.

=item ptytty *pty = ptytty::create ()

Creates new ptytty object. Creation does not yet do anything besides
allocating the structure.

A static method is used because the actual ptytty implementation can
differ at runtime, so you need a dynamic object creation facility.

=back


=head2 DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS

=over 4

=item int pty_fd = pty->pty

=item int tty_fd = pty->tty

These members contain the pty and tty file descriptors, respectively. They
initially contain C<-1> until a successful to C<ptytty::get>.

=item bool success = pty->get ()

Tries to find, allocate and initialise a new pty/tty pair. Returns C<true>
when successful.

=item pty->login (int cmd_pid, bool login_shell, const char *hostname)

Creates an entry in the systems session database(s) (utmp, wtmp, lastlog).
C<cmd_pid> must be the pid of the process representing the session
(such as the login shell), C<login_shell> defines whether the session is
associated with a login, which influences whether wtmp and lastlog entries
are created, and C<hostname> should identify the "hostname" the user logs
in from, which often is the value of the C<DISPLAY> variable or tty line
in case of local logins.

Calling this method is optional. A session starts at the time of the login
call and extends until the ptytty object is destroyed.

=item pty->close_tty ()

Closes the tty. Useful after forking in the parent/pty process.

=item bool success = pty->make_controlling_tty ()

Tries to make the pty/tty pair the controlling terminal of the current
process. Useful after forking in the child/tty process.

=item pty->set_utf8_mode (bool on)

On systems supporting special UTF-8 line disciplines (e.g. Linux), this
tries to enable this discipline for the given pty. Can be called at any
time to change the mode.

=back


=head1 C INTERFACE: THE ptytty FAMILY OF FUNCTIONS

=over 4

=item ptytty_init ()

See C<ptytty::init ()>.

=item PTYTTY ptytty_create ()

Creates a new opaque PTYTTY object and returns it. Do not try to access it
in any way except by testing it for truthness (e.g. C<if (pty) ....>). See
C<ptytty::create ()>.

=item int ptytty_pty (PTYTTY ptytty)

Return the pty file descriptor. See C<< pty->pty >>.

=item int ptytty_tty (PTYTTY ptytty)

Return the tty file descriptor. See C<< pty->tty >>.

=item void ptytty_delete (PTYTTY ptytty)

Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up the
utmp/wtmp/lastlog databases, if initialised/used. Same as C<delete pty> in
C++.

=item int ptytty_get (PTYTTY ptytty)

See C<< pty->get >>, returns 0 in case of an error, non-zero otherwise.

=item void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const char *hostname)

See C<< pty->login >>.

=item void ptytty_close_tty (PTYTTY ptytty)

See C<< pty->close_tty >>.

=item int ptytty_make_controlling_tty (PTYTTY ptytty)

See C<< pty->make_controlling_tty >>.

=item void ptytty_set_utf8_mode (PTYTTY ptytty, int on)

See C<< pty->set_utf8_mode >>.

=item void ptytty_drop_privileges ()

See C<< ptytty::drop_privileges >>.

=item void ptytty_use_helper ()

See C<< ptytty::use_helper >>.

=back


=head1 BUGS

You kiddin'?

=head1 AUTHORS

Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
<rxvt-unicode@schmorp.de>.

Revision:	1.18
Committed:	Thu May 30 18:54:45 2013 UTC (13 years, 3 months ago) by root
Branch:	MAIN
CVS Tags:	rxvt-unicode-rel-9_20, rxvt-unicode-rel-9_21, rxvt-unicode-rel-9_19, rel-1_7
Changes since 1.17:	+3 -2 lines
Log Message:	L really wnats urls
#	Content
1	=head1 NAME
2
3	libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog handling
4
5	=head1 SYNOPSIS
6
7	cc ... -lptytty
8
9	#include <libptytty.h>
10
11
12	// C++
13	ptytty *pty = ptytty::create ();
14
15	if (!pty->get ())
16	// error allocating pty
17
18	if (we want utmp)
19	pty->login (process_pid, 0, "remote.host");
20	else if (we want utmp AND wtmp/lastlog)
21	pty->login (process_pid, 1, "remote.host");
22
23	// we are done with it
24	delete pty;
25
26
27	// C
28	PTYTTY pty = ptytty_create ();
29
30	if (!ptytty_get (pty))
31	// error allocating pty
32
33	if (we want utmp)
34	ptytty_login (pty, process_pid, 0, "remote.host");
35	else if (we want utmp AND wtmp/lastlog)
36	ptytty_login (pty, process_pid, 1, "remote.host");
37
38	// we are done with it
39	ptytty_delete (pty);
40
41	See also the F<eg/> directory, which currently contains the F<c-sample.c>
42	file that spawns a login shell from C using libptytty.
43
44	=head1 DESCRIPTION
45
46	Libptytty is a small library that offers pseudo-tty management in an
47	OS-independent way. It was created out of frustration over the many
48	differences of pty/tty handling in different operating systems for the use
49	inside C<rxvt-unicode>.
50
51	In addition to offering mere pty/tty management, it also offers session
52	database support (utmp and optional wtmp/lastlog updates for login
53	shells).
54
55	It also supports fork'ing after startup and dropping privileges in the
56	calling process, so in case the calling process gets compromised by the
57	user starting the program there is less to gain, as only the helper
58	process runs with privileges (e.g. setuid/setgid), which reduces the area
59	of attack immensely.
60
61	Libptytty is written in C++, but it also offers a C-only API.
62
63	=head1 SECURITY CONSIDERATIONS
64
65	I<< B<It is of paramount importance that you at least read the following
66	paragraph!> >>
67
68	If you write a typical terminal-like program that just wants one or more
69	ptys, you should call the C<ptytty::init ()> method (C: C<ptytty_init ()>
70	function) as the very first thing in your program:
71
72	int main (int argc, char *argv[])
73	{
74	// do nothing here
75	ptytty::init ();
76	// in C: ptytty_init ();
77
78	// initialise, parse arguments, etc.
79	}
80
81	This checks whether the program runs setuid or setgid. If yes then it will
82	fork a helper process and drop privileges.
83
84	Some programs need finer control over if and when this helper process
85	is started, and if and how to drop privileges. For those programs, the
86	methods C<ptytty::use_helper> and C<ptytty::drop_privileges> (and possibly
87	C<ptytty::sanitise_stdfd>) are more useful.
88
89	=head1 C++ INTERFACE: THE ptytty CLASS
90
91	=head2 STATIC METHODS
92
93	=over 4
94
95	=item ptytty::init ()
96
97	The default way to initialise libptytty. Must be called immediately as
98	the first thing in the C<main> function, or earlier e.g. during static
99	construction time. The earlier, the better.
100
101	This method calls C<sanitise_stdfd> and then checks whether the program runs
102	with setuid/setgid permissions and, if yes, spawns a helper process for
103	pty/tty management. It then drops the privileges completely, so the actual
104	program runs without setuid/setgid privileges.
105
106	=item ptytty::use_helper ()
107
108	Tries to start a helper process that retains privileges even when the
109	calling process does not. This is usually called from C<ptytty::init> when
110	it detects that the program is running setuid or setgid, but can be called
111	manually if it is inconvenient to drop privileges at startup, or when
112	you are not running setuid/setgid but want to drop privileges (e.g. when
113	running as a root-started daemon).
114
115	This method will try not to start more than one helper process. The same
116	helper process can usually be used both from the process starting it and
117	all its fork'ed (not exec'ed) children.
118
119	=item ptytty::drop_privileges ()
120
121	Drops privileges completely, i.e. sets real, effective and saved user id
122	to the real user id. Also aborts if this cannot be achieved. Useful to
123	make sure that the process doesn't run with special privileges.
124
125	=item ptytty::sanitise_stdfd ()
126
127	Checks whether file descriptors 0, 1 and 2 (stdin, stdout and stderr) are
128	valid (open) and, if not, connects them to F</dev/tty> or F</dev/null> if
129	possible (and aborts otherwise). This is necessary because libptytty might
130	want to output error messages to those descriptors, which at the time of
131	outputting the error message, might be connected to something unsuitable
132	opened by the unsuspecting program itself (this can be a security issue).
133
134	=item bool success = ptytty::send_fd (int socket, int fd)
135
136	Utility method to send a file descriptor over a unix domain
137	socket. Returns true if successful, false otherwise. This method is only
138	exposed for your convenience and is not required for normal operation.
139
140	=item int fd = ptytty::recv_fd (int socket)
141
142	Utility method to receive a file descriptor over a unix domain
143	socket. Returns the fd if successful and C<-1> otherwise. This method
144	is only exposed for your convenience and is not required for normal
145	operation.
146
147	=item ptytty *pty = ptytty::create ()
148
149	Creates new ptytty object. Creation does not yet do anything besides
150	allocating the structure.
151
152	A static method is used because the actual ptytty implementation can
153	differ at runtime, so you need a dynamic object creation facility.
154
155	=back
156
157
158	=head2 DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
159
160	=over 4
161
162	=item int pty_fd = pty->pty
163
164	=item int tty_fd = pty->tty
165
166	These members contain the pty and tty file descriptors, respectively. They
167	initially contain C<-1> until a successful to C<ptytty::get>.
168
169	=item bool success = pty->get ()
170
171	Tries to find, allocate and initialise a new pty/tty pair. Returns C<true>
172	when successful.
173
174	=item pty->login (int cmd_pid, bool login_shell, const char *hostname)
175
176	Creates an entry in the systems session database(s) (utmp, wtmp, lastlog).
177	C<cmd_pid> must be the pid of the process representing the session
178	(such as the login shell), C<login_shell> defines whether the session is
179	associated with a login, which influences whether wtmp and lastlog entries
180	are created, and C<hostname> should identify the "hostname" the user logs
181	in from, which often is the value of the C<DISPLAY> variable or tty line
182	in case of local logins.
183
184	Calling this method is optional. A session starts at the time of the login
185	call and extends until the ptytty object is destroyed.
186
187	=item pty->close_tty ()
188
189	Closes the tty. Useful after forking in the parent/pty process.
190
191	=item bool success = pty->make_controlling_tty ()
192
193	Tries to make the pty/tty pair the controlling terminal of the current
194	process. Useful after forking in the child/tty process.
195
196	=item pty->set_utf8_mode (bool on)
197
198	On systems supporting special UTF-8 line disciplines (e.g. Linux), this
199	tries to enable this discipline for the given pty. Can be called at any
200	time to change the mode.
201
202	=back
203
204
205	=head1 C INTERFACE: THE ptytty FAMILY OF FUNCTIONS
206
207	=over 4
208
209	=item ptytty_init ()
210
211	See C<ptytty::init ()>.
212
213	=item PTYTTY ptytty_create ()
214
215	Creates a new opaque PTYTTY object and returns it. Do not try to access it
216	in any way except by testing it for truthness (e.g. C<if (pty) ....>). See
217	C<ptytty::create ()>.
218
219	=item int ptytty_pty (PTYTTY ptytty)
220
221	Return the pty file descriptor. See C<< pty->pty >>.
222
223	=item int ptytty_tty (PTYTTY ptytty)
224
225	Return the tty file descriptor. See C<< pty->tty >>.
226
227	=item void ptytty_delete (PTYTTY ptytty)
228
229	Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up the
230	utmp/wtmp/lastlog databases, if initialised/used. Same as C<delete pty> in
231	C++.
232
233	=item int ptytty_get (PTYTTY ptytty)
234
235	See C<< pty->get >>, returns 0 in case of an error, non-zero otherwise.
236
237	=item void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const char *hostname)
238
239	See C<< pty->login >>.
240
241	=item void ptytty_close_tty (PTYTTY ptytty)
242
243	See C<< pty->close_tty >>.
244
245	=item int ptytty_make_controlling_tty (PTYTTY ptytty)
246
247	See C<< pty->make_controlling_tty >>.
248
249	=item void ptytty_set_utf8_mode (PTYTTY ptytty, int on)
250
251	See C<< pty->set_utf8_mode >>.
252
253	=item void ptytty_drop_privileges ()
254
255	See C<< ptytty::drop_privileges >>.
256
257	=item void ptytty_use_helper ()
258
259	See C<< ptytty::use_helper >>.
260
261	=back
262
263
264	=head1 BUGS
265
266	You kiddin'?
267
268	=head1 AUTHORS
269
270	Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
271	<rxvt-unicode@schmorp.de>.
272