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 loginshell 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 wether 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_stdf> and then checks wether 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 inconvinient 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 wether 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 convinience 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 sucecssful and C<-1> otherwise. This method
is only exposed for your convinience 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 wether the session is
associated with a login, which influences wether 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 L<< <e.giaquinta@glauco.it> >>, Marc Alexander Lehmann
L<< <rxvt-unicode@schmorp.de> >>.
Revision:	1.16
Committed:	Wed Dec 12 22:17:45 2007 UTC (18 years, 9 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_3
Changes since 1.15:	+6 -5 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3	root	1.2	libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog handling
4	root	1.1
5			=head1 SYNOPSIS
6
7	root	1.11	cc ... -lptytty
8	root	1.9
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	root	1.12	See also the F<eg/> directory, which currently contains the F<c-sample.c>
42			file that spawns a loginshell from C using libptytty.
43	root	1.1
44			=head1 DESCRIPTION
45
46	root	1.9	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	root	1.6
63	root	1.4	=head1 SECURITY CONSIDERATIONS
64	root	1.3
65	ayin	1.5	I<< B<It is of paramount importance that you at least read the following
66	root	1.4	paragraph!> >>
67
68	root	1.16	If you write a typical terminal-like program that just wants one or more
69	root	1.8	ptys, you should call the C<ptytty::init ()> method (C: C<ptytty_init ()>
70	root	1.7	function) as the very first thing in your program:
71	root	1.4
72			int main (int argc, char *argv[])
73			{
74			// do nothing here
75			ptytty::init ();
76	root	1.7	// in C: ptytty_init ();
77	root	1.4
78			// initialise, parse arguments, etc.
79			}
80
81			This checks wether the program runs setuid or setgid. If yes then it will
82			fork a helper process and drop privileges.
83
84	root	1.6	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	root	1.14	methods C<ptytty::use_helper> and C<ptytty::drop_privileges> (and possibly
87			C<ptytty::sanitise_stdfd>) are more useful.
88	root	1.6
89	root	1.7	=head1 C++ INTERFACE: THE ptytty CLASS
90	root	1.6
91			=head2 STATIC METHODS
92
93			=over 4
94
95			=item ptytty::init ()
96
97	root	1.14	The default way to initialise libptytty. Must be called immediately as
98	root	1.6	the first thing in the C<main> function, or earlier e.g. during static
99			construction time. The earlier, the better.
100
101	root	1.14	This method calls C<sanitise_stdf> and then checks wether 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	root	1.6
106			=item ptytty::use_helper ()
107
108	root	1.7	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 inconvinient 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	root	1.13	helper process can usually be used both from the process starting it and
117			all its fork'ed (not exec'ed) children.
118	root	1.6
119			=item ptytty::drop_privileges ()
120
121			Drops privileges completely, i.e. sets real, effective and saved user id
122	root	1.13	to the real user id. Also aborts if this cannot be achieved. Useful to
123	root	1.6	make sure that the process doesn't run with special privileges.
124
125	root	1.14	=item ptytty::sanitise_stdfd ()
126
127			Checks wether file descriptors 0, 1 and 2 (stdin, stdout and stderr) are
128	root	1.16	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	root	1.14
134	root	1.6	=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 convinience 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 sucecssful and C<-1> otherwise. This method
144			is only exposed for your convinience and is not required for normal
145			operation.
146	root	1.4
147	root	1.6	=item ptytty *pty = ptytty::create ()
148	root	1.3
149	root	1.6	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	root	1.7
158	root	1.6	=head2 DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
159	root	1.1
160			=over 4
161
162	root	1.6	=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 wether the session is
179			associated with a login, which influences wether 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	root	1.1
198	root	1.13	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	root	1.1
202			=back
203
204	root	1.7
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	ayin	1.15
213	root	1.7	=item PTYTTY ptytty_create ()
214
215			Creates a new opaque PTYTTY object and returns it. Do not try to access it
216	root	1.13	in any way except by testing it for truthness (e.g. C<if (pty) ....>). See
217	root	1.7	C<ptytty::create ()>.
218
219			=item int ptytty_pty (PTYTTY ptytty)
220
221			Return the pty file descriptor. See C<< pty->pty >>.
222	ayin	1.15
223	root	1.7	=item int ptytty_tty (PTYTTY ptytty)
224
225			Return the tty file descriptor. See C<< pty->tty >>.
226	ayin	1.15
227	root	1.7	=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	ayin	1.15
245	root	1.7	=item int ptytty_make_controlling_tty (PTYTTY ptytty)
246
247			See C<< pty->make_controlling_tty >>.
248	ayin	1.15
249	root	1.7	=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	ayin	1.15
257	root	1.7	=item void ptytty_use_helper ()
258
259			See C<< ptytty::use_helper >>.
260
261			=back
262
263
264	root	1.1	=head1 BUGS
265
266			You kiddin'?
267
268			=head1 AUTHORS
269
270			Emanuele Giaquinta L<< <e.giaquinta@glauco.it> >>, Marc Alexander Lehmann
271			L<< <rxvt-unicode@schmorp.de> >>.