[ViewVC] Annotation of: cvs/libptytty/README

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

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

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 "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.

SECURITY CONSIDERATIONS
    *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 "ptytty::init ()" method (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 "ptytty::use_helper" and "ptytty::drop_privileges" (and possibly
    "ptytty::sanitise_stdfd") are more useful.

C++ INTERFACE: THE ptytty CLASS
  STATIC METHODS
    ptytty::init ()
        The default way to initialise libptytty. Must be called immediately
        as the first thing in the "main" function, or earlier e.g. during
        static construction time. The earlier, the better.

        This method calls "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.

    ptytty::use_helper ()
        Tries to start a helper process that retains privileges even when
        the calling process does not. This is usually called from
        "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.

    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.

    ptytty::sanitise_stdfd ()
        Checks whether file descriptors 0, 1 and 2 (stdin, stdout and
        stderr) are valid (open) and, if not, connects them to /dev/tty or
        /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).

    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.

    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 -1 otherwise. This method
        is only exposed for your convenience and is not required for normal
        operation.

    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.

  DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
    int pty_fd = pty->pty
    int tty_fd = pty->tty
        These members contain the pty and tty file descriptors,
        respectively. They initially contain -1 until a successful to
        "ptytty::get".

    bool success = pty->get ()
        Tries to find, allocate and initialise a new pty/tty pair. Returns
        "true" when successful.

    pty->login (int cmd_pid, bool login_shell, const char *hostname)
        Creates an entry in the systems session database(s) (utmp, wtmp,
        lastlog). "cmd_pid" must be the pid of the process representing the
        session (such as the login shell), "login_shell" defines whether the
        session is associated with a login, which influences whether wtmp
        and lastlog entries are created, and "hostname" should identify the
        "hostname" the user logs in from, which often is the value of the
        "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.

    pty->close_tty ()
        Closes the tty. Useful after forking in the parent/pty process.

    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.

    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.

C INTERFACE: THE ptytty FAMILY OF FUNCTIONS
    ptytty_init ()
        See "ptytty::init ()".

    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. "if
        (pty) ...."). See "ptytty::create ()".

    int ptytty_pty (PTYTTY ptytty)
        Return the pty file descriptor. See "pty->pty".

    int ptytty_tty (PTYTTY ptytty)
        Return the tty file descriptor. See "pty->tty".

    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
        "delete pty" in C++.

    int ptytty_get (PTYTTY ptytty)
        See "pty->get", returns 0 in case of an error, non-zero otherwise.

    void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const
    char *hostname)
        See "pty->login".

    void ptytty_close_tty (PTYTTY ptytty)
        See "pty->close_tty".

    int ptytty_make_controlling_tty (PTYTTY ptytty)
        See "pty->make_controlling_tty".

    void ptytty_set_utf8_mode (PTYTTY ptytty, int on)
        See "pty->set_utf8_mode".

    void ptytty_drop_privileges ()
        See "ptytty::drop_privileges".

    void ptytty_use_helper ()
        See "ptytty::use_helper".

BUGS
    You kiddin'?

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

Revision:	1.7
Committed:	Wed Dec 21 00:48:52 2011 UTC (12 years, 5 months ago) by sf-exg
Branch:	MAIN
CVS Tags:	rxvt-unicode-rel-9_20, rxvt-unicode-rel-9_21, rel-9_14, rxvt-unicode-rel-9_19, rxvt-unicode-rel-9_18, rxvt-unicode-rel-9_17, rel-1_5, rxvt-unicode-rel-9_16, rel-1_7, rxvt-unicode-rel-9_15, rel-1_6
Changes since 1.6:	+27 -15 lines
Log Message:	Regenerate libptytty.3 and README.
#	User	Rev	Content
1	root	1.1	NAME
2	root	1.2	libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog
3	root	1.1	handling
4
5			SYNOPSIS
6	root	1.4	cc ... -lptytty
7
8			#include <libptytty.h>
9
10	sf-exg	1.7
11	root	1.4	// C++
12			ptytty *pty = ptytty::create ();
13
14			if (!pty->get ())
15			// error allocating pty
16
17			if (we want utmp)
18			pty->login (process_pid, 0, "remote.host");
19			else if (we want utmp AND wtmp/lastlog)
20			pty->login (process_pid, 1, "remote.host");
21
22			// we are done with it
23			delete pty;
24
25	sf-exg	1.7
26	root	1.4	// C
27			PTYTTY pty = ptytty_create ();
28
29			if (!ptytty_get (pty))
30			// error allocating pty
31
32			if (we want utmp)
33			ptytty_login (pty, process_pid, 0, "remote.host");
34			else if (we want utmp AND wtmp/lastlog)
35			ptytty_login (pty, process_pid, 1, "remote.host");
36
37			// we are done with it
38			ptytty_delete (pty);
39	root	1.1
40	root	1.5	See also the eg/ directory, which currently contains the c-sample.c file
41	sf-exg	1.7	that spawns a login shell from C using libptytty.
42	root	1.5
43	root	1.1	DESCRIPTION
44	root	1.4	Libptytty is a small library that offers pseudo-tty management in an
45			OS-independent way. It was created out of frustration over the many
46			differences of pty/tty handling in different operating systems for the
47			use inside "rxvt-unicode".
48
49			In addition to offering mere pty/tty management, it also offers session
50			database support (utmp and optional wtmp/lastlog updates for login
51			shells).
52
53			It also supports fork'ing after startup and dropping privileges in the
54			calling process, so in case the calling process gets compromised by the
55			user starting the program there is less to gain, as only the helper
56			process runs with privileges (e.g. setuid/setgid), which reduces the
57			area of attack immensely.
58
59			Libptytty is written in C++, but it also offers a C-only API.
60	root	1.3
61			SECURITY CONSIDERATIONS
62			*It is of paramount importance that you at least read the following
63			paragraph!*
64
65	sf-exg	1.7	If you write a typical terminal-like program that just wants one or more
66	root	1.3	ptys, you should call the "ptytty::init ()" method (C: "ptytty_init ()"
67			function) as the very first thing in your program:
68
69			int main (int argc, char *argv[])
70			{
71			// do nothing here
72			ptytty::init ();
73			// in C: ptytty_init ();
74
75			// initialise, parse arguments, etc.
76			}
77
78	ayin	1.6	This checks whether the program runs setuid or setgid. If yes then it
79	root	1.3	will fork a helper process and drop privileges.
80
81			Some programs need finer control over if and when this helper process is
82			started, and if and how to drop privileges. For those programs, the
83	sf-exg	1.7	methods "ptytty::use_helper" and "ptytty::drop_privileges" (and possibly
84			"ptytty::sanitise_stdfd") are more useful.
85	root	1.3
86			C++ INTERFACE: THE ptytty CLASS
87			STATIC METHODS
88			ptytty::init ()
89	sf-exg	1.7	The default way to initialise libptytty. Must be called immediately
90	root	1.3	as the first thing in the "main" function, or earlier e.g. during
91			static construction time. The earlier, the better.
92
93	sf-exg	1.7	This method calls "sanitise_stdfd" and then checks whether the
94			program runs with setuid/setgid permissions and, if yes, spawns a
95			helper process for pty/tty management. It then drops the privileges
96			completely, so the actual program runs without setuid/setgid
97			privileges.
98	root	1.3
99			ptytty::use_helper ()
100			Tries to start a helper process that retains privileges even when
101			the calling process does not. This is usually called from
102			"ptytty::init" when it detects that the program is running setuid or
103	sf-exg	1.7	setgid, but can be called manually if it is inconvenient to drop
104	root	1.3	privileges at startup, or when you are not running setuid/setgid but
105			want to drop privileges (e.g. when running as a root-started
106			daemon).
107
108			This method will try not to start more than one helper process. The
109	root	1.5	same helper process can usually be used both from the process
110			starting it and all its fork'ed (not exec'ed) children.
111	root	1.3
112			ptytty::drop_privileges ()
113			Drops privileges completely, i.e. sets real, effective and saved
114	root	1.5	user id to the real user id. Also aborts if this cannot be achieved.
115	root	1.3	Useful to make sure that the process doesn't run with special
116			privileges.
117
118	sf-exg	1.7	ptytty::sanitise_stdfd ()
119			Checks whether file descriptors 0, 1 and 2 (stdin, stdout and
120			stderr) are valid (open) and, if not, connects them to /dev/tty or
121			/dev/null if possible (and aborts otherwise). This is necessary
122			because libptytty might want to output error messages to those
123			descriptors, which at the time of outputting the error message,
124			might be connected to something unsuitable opened by the
125			unsuspecting program itself (this can be a security issue).
126
127	root	1.3	bool success = ptytty::send_fd (int socket, int fd)
128			Utility method to send a file descriptor over a unix domain socket.
129			Returns true if successful, false otherwise. This method is only
130	sf-exg	1.7	exposed for your convenience and is not required for normal
131	root	1.3	operation.
132
133			int fd = ptytty::recv_fd (int socket)
134			Utility method to receive a file descriptor over a unix domain
135	sf-exg	1.7	socket. Returns the fd if successful and -1 otherwise. This method
136			is only exposed for your convenience and is not required for normal
137	root	1.3	operation.
138
139			ptytty *pty = ptytty::create ()
140			Creates new ptytty object. Creation does not yet do anything besides
141			allocating the structure.
142
143			A static method is used because the actual ptytty implementation can
144			differ at runtime, so you need a dynamic object creation facility.
145
146			DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
147			int pty_fd = pty->pty
148			int tty_fd = pty->tty
149			These members contain the pty and tty file descriptors,
150			respectively. They initially contain -1 until a successful to
151			"ptytty::get".
152
153			bool success = pty->get ()
154			Tries to find, allocate and initialise a new pty/tty pair. Returns
155			"true" when successful.
156
157			pty->login (int cmd_pid, bool login_shell, const char *hostname)
158			Creates an entry in the systems session database(s) (utmp, wtmp,
159			lastlog). "cmd_pid" must be the pid of the process representing the
160	ayin	1.6	session (such as the login shell), "login_shell" defines whether the
161	sf-exg	1.7	session is associated with a login, which influences whether wtmp
162			and lastlog entries are created, and "hostname" should identify the
163	root	1.3	"hostname" the user logs in from, which often is the value of the
164			"DISPLAY" variable or tty line in case of local logins.
165
166			Calling this method is optional. A session starts at the time of the
167			login call and extends until the ptytty object is destroyed.
168
169			pty->close_tty ()
170			Closes the tty. Useful after forking in the parent/pty process.
171
172			bool success = pty->make_controlling_tty ()
173			Tries to make the pty/tty pair the controlling terminal of the
174			current process. Useful after forking in the child/tty process.
175
176			pty->set_utf8_mode (bool on)
177			On systems supporting special UTF-8 line disciplines (e.g. Linux),
178	root	1.5	this tries to enable this discipline for the given pty. Can be
179			called at any time to change the mode.
180	root	1.3
181			C INTERFACE: THE ptytty FAMILY OF FUNCTIONS
182			ptytty_init ()
183			See "ptytty::init ()".
184
185			PTYTTY ptytty_create ()
186			Creates a new opaque PTYTTY object and returns it. Do not try to
187	root	1.5	access it in any way except by testing it for truthness (e.g. "if
188	root	1.3	(pty) ...."). See "ptytty::create ()".
189
190			int ptytty_pty (PTYTTY ptytty)
191			Return the pty file descriptor. See "pty->pty".
192
193			int ptytty_tty (PTYTTY ptytty)
194			Return the tty file descriptor. See "pty->tty".
195
196			void ptytty_delete (PTYTTY ptytty)
197			Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up
198			the utmp/wtmp/lastlog databases, if initialised/used. Same as
199			"delete pty" in C++.
200
201			int ptytty_get (PTYTTY ptytty)
202			See "pty->get", returns 0 in case of an error, non-zero otherwise.
203
204			void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const
205			char *hostname)
206			See "pty->login".
207
208			void ptytty_close_tty (PTYTTY ptytty)
209			See "pty->close_tty".
210
211			int ptytty_make_controlling_tty (PTYTTY ptytty)
212			See "pty->make_controlling_tty".
213
214			void ptytty_set_utf8_mode (PTYTTY ptytty, int on)
215			See "pty->set_utf8_mode".
216
217			void ptytty_drop_privileges ()
218			See "ptytty::drop_privileges".
219
220			void ptytty_use_helper ()
221			See "ptytty::use_helper".
222	root	1.1
223			BUGS
224			You kiddin'?
225
226			AUTHORS
227			Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
228			<rxvt-unicode@schmorp.de>.
229