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.

INSTALLATION

       libptytty uses "CMake" as build system. To build libptytty, install "CMake" and run the following
       commands from either the libptytty source directory or a separate build directory:

          cmake -DCMAKE_INSTALL_PREFIX=<prefix> -DBUILD_SHARED_LIBS=ON <path/to/libptytty>
          cmake --build .
          cmake --install .

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.

           On failure, this method throws a "ptytty_error" exception.

       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.

           On failure, this method throws a "ptytty_error" exception.

       ptytty::drop_privileges ()
           Drops  privileges completely, i.e. sets real, effective and saved user id to the real user id. Useful
           to make sure that the process doesn't run with special privileges.

           On failure, this method throws a "ptytty_error" exception.

       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. 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).

           On failure, this method throws a "ptytty_error" exception.

       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 call to "ptytty::get".

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

           If the helper process is running and there is a protocol error, this method throws  a  "ptytty_error"
           exception.

       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".

PORTABILITY

       To date, libptytty has been tested on the following platforms:

       GNU/Linux
       FreeBSD
       NetBSD
       OpenBSD
       macOS
       Solaris
       AIX

BUGS

       You kiddin'?

AUTHORS

       Emanuele Giaquinta <emanuele.giaquinta@gmail.com>, Marc Alexander Lehmann <rxvt-unicode@schmorp.de>.

2.0                                                2021-07-27                                       libptytty(3)