The TCP socket implementation of sesman has a number of limitations,
namely that it is affected by firewalls, and also that determining the
user on the other end requires a full authentication process.
The advantage of the TCP socket is that sesman and xrdp can be run on
separate machines. This is however not supported by the xorgxrdp
backend (shared memory), and is insecure, in that passwords are sent
in-the-clear, and the connection is susceptible to MitM attacks. This
architecture has been deprecated in release notes since xrdp v0.9.17,
and although it will continue to be supported in any further releases
in the x0.9.x series, it will not be supported in the next major
version.
There are a number of ways the existing transport connect logic in
trans_connect could be improved for POSIX compatibility, and also
slightly tidied up:-
1) The same socket is re-used for multiple connect attempts following
failure which isn't behaviour defined by POSIX.1-2017 (although it
works on Linux).
2) An asynchronous connect is started, and then after a short
delay connect() is called again on the same socket. POSIX.1-2017
is clear that in this situation EALREADY is returned before the
connection is established, but is silent on the behaviour expected
when the connection is established. Returning success is an option,
but so is returning EISCONN. The current code assumes the connect()
call will succeed.
3) The code contains two virtually identical, quite complex loops for
TCP and UNIX sockets, differing only in the calls to create a socket
and connect it.
4) trans_connect() contains looping and retry logic, but this isn't
seen as sufficient by the chansrv connect code in xrdp/xrdp_mm.c and
the Xorg connect code in xup/xup.c. Both of these implement their own
looping and retry logic on top of the logic in trans_connect(),
resulting in slightly unpredictable behaviour with regard to
timeouts.
5) A socket number can technically be zero, but in a couple of places
this isn't allowed for.
This PR attempts to correct the implementation of trans_connect(),
and also to simplify the areas it is called from.
As part of the PR, the signature of the server_is_term member of the
xrdp module interface is changed to match the signature expected by the
is_term member of a struct trans. This allows for trans_connect()
in xrdp modules to directly access g_is_term() within the main xrdp
executable. At the moment this functionality is only used by the xup
module.
