Detecting a disconnected RDP session in a unix / linux / bsd server.

explanation.md

Here is a way to obtain a list of disconnected xrdp sessions. It relies on the fact that the xrdp server is, in normal X session manager usage, the only client that establishes a TCP connection to the Xvnc X Window System display server. (The other client programs -- the ones doing things for the user -- use UNIX-domain sockets for their display connections).

When an xrdp session is active, the associated Xvnc display server has two TCP connections, one in the ESTABLISHED state, and the other in the LISTEN state. That looks something like this using the lsof(1) program.

$ sudo lsof  -b -w -n -c /^Xvnc$/b -a -iTCP:5900-5999 
COMMAND  PID  USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
Xvnc    1625 guest    1u  IPv4 252910      0t0  TCP 127.0.0.1:5910 (LISTEN)
Xvnc    1625 guest    9u  IPv4 261226      0t0  TCP 127.0.0.1:5910->127.0.0.1:35242 (ESTABLISHED)

If the user of the remote session abandons it by closing the RDP connection (or, in the case of an Apache Guacamole RDP session, by closing the browser window) it looks something like this:

COMMAND  PID  USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
Xvnc    1625 guest    1u  IPv4 252910      0t0  TCP 127.0.0.1:5910 (LISTEN)

Notice there's no ESTABLISHED connection on this disconnected Xvnc display server process. So, any Xvnc process that's only listening is a disconnected session.

Here's a shell script (named lsdisconnected) that displays the PID and USER for each disconnected remote session. It uses lsof(1) and gawk(1) to implement the connection logic.

This is a handy way to find disconnected remote desktop sessions; it works immediately upon disconnection, without needing to use an idle time.

For those who may not be familiar with lsof(1) here's an explanation of the command line parameters in this example.

• -b -w avoids lsof kernel waits. They're not needed here.
• -n avoids DNS lookups for hostnames.
• -c /^Xvnc$/b looks for processes with the exact command name Xvnc, using a regex.
• -a tells lsof to use AND, not OR, when filtering.
• -iTCP:5900-5999 filters by TCP ports numbered 5900 - 5999, the ones used for X display connections.)

lsdisconnected

#!/bin/bash
sudo lsof -FRgpLT -b -w -n -c /^Xvnc$/b -a -iTCP:5900-5999  |
gawk '
  match($0,/^p([0-9]+)/,       p) {pid = p[1]; pids[pid]=0; } ;
  match($0,/^L([A-Za-z0-9]+)/, p) {user[pid] = p[1]; } ;
  /TST=LISTEN/                    {pids[pid] = pids[pid] - 1 ;};
  /TST=ESTABLISHED/               {pids[pid] = pids[pid] + 1};
  END             {
                    for (pid in pids){
                      if (pids[pid] < 0) {
                        print pid, user[pid];
                      }
                    }
                  };
  '