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Debugging OpenVPN 3 Linux

Since all of the backend processes of the OpenVPN 3 Linux client is started automatically by D-Bus (through the auto-start service), it gets harder to debug what is happening straight out-of-the-box. But several tweaks have been added to ease this.

First, the OpenVPN 3 Linux client must be compiled with debug options. This is done by running ./configure with the --enable-debug-options argument. It is also advisable to ensure the various tools under src/tests/dbus have been built as well.

There are six backend services to beware of.

  1. openvpn3-service-configmgr
  2. openvpn3-service-sessionmgr
  3. openvpn3-service-backendstart
  4. openvpn3-service-client
  5. openvpn3-service-netcfg
  6. openvpn3-service-logger

Running backend services in the terminal

All of these, with the exception of 4. openvpn3-service-client can be started from the command line. They will all have a idle time-out mechanism, which means it will exit after some idle time if the service is not in use. This can be disabled by adding --idle-exit 0 to the command line.

All of these services can be started as openvpn with the default D-Bus policy, with the exception of openvpn3-service-netcfg which must be started as root - but it will fairly quickly drop all but the needed capabilities and switch to the openvpn user as well. If the other services are being started as root, they will also switch to openvpn automatically.

This also means five of backend services can be run via external debug tools directly on the command line. The openvpn3-service-client is different, as that needs to be started via openvpn3-service-backendstart.

Debugging openvpn3-service-client

It is possible to attach a debugger to openvpn3-service-client as well, by running openvpn3-service-backendstart on the command line together with a few extra arguments. Just remember, this service must be started and run as openvpn for everything to function correctly.

 # openvpn3-service-backendstart --idle-exit 0 \
            --run-via $DEBUGGER               \
            --debugger-arg $DBG_ARG1          \
            --debugger-arg $DBG_ARG2          \
            --debugger-arg $DBG_ARG3

In some situations, it might not be wanted to have the openvpn3-service-client to daemonize and start a new process session id (setsid(3P)). This can be avoided by also adding --client-no-fork and --client-no-setsid to the command line above.

To run openvpn3-service-client via valgrind, you could do like this:

# openvpn3-service-backendstart --idle-exit 0 \
            --run-via /usr/bin/valgrind       \
            --debugger-arg "--leak-check=full"

To run openvpn3-service-client via GDB, a different approach needs to be taken - by using the remote debugging feature of GDB.

# openvpn3-service-backendstart --idle-exit 0 \
            --client-no-fork                   \
            --client-no-setsid                 \
            --run-via /usr/bin/gdbserver       \
            --debugger-arg localhost:9944

In a different terminal, start gdb like this:

# gdb openvpn3-service-client
[...snip...]
(gdb) target remote localhost:9944

When the session manager (openvpn3-service-sessionmgr) starts a new tunnel, the terminal with gdb running will come back with a prompt where you can continue the execution.

Caveats with GDB

D-Bus is fairly sensitive to time-outs. These time-outs are normally reasonable, but you might hit several time-outs when using this way of debugging. Further, by using --client-no-fork it may also happen that various openvpn3 front-ends will not respond as expected. In these cases, using the openvpn3 Python module might be of help, where it is possible to step through each of the various steps in a more controlled manner; see below for details.

D-Bus debugging via glib2

By defining the G_DBUS_DEBUG environment variable, it is possible to inspect the various D-Bus messages being passed to/from a glib2 based D-Bus service or client. The most verbose debug logging is gained with using G_DBUS_DEBUG=all. For more details see the upstream Glib2 Running GIO applications: GIO Reference Manual documentation.

More fine grained session management control

It is fully possible to get a more fine grained control of starting tunnels. The easiest way is by using Python and the openvpn3 module. It is advisable to first import the OpenVPN configuration profile via openvpn3 config-import. This will give you a configuration D-Bus path which can be easily used further.

#!/usr/bin/python3

import dbus
import openvpn3

# Shared D-Bus System Bus connection
sysbus = dbus.SystemBus()

# Get access to the configuration manager
cfgmgr = openvpn3.ConfigurationManager(sysbus)

# Retrieve access to the configuration profile
cfg = cfgmgr.Retrieve('/net/openvpn/v3/configuration/some-path')


# Get access to the session manager
sessmgr = openvpn3.SessionManager(sysbus)

# Create a new VPN session, based on the retrieved configuration
sess = sessmgr.NewTunnel(cfg)
print("Session path: %s" % sess.GetPath())

# Various actions to do on the session object
sess.Ready()      # Ready for connection?
sess.Connect()    # Start a connection
sess.Pause()      # Pause the connection
sess.Resume()     # Resume the connection
sess.Disconnect() # Disconnect and close the session.  The session
                  # object is invalid after this call.

The methods available in the configuration manager object (cfgmgr), configuration object (cfg), session manager object (sessmgr) and the session object (sess) mostly works in the same way and with the same names as the D-Bus raw API for these objects. The most noticeable difference is the Retrieve(path) and GetPath()methods as well as the interface for providing username/password credentials to a session object. See src/tests/python for more examples.

Other challenges

D-Bus by design is quite strict when it comes to services using the system bus. This means it can quite often happen that D-Bus calls or signals are being rejected by the D-Bus daemon. The best way to detect these issues, is to look into the D-Bus logs. On systems with systemd, this is easily done via journalctl --since today -u dbus. All log events processed by openvpn3-service-logger will typically also be present here.

Logging

Almost all log events happens exclusively over D-Bus. Some of these log events are targeted to either the session manager or the openvpn3-service-logger service. To retrieve logs it is therefore needed to run openvpn3-service-logger with the --service argument as the openvpn user. This ensures that the services will fetch log entries directly.

If no log events happens with openvpn3-service-logger, the openvpn3-service-backendstart can be run with --client-signal-broadcast. Enabling this will send all backend client signals as system wide D-Bus broadcast signals.

In addition there are more log tools under ./src/tests/dbus.

  • signal-listener: Dumps almost all D-Bus signals broadcasts on the system. This is quite low-level and will not show any "targeted" signals towards a specific recipient. It will also decode some of the OpenVPN 3 specific D-Bus signals.

  • openvpn3 log --log-level 6 --session-path ${SESSION_DBUS_PATH}: This will enable the debug logging for a specific running VPN session.

  • log-listener, log-listener2: Variants of signal-listener which only listens for Log signals.

  • logservice1 and openvpn3 log-service: Can be used to query and modify properties in the openvpn3-service-logger service. The logservice1 can also be used to generate some log events.

  • enable-logging: Used to enable Log forwarding from the openvpn3-service-client backend to the session manager. The session manager will then proxy these log events to a front-end user (as Log signals).

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