Hardening consulting

Using FreeRDP logging facility

When you're working on FreeRDP, it's quite usual to increase the log level and to have to collect a massive amount of logs. And most often it doesn't fit in the terminal backscroll history, or it is so slow (terminal rendering is CPU intensive) that you need a file storage. Another case is when you're on a remote host and you want to retrieve the log over the network.

As each time I want to use the WLog capacities I'm looking at the source code, I had the idea to write that post on the subject, so that next time I will look at this text.

Using Wlog

Log verbosity level is controlled by the WLOG_LEVEL environment variable (in FreeRDP you can also use the /log-level command line switch). It accepts values with FATAL, ERROR, WARN, INFO, DEBUG or TRACE.

Wlog is very similar to log4j and uses appenders attached to loggers. The appenders available for Wlog are: file, standard output, network , syslog or systemd. The kind of instancied appender is given by the WLOG_APPENDER variable.

Logging to a file

To achieve this, you must use a FILE appender, and the WLOG_FILEAPPENDER_OUTPUT_FILE_PATH and WLOG_FILEAPPENDER_OUTPUT_FILE_NAME variables give the directory and the name of the file where the logs will be stored.

For instance if you want logs in /tmp/output.log:


Logging through the network

I've already used that feature when I was running FreeRDP on a thinclient (by the way I've coded this appender for that precise case), and the hardware was short in terms of CPU and storage. So exporting the logs solved both problems.

To use the network appender, you use a udp appender (WLOG_APPENDER) and WLOG_UDP_TARGET gives the target host for the packets, they will be send in UDP datagrams.

For example, you run FreeRDP this way:

WLOG_APPENDER=udp WLOG_UDP_TARGET= xfreerdp /v:....

And on the host that may receive the logs (at ip address

nc -ul -p20000


Now you know almost everything about wlog.