KRB5_TRACE documentation for gssproxy

Added KRB5_TRACE.md to provide a brief overview on how to enable low
level Kerberos tracing below gssapi via KRB5_TRACE.

Updated gssproxy.conf man page to cite newly added doc when mentioning
KRB5_TRACE behavior.

Signed-off-by: Jacob Shivers <jacob.shivers@redhat.com>

docs/KRB5_TRACE.md

@@ -0,0 +1,36 @@
+# Setting KRB5_TRACE for gssproxy
+
+Enabling `KRB5_TRACE` output as described in `# man gssproxy.conf`:
+
+**At level 3 and above, KRB5_TRACE output is logged. If KRB5_TRACE was already set in the execution environment, trace output is sent to its value instead.**
+
+can be done either by including `KRB5_TRACE=/path/to/location` when executing gssproxy at the command line or by including a location within a drop file loaded by systemd. Systemd does not open `/dev/stderr` and instead creates a file descriptor for stderr when gssproxy is started. A drop file is necessary to record `KRB5_TRACE` information as gssproxy directs `KRB5_TRACE` to `/dev/stderr` at this time.
+
+The output can be directed to any location, but gssproxy only has write access to `/var/lib/gssproxy` by default. This means that for a host system using SELinux either a custom module policy will need to be created or SELinux will need to be put into permissive mode.
+
+As setting `KRB5_TRACE` output is not designed to be continually logged nor treated as traditional log output, it is recommended to direct the trace output to `/var/lib/gssproxy`.
+
+- Increase gssproxy debugging so that `KRB5_TRACE` information is logged as described in `# man gssproxy.conf`.
+
+~~~~
+# echo ' debug_level = 3' >> /etc/gssproxy/gssproxy.conf
+~~~~
+
+ - Create a drop file for gssproxy to log `KRB5_TRACE` output to a file under `/var/lib/gssproxy`
+
+~~~~
+# mkdir /etc/systemd/system/gssproxy.service.d
+
+# cat <<EOF > /etc/systemd/system/gssproxy.service.d/99-trace.conf
+[Service]
+Environment=KRB5_TRACE=/var/lib/gssproxy/gssproxy.krb5_trace
+EOF
+~~~~
+
+ - Reload the service files and restart the `gssproxy` service
+
+~~~~
+# systemctl daemon-reload
+
+# systemctl restart gssproxy
+~~~~

man/gssproxy.conf.5.xml

@@ -216,7 +216,7 @@
                             At level 3 and above, KRB5_TRACE output is logged.
                             If KRB5_TRACE was already set in the execution
                             environment, trace output is sent to its value
-                            instead.
+				instead. Refer to docs/KRB5_TRACE.md for more information.
                         </para>
                         <para>Default: 1 if debug is true, otherwise 0</para>
                     </listitem>