Add SELinux instructions for systemd monitoring (#14809)

doc/Extensions/Applications.md

@@ -2186,6 +2186,29 @@ chmod +x /etc/snmp/redis.py
 extend redis /etc/snmp/redis.py
 ```
 
+4. (Optional) If you have SELinux in Enforcing mode, you must add a module so the script can get redis informations and write them:
+```
+cat << EOF > snmpd_redis.te
+module snmpd_redis 1.0;
+
+require {
+        type tmp_t;
+        type redis_port_t;
+        type snmpd_t;
+        class tcp_socket name_connect;
+        class dir { add_name write };
+}
+
+#============= snmpd_t ==============
+
+allow snmpd_t redis_port_t:tcp_socket name_connect;
+allow snmpd_t tmp_t:dir { write add_name };
+EOF
+checkmodule -M -m -o snmpd_redis.mod snmpd_redis.te
+semodule_package -o snmpd_redis.pp -m snmpd_redis.mod
+semodule -i snmpd_redis.pp
+```
+
 ### Agent
 
 [Install the agent](Agent-Setup.md) on this device if it isn't already