use the network trace, from msdn.

<system.diagnostics>
  <trace autoflush="true" indentsize="4">
    <listeners>
      <add name="file" type="System.Diagnostics.TextWriterTraceListener" initializeData="trace.log"/>
    </listeners>
  </trace>
</system.diagnostics>

http://msdn2.microsoft.com/en-us/library/a6sbz1dx.aspx