Occasionally you will have trouble connecting to a WMS service; here is how to enable tracing.
#Master Tracing Options #Tue Aug 05 14:56:31 PDT 2008 net.refractions.udig.catalog.wms/debug=true net.refractions.udig.catalog.wms/debug/request=true
When connecting to the JPL server and choosing the blue marble layer the following trace was produced:
!ENTRY net.refractions.udig.catalog.internal.wms 4 0 2008-09-05 08:26:15.561
!MESSAGE GetCapabilities: http://wms.jpl.nasa.gov/wms.cgi?REQUEST=GetCapabilities&VERSION=1.1.1&SERVICE=WMS
!ENTRY net.refractions.udig.catalog.internal.wms 4 0 2008-09-05 08:26:24.781
!MESSAGE GetMap: http://wms.jpl.nasa.gov/wms.cgi? SERVICE=WMS&LAYERS=BMNG&EXCEPTIONS=application/vnd.ogc.se_xml& FORMAT=image/png&HEIGHT=357&TRANSPARENT=TRUE&REQUEST=GetMap&BBOX=-180.0,-90.0,180.0,90.0&WIDTH=714&STYLES=default& SRS=EPSG:4326&VERSION=1.1.1
The URLs mentioned during tracing can be cut and pasted into a browser to verify an error message, or see the same image that uDig displayed.
Please keep the following in mind when connecting to a WMS. The URL you provide should be a valid GetCapabilities URL with the following elements:
The uDig will enter into "version negotiation" with the server using the above information.