There is only one file type that uses the .UDL extension:
Microsoft Universal Data Link File
Universal Data Link (UDL) file used by Windows applications for specifying connection information to a data provider; defines the data provider type, the connection string, user name and password, and other properties, such as connection timeouts; can also be used for testing a connection to a data source.
UDL files can be edited using a Windows built-in utility called Data Link Properties. They can also be edited using a text editor since they are saved in a plain text format.
You can create a UDL file by creating a blank text file and changing its extension to ".udl." To edit the resulting file, double-click it in Windows, which will open the Data Link Properties dialog box.
Updated: January 24, 2011