External DTD
An external DTD is one that resides in a separate document. To use the DTD, you need to link to it from your XML document by providing the URI of the DTD file. This URI is typically in the form of a URL. The URL can point to a local file using a relative reference, or a remote one (i.e. using HTTP) using an absolute reference.
Example External DTD
Here's an example of an XML document that uses an external DTD. Note that the "standalone" attribute is set to "no". This is because the document relies on an external resource (the DTD):
<?xml version="1.0" standalone="no"?>
<!DOCTYPE tutorials SYSTEM "tutorials.dtd">
<tutorials>
<tutorial>
<name>XML Tutorial</name>
<url>http://www.quackit.com/xml/tutorial</url>
</tutorial>
<tutorial>
<name>HTML Tutorial</name>
<url>http://www.quackit.com/html/tutorial</url>
</tutorial>
</tutorials>
And, using the above XML document as an example, here's an example of what "tutorials.dtd" (the external DTD file) could look like. Note that the external DTD file doesn't need the DOCTYPE declaration - it is already on the XML file that is using this DTD:
<!ELEMENT tutorials (tutorial)+>
<!ELEMENT tutorial (name,url)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT url (#PCDATA)>
<!ATTLIST tutorials type CDATA #REQUIRED>