How to Validate XML Against DTD Locally and Securely

Ensuring your XML files conform to their Document Type Definitions (DTD) without risking data exposure is crucial. Here are two effective methods for local, secure XML validation:

1. Command Line Tool: xmllint

xmllint, part of the libxml2 library, offers offline validation with no network risks.

Validation Steps:

# Basic syntax check
xmllint --noout your_file.xml

# DTD validation (XML must declare DOCTYPE)
xmllint --dtdvalid your_dtd.dtd --noout your_file.xml

Key Parameters:

• --noout: Prevents output of XML content.
• --dtdvalid: Specifies the path to the external DTD file.
• --nonet: Forces disabling network connections, enhancing security.

Common Issues and Solutions:

1. DTD Not Linked Error Add the DTD declaration in your XML's header:

   <!DOCTYPE root_element SYSTEM "your_dtd.dtd">

1. DTD Syntax Errors
   
   Ensure your DTD file does not include a <!DOCTYPE> declaration, only element/attribute definitions.

2. Batch Validation Script
   

   find ./xml_files -name "*.xml" -exec xmllint --dtdvalid schema.dtd --noout {} \;

---

2. VS Code XML Extension

The XML extension by Red Hat for VS Code provides real-time validation across Windows, Mac, and Linux.

Setup Process:

1. Install the Extension
   
   Search for "XML" by Red Hat in VS Code's extension marketplace and install.

2. Link DTD File
   
   Add to settings.json:
   

   "xml.fileAssociations": [{
     "pattern": "**/*.xml",
     "systemId": "/path/to/your.dtd"
   }]

1. XML Catalog Support Create catalog.xml for mapping public identifiers:

   <catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
     <public publicId="-//Your//DTD" uri="your.dtd"/>
   </catalog>

Feature Comparison:

Feature	xmllint	VS Code XML Extension
Real-time Validation	❌ Requires command execution	✅ Automatic during typing
Error Localization	❌ Line numbers in CLI	✅ Visual markers in editor
Auto-completion	❌	✅ Based on DTD
Batch Processing	✅ Scriptable	❌ Single file operations
Cross-platform	✅ Linux/Mac/Windows	✅ All platforms

---

Recommendations for Choosing a Method

• Development & Debugging: Opt for the VS Code extension for real-time feedback and auto-completion.
• CI/CD Pipelines: Use xmllint for scripting in automated workflows like Jenkins or GitHub Actions.
• Sensitive Data Validation: Both methods support offline operation, but xmllint with --nonet adds an extra layer of security.

References

• libxml2 Manual Page for xmllint
• Red Hat XML Extension for VSCode

---