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How to Validate XML Against DTD Locally and Securely

#xml #dtd #xmllint #redhat

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
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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">
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  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 {} \;
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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"
   }]
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  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>
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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

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