How to Send HTTP Headers with cURL?

Sending custom HTTP headers with cURL is essential for testing and debugging web applications. In this article, we’ll explore how to send HTTP headers using cURL, a powerful command-line tool for transferring data with URLs.

Understanding HTTP Headers

HTTP headers are key-value pairs sent in requests and responses between clients and servers. They convey meta-information about the request or response, such as content type, authorization credentials, and caching directives. Common HTTP headers include:

Why Send Custom HTTP Headers with cURL?

Custom HTTP headers allow you to:

• Simulate requests from different browsers or devices.
• Test APIs that require specific headers.
• Debug issues by modifying headers.
• Control caching and content negotiation.
• Authenticate requests to protected resources.

Sending HTTP Headers with cURL

Using the -H or --header Option

To send a custom HTTP header with cURL, use the -H or --header option followed by the header:

curl -H "Header-Name: Header-Value" http://example.com

Multiple Headers

You can send multiple headers by specifying multiple -H options:

curl -H "First-Header: Value1" -H "Second-Header: Value2" http://example.com

Replacing or Modifing Default Headers

cURL automatically sends certain headers like Host and User-Agent. To replace a default header, specify it with a new value. For example, to change the User-Agent:

curl -H "User-Agent: MyCustomAgent" http://example.com

If you want to remove a header, set it to an empty string:

curl -H "Accept:" http://example.com

Examples of Sending HTTP Headers with cURL

Example: Setting User-Agent

The User-Agent header identifies the client software. To set it:

curl -H "User-Agent: Mozilla/5.0" http://example.com

Alternatively, you can use the -A option:

curl -A "Mozilla/5.0" http://example.com

Example: Adding Authorization Header

For APIs that require authentication, you might need to send an Authorization header:

curl -H "Authorization: Bearer your_token_here" http://api.example.com

Example: Setting Content-Type for POST Requests

When sending data in a POST request, you may need to specify the Content-Type header:

curl -d '{"key":"value"}' -H "Content-Type: application/json" http://example.com/api

Example: Using Custom Headers with Proxies

If you’re using a proxy to send out your requests, you can combine proxy options with custom headers:

curl -x http://proxyelite.info:8080 -H "Custom-Header: Value" http://example.com

Tips and Best Practices

• Enclose Headers in Quotes: Always enclose header values in quotes to prevent shell interpretation, especially when they contain spaces or special characters.
• Verbose Mode: Use verbose mode -v to see the headers sent and received. This is useful for debugging:

curl -v -H "Header: Value" http://example.com

• Sensitive Data: Be cautious when sending sensitive data in headers, especially when sharing command examples or logs.
• Default Headers: Remember that cURL send some default headers. Overwriting them can affect how the server responds.

Common Mistakes to Avoid

• Missing Quotes: Forgetting quotes around headers can lead to errors or unexpected behavior.
• Incorrect Header Syntax: Headers should be in the format Header-Name: Value.
• Case Sensitivity: While header names are case-insensitive, it’s good practise to use standard capitalization.

Conclusion

Sending HTTP headers with cURL is straightforward and powerful. Whether you’re testing APIs or simulating different clients, mastering cURL’s header options enhances your ability to interact with web servers effectively.