Mastering HTTP Headers with cURL: The Key to Smarter Web Interactions

When interacting with web APIs or making complex web requests, including additional HTTP headers in your cURL commands is crucial. These headers provide the server with critical context about the request, such as specifying the content type, providing authentication credentials, or including custom data. For developers, understanding how to manipulate these headers is essential to ensure that the server processes their requests correctly and efficiently. Let’s explore how to effectively use and customize HTTP headers in cURL requests, which can greatly benefit users of services like ProxyTee for enhanced data gathering and API interaction.

How to Send HTTP Headers with cURL?

To send HTTP headers with cURL, you can use the -H or --header option followed by the header you want to include. A typical cURL request with headers might look like this:

curl -X GET "https://api.example.com/resource" \
     -H "Authorization: Bearer YOUR_ACCESS_TOKEN" \
     -H "Content-Type: application/json"

These headers provide the server with essential context, such as what kind of content to expect (Content-Type) and how to authenticate the request (Authorization). This is particularly useful when you’re leveraging proxies for web scraping or accessing specific resources.

What Are Default cURL Headers?

By default, cURL does not include many special headers in its HTTP requests besides those automatically set by the HTTP protocol. The most common default headers that cURL might send are:

• User-Agent: cURL sends a User-Agent header that identifies the client software, usually in the form of curl/ followed by the version number (e.g., curl/7.69.0).
• Host: This header specifies the domain name of the server you’re trying to reach. It’s automatically set based on the URL provided in the request.
• Accept: By default, cURL sends Accept: */*, indicating that the client is willing to accept any response.

These default headers provide basic information that helps the server understand and process the request. They can be overridden or supplemented with additional headers using various header options with cURL.

How to Send Custom Headers with cURL?

To add a custom header to a cURL request, use the -H or --header flag followed by the header name and value:

curl -H "Content-Type: application/json" -H "X-Custom-Header: Value" http://example.com/api/resource

• Content-Type: application/json tells the server that the body of the request contains JSON data.
• X-Custom-Header: Value is an example of a custom header that could be used for application-specific purposes.

Custom headers are invaluable when using services like ProxyTee, allowing you to send precise instructions along with your proxy requests, and enabling effective integration with various systems.

How to Pass Multiple Headers with cURL?

To pass multiple headers with a cURL request, use the -H or --header flag multiple times, each followed by a different header name and value. This allows you to include various types of information in your request. Here’s an example:

curl -H "Content-Type: application/json" \
     -H "Authorization: Bearer YOUR_ACCESS_TOKEN" \
     -H "X-Custom-Header: Value" \
     http://example.com/api/resource

• Content-Type: application/json specifies that the request body is in JSON format.
• Authorization: Bearer YOUR_ACCESS_TOKEN adds an authorization token for secure API access.
• X-Custom-Header: Value demonstrates including an additional custom header for specific application needs.

When using ProxyTee’s Unlimited Residential Proxies, these header manipulation capabilities become crucial to avoid bot detection.

How to Show HTTP Response Headers with cURL?

To display the HTTP response headers when making a cURL request, use the -i or --include flag. This flag tells cURL to include the response headers in the output, allowing you to see important information such as status codes, server details, and cookies:

curl -i http://example.com/api/resource

Here’s an example of how the response would look like:

HTTP/1.1 200 OK
Date: Tue, 27 Aug 2025 12:34:56 GMT
Content-Type: application/json
Content-Length: 123
Connection: keep-alive
Server: Apache/2.4.41 (Ubuntu)
X-Powered-By: PHP/7.4.3
Set-Cookie: sessionid=abc123; Path=/; HttpOnly

{
    "id": 1,
    "name": "Example Resource",
    "status": "active"
}

The response body follows these headers containing details about the requested resource. This ability can be helpful in debugging issues or understanding what server parameters are set for users of ProxyTee’s services, including our Static Residential Proxies, ensuring a stable connection and proper functionality.

How to Print Both Request and Response Headers?

For debugging, seeing what’s happening when sending a request with cURL might be helpful. To have cURL display the HTTP headers for both the request and the response, along with the response body and additional information provided by cURL, use the -v or --verbose flag:

curl -v http://example.com/api/resource

The -v flag instructs cURL to print detailed information about the request and response, including the full set of HTTP headers. This is beneficial for anyone needing in-depth info for effective use of our Datacenter Proxies, which guarantees superior connection and stability.

Common Use Cases for cURL Headers

Even though there are many possible use cases for cURL headers, here are a few of the most popular examples of HTTP headers in cURL requests.

Sending Authenticated Requests

When interacting with secured resources, you might need to include an authorization header. This header often carries tokens or credentials needed to authenticate the request:

curl -H "Authorization: Bearer YOUR_ACCESS_TOKEN" http://example.com/api/protected/resource

In this example, the Authorization header carries a bearer token, a common method used in OAuth2 authentication. This is particularly relevant for those utilizing ProxyTee for accessing specific API resources, where you need to correctly use header authentication.

Changing the Response Format

The Accept header tells the server the type of content you’re willing to receive in response. This is crucial for APIs that can return data in multiple formats:

curl -H "Accept: application/xml" http://example.com/api/resource

Here, the request specifies that it prefers to receive XML data in response.

Specifying the User-Agent

Some servers return different data based on the client’s User-Agent string. You can customize this header to mimic different devices or browsers:

curl -H "User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 10_3 like Mac OS X) AppleWebKit/603.1.30 (KHTML, like Gecko) Version/10.0 Mobile/14E5239e Safari/602.1" http://example.com

This can be particularly useful for testing how your API responds to requests from different browsers or devices.

Sending the Content-Length Header

While cURL automatically calculates and adds the Content-Length header, there might be cases where you need to override this behavior, especially when dealing with servers that have specific requirements:

curl -H "Content-Length: 0" http://example.com/api/empty-post

Customizing the Connection Header

You can control aspects of the HTTP connection itself using headers like Connection: keep-alive or Connection: close, although such cases are rare since cURL and the server usually manage connection behavior efficiently:

curl -H "Connection: close" http://example.com/api/resource

Key Takeaways: Mastering HTTP Headers with cURL

Effectively managing HTTP headers with cURL is critical for seamless API interactions and complex web requests. By mastering header customization—like setting Content-Type or authentication tokens—developers ensure servers process requests accurately. This precision leads to more efficient, secure, and targeted communication.

The integration between the powerful tools like curl and proxy services such as ProxyTee becomes very valuable. By adding custom headers you can control the behaviour and interaction between clients and servers. ProxyTee helps developers perform more precise requests in data collection, adtech or many other scenarios.