Emmanuel Oyibo

Posted on Mar 13 • Originally published at emminex.Medium

Using cURL Custom Headers: A Simple Guide

#beginners #tutorial #linux #header

In the world of web communication, cURL custom headers are versatile tools that allow you to tweak your web requests to your taste. Think of them as special notes you attach to your web requests.

They help you customize how your requests work. For example, cURL custom headers enable you to customize adding passwords, choosing data types, or sending extra information.

In this guide, you’ll explore the world of cURL custom headers. You’ll learn:

What cURL custom headers are
Why cURL custom headers are important
How to use cURL custom headers effectively
How proxy servers can enhance the functionality of cURL custom servers

What Are cURL Custom Headers?

cURL is a powerful command line tool that lets you talk to websites and servers. It’s like a super-advanced messenger for sending and receiving all web data.

On the other hand, Hypertext Transfer Protocol (HTTP) is the set of rules websites and servers use to communicate. Think of it as the language they speak. When you send a web request, hidden messages are attached called HTTP headers. These headers tell the server what browser you’re using or what type of data to expect back.

cURL custom headers let you add your own notes to these hidden messages, giving you more control over your web page requests.

Why Are cUrl Custom Headers Important?

Let’s consider some key reasons why cURL custom headers are an invaluable tool:

Authentication: Many web services require an authentication token or API keys to let you in. With custom headers (like the “Authorization” header), you can streamline authentication within your cURL requests.
Choosing Formats: You can tell the server what kind of data you want back (like JSON and XML), using headers like 'Accept' and 'Content-Type'. It’s necessary for handling data of different formats.
Metadata: Using your own descriptive headers, you can pass custom information to the server. Such information are application version, user identifier, or unique request IDs.
Debugging: You can inspect incoming and outgoing headers for troubleshooting networking issues and communication errors.
Advanced Control: Custom headers offer you precise control over how your cURL requests behave. This feature may open doors to more complex interactions.

How to Use cURL Custom Headers

cURL offers a simple way to add custom headers. You can achieve this using the -H or --header flag. Below is the basic syntax:

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

Let’s illustrate some practical use cases:

1. Setting an Authorization Header

curl -H "Authorization: Bearer your_api_key" http://httpbin.org/headers

Below is a breakdown of the command:

curl: This is the core command-line tool used for sending and receiving over various protocols including HTTP
-H "Authorization: Bearer your_api_key": This part sets a custom HTTP header within the request. Let’s break it down further:
- -H: This flag tells cURL you want to include a custom header
- "Authorization: Bearer your_api_key": This is the header itself with the following structure:
  - Authorization: This is the standard name for headers used to provide authentication credentials.
  - Bearer: This indicates the authentication scheme is a “bearer token”. Bearer tokens are a type of security token where simply possessing the token grants you access.
  - Your_api_key: This is a placeholder. You would replace it with your actual unique API key. It’s often issued by the service you want to access.
http://httpbin.org/headers: This is the URL of the web resource you’re trying to access.

2. Specifying Content Type

curl -H "Content-Type: application/json" -d '{"data": "value"}' http://httpbin.org/headers

Let’s examine the components of the command:

curl: same as the example above
-H "Content-Type: application/json": This sets a custom HTTP header. Here’s the breakdown:
- -H:The flag that introduces the custom header
- "Content-Type: application/json": The header itself with:
  - Content-Type: The standard name for headers specifying the type of data being sent
  - application/json: This declares the data format as JSON (JavaScript Object Notation)
-d ‘{"data": "value"}’: This part indicates the actual data payload you want to send:
- -d: This flag tells cURL there’s data to include in the request
- '{"data": "value"}': This is a simple JSON object with a single key-value pair
http://httpbin.org/headers: This is the URL of a web API you’re interacting with

3. Custom Metadata

curl -H "X-Application-Version: 1.2.5" http://httpbin.org/headers

From the command above:

-H "X-Application-Version: 1.2.5": This sets up a custom HTTP header. Here’s the breakdown:
- -H: The flag that tells cURL you’re adding a custom header.
- "X-Application-Version: 1.2.5": This is the header. It has the following parts:
  - X-: It’s common practice to prefix custom headers with “X-” to clearly distinguish them from standard headers.
  - Application-Version: This is a descriptive name for the header to indicate its purpose.
  - 1.2.5: The version number of your application.
http://httpbin.org/headers: The target URL

Using Proxy Servers with cURL Custom Headers

When you combine cURL custom headers with proxy servers, you get a powerful combination. Think of proxy servers as middlemen for your internet requests. Rather than having your computer connect directly to the website you want to visit, you can have your request pass through a proxy server.

You might use a server for the following reasons:

To mask your real IP address and make it difficult to track your online activity
To bypass regional restrictions
Adds an extra layer of security
Improve loading time by caching frequently accessed websites

How to Use a Proxy with cURL

You can direct cURL to use a proxy server using the -x (or --proxy) command-line option:

curl -x socks5h://localhost:9050 https://example.com

In this example:

socks5h:// indicates the proxy type (SOCKS5 with hostname resolution)
localhost:9050 is the address and port of your proxy server

Practical Examples

1. Making Anonymous Requests:

curl -x socks5://proxyprovider.com:1080 -H "X-Forwarded-For: 123.123.123.123" https://api.ipify.org

This masks your true IP address with the proxy’s
The X-Forwarded-For is used to further hide your origin

2. Geolocation Testing:

curl -x http://uk-proxy.example:8080 -H "X-Client-Location: London" https://whatismycountry.com

Simulates a request from the UK through proxy
Sets headers to mimic device details expected from a UK location

3. Load Distribution:

# Assuming you have a list of proxies in proxies.txt

for proxy in $(cat proxies.txt); do
  curl -x $proxy https://api.example.com/data >> results.json
done

Distributes requests across multiple proxies in a list
Helps to avoid overwhelming a single server

Conclusion

In this simple guide, you have explored the following:

What cURL custom headers are
Why cURL custom headers are important
How to use cURL custom headers effectively
How proxy servers can enhance the functionality of cURL custom servers

cURL custom headers offer remarkable flexibility in tailoring how you interact with websites and APIs.

FAQs

Can I use cURL custom headers even if I don't need authentication?

Absolutely! Custom headers are valuable for many purposes beyond authentication. Use them to specify data formats (like JSON or XML), send metadata about your application, or even help with troubleshooting issues on the server side.

Are there any risks to using proxy servers with cURL?

It's important to select reliable proxy providers. Free proxies may be less secure, might log your activity, or could have slower connections. Always research a proxy service before using it, especially with sensitive requests.

Can I combine multiple custom headers and a proxy in a single cURL command?

Yes! You can easily chain together multiple -H flags to add several custom headers and use the -x flag to specify a proxy server – all within the same cURL command.

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