How to send HTTP header using cURL? | ScrapingBee

How to Send HTTP Headers Using cURL: The Complete Guide

If you‘re a web developer, API tester, or just someone who likes to tinker with web requests, you‘ve probably heard of cURL. This handy command-line tool allows you to send HTTP requests and receive responses right from your terminal. One of cURL‘s most powerful features is the ability to easily set custom HTTP headers on your requests.

In this guide, we‘ll take an in-depth look at how to send HTTP headers using cURL. Whether you‘re a beginner just getting started or an experienced user looking to expand your cURL skills, read on to learn everything you need to know. We‘ll cover what HTTP headers are, the basic syntax for setting headers in cURL, advanced header options and troubleshooting, and examples of real-world use cases. Let‘s get started!

What are HTTP Headers?

Before we dive into the specifics of setting headers with cURL, let‘s start with a quick refresher on what HTTP headers are. Headers are key-value pairs that are sent along with an HTTP request or response. They provide additional information and instructions on how the client and server should handle the request/response.

Some common HTTP header fields include:

• User-Agent: Identifies the client application
• Accept: Specifies the media types the client can handle
• Content-Type: Indicates the media type of the request body
• Authorization: Sends credentials for authentication
• Cookie: Passes cookies between client and server

Headers are an essential part of the HTTP protocol and can be used for many purposes such as authentication, caching, compression, redirects, and more. Now that we know what headers are, let‘s see how to set them using cURL.

Setting Headers with cURL

The basic syntax for sending an HTTP header with cURL is:

curl -H "HeaderName: HeaderValue" URL

The -H flag allows you to pass one or more headers to the request. Simply provide the header name and value, separated by a colon. Here‘s an example that sets the User-Agent header:

curl -H "User-Agent: My-Custom-User-Agent/1.0" https://example.com

You can set multiple headers by specifying the -H flag multiple times:

curl -H "User-Agent: My-Custom-User-Agent/1.0" 
     -H "Accept: application/json"
     -H "Authorization: Bearer abc123" 
     https://example.com

cURL will send all the provided headers along with the request to the specified URL. The server can then read these headers and use them to modify its response accordingly.

Common Header Use Cases

Now that you know how to set headers with cURL, let‘s look at some common use cases and examples.

Spoofing the User-Agent Header
The User-Agent header identifies the client making the request, usually a web browser. However, some websites may block requests from certain user agents, like scrapers or bots. You can use cURL to spoof the User-Agent and disguise your request as coming from a browser:

curl -H "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:93.0) Gecko/20100101 Firefox/93.0" https://example.com  

This makes the request appear to originate from a Firefox browser, which can help bypass blocks and scrape data successfully.

Setting Content-Type for POST Requests
When sending POST requests with a request body, you need to specify the Content-Type header so the server knows how to parse the data. For example, to send JSON data:

curl -X POST 
     -H "Content-Type: application/json"
     -d ‘{"username":"johndoe", "password":"secret"}‘
     https://api.example.com/login

The -d flag sends the specified JSON data in the request body, while the Content-Type header tells the API to expect JSON.

Adding Authentication Headers
Many APIs and websites require authentication to access protected resources. You can pass credentials using the Authorization header:

curl -H "Authorization: Bearer abc123" https://api.example.com/user 

This example uses a bearer token for authentication, but other schemes like Basic auth are also possible.

Sending Custom Headers
In addition to the standard HTTP headers, you can also send your own custom headers using cURL:

curl -H "X-MyCustom-Header: SomeValue" https://example.com

Custom headers are often used by APIs for app-specific data and configuration.

Viewing Response Headers

So far we‘ve focused on setting request headers sent by cURL. But it‘s equally important to be able to view the headers returned by the server in the response. You can use cURL‘s –dump-header option to save the response headers to a file:

curl --dump-header headers.txt https://example.com

This will write the HTTP response headers to headers.txt, allowing you to inspect them. Alternatively, you can use -v or –verbose to print both request and response headers to the console for debugging purposes.

Troubleshooting Header Issues

If you run into issues sending headers with cURL, here are a few tips to troubleshoot the problem:

Double-check your header syntax. Make sure header names/values are spelled correctly and separated by a colon and space.

Look out for special characters in header values. If your header value contains spaces, quotes, or other special characters, you may need to escape them or put the value in quotes.

Inspect the raw HTTP data. Use –trace or –trace-ascii to log the raw bytes sent and received, including headers. This can reveal formatting mistakes.

Check the protocol version. Rarely, sending certain headers may require a newer HTTP version. Try –http1.1 or –http2 to change the default HTTP version used by cURL.

Consult cURL‘s documentation. cURL has excellent man pages and online documentation with all the details about header options and the -H flag.

Best Practices for Setting Headers

Here are a few best practices to keep in mind when setting HTTP headers with cURL:

Only set headers you actually need. Don‘t add headers just for the sake of it, as this can bloat your requests unnecessarily.

Use the right case for header names. The HTTP spec requires header names to be case-insensitive, but some servers may be picky. Stick to the canonical spelling to be safe.

URL-encode special characters in header values. If your header value contains reserved characters like & or =, make sure to URL-encode them properly.

Test with and without headers. When debugging, try the request with no special headers to isolate the problem. Add headers back in one at a time to identify the culprit.

Be mindful of security. Headers like Authorization carry sensitive credentials, so be careful not to leak them in logs or error messages. Use HTTPS to encrypt them in transit.

Conclusion

Sending HTTP headers with cURL is a critical skill for anyone working with web APIs, scraping websites, or debugging HTTP. By using the -H flag and following the tips above, you can set custom headers to control how your requests are processed by servers.

Whether you need to spoof your user agent, set content types for POST bodies, add authentication, or just experiment with custom headers, cURL has you covered. You can even inspect response headers to debug issues and understand how the server handles your requests.

So the next time you‘re tinkering with a stubborn API or website, remember the power of HTTP headers and cURL. With a little bit of experimentation and the techniques covered here, you‘ll be able to tackle even the trickiest HTTP challenges. Happy cURLing!