Question 1

What does the -d flag mean in curl?

Accepted Answer

The -d (or --data) flag sends data in the request body, which also automatically sets the method to POST. It is used to send form data or JSON payloads to an API endpoint.

Question 2

What does -H mean in a curl command?

Accepted Answer

The -H (or --header) flag adds a custom HTTP header to the request. Common examples: -H 'Content-Type: application/json' to declare a JSON body, or -H 'Authorization: Bearer TOKEN' to authenticate with an API.

Question 3

What does Cache-Control: max-age mean?

Accepted Answer

Cache-Control: max-age=N tells the browser (and any intermediate caches) that this response can be stored and reused for N seconds without checking the server again. After N seconds the cached copy is considered stale.

Question 4

What does -L do in curl?

Accepted Answer

The -L (or --location) flag tells curl to follow HTTP redirects automatically. Without it, if the server responds with a 301 or 302, curl stops and shows the redirect response instead of following it.

Question 5

What does Strict-Transport-Security mean?

Accepted Answer

The Strict-Transport-Security header (HSTS) tells browsers to only access this site over HTTPS for the specified duration. Once seen, the browser will refuse plain HTTP connections for that period, protecting against protocol downgrade attacks.

Question 6

What does the -k flag do in curl?

Accepted Answer

The -k (or --insecure) flag disables SSL/TLS certificate verification. This allows curl to connect to servers with self-signed, expired, or otherwise invalid certificates. Only use it for testing — never in production scripts.

-X METHOD	Set HTTP method (GET, POST, PUT, DELETE…)
-H "K: V"	Add a request header
-d DATA	Send data in body; implies POST
-F KEY=VAL	Send multipart/form-data field
-u user:pass	HTTP Basic Authentication
-b COOKIE	Send cookie(s)

-L	Follow redirects automatically
-k	Skip SSL certificate verification
-s	Silent — suppress progress and errors
-v	Verbose — show full request and response
-o FILE	Write response body to file
--compressed	Request gzip-compressed response

--max-time N	Total time limit in seconds
--connect-timeout N	Connection phase time limit
--retry N	Retry N times on transient failure
--retry-delay N	Seconds between retries

-I	HEAD request — fetch headers only
-i	Include response headers in output
-w FORMAT	Custom output format (e.g. %{http_code})
-A AGENT	Set User-Agent string
-e URL	Set Referer header
--proxy URL	Use HTTP proxy

curl & HTTP Decoder

Common curl flags

Request control

Behavior flags

Timeouts & retry

Output control