libcurl/opts/CURLOPT_HTTPHEADER.md

---
c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_HTTPHEADER
Section: 3
Source: libcurl
See-also:
  - CURLOPT_CUSTOMREQUEST (3)
  - CURLOPT_HEADER (3)
  - CURLOPT_HEADEROPT (3)
  - CURLOPT_MIMEPOST (3)
  - CURLOPT_PROXYHEADER (3)
  - curl_mime_init (3)
---

# NAME

CURLOPT_HTTPHEADER - set of HTTP headers

# SYNOPSIS

~~~c
#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTPHEADER,
                          struct curl_slist *headers);
~~~

# DESCRIPTION

Pass a pointer to a linked list of HTTP headers to pass to the server and/or
proxy in your HTTP request. The same list can be used for both host and proxy
requests!

When used within an IMAP or SMTP request to upload a MIME mail, the given
header list establishes the document-level MIME headers to prepend to the
uploaded document described by CURLOPT_MIMEPOST(3). This does not affect
raw mail uploads.

The linked list should be a fully valid list of **struct curl_slist**
structs properly filled in. Use curl_slist_append(3) to create the list
and curl_slist_free_all(3) to clean up an entire list. If you add a
header that is otherwise generated and used by libcurl internally, your added
header is used instead. If you add a header with no content as in 'Accept:'
(no data on the right side of the colon), the internally used header is
disabled/removed. With this option you can add new headers, replace internal
headers and remove internal headers. To add a header with no content (nothing
to the right side of the colon), use the form 'name;' (note the ending
semicolon).

The headers included in the linked list **must not** be CRLF-terminated,
because libcurl adds CRLF after each header item itself. Failure to comply
with this might result in strange behavior. libcurl passes on the verbatim
strings you give it, without any filter or other safe guards. That includes
white space and control characters.

The first line in an HTTP request (containing the method, usually a GET or
POST) is not a header and cannot be replaced using this option. Only the lines
following the request-line are headers. Adding this method line in this list
of headers only causes your request to send an invalid header. Use
CURLOPT_CUSTOMREQUEST(3) to change the method.

When this option is passed to curl_easy_setopt(3), libcurl does not copy
the entire list so you **must** keep it around until you no longer use this
*handle* for a transfer before you call curl_slist_free_all(3) on
the list.

Pass a NULL to this option to reset back to no custom headers.

The most commonly replaced HTTP headers have "shortcuts" in the options
CURLOPT_COOKIE(3), CURLOPT_USERAGENT(3) and
CURLOPT_REFERER(3). We recommend using those.

There is an alternative option that sets or replaces headers only for requests
that are sent with CONNECT to a proxy: CURLOPT_PROXYHEADER(3). Use
CURLOPT_HEADEROPT(3) to control the behavior.

# SPECIFIC HTTP HEADERS

Setting some specific headers causes libcurl to act differently.

## Host:

The specified hostname is used for cookie matching if the cookie engine is
also enabled for this transfer. If the request is done over HTTP/2 or HTTP/3,
the custom hostname is instead used in the ":authority" header field and
Host: is not sent at all over the wire.

## Transfer-Encoding: chunked

Tells libcurl the upload is to be done using this chunked encoding instead of
providing the Content-Length: field in the request.

# SPECIFIC MIME HEADERS

When used to build a MIME email for IMAP or SMTP, the following document-level
headers can be set to override libcurl-generated values:

## Mime-Version:

Tells the parser at the receiving site how to interpret the MIME framing.
It defaults to "1.0" and should normally not be altered.

## Content-Type:

Indicates the document's global structure type. By default, libcurl sets it
to "multipart/mixed", describing a document made of independent parts. When a
MIME mail is only composed of alternative representations of the same data
(i.e.: HTML and plain text), this header must be set to "multipart/alternative".
In all cases the value must be of the form "multipart/*" to respect the
document structure and may not include the "boundary=" parameter.

Other specific headers that do not have a libcurl default value but are
strongly desired by mail delivery and user agents should also be included.
These are "From:", "To:", "Date:" and "Subject:" among others and their
presence and value is generally checked by anti-spam utilities.

# SECURITY CONCERNS

By default, this option makes libcurl send the given headers in all HTTP
requests done by this handle. You should therefore use this option with
caution if you for example connect to the remote site using a proxy and a
CONNECT request, you should to consider if that proxy is supposed to also get
the headers. They may be private or otherwise sensitive to leak.

Use CURLOPT_HEADEROPT(3) to make the headers only get sent to where you
intend them to get sent.

Custom headers are sent in all requests done by the easy handle, which implies
that if you tell libcurl to follow redirects
(CURLOPT_FOLLOWLOCATION(3)), the same set of custom headers is sent in
the subsequent request. Redirects can of course go to other hosts and thus
those servers get all the contents of your custom headers too.

Starting in 7.58.0, libcurl specifically prevents "Authorization:" headers
from being sent to other hosts than the first used one, unless specifically
permitted with the CURLOPT_UNRESTRICTED_AUTH(3) option.

Starting in 7.64.0, libcurl specifically prevents "Cookie:" headers from being
sent to other hosts than the first used one, unless specifically permitted
with the CURLOPT_UNRESTRICTED_AUTH(3) option.

# DEFAULT

NULL

# PROTOCOLS

HTTP, IMAP and SMTP

# EXAMPLE

~~~c
int main(void)
{
  CURL *curl = curl_easy_init();

  struct curl_slist *list = NULL;

  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    list = curl_slist_append(list, "Shoesize: 10");
    list = curl_slist_append(list, "Accept:");

    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, list);

    curl_easy_perform(curl);

    curl_slist_free_all(list); /* free the list */
  }
}
~~~

# AVAILABILITY

As long as HTTP is enabled. Use in MIME mail added in 7.56.0.

# RETURN VALUE

Returns CURLE_OK if HTTP is supported, and CURLE_UNKNOWN_OPTION if not.