CURLOPT_HTTPHEADER(3) Library Functions Manual CURLOPT_HTTPHEADER(3)
NAME
CURLOPT_HTTPHEADER - set of HTTP headers
SYNOPSIS
#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.
When used with HTTP, this option can add new headers, replace internal
headers and remove internal headers.
The linked list should be a valid list of struct curl_slist entries
properly filled in. Use curl_slist_append(3) to create the list and
curl_slist_free_all(3) to free it again after use.
If you provide a header that is otherwise generated and used by libcurl
internally, your header alternative is used instead. If you provide a
header without content (no data on the right side of the colon) as in
Accept:, the internally used header is removed. To forcibly add a header
without content (nothing after the colon), use the form name; (using a
trailing semicolon).
The headers included in the linked list must not be CRLF-terminated,
since libcurl adds CRLF after each header item itself. Failure to comply
with this might result in strange behavior. libcurl passes on the verba-
tim 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.
Using this option multiple times makes the last set list override the
previous ones. Set it to NULL to disable its use again.
The most commonly replaced HTTP headers have "shortcuts" in the options
CURLOPT_COOKIE(3), CURLOPT_USERAGENT(3) and CURLOPT_REFERER(3). We rec-
ommend 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 encod-
ing instead of providing the Content-Length: field in the re-
quest.
SPECIFIC MIME HEADERS
When used to build a MIME email for IMAP or SMTP, the following docu-
ment-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 al-
tered.
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 alter-
native 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=" para-
meter.
Other specific headers that do not have a libcurl default value but are
strongly desired by mail delivery and user agents should also be in-
cluded. 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_FOLLOWLO-
CATION(3)), the same set of custom headers is sent in the subsequent re-
quest. 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:" head-
ers 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
This functionality affects http, imap and smtp
EXAMPLE
int main(void)
{
CURL *curl = curl_easy_init();
struct curl_slist *list = NULL;
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
/* add this header */
list = curl_slist_append(list, "Shoesize: 10");
/* remove this header */
list = curl_slist_append(list, "Accept:");
/* change this header */
list = curl_slist_append(list, "Host: example.net");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, list);
curl_easy_perform(curl);
curl_slist_free_all(list); /* free the list */
}
}
HISTORY
Use for MIME mail added in 7.56.0.
AVAILABILITY
Added in curl 7.1
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_CUSTOMREQUEST(3), CURLOPT_HEADER(3), CURLOPT_HEADEROPT(3), CUR-
LOPT_MIMEPOST(3), CURLOPT_PROXYHEADER(3), curl_mime_init(3)
libcurl 2026-07-12 CURLOPT_HTTPHEADER(3)
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