curl/docs/cmdline-opts/header.md

---

c: Copyright (C) Daniel Stenberg, daniel@haxx.se, et al. SPDX-License-Identifier: curl Long: header Short: H Arg: Help: Pass custom header(s) to server Protocols: HTTP IMAP SMTP Category: http imap smtp Added: 5.0 Multi: append See-also:

• user-agent
• referer Example:
• -H "X-First-Name: Joe" $URL
• -H "User-Agent: yes-please/2000" $URL
• -H "Host:" $URL
• -H @headers.txt $URL ---

--header

Extra header to include in information sent. When used within an HTTP request, it is added to the regular request headers.

For an IMAP or SMTP MIME uploaded mail built with --form options, it is prepended to the resulting MIME document, effectively including it at the mail global level. It does not affect raw uploaded mails (Added in 7.56.0).

You may specify any number of extra headers. Note that if you should add a custom header that has the same name as one of the internal ones curl would use, your externally set header is used instead of the internal one. This allows you to make even trickier stuff than curl would normally do. You should not replace internally set headers without knowing perfectly well what you are doing. Remove an internal header by giving a replacement without content on the right side of the colon, as in: -H Host:. If you send the custom header with no-value then its header must be terminated with a semicolon, such as -H X-Custom-Header; to send X-Custom-Header:.

curl makes sure that each header you add/replace is sent with the proper end-of-line marker, you should thus not add that as a part of the header content: do not add newlines or carriage returns, they only mess things up for you. curl passes on the verbatim string you give it without any filter or other safe guards. That includes white space and control characters.

This option can take an argument in @filename style, which then adds a header for each line in the input file. Using @- makes curl read the header file from stdin. Added in 7.55.0.

Please note that most anti-spam utilities check the presence and value of several MIME mail headers: these are From:, To:, Date: and Subject: among others and should be added with this option.

You need --proxy-header to send custom headers intended for an HTTP proxy. Added in 7.37.0.

Passing on a "Transfer-Encoding: chunked" header when doing an HTTP request with a request body, makes curl send the data using chunked encoding.

WARNING: headers set with this option are set in all HTTP requests - even after redirects are followed, like when told with --location. This can lead to the header being sent to other hosts than the original host, so sensitive headers should be used with caution combined with following redirects.