Apache module mod_headers

### Apache HTTP Server Version 1.3

# Module mod_headers

This module provides for the customization of HTTP response headers.

Status: Extension
Source File: mod_headers.c
Module Identifier: headers_module
Compatibility: Available in Apache 1.2 and later.

## Summary

This module provides a directive to control the sending of HTTP headers. Headers can be merged, replaced or removed.

## Directives

- Header

## Header directive

Syntax: Header set|append|add header value
Syntax: Header unset header
Context: server config, virtual host, access.conf, .htaccess
Override: FileInfo
Status: Extension
Module: mod_header

This directive can replace, merge or remove HTTP response headers. The action it performs is determined by the first argument. This can be one of the following values:

- set
  The response header is set, replacing any previous header with this name
- append
  The response header is appended to any existing header of the same name. When a new value is merged onto an existing header it is separated from the existing header with a comma. This is the HTTP standard way of giving a header multiple values.
- add
  The response header is added to the existing set of headers, even if this header already exists. This can result in two (or more) headers having the same name. This can lead to unforeseen consequences, and in general "append" should be used instead.
- unset
  The response header of this name is removed, if it exists. If there are multiple headers of the same name, all will be removed.

This argument is followed by a header name, which can include the final colon, but it is not required. Case is ignored. For add, append and set a value is given as the third argument. If this value contains spaces, it should be surrounded by double quotes. For unset, no value should be given.

### Order of Processing

The Header directive can occur almost anywhere within the server configuration. It is valid in the main server config and virtual host sections, inside <Directory>, <Location> and <Files> sections, and within .htaccess files.

The Header directives are processed in the following order:

1. main server
2. virtual host
3. <Directory> sections and .htaccess
4. <Location>
5. <Files>

Order is important. These two headers have a different effect if reversed:

    Header append Author "John P. Doe"
    Header unset Author

This way round, the Author header is not set. If reversed, the Author header is set to "John P. Doe".

The Header directives are processed just before the response is sent by its handler. These means that some headers that are added just before the response is sent cannot be unset or overridden. This includes headers such as "Date" and "Server".

### Apache HTTP Server Version 1.3