Module mod_headers

The Header directive can replace, merge or remove HTTP response headers. The action performed by this module is determined by the action parameter. This parameter is followed by a header name, which can include the final colon, but it is not required. Case is also ignored. For add, append, and set, a value is given as the next parameter. If this value contains spaces, it should be surrounded by double quotes. For "unset", no value should be given.

Note:The [condition] parameter was introduced with Apache 2.0.52. It did not exist prior to this version on iSeries™.

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 contexts, inside <Directory>, <Location>, and <Files> contexts 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. For example:

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

This way 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. This means that some headers, that are added just before the response is sent, cannot be changed or overridden. This includes headers such as Date and Server.

Parameter One: condition

• The condition parameter is an optional parameter which can be one of the following values:

  Condition	Description
  onsuccess	The Header directive will only effect responses with a status code of 2xx.
  always	The Header directive will effect all responses, including 2xx.

Parameter Two: action

• The action parameter can be one of the following values:

  Action	Description
  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 consequence, 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.
  echo	Request headers with this name are echoed back in the response headers. Header may be a regular expression. Echo without any parameters echoes back all the request headers in the response.

Parameter Three: header

• The HTTP Response header parameter to be set, appended, or unset with this directive. There is no validity checking of the header, which allows the use of experimental headers.

Parameter Four: value

• The value parameter may be a character string, a string containing format specifiers or a combination of both and specifies the value of the header to be set. It is only valid for set, add and append. There is no validity checking of the value specified, which allows the use of experimental headers values. All characters and escaped characters, such as '\n', are allowed in the value string. If value contains spaces, it should be surrounded by double quotes. The following format specifiers are supported in value:

  %t

  The time the request was received in Universal Coordinated Time since the epoch (Jan. 1, 1970) measured in microseconds. The value is preceded by "t=".

  %D

  The time from when the request was received to the time the headers are sent on the wire. This is a measure of the duration of the request. The value is preceded by "D=".

  %{ENVVAR}e

  The contents of the environment variable ENVVAR.

  Other format strings, such as %s, will receive an error and the server will not start.

Parameter Five: env=[!]environment-variable

• Optional: The envvar parameter specified in the env=[!]envvar clause can be any character or numeric value. When the Header directive is used with the add, append, or set argument, a fourth argument may be used to specify conditions under which the action will be taken. If the environment variable specified in the env=... argument exists (or if the environment variable does not exist and env=!... is specified) then the action specified by the Header directive will take effect. Otherwise, the directive will have no effect on the request. See the environment variable directives for details on naming environment variables.

The Header directives are processed just before the response is sent to the network. This means that it is possible to set or override most headers, except for those headers added by the header filter.

The RequestHeader directive can replace, merge or remove HTTP request headers. The header is modified just before the content handler is run, allowing incoming headers to be modified. The action it performs is determined by the first argument.

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 RequestHeader (and Header) directives 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 whithin .htaccess files.

The RequestHeader 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:

RequestHeader append MirrorID "mirror 12"
RequestHeader unset MirrorID

This way round, the MirrorID header is not set. If reversed, the MirrorID header is set to "mirror 12".

The RequestHeader directive is processed just before the request is run by its handler in the fixup phase. This should allow headers generated by the browser, or by Apache input filters to be overridden or modified. For example,

RequestHeader append MirrorID "mirror 12"

Parameter One: action

• The action parameter can be one of the following values:

  Action	Description
  set	The request header is set, replacing any previous header with this name.
  append	The request header is appended to any existing header of the same name. When a new value is merged into 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 request 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 request header of this name is removed, if it exists. If there are multiple headers of the same name, all will be removed.

Parameter Two: header

• The HTTP Response header parameter to be set, appended, or unset with this directive. There is no validity checking of the header, which allows the use of experimental headers.

Parameter Three: value

• The value parameter can have any valid character string as a value and specifies the value of the header to be set. It is only valid for set, add and append. There is no validity checking of the value specified, which allows the use of experimental headers values. All characters and escaped characters are allowed in this string. If this value contains spaces, it should be surrounded by double quotes.