Apache Module mod_headers

Summary

This module provides directives to control and modify HTTP request and response headers. Headers can be merged, replaced or removed.

Topics

• Order of Processing
• Early and Late Processing
• Examples

Directives

• Header
• RequestHeader

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Comments

Order of Processing

The directives provided bymod_headers can occur almost anywhere within the server configuration, and can be limited in scope by enclosing them inconfiguration sections.

Order of processing is important and is affected both by the order in the configuration file and by placement inconfiguration sections. These two directives have a different effect if reversed:

RequestHeader append MirrorID "mirror 12"RequestHeader unset MirrorID

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

Early and Late Processing

mod_headers can be applied either early or late in the request. The normal mode is late, whenRequest Headers are set immediately before running the content generator andResponse Headers just as the response is sent down the wire. Always use Late mode in an operational server.

Early mode is designed as a test/debugging aid for developers. Directives defined using theearly keyword are set right at the beginning of processing the request. This means they can be used to simulate different requests and set up test cases, but it also means that headers may be changed at any time by other modules before generating a Response.

Because early directives are processed before the request path's configuration is traversed, early headers can only be set in a main server or virtual host context. Early directives cannot depend on a request path, so they will fail in contexts such as<Directory> or<Location>.

Examples

1. Copy all request headers that begin with "TS" to the response headers:

   Header echo ^TS

2. Add a header,MyHeader, to the response including a timestamp for when the request was received and how long it took to begin serving the request. This header can be used by the client to intuit load on the server or in isolating bottlenecks between the client and the server.

   Header set MyHeader "%D %t"

   results in this header being added to the response:

   MyHeader: D=3775428 t=991424704447256

3. Say hello to Joe

   Header set MyHeader "Hello Joe. It took %D microseconds for Apache to serve this request."

   results in this header being added to the response:

   MyHeader: Hello Joe. It took D=3775428 microseconds for Apache to serve this request.

4. Conditionally sendMyHeader on the response if and only if headerMyRequestHeader is present on the request. This is useful for constructing headers in response to some client stimulus. Note that this example requires the services of themod_setenvif module.

   SetEnvIf MyRequestHeader myvalue HAVE_MyRequestHeaderHeader set MyHeader "%D %t mytext" env=HAVE_MyRequestHeader

   If the headerMyRequestHeader: myvalue is present on the HTTP request, the response will contain the following header:

   MyHeader: D=3775428 t=991424704447256 mytext

5. Enable DAV to work with Apache running HTTP through SSL hardware (problem description) by replacinghttps: withhttp: in theDestination header:

   RequestHeader edit Destination ^https: http: early

6. Set the same header value under multiple nonexclusive conditions, but do not duplicate the value in the final header. If all of the following conditions applied to a request (i.e., if theCGI,NO_CACHE andNO_STORE environment variables all existed for the request):

   Header merge Cache-Control no-cache env=CGIHeader merge Cache-Control no-cache env=NO_CACHEHeader merge Cache-Control no-store env=NO_STORE

   then the response would contain the following header:

   Cache-Control: no-cache, no-store

   Ifappend was used instead ofmerge, then the response would contain the following header:

   Cache-Control: no-cache, no-cache, no-store

7. Set a test cookie if and only if the client didn't send us a cookie

   Header set Set-Cookie testcookie "expr=-z %{req:Cookie}"

8. Append a Caching header for responses with a HTTP status code of 200

   Header append Cache-Control s-maxage=600 "expr=%{REQUEST_STATUS} == 200"

HeaderDirective

This directive can replace, merge or remove HTTP response headers. The header is modified just after the content handler and output filters are run, allowing outgoing headers to be modified.

The optionalcondition argument determines which internal table of responses headers this directive will operate against:onsuccess (default, can be omitted) oralways. The difference between the two lists is that the headers contained in the latter are added to the response even on error, and persisted across internal redirects (for example, ErrorDocument handlers). Note also that repeating this directive with both conditions makes sense in some scenarios becausealways is not a superset ofonsuccess with respect to existing headers:

• You're adding a header to a locally generated non-success (non-2xx) response, such as a redirect, in which case only the table corresponding toalways is used in the ultimate response.
• You're modifying or removing a header generated by a CGI script or bymod_proxy_fcgi, in which case the CGI scripts' headers are in the table corresponding toalways and not in the default table.
• You're modifying or removing a header generated by some piece of the server but that header is not being found by the defaultonsuccess condition.

This difference betweenonsuccess andalways is a feature that resulted as a consequence of how httpd internally stores headers for a HTTP response, since it does not offer any "normalized" single list of headers. The main problem that can arise if the following concept is not kept in mind while writing the configuration is that some HTTP responses might end up with the same header duplicated (confusing users or sometimes even HTTP clients). For example, suppose that you have a simple PHP proxy setup withmod_proxy_fcgi and your backend PHP scripts adds theX-Foo: bar header to each HTTP response. As described above,mod_proxy_fcgi uses thealways table to store headers, so a configuration like the following ends up in the wrong result, namely having the header duplicated with both values:

# X-Foo's value is set in the 'onsuccess' headers tableHeader set X-Foo: baz

To circumvent this limitation, there are some known configuration patterns that can help, like the following:

# 'onsuccess' can be omitted since it is the defaultHeader onsuccess unset X-FooHeader always set X-Foo "baz"

Separately from thecondition parameter described above, you can limit an action based on HTTP status codes for e.g. proxied or CGI requests. See the example that uses %{REQUEST_STATUS} in the section above.

The action it performs is determined by the first argument (second argument if acondition is specified). This can be one of the following 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 generalset,append ormerge should be used instead.

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.

echo

Request headers with this name are echoed back in the response headers.header may be aregular expression.value must be omitted.

edit

edit*

If this response header exists, its value is transformed according to aregular expression search-and-replace. Thevalue argument is aregular expression, and thereplacement is a replacement string, which may contain backreferences or format specifiers. Theedit form will match and replace exactly once in a header value, whereas theedit* form will replaceevery instance of the search pattern if it appears more than once.

merge

The response header is appended to any existing header of the same name, unless the value to be appended already appears in the header's comma-delimited list of values. 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. Values are compared in a case sensitive manner, and after all format specifiers have been processed. Values in double quotes are considered different from otherwise identical unquoted values.

set

The response header is set, replacing any previous header with this name. Thevalue may be a format string.

setifempty

The request header is set, but only if there is no previous header with this name.

The Content-Type header is a special use case since there might be the chance that its value have been determined but the header is not part of the response whensetifempty is evaluated. It is safer to useset for this use case like in the following example:

Header set Content-Type "text/plain" "expr=-z %{CONTENT_TYPE}"

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.value must be omitted.

note

The value of the named responseheader is copied into an internal note whose name is given byvalue. This is useful if a header sent by a CGI or proxied resource is configured to be unset but should also be logged.
Available in 2.4.7 and later.

This argument is followed by aheader name, which can include the final colon, but it is not required. Case is ignored forset,append,merge,add,unset andedit. Theheader name forecho is case sensitive and may be aregular expression.

Forset,append,merge andadd avalue is specified as the next argument. Ifvalue contains spaces, it should be surrounded by double quotes.value may be a character string, a string containingmod_headers specific format specifiers (and character literals), or anap_expr expression prefixed withexpr=

The following format specifiers are supported invalue:

Foredit there is both avalue argument which is aregular expression, and an additionalreplacement string. As of version 2.4.7 the replacement string may also contain format specifiers.

TheHeader directive may be followed by an additional argument, which may be any of:

early

Specifiesearly processing.

env=[!]varname

The directive is applied if and only if theenvironment variablevarname exists. A! in front ofvarname reverses the test, so the directive applies only ifvarname is unset.

expr=expression

The directive is applied if and only ifexpression evaluates to true. Details of expression syntax and evaluation are documented in theap_expr documentation.

# This delays the evaluation of the condition clause compared to <If>Header always set CustomHeader my-value "expr=%{REQUEST_URI} =~ m#^/special_path.php$#"

Except inearly mode, theHeader directives are processed just before the response is sent to the network. This means that it is possible to set and/or override most headers, except for some headers added by the HTTP header filter. Prior to 2.2.12, it was not possible to change the Content-Type header with this directive.

RequestHeaderDirective

This directive can replace, merge, change 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 can be one of the following 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 generalset,append ormerge should be used instead.

append

The request 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.

edit

edit*

If this request header exists, its value is transformed according to aregular expression search-and-replace. Thevalue argument is aregular expression, and thereplacement is a replacement string, which may contain backreferences or format specifiers. Theedit form will match and replace exactly once in a header value, whereas theedit* form will replaceevery instance of the search pattern if it appears more than once.

merge

The request header is appended to any existing header of the same name, unless the value to be appended already appears in the existing header's comma-delimited list of values. 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. Values are compared in a case sensitive manner, and after all format specifiers have been processed. Values in double quotes are considered different from otherwise identical unquoted values.

set

The request header is set, replacing any previous header with this name

setifempty

The request header is set, but only if there is no previous header with this name.
Available in 2.4.7 and later.

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.value must be omitted.

This argument is followed by a header name, which can include the final colon, but it is not required. Case is ignored. Forset,append,merge andadd avalue is given as the third argument. If avalue contains spaces, it should be surrounded by double quotes. Forunset, novalue should be given.value may be a character string, a string containing format specifiers or a combination of both. The supported format specifiers are the same as for theHeader, please have a look there for details. Foredit both avalue and areplacement are required, and are aregular expression and a replacement string respectively.

TheRequestHeader directive may be followed by an additional argument, which may be any of:

early

Specifiesearly processing.

env=[!]varname

The directive is applied if and only if theenvironment variablevarname exists. A! in front ofvarname reverses the test, so the directive applies only ifvarname is unset.

expr=expression

The directive is applied if and only ifexpression evaluates to true. Details of expression syntax and evaluation are documented in theap_expr documentation.

Except inearly mode, theRequestHeader 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.