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mod_headers - Apache HTTP Server Version 2.4









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Apache HTTP Server Version 2.4



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Apache Module mod_headers

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Description:Customization of HTTP request and response
headers
Status:Extension
ModuleIdentifier:headers_module
SourceFile:mod_headers.c
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 checklisthttpd changelogKnown issuesReport a bugSee also

Comments


Order of Processing

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

    Order of processing is important and is affected both by the
    order in the configuration file and by placement in configuration sections. These
    two directives 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".


Early and Late Processing
    mod_headers can be applied either early or late
    in the request.  The normal mode is late, when Request Headers are
    set immediately before running the content generator and Response
    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 the early 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

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

        Header echo ^TS

      

      
        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
        
      

      
        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.
        
      

      
        Conditionally send MyHeader on the response if and
        only if header MyRequestHeader 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 the
        mod_setenvif module.

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


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

        
          MyHeader: D=3775428 t=991424704447256 mytext
        
      

      
        Enable DAV to work with Apache running HTTP through SSL hardware
        (problem
        description) by replacing https: with
        http: in the Destination header:

        RequestHeader edit Destination ^https: http: early

      

      
        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 the CGI, NO_CACHE and
        NO_STORE environment variables all existed for the
        request):

        Header merge Cache-Control no-cache env=CGI
Header merge Cache-Control no-cache env=NO_CACHE
Header merge Cache-Control no-store env=NO_STORE


        then the response would contain the following header:

        
          Cache-Control: no-cache, no-store
        

        If append was used instead of merge,
        then the response would contain the following header:

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

      
      
        Append a Caching header for responses with a HTTP status code of 200
        Header append Cache-Control s-maxage=600 "expr=%{REQUEST_STATUS} == 200"

      

    


Header Directive

Description:Configure HTTP response headers
Syntax:Header [condition] add|append|echo|edit|edit*|merge|set|setifempty|unset|note
header [[expr=]value [replacement]
[early|env=[!]varname|expr=expression]]

Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Extension
Module:mod_headers
Compatibility:SetIfEmpty available in 2.4.7 and later, expr=value 
available in 2.4.10 and later

    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 optional condition argument determines which internal
    table of responses headers this directive will operate against. Despite the
    name, the default value of onsuccess does not limit 
    an action to responses with a 2xx status code.  Headers set under
    this condition are still used when, for example, a request is successfully
    proxied or generated by CGI, even when they have generated a failing status code.

    When your action is a function of an existing header, you may need to specify
    a condition of always, depending on which internal table the
    original header was set in.  The table that corresponds to always is
    used for locally generated error responses as well as successful responses.  
    Note also that repeating this directive with both conditions makes sense in
    some scenarios because always is not a superset of
    onsuccess 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 to
            always is used in the ultimate response.
        You're modifying or removing a header generated by a CGI script,
            in which case the CGI scripts are in the table corresponding to
            always 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 default
            onsuccess condition.
    

    Separately from the condition 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 a condition 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 general set,
    append or merge 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 a
    regular expression.
    value must be omitted.

    edit
    edit*
    If this response header exists, its value is transformed according
    to a regular expression
    search-and-replace.  The value argument is a regular expression, and the replacement
    is a replacement string, which may contain backreferences or format specifiers.
    The edit form will match and replace exactly once
    in a header value, whereas the edit* form will replace
    every 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. The value 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 when setifempty is evaluated.
    It is safer to use set 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 response header is copied into an
    internal note whose name is given by value.  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 a header name, which
    can include the final colon, but it is not required. Case is
    ignored for set, append, merge,
    add, unset and edit.
    The header name for echo
    is case sensitive and may be a regular
    expression.

    For set, append, merge and
    add a value is specified as the next argument.
    If value
    contains spaces, it should be surrounded by double quotes.
    value may be a character string, a string containing 
    mod_headers specific format specifiers (and character 
    literals), or an ap_expr expression prefixed
    with expr=
    
     The following format specifiers are supported in value:

    FormatDescription
%%
        The percent sign
%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=.
        The value is measured in microseconds.
%l
        The current load averages of the actual server itself. It is
        designed to expose the values obtained by getloadavg()
        and this represents the current load average, the 5 minute average, and
        the 15 minute average. The value is preceded by l= with each
        average separated by /.
        Available in 2.4.4 and later.
        
%i
        The current idle percentage of httpd (0 to 100) based on available
        processes and threads. The value is preceded by i=.
        Available in 2.4.4 and later.
        
%b
        The current busy percentage of httpd (0 to 100) based on available
        processes and threads. The value is preceded by b=.
        Available in 2.4.4 and later.
        
%{VARNAME}e
        The contents of the environment
        variable VARNAME.
%{VARNAME}s
        The contents of the SSL environment
        variable VARNAME, if mod_ssl is enabled.


    Note
      The %s format specifier is only available in
      Apache 2.1 and later; it can be used instead of %e
      to avoid the overhead of enabling SSLOptions
      +StdEnvVars.  If SSLOptions +StdEnvVars must
      be enabled anyway for some other reason, %e will be
      more efficient than %s.
    

    Note on expression values
     When the value parameter uses the ap_expr
    parser, some expression syntax will differ from examples that evaluate
    boolean expressions such as <If>:
    
      The starting point of the grammar is 'string' rather than 'expr'.
      Function calls use the %{funcname:arg} syntax rather than 
          funcname(arg).
      Multi-argument functions are not currently accessible from this
          starting point
      Quote the entire parameter, such as 
          Header set foo-checksum "expr=%{md5:foo}"

       
                   
    
    

    For edit there is both a value argument
    which is a regular expression,
    and an additional replacement string. As of version 2.4.7
    the replacement string may also contain format specifiers.

    The Header directive may be followed by
    an additional argument, which may be any of:
    
    early
    Specifies early processing.
    env=[!]varname
    The directive is applied if and only if the environment variable varname exists.
        A ! in front of varname reverses the test,
        so the directive applies only if varname is unset.
    expr=expression
    The directive is applied if and only if expression
        evaluates to true. Details of expression syntax and evaluation are
        documented in the ap_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 in early mode, the
    Header 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.




RequestHeader Directive

Description:Configure HTTP request headers
Syntax:RequestHeader add|append|edit|edit*|merge|set|setifempty|unset
header [[expr=]value [replacement]
[early|env=[!]varname|expr=expression]]

Context:server config, virtual host, directory, .htaccess
Override:FileInfo
Status:Extension
Module:mod_headers
Compatibility:SetIfEmpty available in 2.4.7 and later, expr=value 
available in 2.4.10 and later

    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 general set,
    append or merge 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 a regular expression
    search-and-replace.  The value argument is a regular expression, and the replacement
    is a replacement string, which may contain backreferences or format specifiers.
    The edit form will match and replace exactly once
    in a header value, whereas the edit* form will replace
    every 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. For set, append, merge and
    add a value is given as the third argument. If a
    value contains spaces, it should be surrounded by double
    quotes. For unset, no value 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 the Header,
    please have a look there for details.  For edit both
    a value and a replacement are required, and are
    a regular expression and a
    replacement string respectively.

    The RequestHeader directive may be followed by
    an additional argument, which may be any of:
    
    early
    Specifies early processing.
    env=[!]varname
    The directive is applied if and only if the environment variable varname exists.
        A ! in front of varname reverses the test,
        so the directive applies only if varname is unset.
    expr=expression
    The directive is applied if and only if expression
        evaluates to true. Details of expression syntax and evaluation are
        documented in the ap_expr documentation.
    

    Except in early mode, 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.




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