Apache Module mod_headers

Header Directive

Header [condition] add|append|echo|edit|edit*|merge|set|setifempty|unset|note header [[expr=]value [replacement] [early|env=[!]varname|expr=expression]]

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: onsuccess (default, can be omitted) or always. 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 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 or by mod_proxy_fcgi, in which case the CGI scripts' headers 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.

This difference between onsuccess and always 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 with mod_proxy_fcgi and your backend PHP scripts adds the X-Foo: bar header to each HTTP response. As described above, mod_proxy_fcgi uses the always 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 table
Header 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 default
Header onsuccess unset X-Foo
Header always set X-Foo "baz"

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:

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

RequestHeader add|append|edit|edit*|merge|set|setifempty|unset header [[expr=]value [replacement] [early|env=[!]varname|expr=expression]]

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.