Apache HTTP Server Version 2.5
Historically, there are several syntax variants for expressions used to express a condition in the different modules of the Apache HTTP Server. There is some ongoing effort to only use a single variant, called ap_expr, for all configuration directives. This document describes the ap_expr expression parser.
The ap_expr expression is intended to replace most other
expression variants in HTTPD. For example, the deprecated SSLRequire
expressions can be replaced
by Require expr.
<If>
<ElseIf>
<Else>
ErrorDocument
Alias
ScriptAlias
Redirect
AuthBasicFake
AuthFormLoginRequiredLocation
AuthFormLoginSuccessLocation
AuthFormLogoutLocation
AuthName
AuthType
RewriteCond
SetEnvIfExpr
Header
RequestHeader
FilterProvider
CryptoKey
CryptoIV
SSLRequire
LogMessage
mod_include
Backus-Naur
Form (BNF) is a notation technique for context-free grammars,
often used to describe the syntax of languages used in computing.
In most cases, expressions are used to express boolean values.
For these, the starting point in the BNF is cond
.
Directives like ErrorDocument
,
Require
,
AuthName
,
Redirect
,
Header
,
CryptoKey
or
LogMessage
accept expressions
that evaluate to a string value. For those, the starting point in
the BNF is string
.
expr ::= cond | string string ::= substring | string substring cond ::= "true" | "false" | "!" cond | cond "&&" cond | cond "||" cond | comp | "(" cond ")" comp ::= stringcomp | integercomp | unaryop word | word binaryop word | word "in" listfunc | word "=~" regex | word "!~" regex | word "in" "{" list "}" stringcomp ::= word "==" word | word "!=" word | word "<" word | word "<=" word | word ">" word | word ">=" word integercomp ::= word "-eq" word | word "eq" word | word "-ne" word | word "ne" word | word "-lt" word | word "lt" word | word "-le" word | word "le" word | word "-gt" word | word "gt" word | word "-ge" word | word "ge" word word ::= digits | "'" string "'" | '"' string '"' | word "." word | variable | sub | join | function | "(" word ")" list ::= split | listfunc | "{" words "}" | "(" list ")" substring ::= cstring | variable variable ::= "%{" varname "}" | "%{" funcname ":" funcargs "}" | "%{:" word ":}" | "%{:" cond ":}" | rebackref sub ::= "sub" ["("] regsub "," word [")"] join ::= "join" ["("] list [")"] | "join" ["("] list "," word [")"] split ::= "split" ["("] regany "," list [")"] | "split" ["("] regany "," word [")"] function ::= funcname "(" words ")" listfunc ::= listfuncname "(" words ")" words ::= word | word "," list regex ::= "/" regpattern "/" [regflags] | "m" regsep regpattern regsep [regflags] regsub ::= "s" regsep regpattern regsep string regsep [regflags] regany ::= regex | regsub regsep ::= "/" | "#" | "$" | "%" | "^" | "|" | "?" | "!" | "'" | '"' | "," | ";" | ":" | "." | "_" | "-" regflags ::= 1*("i" | "s" | "m" | "g") regpattern ::= cstring ; except enclosing regsep rebackref ::= "$" DIGIT digits ::= 1*(DIGIT) cstring ::= 0*(TEXT) TEXT ::= <any OCTET except CTLs> DIGIT ::= <any US-ASCII digit "0".."9">
The expression parser provides a number of variables of the form
%{HTTP_HOST}
. Note that the value of a variable may depend
on the phase of the request processing in which it is evaluated. For
example, an expression used in an <If >
directive is evaluated before authentication is done. Therefore,
%{REMOTE_USER}
will not be set in this case.
The following variables provide the values of the named HTTP request
headers. The values of other headers can be obtained with the
req
function. Using these
variables may cause the header name to be added to the Vary
header of the HTTP response, except where otherwise noted for the
directive accepting the expression. The req_novary
function may be used to circumvent this
behavior.
Name |
---|
HTTP_ACCEPT |
HTTP_COOKIE |
HTTP_FORWARDED |
HTTP_HOST |
HTTP_PROXY_CONNECTION |
HTTP_REFERER |
HTTP_USER_AGENT |
Other request related variables
Name | Description |
---|---|
REQUEST_METHOD |
The HTTP method of the incoming request (e.g.
GET ) |
REQUEST_SCHEME |
The scheme part of the request's URI |
REQUEST_URI |
The path part of the request's URI |
DOCUMENT_URI |
Same as REQUEST_URI |
REQUEST_FILENAME |
The full local filesystem path to the file or script matching the
request, if this has already been determined by the server at the
time REQUEST_FILENAME is referenced. Otherwise, such
as when used in virtual host context, the same value as
REQUEST_URI |
SCRIPT_FILENAME |
Same as REQUEST_FILENAME |
LAST_MODIFIED |
The date and time of last modification of the file in the format
20101231235959 , if this has already been determined by
the server at the time LAST_MODIFIED is referenced.
|
SCRIPT_USER |
The user name of the owner of the script. |
SCRIPT_GROUP |
The group name of the group of the script. |
PATH_INFO |
The trailing path name information, see
AcceptPathInfo |
QUERY_STRING |
The query string of the current request |
IS_SUBREQ |
"true " if the current request is a subrequest,
"false " otherwise |
THE_REQUEST |
The complete request line (e.g.,
"GET /index.html HTTP/1.1 ") |
REMOTE_ADDR |
The IP address of the remote host |
REMOTE_PORT |
The port of the remote host (2.4.26 and later) |
REMOTE_HOST |
The host name of the remote host |
REMOTE_USER |
The name of the authenticated user, if any (not available during <If> ) |
REMOTE_IDENT |
The user name set by mod_ident |
SERVER_NAME |
The ServerName of
the current vhost |
SERVER_PORT |
The server port of the current vhost, see
ServerName |
SERVER_ADMIN |
The ServerAdmin of
the current vhost |
SERVER_PROTOCOL |
The protocol used by the request (e.g. HTTP/1.1). In some types of
internal subrequests, this variable has the value
INCLUDED . |
SERVER_PROTOCOL_VERSION |
A number that encodes the HTTP version of the request:
1000 * major + minor . For example, 1001
corresponds to HTTP/1.1 and 9 corresponds
to HTTP/0.9 |
SERVER_PROTOCOL_VERSION_MAJOR |
The major version part of the HTTP version of the request,
e.g. 1 for HTTP/1.0 |
SERVER_PROTOCOL_VERSION_MINOR |
The minor version part of the HTTP version of the request,
e.g. 0 for HTTP/1.0 |
DOCUMENT_ROOT |
The DocumentRoot of
the current vhost |
AUTH_TYPE |
The configured AuthType (e.g.
"basic ") |
CONTENT_TYPE |
The content type of the response (not available during <If> ) |
HANDLER |
The name of the handler creating the response |
HTTP2 |
"on " if the request uses http/2,
"off " otherwise |
HTTPS |
"on " if the request uses https,
"off " otherwise |
IPV6 |
"on " if the connection uses IPv6,
"off " otherwise |
REQUEST_STATUS |
The HTTP error status of the request (not available during <If> ) |
REQUEST_LOG_ID |
The error log id of the request (see
ErrorLogFormat ) |
CONN_LOG_ID |
The error log id of the connection (see
ErrorLogFormat ) |
CONN_REMOTE_ADDR |
The peer IP address of the connection (see the
mod_remoteip module) |
CONTEXT_PREFIX |
|
CONTEXT_DOCUMENT_ROOT |
Misc variables
Name | Description |
---|---|
TIME_YEAR |
The current year (e.g. 2010 ) |
TIME_MON |
The current month (01 , ..., 12 ) |
TIME_DAY |
The current day of the month (01 , ...) |
TIME_HOUR |
The hour part of the current time
(00 , ..., 23 ) |
TIME_MIN |
The minute part of the current time |
TIME_SEC |
The second part of the current time |
TIME_WDAY |
The day of the week (starting with 0
for Sunday) |
TIME |
The date and time in the format
20101231235959 |
SERVER_SOFTWARE |
The server version string |
API_VERSION |
The date of the API version (module magic number) |
Some modules register additional variables, see e.g.
mod_ssl
.
Any variable can be embedded in a string, both in quoted strings from boolean expressions but also in string expressions, resulting in the concatenation of the constant and dynamic parts as expected.
There exists another form of variables (temporaries) expressed like
%{:word:}
and which allow embedding of the more
powerful word syntax (and constructs) in both type of expressions,
without colliding with the constant part of such strings. They are mainly
useful in string expressions though, since the word is directly
available in boolean expressions already. By using this form of variables,
one can evaluate regexes, substitutions, join and/or split strings and
lists in the scope of string expressions, hence construct complex strings
dynamically.
With the exception of some built-in comparison operators, binary
operators have the form "-[a-zA-Z][a-zA-Z0-9_]+
", i.e. a
minus and at least two characters. The name is not case sensitive.
Modules may register additional binary operators.
Name | Alternative | Description |
---|---|---|
== |
= |
String equality |
!= |
String inequality | |
< |
String less than | |
<= |
String less than or equal | |
> |
String greater than | |
>= |
String greater than or equal | |
=~ |
String matches the regular expression | |
!~ |
String does not match the regular expression | |
-eq |
eq |
Integer equality |
-ne |
ne |
Integer inequality |
-lt |
lt |
Integer less than |
-le |
le |
Integer less than or equal |
-gt |
gt |
Integer greater than |
-ge |
ge |
Integer greater than or equal |
Name | Description |
---|---|
-ipmatch |
IP address matches address/netmask |
-strmatch |
left string matches pattern given by right string (containing wildcards *, ?, []) |
-strcmatch |
same as -strmatch , but case insensitive |
-fnmatch |
same as -strmatch , but slashes are not matched by
wildcards |
Unary operators take one argument and have the form
"-[a-zA-Z]
", i.e. a minus and one character.
The name is case sensitive.
Modules may register additional unary operators.
Name | Description | Restricted |
---|---|---|
-d |
The argument is treated as a filename. True if the file exists and is a directory | yes |
-e |
The argument is treated as a filename. True if the file (or dir or special) exists | yes |
-f |
The argument is treated as a filename. True if the file exists and is regular file | yes |
-s |
The argument is treated as a filename. True if the file exists and is not empty | yes |
-L |
The argument is treated as a filename. True if the file exists and is symlink | yes |
-h |
The argument is treated as a filename.
True if the file exists and is symlink
(same as -L ) | yes |
-F |
True if string is a valid file, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact your server's performance! | |
-U |
True if string is a valid URL, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact your server's performance! | |
-A |
Alias for -U | |
-n |
True if string is not empty | |
-z |
True if string is empty | |
-T |
False if string is empty, "0 ", "off ",
"false ", or "no " (case insensitive).
True otherwise. | |
-R |
Same as "%{REMOTE_ADDR} -ipmatch ... ", but more
efficient
|
The operators marked as "restricted" are not available in some modules
like mod_include
.
Normal string-valued functions take one string as argument and return a string. Functions names are not case sensitive. Modules may register additional functions.
Name | Description | Special notes |
---|---|---|
req , http |
Get HTTP request header; header names may be added to the Vary header, see below | |
req_novary |
Same as req , but header names will not be added to the
Vary header | |
resp |
Get HTTP response header (most response headers will not yet be set
during <If> ) | |
reqenv |
Lookup request environment variable (as a shortcut,
v can also be used to access variables).
|
ordering |
osenv |
Lookup operating system environment variable | |
note |
Lookup request note | ordering |
env |
Return first match of note , reqenv ,
osenv | ordering |
tolower |
Convert string to lower case | |
toupper |
Convert string to upper case | |
escape |
Escape special characters in %hex encoding | |
unescape |
Unescape %hex encoded string, leaving encoded slashes alone; return empty string if %00 is found | |
base64 |
Encode the string using base64 encoding | |
unbase64 |
Decode base64 encoded string, return truncated string if 0x00 is found | |
md5 |
Hash the string using MD5, then encode the hash with hexadecimal encoding | |
sha1 |
Hash the string using SHA1, then encode the hash with hexadecimal encoding | |
file |
Read contents from a file (including line endings, when present) | restricted |
filemod |
Return last modification time of a file (or 0 if file does not exist or is not regular file) | restricted |
filesize |
Return size of a file (or 0 if file does not exist or is not regular file) | restricted |
ldap |
Escape characters as required by LDAP distinguished name escaping
(RFC4514) and LDAP filter escaping (RFC4515). (Available in httpd 2.4.53 and later) | |
replace |
replace(string, "from", "to") replaces all occurrences of "from" in the string with "to". The first parameter must not be a literal string. |
The functions marked as "restricted" in the final column are not
available in some modules like mod_include
.
The functions marked as "ordering" in the final column require some
consideration for the ordering of different components of the server,
especially when the function is used within the
<If
> directive which is
evaluated relatively early.
If
> condition, it's important
to consider how extremely early in request processing that this
resolution occurs. As a guideline, any directive defined outside of virtual host
context (directory, location, htaccess) is not likely to have yet had a
chance to execute. SetEnvIf
in virtual host scope is one directive that runs prior to this resolution
reqenv
is used outside of <If
>, the resolution will generally occur later, but the
exact timing depends on the directive the expression has been used within.
When the functions req
or http
are used,
the header name will automatically be added to the Vary header of the
HTTP response, except where otherwise noted for the directive accepting
the expression. The req_novary
function can be used to
prevent names from being added to the Vary header.
In addition to string-valued functions, there are also
list-valued functions which take one string as argument and return a
list, i.e. a list of strings. The list can be used with the
special -in
operator. Functions names are not case
sensitive. Modules may register additional functions.
There are no built-in list-valued functions. mod_ssl
provides PeerExtList
. See the description of
SSLRequire
for details
(but PeerExtList
is also usable outside
of SSLRequire
).
Name | Alternative | Description |
---|---|---|
-in |
in |
string contained in list |
/regexp/ |
m#regexp# |
Regular expression (the second form allows different delimiters than /) |
/regexp/i |
m#regexp#i |
Case insensitive regular expression |
$0 ... $9 |
Regular expression backreferences |
The strings $0
... $9
allow to reference
the capture groups from a previously executed, successfully
matching regular expressions. They can normally only be used in the
same expression as the matching regex, but some modules allow special
uses.
The ap_expr syntax is mostly a superset of the syntax of the
deprecated SSLRequire
directive.
The differences are described in SSLRequire
's documentation.
The req_novary
function
is available for versions 2.4.4 and later.
The SERVER_PROTOCOL_VERSION
,
SERVER_PROTOCOL_VERSION_MAJOR
and
SERVER_PROTOCOL_VERSION_MINOR
variables
are available for versions 2.5.0 and later.
The following examples show how expressions might be used to evaluate requests:
# Compare the host name to example.com and redirect to www.example.com if it matches <If "%{HTTP_HOST} == 'example.com'"> Redirect permanent "/" "http://www.example.com/" </If> # Force text/plain if requesting a file with the query string contains 'forcetext' <If "%{QUERY_STRING} =~ /forcetext/"> ForceType text/plain </If> # Only allow access to this content during business hours <Directory "/foo/bar/business"> Require expr %{TIME_HOUR} -gt 9 && %{TIME_HOUR} -lt 17 </Directory> # Check a HTTP header for a list of values <If "%{HTTP:X-example-header} in { 'foo', 'bar', 'baz' }"> Header set matched true </If> # Check an environment variable for a regular expression, negated. <If "! reqenv('REDIRECT_FOO') =~ /bar/"> Header set matched true </If> # Check result of URI mapping by running in Directory context with -f <Directory "/var/www"> AddEncoding x-gzip gz <If "-f '%{REQUEST_FILENAME}.unzipme' && ! %{HTTP:Accept-Encoding} =~ /gzip/"> SetOutputFilter INFLATE </If> </Directory> # Check against the client IP <If "-R '192.168.1.0/24'"> Header set matched true </If> # Function examples in boolean context <If "md5('foo') == 'acbd18db4cc2f85cedef654fccc4a4d8'"> Header set checksum-matched true </If> Require expr replace(%{REQUEST_METHOD}, 'E', 'O') == 'GET'" # Function example in string context Header set foo-checksum "expr=%{md5:foo}" # This delays the evaluation of the condition clause compared to <If> Header always set CustomHeader my-value "expr=%{REQUEST_URI} =~ m#^/special_path\.php$#" # Add a header to forward client's certificate SAN to some backend RequestHeader set X-Client-SAN "expr=%{:join PeerExtList('subjectAltName'):}" # Require that the remote IP be in the client's certificate SAN Require expr %{REMOTE_ADDR} -in split s/.*?IP Address:([^,]+)/$1/, PeerExtList('subjectAltName') # or alternatively: Require expr "IP Address:%{REMOTE_ADDR}" -in split/, /, join PeerExtList('subjectAltName') # Conditional logging CustomLog logs/access-errors.log common "expr=%{REQUEST_STATUS} >= 400" CustomLog logs/access-errors-specific.log common "expr=%{REQUEST_STATUS} -in {'405','410'}"