## The rewrite Valve

### Introduction

The rewrite valve implements URL rewrite functionality in a way that is very similar to mod_rewrite from Apache HTTP Server.

### Configuration

The rewrite valve is configured as a valve using the `org.apache.catalina.valves.rewrite.RewriteValve` class name.

The rewrite valve can be configured as a valve added in a Host. See virtual-server documentation for informations how to configure it. It will use a `rewrite.config` file containing the rewrite directives, it must be placed in the Host configuration folder.

It can also be in the context.xml of a webapp. The valve will then use a `rewrite.config` file containing the rewrite directives, it must be placed in the WEB-INF folder of the web application

### Directives

The rewrite.config file contains a list of directives which closely resemble the directives used by mod_rewrite, in particular the central RewriteRule and RewriteCond directives. Lines that start with a `#` character are treated as comments and will be ignored.

Note: This section is a modified version of the mod_rewrite documentation, which is Copyright 1995-2006 The Apache Software Foundation, and licensed under the under the Apache License, Version 2.0.

#### RewriteCond

Syntax: `RewriteCond TestString CondPattern`

The RewriteCond directive defines a rule condition. One or more RewriteCond can precede a RewriteRule directive. The following rule is then only used if both the current state of the URI matches its pattern, and if these conditions are met.

TestString is a string which can contain the following expanded constructs in addition to plain text:

- RewriteRule backreferences: These are backreferences of the form `$N` (0 <= N <= 9), which provide access to the grouped parts (in parentheses) of the pattern, from the `RewriteRule` which is subject to the current set of `RewriteCond` conditions..
- RewriteCond backreferences: These are backreferences of the form `%N` (1 <= N <= 9), which provide access to the grouped parts (again, in parentheses) of the pattern, from the last matched `RewriteCond` in the current set of conditions.
- RewriteMap expansions: These are expansions of the form `${mapname:key|default}`. See the documentation for RewriteMap for more details.
- Server-Variables: These are variables of the form `%{` NAME_OF_VARIABLE `}` where NAME_OF_VARIABLE can be a string taken from the following list:

  - HTTP headers:

    HTTP_USER_AGENT
    HTTP_REFERER
    HTTP_COOKIE
    HTTP_FORWARDED
    HTTP_HOST
    HTTP_PROXY_CONNECTION
    HTTP_ACCEPT

  - connection & request:

    REMOTE_ADDR
    REMOTE_HOST
    REMOTE_PORT
    REMOTE_USER
    REMOTE_IDENT
    REQUEST_METHOD
    SCRIPT_FILENAME
    REQUEST_PATH
    CONTEXT_PATH
    SERVLET_PATH
    PATH_INFO
    QUERY_STRING
    AUTH_TYPE

  - server internals:

    DOCUMENT_ROOT
    SERVER_NAME
    SERVER_ADDR
    SERVER_PORT
    SERVER_PROTOCOL
    SERVER_SOFTWARE

  - date and time:

    TIME_YEAR
    TIME_MON
    TIME_DAY
    TIME_HOUR
    TIME_MIN
    TIME_SEC
    TIME_WDAY
    TIME

  - specials:

    THE_REQUEST
    REQUEST_URI
    REQUEST_FILENAME
    HTTPS

  These variables all correspond to the similarly named HTTP MIME-headers and Servlet API methods. Most are documented elsewhere in the Manual or in the CGI specification. Those that are special to the rewrite valve include those below.

  `REQUEST_PATH`
    Corresponds to the full path that is used for mapping.
  `CONTEXT_PATH`
    Corresponds to the path of the mapped context.
  `SERVLET_PATH`
    Corresponds to the servlet path.
  `THE_REQUEST`
    The full HTTP request line sent by the browser to the server (e.g., "`GET /index.html HTTP/1.1`"). This does not include any additional headers sent by the browser.
  `REQUEST_URI`
    The resource requested in the HTTP request line. (In the example above, this would be "/index.html".)
  `REQUEST_FILENAME`
    The full local file system path to the file or script matching the request.
  `HTTPS`
    Will contain the text "on" if the connection is using SSL/TLS, or "off" otherwise.

Other things you should be aware of:

1. The variables SCRIPT_FILENAME and REQUEST_FILENAME contain the same value - the value of the `filename` field of the internal `request_rec` structure of the Apache server. The first name is the commonly known CGI variable name while the second is the appropriate counterpart of REQUEST_URI (which contains the value of the `uri` field of `request_rec`).
2. `%{ENV:variable}`, where variable can be any Java system property, is also available.
3. `%{SSL:variable}`, where variable is the name of an SSL environment variable, are not implemented yet. Example: `%{SSL:SSL_CIPHER_USEKEYSIZE}` may expand to `128`.
4. `%{HTTP:header}`, where header can be any HTTP MIME-header name, can always be used to obtain the value of a header sent in the HTTP request. Example: `%{HTTP:Proxy-Connection}` is the value of the HTTP header '`Proxy-Connection:`'.

CondPattern is the condition pattern, a regular expression which is applied to the current instance of the TestString. TestString is first evaluated, before being matched against CondPattern.

Remember: CondPattern is a perl compatible regular expression with some additions:

1. You can prefix the pattern string with a '`!`' character (exclamation mark) to specify a non-matching pattern.
2. There are some special variants of CondPatterns. Instead of real regular expression strings you can also use one of the following:
   - '<CondPattern' (lexicographically precedes)
     Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString lexicographically precedes CondPattern.
   - '>CondPattern' (lexicographically follows)
     Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString lexicographically follows CondPattern.
   - '=CondPattern' (lexicographically equal)
     Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString is lexicographically equal to CondPattern (the two strings are exactly equal, character for character). If CondPattern is `""` (two quotation marks) this compares TestString to the empty string.
   - '-d' (is directory)
     Treats the TestString as a pathname and tests whether or not it exists, and is a directory.
   - '-f' (is regular file)
     Treats the TestString as a pathname and tests whether or not it exists, and is a regular file.
   - '-s' (is regular file, with size)
     Treats the TestString as a pathname and tests whether or not it exists, and is a regular file with size greater than zero.
   Note: All of these tests can also be prefixed by an exclamation mark ('!') to negate their meaning.
3. You can also set special flags for CondPattern by appending `[`flags`]` as the third argument to the `RewriteCond` directive, where flags is a comma-separated list of any of the following flags:
   - '`nocase|NC`' (no case)
     This makes the test case-insensitive - differences between 'A-Z' and 'a-z' are ignored, both in the expanded TestString and the CondPattern. This flag is effective only for comparisons between TestString and CondPattern. It has no effect on file system and subrequest checks.
   - '`ornext|OR`' (or next condition)
     Use this to combine rule conditions with a local OR instead of the implicit AND. Typical example:

         RewriteCond %{REMOTE_HOST}  ^host1.*  [OR]
         RewriteCond %{REMOTE_HOST}  ^host2.*  [OR]
         RewriteCond %{REMOTE_HOST}  ^host3.*
         RewriteRule ...some special stuff for any of these hosts...

     Without this flag you would have to write the condition/rule pair three times.

Example:

To rewrite the Homepage of a site according to the '`User-Agent:`' header of the request, you can use the following:

    RewriteCond  %{HTTP_USER_AGENT}  ^Mozilla.*
    RewriteRule  ^/$                 /homepage.max.html  [L]

    RewriteCond  %{HTTP_USER_AGENT}  ^Lynx.*
    RewriteRule  ^/$                 /homepage.min.html  [L]

    RewriteRule  ^/$                 /homepage.std.html  [L]

Explanation: If you use a browser which identifies itself as 'Mozilla' (including Netscape Navigator, Mozilla etc), then you get the max homepage (which could include frames, or other special features). If you use the Lynx browser (which is terminal-based), then you get the min homepage (which could be a version designed for easy, text-only browsing). If neither of these conditions apply (you use any other browser, or your browser identifies itself as something non-standard), you get the std (standard) homepage.

#### RewriteMap

Syntax: `RewriteMap name rewriteMapClassName optionalParameters`

The maps are implemented using an interface that users must implement. Its class name is `org.apache.catalina.valves.rewrite.RewriteMap`, and its code is:

    package org.apache.catalina.valves.rewrite;

    public interface RewriteMap {
        default String setParameters(String params...); // calls setParameters(String) with the first parameter if there is only one
        public String setParameters(String params);
        public String lookup(String key);
    }

The referenced implementation of such a class – in our example `rewriteMapClassName` – will be instantiated and initialized with the optional parameter – `optionalParameters` from above (be careful with whitespace) – by calling `setParameters(String)`. That instance will then be registered under the name given as the first paramter of `RewriteMap` rule.

Note: Starting with Tomcat 9 you can use more than one parameter. These have to be separated by spaces. Parameters can be quoted with ". This enables space characters inside parameters.

That map instance will be given the the lookup value that is configured in the corresponding `RewriteRule` by calling `lookup(String)`. Your implementation is free to return `null` to indicate, that the given default should be used, or to return a replacement value.

Say, you want to implement a rewrite map function that converts all lookup keys to uppercase. You would start by implementing a class that implements the `RewriteMap` interface.

    package example.maps;

    import org.apache.catalina.valves.rewrite.RewriteMap;

    public class UpperCaseMap implements RewriteMap {

      @Override
      public String setParameters(String params) {
        // nothing to be done here
        return null;
      }

      @Override
      public String lookup(String key) {
        if (key == null) {
          return null;
        }
        return key.toUpperCase();
      }

    }

Compile this class, put it into a jar and place that jar in `${CATALINA_BASE}/lib`.

Having done that, you can now define a map with the `RewriteMap` directive and further on use that map in a `RewriteRule`.

    RewriteMap uc example.maps.UpperCaseMap

    RewriteRule ^/(.*)$ ${uc:$1}

With this setup a request to the url path `/index.html` would get routed to `/INDEX.HTML`.

#### RewriteRule

Syntax: `RewriteRule Pattern Substitution`

The RewriteRule directive is the real rewriting workhorse. The directive can occur more than once, with each instance defining a single rewrite rule. The order in which these rules are defined is important - this is the order in which they will be applied at run-time.

Pattern is a perl compatible regular expression, which is applied to the current URL. 'Current' means the value of the URL when this rule is applied. This may not be the originally requested URL, which may already have matched a previous rule, and have been altered.

Security warning: Due to the way Java's regex matching is done, poorly formed regex patterns are vulnerable to "catastrophic backtracking", also known as "regular expression denial of service" or ReDoS. Therefore, extra caution should be used for RewriteRule patterns. In general it is difficult to automatically detect such vulnerable regex, and so a good defense is to read a bit on the subject of catastrophic backtracking. A good reference is the OWASP ReDoS guide.

Some hints on the syntax of regular expressions:

    Text:
      .           Any single character
      [chars]     Character class: Any character of the class 'chars'
      [^chars]    Character class: Not a character of the class 'chars'
      text1|text2 Alternative: text1 or text2

    Quantifiers:
      ?           0 or 1 occurrences of the preceding text
      *           0 or N occurrences of the preceding text (N > 0)
      +           1 or N occurrences of the preceding text (N > 1)

    Grouping:
      (text)      Grouping of text
                  (used either to set the borders of an alternative as above, or
                  to make backreferences, where the Nth group can
                  be referred to on the RHS of a RewriteRule as $N)

    Anchors:
      ^           Start-of-line anchor
      $           End-of-line anchor

    Escaping:
      \char       escape the given char
                  (for instance, to specify the chars ".[]()" etc.)

For more information about regular expressions, have a look at the perl regular expression manpage ("perldoc perlre"). If you are interested in more detailed information about regular expressions and their variants (POSIX regex etc.) the following book is dedicated to this topic:

Mastering Regular Expressions, 2nd Edition
Jeffrey E.F. Friedl
O'Reilly & Associates, Inc. 2002
ISBN 978-0-596-00289-3

In the rules, the NOT character ('`!`') is also available as a possible pattern prefix. This enables you to negate a pattern; to say, for instance: 'if the current URL does NOT match this pattern'. This can be used for exceptional cases, where it is easier to match the negative pattern, or as a last default rule.

Note: When using the NOT character to negate a pattern, you cannot include grouped wildcard parts in that pattern. This is because, when the pattern does NOT match (i.e., the negation matches), there are no contents for the groups. Thus, if negated patterns are used, you cannot use `$N` in the substitution string!

The substitution of a rewrite rule is the string which is substituted for (or replaces) the original URL which Pattern matched. In addition to plain text, it can include

1. back-references (`$N`) to the RewriteRule pattern
2. back-references (`%N`) to the last matched RewriteCond pattern
3. server-variables as in rule condition test-strings (`%{VARNAME}`)
4. mapping-function calls (`${mapname:key|default}`)

Back-references are identifiers of the form `$`N (N=0..9), which will be replaced by the contents of the Nth group of the matched Pattern. The server-variables are the same as for the TestString of a `RewriteCond` directive. The mapping-functions come from the `RewriteMap` directive and are explained there. These three types of variables are expanded in the order above.

As already mentioned, all rewrite rules are applied to the Substitution (in the order in which they are defined in the config file). The URL is completely replaced by the Substitution and the rewriting process continues until all rules have been applied, or it is explicitly terminated by a `L` flag.

The special characters `$` and `%` can be quoted by prepending them with a backslash character `\`.

There is a special substitution string named '`-`' which means: NO substitution! This is useful in providing rewriting rules which only match URLs but do not substitute anything for them. It is commonly used in conjunction with the C (chain) flag, in order to apply more than one pattern before substitution occurs.

Unlike newer mod_rewrite versions, the Tomcat rewrite valve does not automatically support absolute URLs (the specific redirect flag must be used to be able to specify an absolute URLs, see below) or direct file serving.

Additionally you can set special flags for Substitution by appending `[`flags`]` as the third argument to the `RewriteRule` directive. Flags is a comma-separated list of any of the following flags:

- '`chain|C`' (chained with next rule)
  This flag chains the current rule with the next rule (which itself can be chained with the following rule, and so on). This has the following effect: if a rule matches, then processing continues as usual - the flag has no effect. If the rule does not match, then all following chained rules are skipped. For instance, it can be used to remove the '`.www`' part, inside a per-directory rule set, when you let an external redirect happen (where the '`.www`' part should not occur!).
- '`cookie|CO=`NAME:VAL:domain[:lifetime[:path]]' (set cookie)
  This sets a cookie in the client's browser. The cookie's name is specified by NAME and the value is VAL. The domain field is the domain of the cookie, such as '.apache.org', the optional lifetime is the lifetime of the cookie in minutes, and the optional path is the path of the cookie
- '`env|E=`VAR:VAL' (set environment variable)
  This forces a request attribute named VAR to be set to the value VAL, where VAL can contain regexp backreferences (`$N` and `%N`) which will be expanded. You can use this flag more than once, to set more than one variable.
- '`forbidden|F`' (force URL to be forbidden)
  This forces the current URL to be forbidden - it immediately sends back an HTTP response of 403 (FORBIDDEN). Use this flag in conjunction with appropriate RewriteConds to conditionally block some URLs.
- '`gone|G`' (force URL to be gone)
  This forces the current URL to be gone - it immediately sends back an HTTP response of 410 (GONE). Use this flag to mark pages which no longer exist as gone.
- '`host|H`=Host' (apply rewriting to host)
  Rather that rewrite the URL, the virtual host will be rewritten.
- '`last|L`' (last rule)
  Stop the rewriting process here and don't apply any more rewrite rules. This corresponds to the Perl `last` command or the `break` command in C. Use this flag to prevent the currently rewritten URL from being rewritten further by following rules. For example, use it to rewrite the root-path URL ('`/`') to a real one, e.g., '`/e/www/`'.
- '`next|N`' (next round)
  Re-run the rewriting process (starting again with the first rewriting rule). This time, the URL to match is no longer the original URL, but rather the URL returned by the last rewriting rule. This corresponds to the Perl `next` command or the `continue` command in C. Use this flag to restart the rewriting process - to immediately go to the top of the loop.
  Be careful not to create an infinite loop!
- '`nocase|NC`' (no case)
  This makes the Pattern case-insensitive, ignoring difference between 'A-Z' and 'a-z' when Pattern is matched against the current URL.
- '`noescape|NE`' (no URI escaping of output)
  This flag prevents the rewrite valve from applying the usual URI escaping rules to the result of a rewrite. Ordinarily, special characters (such as '%', '$', ';', and so on) will be escaped into their hexcode equivalents ('%25', '%24', and '%3B', respectively); this flag prevents this from happening. This allows percent symbols to appear in the output, as in

      RewriteRule /foo/(.*) /bar?arg=P1\%3d$1 [R,NE]

  which would turn '`/foo/zed`' into a safe request for '`/bar?arg=P1=zed`'.
- '`qsappend|QSA`' (query string append)
  This flag forces the rewrite engine to append a query string part of the substitution string to the existing string, instead of replacing it. Use this when you want to add more data to the query string via a rewrite rule.
- '`redirect|R` [=code]' (force redirect)
  Prefix Substitution with `http://thishost[:thisport]/` (which makes the new URL a URI) to force an external redirection. If no code is given, an HTTP response of 302 (FOUND, previously MOVED TEMPORARILY) will be returned. If you want to use other response codes in the range 300-399, simply specify the appropriate number or use one of the following symbolic names: `temp` (default), `permanent`, `seeother`. Use this for rules to canonicalize the URL and return it to the client - to translate '`/~`' into '`/u/`', or to always append a slash to `/u/`user, etc.
  Note: When you use this flag, make sure that the substitution field is a valid URL! Otherwise, you will be redirecting to an invalid location. Remember that this flag on its own will only prepend `http://thishost[:thisport]/` to the URL, and rewriting will continue. Usually, you will want to stop rewriting at this point, and redirect immediately. To stop rewriting, you should add the 'L' flag.
- '`skip|S`=num' (skip next rule(s))
  This flag forces the rewriting engine to skip the next num rules in sequence, if the current rule matches. Use this to make pseudo if-then-else constructs: The last rule of the then-clause becomes `skip=N`, where N is the number of rules in the else-clause. (This is not the same as the 'chain|C' flag!)
- '`type|T`=MIME-type' (force MIME type)
  Force the MIME-type of the target file to be MIME-type. This can be used to set up the content-type based on some conditions. For example, the following snippet allows `.php` files to be displayed by `mod_php` if they are called with the `.phps` extension:

      RewriteRule ^(.+\.php)s$ $1 [T=application/x-httpd-php-source]