Running a Reverse Proxy in Apache

In 2003, Nick Kew released a new module that complements Apache's mod_proxy and is essential for reverse-proxying. Since then he gets regular questions and requests for help on proxying with Apache. In this article he attempts to give a comprehensive overview of the proxying and mod_proxy_html

This article was originally published at ApacheWeek in January 2004, and moved to ApacheTutor with minor updates in October 2006. The current revision was made in October 2009 and incorporates updates in Apache 2.2 and mod_proxy_html 3.1.


Sorry, I've turned off anonymous annotations, due to an unusual level of abuse. Existing non-spam annotations are preserved.

Web Proxies

A proxy server is a gateway for users to the Web at large. Users configure the proxy in their browser settings, and all HTTP requests are routed via the proxy. Proxies are typically operated by ISPs and network administrators, and serve several purposes: for example,

  • to speed access to the Web by caching pages fetched, so that popular pages don't have to be re-fetched for every user who views them.
  • to enable controlled access to the web for users behind a firewall.
  • to filter or transform web content.

Reverse Proxies

A reverse proxy is a gateway for servers, and enables one web server to provide content from another transparently. As with a standard proxy, a reverse proxy may serve to improve performance of the web by caching; this is a simple way to mirror a website. Loadbalancing a heavy-duty application, or protecting a vulnerable one, are other common usages. But the most common reason to run a reverse proxy is to enable controlled access from the Web at large to servers behind a firewall.

The proxied server may be a webserver itself, or it may be an application server using a different protocol, or an application server with just rudimentary HTTP that needs to be shielded from the web at large. Since 2004, reverse proxying has been the preferred method of deploying JAVA/Tomcat applications on the Web, replacing the old mod_jk (itself a special-purpose reverse proxy module).

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Note by anonymous, Thu Feb 1 11:47:50 2007

The Funda of Reverse Proxy - The web server will service any HTTP or HTTPS requests and CAN operate in reverse proxy mode. In this mode, the destination server will be hidden from the user and all requests will appear as though they are being fulfilled at the proxy. The web agent acts as a filter for requests directed to the proxy server. The web agent will intercept all requests directed to the web server where it is loaded and will communicate with SiteMinder to determine if the requested resource is protected. If the resource is protected, the web agent will challenge the user to provide an authorised set of credentials. Otherwise, the request is release to the web server for processing.

Proxying with Apache

The standard Apache module mod_proxy supports both types of proxy operation. Under Apache 1.x, mod_proxy only supported HTTP/1.0, but from Apache 2.0, it supports HTTP/1.1. This distinction is particularly important in a proxy, because one of the most significant changes between the two protocol versions is that HTTP/1.1 introduces rich new cache control mechanisms.

Apache 2.2 brings major improvements over Apache 2.0 in both proxying and cacheing, and is also the first version to support load-balancing as standard. If you are using an older Apache version, it is strongly recommended you upgrade.

For users of Unix-family platforms, you have a choice of MPM. A threaded MPM (Worker or Event) is likely to perform best in a proxy, especially if you need to support large numbers of clients. If you have an application that is not compatible with a threaded MPM, you may want to consider putting that on a different server (which could be another Apache instance on the same hardware), unless your load is too low to matter.

The Apache Proxy Modules

So far, we have spoken loosely of mod_proxy. However, it's a little more complicated than that. In keeping with Apache's modular architecture, mod_proxy is itself modular, and a typical proxy server will need to enable several modules. Those relevant to proxying and this article include:

  • mod_proxy: The core module deals with proxy infrastructure and configuration and managing a proxy request.
  • mod_proxy_http: This handles fetching documents with HTTP and HTTPS.
  • mod_proxy_ftp: This handles fetching documents with FTP.
  • mod_proxy_connect: This handles the CONNECT method for secure (SSL) tunneling.
  • mod_proxy_ajp: This handles the AJP protocol for Tomcat and similar backend servers.
  • mod_proxy_balancer implements clustering and load-balancing over multiple backends.
  • mod_cache, mod_disk_cache, mod_mem_cache: These deal with managing a document cache. To enable caching requires mod_cache and one or both of disk_cache and mem_cache.
  • mod_proxy_html: This rewrites HTML links into a proxy's address space.
  • mod_xml2enc: This supports internationalisation (i18n) on behalf of mod_proxy_html and other markup-filtering modules. space.
  • mod_headers: This modifies HTTP request and response headers.
  • mod_deflate: Negotiates compression with clients and backends.

Having mentioned the modules, I'm going to ignore caching for the remainder of this article. You may want to add it if you are concerned about the load on your network or origin servers, but the details are outside the scope of this article. I'm also going to ignore all non-HTTP protocols, and load balancing.

Building Apache for Proxying

Note: if you are installing Apache from a package, you will just need to install packages for Apache, libxml2 and third-party modules according to your distributor's conventions, which may differ from what is described here.

Most of the above modules are included in the core Apache distribution. They can easily be enabled in the Apache build process. For example:

$ ./configure --enable-so --enable-mods-shared="proxy cache ssl all"
$ make
# make install

Of course, you may want other build options too, and you could just as well build the modules as static.

If you are adding proxying to an existing installation, you should use apxs instead:

# apxs -c -i [module-name].c
noting that mod_proxy itself is in two source files
(mod_proxy.c and proxy_util.c).

This leaves mod_proxy_html and mod_xml2enc, which are third-party modules, and require a third-party library libxml2. At the time of writing, libxml2 is installed as standard or packaged for most operating systems (except Windows - see below). If you don't have it, you can download it from and install it yourself. For the purposes of this article, we'll assume libxml2 is installed as /usr/lib/, with headers in /usr/include/libxml2/libxml/.

  1. Check libxml2 is installed. The version should no longer be an issue, but note that versions before 2.5.10 had a bug that could cause mod_proxy_html to DoS, and version 2.6 is required for some error recovery when parsing data containing invalid byte sequences.
  2. Download mod_proxy_html and mod_xml2enc from
  3. Build mod_proxy_html and mod_xml2enc with apxs:

# apxs -c -I/usr/include/libxml2 -I. -i mod_proxy_html.c
# apxs -c -I/usr/include/libxml2 -I. -i mod_xml2enc.c

A Reverse Proxy Scenario

Company has a website at, which has a public IP address and DNS entry, and can be accessed from anywhere on the Internet.

The company also has a couple of application servers which have private IP addresses and unregistered DNS entries, and are inside the firewall. The application servers are visible within the network - including the webserver, as "" and "", But because they have no public DNS entries, anyone looking at from outside the company network will get a "no such host" error.

A decision is taken to enable Web access to the application servers. But they should not be exposed to the Internet directly, instead they should be integrated with the webserver, so that is mapped internally to and is mapped internally to This is a typical reverse-proxy situation.

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Note by anonymous, Tue Jan 9 15:35:04 2007

If you Explain it diagramatically people will understand more clearly and confidently.

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Note by niq, Thu Mar 1 16:53:50 2007

Good point. Would you like to contribute a diagram?

Configuring the Proxy

As with any modules, the first thing to do is to load them in httpd.conf (this is not necessary if we build them statically into Apache).

LoadModule  proxy_module         modules/
LoadModule  proxy_http_module    modules/
#LoadModule proxy_ftp_module     modules/
#LoadModule proxy_connect_module modules/
LoadModule  headers_module       modules/
LoadModule  deflate_module       modules/
LoadFile    /usr/lib/
LoadModule  xml2enc_module       modules/
LoadModule  proxy_html_module    modules/

For windows users this is slightly different: you'll need to load libxml2.dll rather than, and you'll probably need to load iconv.dll and xlib.dll as prerequisites to libxml2 (you can download them from, the same site that maintains windows binaries of libxml2). The LoadFile directive is the same.

Of course, you may not need all the modules. Two that are not required in our typical scenario are shown commented out above.

Having loaded the modules, we can now configure the Proxy. But before doing so, we have an important security warning:

Do Not set "ProxyRequests On". Setting ProxyRequests On turns your server into an Open Proxy. There are 'bots scanning the Web for open proxies. When they find you, they'll start using you to route around blocks and filters to access questionable or illegal material. At worst, they might be able to route email spam through your proxy. Your legitimate traffic will be swamped, and you'll find your server getting blocked by things like family filters.

Of course, you may also want to run a forward proxy with appropriate security measures, but that lies outside the scope of this article. The author runs both forward and reverse proxies on the same server (but under different Virtual Hosts).

The fundamental configuration directive to set up a reverse proxy is ProxyPass. We use it to set up proxy rules for each of the application servers:

ProxyPass       /app1/
ProxyPass       /app2/

The [P] flag to mod_rewrite offers an alternative to Proxypass, but this is more complex, and may in some instances degrade performance by making it impossible for Apache to use persistent proxy connections.

Now as soon as Apache re-reads the configuration (the recommended way to do this is with "apachectl graceful"), proxy requests will work, so maps to as required.

However, this is not the whole story. ProxyPass just sends traffic straight through. So when the application servers generate references to themselves (or to other internal addresses), they will be passed straight through to the outside world, where they won't work.

For example, an HTTP redirection often takes place when a user (or author) forgets a trailing slash in a URL. So the response to a request for proxies to which generates a response:

        HTTP/1.1 302 Found

But from the outside world, the net effect of this is a "No such host" error. The proxy needs to re-map the Location header to its own address space and return a valid URL

        HTTP/1.1 302 Found

The command to enable such rewrites in the HTTP Headers is ProxyPassReverse. The Apache documentation suggests the form:

ProxyPassReverse /app1/
ProxyPassReverse /app2/

However, there is a slightly more complex alternative form that I recommend as more robust:

<Location /app1/>
        ProxyPassReverse /
<Location /app2/>
        ProxyPassReverse /

Note: this currently fails due to a regression in mod_proxy. It does the right thing with the ProxyPassReverse balancer:/// form if you have a balancer: this is a workaround. Note too that the three slashes are not a typo! Without a balancer, please apply the patch from the bug report or use the other form.

The reason for recommending this is that a problem arises with some application servers. Suppose for example we have a redirect:

        HTTP/1.1 302 Found
        Location: /some/path/to/file.html

This is a violation of the HTTP protocol and so should never happen: HTTP only permits full URLs in Location headers. However, it is also a source of much confusion, not least because the CGI spec has a similar Location header with different semantics where relative paths are allowed. There are a lot of broken servers out there! In this instance, the first form of ProxyPassReverse will return the incorrect response

        HTTP/1.1 302 Found
        Location: /some/path/to/file.html

which, even allowing for error-correcting browsers, is outside the Proxy's address space and won't work. The second form fixes this to

        HTTP/1.1 302 Found
        Location: /app2/some/path/to/file.html

which is still broken, but will at least work in error-correcting browsers. Most browsers will deal with this.

If your backend server uses cookies, you may also need the ProxyPassReverseCookiePath and ProxyPassReverseCookieDomain directives. These are similar to ProxyPassReverse, but deal with the different form of cookie headers. These require mod_proxy from Apache 2.2 (recommended), or a patched version of 2.0.

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Note by anonymous, Tue Jan 9 18:34:12 2007

Is the apache version 2.0.59 for windows is patched to support "ProxyPassReverseCookiePath"?

Fixing HTML Links

As we have seen, ProxyPassReverse remaps URLs in the HTTP headers to ensure they work from outside the company network. There is, however, a separate problem when links appear in HTML pages served. Consider the following cases:

  1. <a href="somefile.html">This link will be resolved by the browser and will work correctly.</a>
  2. <a href="/otherfile.html">This link will be resolved by the browser to, which is incorrect.</a>
  3. <a href="">This link will resolve to "no such host" for the browser.</a>

The same problem of course applies to included content such as images, stylesheets, scripts or applets, and other contexts where URLs occur in HTML.

To fix this requires us to parse the HTML and rewrite the links. This is the purpose of mod_proxy_html. It works as an output filter, parsing the HTML and rewriting links as it is served. Two basic configuration directives are required to set it up:

  • ProxyHTMLEnable On
    This activates mod_proxy_html (and mod_xml2enc if available) for the request, and enables ProxyHTMLURLMap and other directives.
  • ProxyHTMLURLMap from-pattern to-pattern [flags] [cond]
    In its basic form, this has a similar purpose and semantics to ProxyPassReverse. Additionally, an extended form is available to enable search-and-replace rewriting of URLs within Scripts and Stylesheets.

Note that this is a change from earlier versions of mod_proxy_html and of this tutorial. The old method is deprecated. The reason for the change is that ProxyHTMLEnable configures both mod_proxy_html and mod_xml2enc and ensures they interact correctly: a task that would otherwise be far more complex.

How it works

mod_proxy_html is based on a SAX parser: specifically the HTMLparser module from libxml2 running in SAX mode (any other parse mode would of course be very much slower, especially for larger documents). It has full knowledge of all URI attributes that can occur in HTML 4 and XHTML 1. Whenever a URL is encountered, it is matched against applicable ProxyHTMLURLMap directives. If it starts with any from-pattern, that will be rewritten to the to-pattern. Rules are applied in the reverse order to their appearance in httpd.conf, and matching stops as soon as a match is found.

Here's how we set up a reverse proxy for HTML. Firstly, full links to the internal servers should be rewritten regardless of where they arise, so we have:

ProxyHTMLURLMap /app1
ProxyHTMLURLMap /app2

Note that in this instance we omitted the "trailing" slash. Since the matching logic is starts-with, we use the minimal matching pattern. We have now globally fixed case 3 above.

Case 2 above requires a little more care. Because the link doesn't include the hostname, the rewrite rule must be context-sensitive. As with ProxyPassReverse above, we deal with that using <Location>

<Location /app1/>
        ProxyHTMLURLMap / /app1/
<Location /app2/>
        ProxyHTMLURLMap / /app2/

Debugging your Proxy Configuration

The above is a simple case taken from mod_proxy_html version 1. With the more complex URLmapping and rewriting enabled by Version 2, you may need a bit of help setting up a complex ruleset, perhaps involving a series of complex regexps, chained anc blocking rules, etc. To help with setting up and troubleshooting your rulesets, mod_proxy_html 2 provides a "debug" mode, in which all the 'interesting' things it does are written to the Apache error log. To analyse and fix your rulesets, set

        ProxyHTMLLogVerbose On
        LogLevel Info   (or LogLevel Debug)

Now run your testcases through your rulesets, and examine the apache error log for details of exactly how it was processed.

Do not leave ProxyHTMLLogVerbose On for normal use. Although the effect is marginal, it is an overhead.

Extended URL Mapping

The previous section sets up remapping of HTML URLs, but leaves any URL encountered in a Stylesheet or Script untouched. mod_proxy_html doesn't parse Javascript or CSS, so dealing with URLs in them requires text-based search-and-replace. This is enabled by the directive ProxyHTMLExtended On.

Because the extended mode is text-based, it can no longer guarantee to match exact URLs. It's up to you to devise matching rules that can pick out URLs, just as if you were writing an old-fashioned Perl or PHP regexp-based filter (though of course it's still massively more efficient than performing search-and-replace on an entire document in-memory). To help with this, ProxyHTMLExtended supports both simple text-based and regular expression search-and-replace, according to the flags. You can also use the flags to specify rules separately for HTML links, scripting events, and embedded scripts and stylesheets.

A second key consideration with extended URL mapping is that whereas an HTML link contains exactly one URL, a script or stylesheet may contain many. So instead of stopping after a successful match, the processor will apply all applicable mapping rules. This can be stopped with the L (last) flag.

Dealing with multimedia content

We just set up a proxy to parse and where necessary correct HTML. But of course, the web isn't just HTML. Surely feeding non-HTML content through an HTML parser is at best inefficient, if not totally broken?

Yes indeed. mod_proxy_html deals with that by checking the Content-Type header, and removing itself from the processing chain when a document is not HTML (text/html) or XHTML (application/xhtml+xml). This happens in the filter initialisation phase, before any data are processed by the filter.

But that still leaves a problem. Consider compressed HTML:

        Content-Type: text/html
        Content-Encoding: gzip

Feeding that into an HTML parser is clearly broken!

There are two solutions to this. One is to uncompress the incoming data with mod_deflate. Uncompressing and compressing content radically reduces network traffic, but increases the processor load on the proxy. It is worthwhile if and only if bandwidth between the proxy and the backend is at a premium: this is common on the 'net at large, but unlikely to be the case on a company internal network.


(note that ProxyHTMLEnable correctly inserts the proxy-html filter between INFLATE and DEFLATE).

The alternative solution is to refuse to support compression. Stripping any Accept-Encoding request header does the job. So invoking mod_headers, we add a directive

RequestHeader unset Accept-Encoding

This should only apply to the Proxy, so we put it inside our <Location> containers.

A similar situation arises in the case of encrypted (https) content. But in this case, there is no such workaround: if we could decrypt the data to process it then so could any other man-in-the-middle, and the security would be worthless. This can only be circumvented by installing mod_ssl and a certificate on the proxy, so that the actual secure session is between the browser and the proxy, not the origin server.

The Complete Configuration

We are now in a position to write a complete configuration for our reverse proxy. Here is a bare minimum, that ignores extended urlmapping:

LoadModule proxy_module      modules/
LoadModule proxy_http_module modules/
LoadModule headers_module    modules/
LoadFile   /usr/lib/
LoadModule proxy_html_module modules/
LoadModule xml2enc_module modules/

ProxyRequests off
ProxyPass /app1/
ProxyPass /app2/
ProxyHTMLURLMap /app1
ProxyHTMLURLMap /app2

<Location /app1/>
        ProxyPassReverse /
        ProxyHTMLEnable On
        ProxyHTMLURLMap  /      /app1/
        RequestHeader    unset  Accept-Encoding

<Location /app2/>
        ProxyPassReverse /
        ProxyHTMLEnable On
        ProxyHTMLURLMap /       /app2/
        RequestHeader   unset   Accept-Encoding

Of course, there's more than one way to do it. Our configuration would actually have been simpler if we'd used Virtual Hosts for each application server. But that takes you beyond the realm of Apache configuration and into DNS. If you don't fully understand that (or if you think "why can't I see my domain" is a webserver question), then please don't try using virtual hosts for this.


If you are using a mod_proxy_html version older than 3.1, there is no ProxyHTMLEnable directive, and you'll have to insert the filter with Apache's standard directives: for example
SetOutputFilter proxy-html

Further topics


We haven't dealt with cacheing in this article. In a company-intranet situation, the connection from the proxy to the application servers is the local LAN, which is probably fast and has ample capacity. In such cases, caching at the proxy will have little effect, and can probably be omitted.

If we want to cache pages, we can of course do so with mod_cache But that is beyond the scope of this article.

Load Balancing

If the backend is an application that's heavy on the computer, we may wish to spread the load across multiple machines. Apache enables this with mod_proxy_loadbalancer. Current development versions (and in future stable versions 2.4.x) have additional clustering and monitoring support.

Content Transformation and Aggregation

mod_proxy_html is one of many modules that can be deployed on a proxy to rewrite contents on the fly. Other examples include more general-purpose content transformation, aggregation (e.g with server-side includes or edge-side includes), XML processing such as XInclude and XSLT, and even embedded scripting and database queries.

Filtering and Security

A reverse proxy is not the natural place for a "family filter", but is ideal for defining access controls and imposing security restrictions. We could, for example, configure the proxy to recognise a custom header from an origin server and block content based on it. This delegates control to the application servers.

If you are interested in this subject, another third-party module mod_security offers powerful and sophisicated protection.

Questions and Answers

(Q) Where can I get the software?
(A) Most of it from the obvious place,
mod_proxy_html and mod_xml2enc are available from
libxml2 is available from Windows users should read libxml2.dll for, and can obtain it together with the prerequisites iconv.dll and zlib.dll from Igor Zlatkovic's site,
Finally, mod_security is available from
(Q) Can I get a binaries of software ?
(A) If there's no link at the websites above, ask the provider of your operating system or distribution. The author can compile it on different platforms but does not provide a free compilation service.
(Q) What is httpd.conf? My apache has different configuration files.
(A) Some distribution packagers mess about with the Apache configuration. If this applies to you, the details should be documented by your distributor, and have nothing to do with Apache itself! Substitute your distributions choice of configuration file for httpd.conf in the above discussion, or create your own proxy.conf file and Include it.
(Q) You mentioned apxs and apachectl. Where do I find them?
(A) They're part of a standard Apache installation (except on Windows). If you don't have them or can't find them, that's a problem with your installation. The easiest solution is probably to download a complete Apache from
(Q) Does mod_proxy_html deal with Javascript links?
(A) From mod_proxy_html 2.0, yes!
(Q) The proxy appears to change my HTML?
(A) It doesn't really, but it may appear to. Here are the possible causes:
  1. Changing the FPI (the <!DOCTYPE ...> line) may affect some browsers. FIX: set the doctype explicitly if this bothers you.

  2. mod_proxy_html has the side-effect of transforming content to utf-8 (Unicode) encoding. This should not be a problem: utf-8 is well-supported by browsers, and offers comprehensive support for internationalisation. If it appears to cause a problem, that's almost certainly a bug in the application server, or possibly a misconfigured browser. FIX: filter through mod_charset_lite to your chosen charset.

  3. mod_proxy_html will perform some minor normalisations. If your HTML includes elements that are closed implicitly, it will explicitly close them. In other words:

                    <p>Hello, World!

    will be transformed to

                    <p>Hello, World!</p>

    If this affects the rendition in your browser, it almost certainly means you are using malformed HTML and relying on error-correction in a browser. FIX: validate your HTML! The online Page Valet service will both validate and show your markup normalised by the DTD, while a companion tool AccessValet will show markup normalised by the same parser used in the proxy, and highlight other problems. Both are available at

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Note by anonymous, Fri Feb 9 15:08:52 2007

One thing that isn't mentioned too much is what happens to <Script> tags. I tend to have an issue with scripts and the "ProxyHTMLURLMap /" line.

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Note by niq, Thu Mar 1 16:55:51 2007

Agreed. This article was published before mod_proxy_html had the capability to deal with them, and could do with updating.