12.7. Apache's mod_proxy Module
Apache's mod_proxy module implements a proxy and cache for Apache. It implements proxying capabilities for the following protocols: FTP, CONNECT (for SSL), HTTP/0.9, HTTP/1.0, and HTTP/1.1. The module can be configured to connect to other proxy modules for these and other protocols.
mod_proxy is part of Apache, so there is no need to install a separate server—you just have to enable this module during the Apache build process or, if you have Apache compiled as a DSO, you can compile and add this module after you have completed the build of Apache.
A setup with a mod_proxy-enabled server and a mod_perl-enabled server is depicted in Figure 12-6.
Figure 12-6. mod_proxy-enabled Apache and mod_perl-enabled Apache
We do not think the difference in speed between Apache's mod_proxy and Squid is relevant for most sites, since the real value of what they do is buffering for slow client connections. However, Squid runs as a single process and probably consumes fewer system resources.
The trade-off is that mod_rewrite is easy to use if you want to spread parts of the site across different backend servers, while mod_proxy knows how to fix up redirects containing the backend server's idea of the location. With Squid you can run a redirector process to proxy to more than one backend, but there is a problem in fixing redirects in a way that keeps the client's view of both server names and port numbers in all cases.
The difficult case is where you have DNS aliases that map to the same IP address, you want them redirected to port 80 (although the server is on a different port), and you want to keep the specific name the browser has already sent so that it does not change in the client's browser's location window.
The advantages of mod_proxy are:
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No additional server is needed. We keep the plain one plus one mod_perl-enabled Apache server. All you need is to enable mod_proxy in the httpd_docs server and add a few lines to the httpd.conf file.
ProxyPass /perl/ http://localhost:81/perl/ ProxyPassReverse /perl/ http://localhost:81/perl/
The ProxyPass directive triggers the proxying process. A request for http://example.com/perl/ is proxied by issuing a request for http://localhost:81/perl/ to the mod_perl server. mod_proxy then sends the response to the client. The URL rewriting is transparent to the client, except in one case: if the mod_perl server issues a redirect, the URL to redirect to will be specified in a Location header in the response. This is where ProxyPassReverse kicks in: it scans Location headers from the responses it gets from proxied requests and rewrites the URL before forwarding the response to the client.
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It buffers mod_perl output like Squid does.
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It does caching, although you have to produce correct Content-Length, Last-Modified, and Expires HTTP headers for it to work. If some of your dynamic content does not change frequently, you can dramatically increase performance by caching it with mod_proxy.
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ProxyPass happens before the authentication phase, so you do not have to worry about authenticating twice.
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Apache is able to accelerate secure HTTP requests completely, while also doing accelerated HTTP. With Squid you have to use an external redirection program for that.
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The latest mod_proxy module (for Apache 1.3.6 and later) is reported to be very stable.
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