Sunday, July 31, 2016

Minimizing apache and PHP

Recently I was looking at migrating a simple website in which every page but one was static.

The simplest thing here would be to use nginx. It's simple, fast, modern, and should make it dead easy to get an A+ on the Qualys SSL test.

But that non-static page? A trivial contact form. Fill in a box, the back-end sends the content of the box as an email message.

The simplest thing here in days gone by would have been to put together a trivial CGI script. Only nginx doesn't do CGI, at least not directly. Not only that, but writing the CGI script and doing it well is pretty hard.

So, what about PHP? Now, PHP has gotten itself a not entirely favourable reputation on the security front. Given the frequent security updates, not entirely undeserved. But could it be used for this?

For such a task, all you need is the mail() function. Plus maybe a quick regex and some trivial string manipulation. All that is in core, so you don't need very much of PHP at all. For example, you could use the follwoing flags to build:


So, no modules. Far less to go wrong. On top  of that, you can disable a bunch of things in php.ini:

file_uploads = Off [change]
allow_url_fopen = Off [change]
allow_url_include = Off [default]
display_errors = Off [default]
expose_php=Off [change]

Furthermore, you could start disabling functions to your heart's content:

disable_functions = php_uname, getmyuid, getmypid, passthru, leak, listen, diskfreespace, tmpfile, link, ignore_user_abord, shell_exec, dl, set_time_limit, exec, system, highlight_file, source, show_source, fpaththru, virtual, posix_ctermid, posix_getcwd, posix_getegid, posix_geteuid, posix_getgid, posix_getgrgid, posix_getgrnam, posix_getgroups, posix_getlogin, posix_getpgid, posix_getpgrp, posix_getpid, posix, _getppid, posix_getpwnam, posix_getpwuid, posix_getrlimit, posix_getsid, posix_getuid, posix_isatty, posix_kill, posix_mkfifo, posix_setegid, posix_seteuid, posix_setgid, posix_setpgid, posix_setsid, posix_setuid, posix_times, posix_ttyname, posix_uname, proc_open, proc_close, proc_get_status, proc_nice, proc_terminate, phpinfo

Once you've done that, you end up with a pretty hardened PHP install. And if all it does is take in a request and issue a redirect to a static target page, it doesn't even need to create any html output.

Then, how to talk to PHP?  The standard way to integrate PHP with nginx is using FPM. Certainly, if this was a high or even moderate traffic site, then that would be fine. But that involves leaving FPM running permanently, and is a bit of a pain and a resource hog for one page that might get used once a week.

So how about forwarding to apache? Integration using mod_php is an absolute doddle. OK, it's still running permanently, but you can dial down the process count and it's pretty lightweight. But we have a similar issue to the one we faced with PHP - the default build enables a lot of things we don't need. I normally build apache with:


but in this case you can reduce that to:


now, there is the option of --enable-modules=none, but I couldn't actually get apache to start at all with that - some modules appear to be essential (mod_authz_host, mod_dir, mod_mime, and mod_log_config at least), and going below the "few" setting is entering unsupported territory.

You can restrict apache even further with configuration, just enable PHP, return an error for any other page, listen only on localhost. (I like the concept of the currently experimental mod_allowmethods as we might only want POST for this case. Normally disabling methods with current apache version involves mod_rewrite, which is one of the more complex modules.)

In the end, we elected to solve the problem a different way, but it was still an instructive exercise.

The above would be suitable for one particular use case. For a general service, it would be completely useless. Most providers and distributions tend to build with the kitchen sink enabled, because you don't know what your users or customers might need at runtime. They might build everything as a shared module, and could package every module in a separate package (although this ends up being a pain to manage); or you rely on the user to explicitly enable and disable modules as necessary.

In Tribblix, I've tended to avoid breaking something like apache or PHP up into multiple packages.  There's one exception, which is that the PHP interface to postgresql is split out into a separate package. This is simply because it links against the postgresql shared library, so I ship that part separately to avoid forcing postgresql to be installed as a dependency.

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