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Isolating PHP Web Sites

If you have multiple PHP web sites on a server in a default configuration they will all be able to read each other’s files in a default configuration. If you have multiple PHP web sites that have stored data or passwords for databases in configuration files then there are significant problems if they aren’t all trusted. Even if the sites are all trusted (IE the same person configures them all) if there is a security problem in one site it’s ideal to prevent that being used to immediately attack all sites.

mpm_itk

The first thing I tried was mpm_itk [1]. This is a version of the traditional “prefork” module for Apache that has one process for each HTTP connection. When it’s installed you just put the directive “AssignUserID USER GROUP” in your VirtualHost section and that virtual host runs as the user:group in question. It will work with any Apache module that works with mpm_prefork. In my experiment with mpm_itk I first tried running with a different UID for each site, but that conflicted with the pagespeed module [2]. The pagespeed module optimises HTML and CSS files to improve performance and it has a directory tree where it stores cached versions of some of the files. It doesn’t like working with copies of itself under different UIDs writing to that tree. This isn’t a real problem, setting up the different PHP files with database passwords to be read by the desired group is easy enough. So I just ran each site with a different GID but used the same UID for all of them.

The first problem with mpm_itk is that the mpm_prefork code that it’s based on is the slowest mpm that is available and which is also incompatible with HTTP/2. A minor issue of mpm_itk is that it makes Apache take ages to stop or restart, I don’t know why and can’t be certain it’s not a configuration error on my part. As an aside here is a site for testing your server’s support for HTTP/2 [3]. To enable HTTP/2 you have to be running mpm_event and enable the “http2” module. Then for every virtual host that is to support it (generally all https virtual hosts) put the line “Protocols h2 h2c http/1.1” in the virtual host configuration.

A good feature of mpm_itk is that it has everything for the site running under the same UID, all Apache modules and Apache itself. So there’s no issue of one thing getting access to a file and another not getting access.

After a trial I decided not to keep using mpm_itk because I want HTTP/2 support.

php-fpm Pools

The Apache PHP module depends on mpm_prefork so it also has the issues of not working with HTTP/2 and of causing the web server to be slow. The solution is php-fpm, a separate server for running PHP code that uses the fastcgi protocol to talk to Apache. Here’s a link to the upstream documentation for php-fpm [4]. In Debian this is in the php7.3-fpm package.

In Debian the directory /etc/php/7.3/fpm/pool.d has the configuration for “pools”. Below is an example of a configuration file for a pool:

# cat /etc/php/7.3/fpm/pool.d/example.com.conf
[example.com]
user = example.com
group = example.com
listen = /run/php/php7.3-example.com.sock
listen.owner = www-data
listen.group = www-data
pm = dynamic
pm.max_children = 5
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3

Here is the upstream documentation for fpm configuration [5].

Then for the Apache configuration for the site in question you could have something like the following:

ProxyPassMatch "^/(.*\.php(/.*)?)$" "unix:/run/php/php7.3-example.com.sock|fcgi://localhost/usr/share/wordpress/"

The “|fcgi://localhost” part is just part of the way of specifying a Unix domain socket. From the Apache Wiki it appears that the method for configuring the TCP connections is more obvious [6]. I chose Unix domain sockets because it allows putting the domain name in the socket address. Matching domains for the web server to port numbers is something that’s likely to be error prone while matching based on domain names is easier to check and also easier to put in Apache configuration macros.

There was some additional hassle with getting Apache to read the files created by PHP processes (the options include running PHP scripts with the www-data group, having SETGID directories for storing files, and having world-readable files). But this got things basically working.

Nginx

My Google searches for running multiple PHP sites under different UIDs didn’t turn up any good hits. It was only after I found the DigitalOcean page on doing this with Nginx [7] that I knew what to search for to find the way of doing it in Apache.

2 comments to Isolating PHP Web Sites

  • niol

    Similar to what you can achieve with php-fpm, I’ve been very happy with the following setup:
    – apache mpm-event in front for TLS, rewrite, static, etc.
    – uwsgi behind via mod-proxy-uwsgi.
    The advantage of using uwsgi is that you have the same setup for any web application. I use it in the same way for python3, cgi, php webapps. Coupled with systemd socket activation and DynamicUser, it makes for an elegant setup, I think. My setup was documented in the uwsgi documentation[1].

    [1] https://uwsgi-docs.readthedocs.io/en/latest/Systemd.html#one-service-per-app-in-systemd

  • niol: Thanks for that link, what they are doing with systemd is neat. The support for different languages is good. For the moment I’ll stick with php-fpm as pretty much everything that I run uses PHP but I’ll keep uwsgi in mind for future use.

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