Caching the Web, Part 2

Last month we discussed the basic concepts of proxy servers and caching. Now, let's see how to implement this technology in your organization. A few proxy-server programs are on the market, such as MS-PROXY, aka Catapult, available only for Windows NT, and Netscape Proxy Server, available for different UNIX platforms and Windows NT. Both have two main drawbacks: they are commercial software and they don't support ICP. The excellent Apache web server has included a proxy-cache module since its 1.2 version. This module is a very interesting option: it's free, and works with the most popular web server on the Net. However, it doesn't use ICP, and its robustness is not comparable to the best choice for a proxy-cache server—Squid.

Squid is a high-performance proxy-cache server derived from the cache module of the Harvest Research Project, maintained by Duane Wessels. It supports FTP, gopher, WAIS and HTTP objects. It stores hot objects in RAM and maintains a robust database of objects in disk directories. Squid also supports the SSL protocol for proxying secure connections and has a complex access control mechanism. Another interesting feature of Squid is negative caching, which saves “connection refused” and “404 Not Found” replies for a short period of time (usually five minutes).

Squid consists of four programs:

It also provides a CGI program, designed to be run through a web interface, that outputs statistics about its configuration and performance and allows some management capabilities.

Installing Squid is easy. Just download the source archive from http://squid.nlanr.net/ and, in a temporal directory, type:

gzip -dc squid-x.y.z-src.tar.gz | tar xvf -

Next, compile and install the software by typing:

cd squid-x.y.z
./configure
make all
make install

These commands install all needed programs and configuration files to /usr/local/squid. The binary programs are installed in the /bin directory, the configuration files in /conf. Log files are located in the /logs directory, and the object database in the cache directory and its subdirectories. A shell script called RunCache is in the bin directory used to run the squid binary, and assures that if the process dies for any reason, it is restarted automatically. So, put the following line in your rc.local file:

/usr/local/squid/bin/RunCache &

This will generate an error log in /usr/local/squid/squid.out, if Squid could not start because of some configuration problem.

Of course you can choose to install an RPM version of Squid if you use RedHat Linux or another distribution that supports RPM packages.

Squid installs a sample configuration file called squid.conf with many comments for each option. Here you can change the ICP and HTTP ports (3128 by default) and define how much memory and disk space to reserve for caching objects and other parameters such as refresh patterns and access control restrictions. Of course, you need an ICP port only if your cache is going to be the sibling or parent of other caches. The directives for changing these values are http_port, icp_port, cache_dir and cache_swap. Additionally, you can set the maximum object size to be stored in the database; the default is 4MB. Also, you should uncomment the following lines in this file:

cache_effective_user nobody
cache_effective_group nobody

This avoids running Squid as root, a dangerous habit for anyone who runs servers like httpd or gopherd. If you are using a recent version of Squid (at the time of this writing, the current version is 1.1.16), it will not start running as root, but will write an error message to the squid.out file.

To let Squid use 100 MB of your HD, the directive cache_dir should be something like this:

cache_dir /usr/local/squid/cache 100 16 256

Before starting Squid for the first time, create the cache and logs directories. To build the cache and hashed subdirectories, you should execute the commands:

cd /usr/local/squid
mkdir cache
chown -R nobody cache
cd /usr/local/squid/bin
./squid -z

Finally, to create and change the owner of the logs directory:

cd /usr/local/squid
mkdir logs
chown nobody logs

Now Squid can be run safely for the first time, with the above RunCache invocation. It will spawn several dnsserver processes and write its PID in the file logs/squid.pid. Important warning or error messages can be found in the squid.out and logs/cache.log files. Remember, if you want to shut down the cache, you must first kill the RunCache process to avoid an immediate restart and then type:

/usr/local/squid/bin/squid -k shutdown

Never use kill -9 to shut down the cache, because it doesn't close the object database in such a way that it can be recovered—you'll probably lose part of it.