What can Squid do for your site?
Put Squid between the users and the Internet to cache your web pages. Users surf faster, HTTP traffic uses less bandwidth, and you can save on bandwidth fees -- or use the saved bandwidth for other traffic.
Squid source code is available from squid-cache.org. There is also a list of mirror sites. Installation instructions are available in the ReadMe file in the source tar file.
There is also an RPM package for Red Hat Linux, and packages for FreeBSD and NetBSD. Instructions on installing the packages are available from the package sites. All three package sets come with Squid source code and additional instructions on installing from the source.
./configure script has several options. A recommended set is
./configure --enable-heap-replacement --enable-cache-digests --enable-dlmalloc.
The Squid configuration file is in the
$SQUID-HOME/etc/squid.conf directory. The file has extensive comments about each option. This article is a quick reference to which options you are most likely to want to change. Your local network may require configurations not mentioned in this article.
When you have Squid configured, run
squid -z to create the cache directory structure. Then you can start Squid.
htcp_port. 3128 is a good default, 8080 is a reasonable alternative for HTTP. Port 80, or any port normally used by some other service, should be avoided if at all possible.
cache_memat 8 Mbytes at first, unless you have between 0.5 Gbytes and 1 Gbyte of RAM free routinely. If so, set
cache-memto 128 Mbytes. Adjust
cache_memonce local cache patterns are known.
maximum_object_sizeto 40 Mbytes. If larger files are routinely downloaded, increase it to 250 Mbytes or even 700 Mbytes.
cache_dirto an area that has a large amount of space. Technically it belongs under
/var, but you might not want it backed up. Don't set it to use more than 70 percent of the space, Squid uses this directory to store journal files as well.
cache_dir ufs /var/cache/squid 80000 16 256is common.
cache_logon. The former tells you who is doing what, and the latter tells you when things aren't quite right.
cache_swap_logis the location for the journal files mentioned in
cache_dir. The default location is in the same directory as
pid_filenamemust be set.
/var/log/squid/squid.pidis a good location. Squid uses this to shut down, rotate log files, or reread its configuration.
refresh_patternaffects how objects are evaluated for freshness. A reasonable default is
refresh_pattern . 0 20% 10080.
cache_mgris for people that use the cache to report problems. Be sure to use an email address that you will actually read.
cache_effective groupshould be set to a "proxy" user and group. Many distributions ship with this user and group pre-installed.
chownthe log and cache directories to this user before you start Squid. This user must be able to read the configuration file and the directory that it's in.
chown -R proxy.proxy /var/log/squid /var/cache/squid
visible_hostnameto the fully qualified domain name. For example,
dns_testnames. If it can't resolve names like "netscape.com", "internic.net", and "nlanr.net", your system needs fixing.
memory_poolsoff unless there's a lot of free memory on the box.
log_icp_querieson. ICP queries come from other proxies -- if you don't have sibling or parent proxies and you're getting them, you'll want to see these in the
Access-control lists manage the access to your network. This basic example limits access to the proxy to the network
188.8.131.52/24. It matches successfully if a request comes from any of the addresses between
acl our_network src 184.108.40.206/24 http_access allow our_network http_access deny all
ACLs are checked from top to bottom. Clients with IPs in
our_network are permitted, anyone else falls through to the "deny all" and gets a failure message. The format for the class definition is
acl listname src network/netmask.
ACLs have an implicit last line that reverses the rule of the previous line. This protects against forgetting to add the
http_access deny all, but explicitly adding that line makes the ACL more readable and helps ensure that it's not missed when the ACL is changed.
If an object isn't in the cache and marked as fresh, Squid checks with the origin server to see if it is still current and requests a new copy if it isn't. This behavior serves local users well, but is undesirable if the requesting client is a neighboring proxy server. The following ACL lines allow the local network to be passed objects which aren't in the current cache, but deny this service to anyone outside the local network.
miss_access allow our_network miss_access deny all
Caches communicate with ICP messages to find out whether they have fresh content that satisfies a request. The
icp_access ACL lines are used to control the caches Squid can communicate with.
To maximize speed, minimize the number of simultaneous requests Squid has to handle. The more requests Squid has to process in parallel, the longer each request takes. Every bit of latency you can reduce speed of the server.
Aim to have 20 or 30 DNS servers. DNS lookups can be slow -- some continental backbones can take a minute and more to resolve a DNS request.
ipcache. Bigger is better. Stale addresses are less important than many entries and long TTLs. Cache addresses for at least 24 hours, and negative cache for at least 5 minutes.
Saving dollars is a large part of what proxy caches are all about. It's easy to waste your proxy -- and real money in bandwidth -- if you don't understand what's going on. It's also easy to save money with one if you know the issues. Most web servers and web content are operated or produced by people who don't really understand the HTTP protocol. As cache administrators, their errors are going to land on your shoulders.
A cache can always use more disk space, but as the size of your disk-cache grows, you will need more memory to index it. There's a straightforward rule for memory.
Divide the size of your disk cache by 13 Kbytes, and multiply that by 130 bytes. Add the size of
cache_mem, and add about 2.5 Mbytes more for executable files, libraries, and other sundry overhead. For example: We have a 10-Gbyte drive, and a
cache_mem of 8 Mbytes.
10 Gbytes/13 Kbytes = 769,230
769,230 x 130 bytes = 99,999,900 bytes (or 97,656 Kbytes)
97,656 Kbytes + 2.5 Mbytes + 8 Mbytes = 10,849,656 Kbytes or about 108 Mbytes
The example server needs 108 Mbytes available to Squid to support 10 Gbytes of
Provide as much disk space as you can provide RAM to support it. Squid performs very badly when it starts to swap. Remember to set aside memory for anything else on the machine (DNS,
cron, operating system, etc.).
Refresh patterns determine the lifetime of the object. Within an object lifetime, Squid will serve the object without requesting an IMS ("if modified since") request. Once the lifetime is exceeded, Squid will keep the object but will send an IMS request to the origin server. If the object has been modified since it was first cached, Squid requests the new copy. If not, it keeps the old copy. Either way, the object is marked as fresh again.
Here's our (default) basic refresh pattern:
refresh_pattern . 0 20% 4320
The dot (
.) is the the regular expression pattern, and matches anything. It uses POSIX regular expressions. (See
man 7 regex).
The zero (
0) is the minimum freshness time. If it's anything other than zero, it will override any expiration headers given with the object. If the content provider actually provided an expiration header, we should usually honor it.
The last term (
4320) is the maximum freshness time. The object becomes stale after this many minutes in the cache.
The 20 percent is used for our default case, for when there's no information from the content provider about the lifetime of the object. Squid takes x percent (20 percent in this example) of the difference between the last-modified time of the object and the current time, and uses that as the object lifetime. If the object lifetime is less than the minimum set by the
refresh_pattern, it is increased to at least that. If it's greater than the supplied maximum, it's reduced to that.
Some kinds of files can be maintained much longer than others. Zip,
.exe files rarely change content without also changing name. Using regular expressions, we can create a set of refresh patterns like this:
refresh_pattern -i exe$ 0 50% 999999 refresh_pattern -i zip$ 0 50% 999999 refresh_pattern -i tar\.gz$ 0 50% 999999 refresh_pattern -i tgz$ 0 50% 999999
Note that these options violate the HTTP standard. Do not use them lightly.
override-expire pretends there is no expiration header on the object and calculates purely based on last-modified times. This permits you to cache sites that abuse the use of expiration headers, but also inhibits updates of frequently changed content (such as news sites).
ignore-reload prevents the object being refreshed when the user presses the refresh button on their browser. This does not perform well when the object has no content length -- you may wind up with a broken object that the users cannot reload.
reload-into-ims transforms reloads into validations. Beware: Web servers may permit an object to be updated without the last-modified time being altered. The server may then insist that the object is still valid when it actually is not.
maximum_object_size. 40 Mbytes is not too large. 800 Mbytes might cache the large downloads.
calamaristo analyze your logs periodically and look for changes you can make to your
refresh_patterns. There is no single "good" set. Users change their browsing habits and create new files, and new technology is always being developed. Adapt.
In many clients, "reload" forces the cache to reload from the origin server. This can cause testing problems.
Don't test Squid with pages that have "do not cache" headers. Squid will not cache them.
access.log when testing to verify that you're pulling pages from Squid.
Access control lists have an implicit last line which reverses the rule of the last explicit line.
Squid can improve browsing speed and reduce HTTP bandwidth. The
squid.conf file gives great flexibility, but can be initially daunting. These settings let you get started -- but are just a start. Experiment!
Jennifer Vesperman is the author of Essential CVS. She writes for the O'Reilly Network, the Linux Documentation Project, and occasionally Linux.Com.
Return to the Linux DevCenter.
Copyright © 2009 O'Reilly Media, Inc.