Andrew Hudson - Fedora™ Unleashed, 2008 edition

tail -f access.log cache.log

That command reads the last few lines from both files and (thanks to the -f flag) follows them so that any changes appear in there. This allows you to watch what Squid is doing as people access it. We will refer to this window as the log window , so keep it open. In the other window (as root, remember), bring up the file /etc/squid/squid.confin your favorite editor. This window will be referred to as the config editor , and you should keep it open also.

To get started, search for the string acl all — this brings you to the access control section, which is where most of the work needs to be done. There is a lot you can configure else where, but unless you have unusual requirements, you can leave the defaults in place.

The default port for Squid is 3128, but you can change that by editing the http_portline. Alternatively, you can have Squid listen on multiple ports by having multiple http_portlines: 80, 8000, and 8080 are all popular ports for proxy servers.

The acllines make up your access control lists. The first 16 or so lines define the minimum recommended configuration for setting up which ports to listen to, and other fairly standard configuration settings that you can safely ignore. If you scroll down farther (past another short block of comments), you come to the http_accesslines, which are combined with the acllines to dictate who can do what. You can (and should) mix and match acland http_accesslines to keep your configuration file easy to read.

Just below the first block of http_accesslines is a comment like # INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS. This is just what we are going to do. First, though, scroll just a few lines farther and you should see these two lines:

http_access allow localhost

http_access deny all

The first says, "allow HTTP access to the local computer, but deny everyone else." This is the default rule, as mentioned earlier. Leave that in place for now, and run service squid startto start the server with the default settings. If you have not yet configured the local web browser to use your Squid server, do so now so you can test the default rules.

In your web browser (Firefox is assumed from here on, but it makes little difference), go to the URL http://fedora.redhat.com. You should see it appear as normal in the browser, but in the log window you should see a lot of messages scroll by as Squid downloads the site for you and stores it in its cache. This is all allowed because the default configuration allows access to the localhost.

Go back to the config editor window and add the following before the last two http_accesslines:

http_access deny localhost

So the last three lines should look like this:

http_access deny localhost

http_access allow localhost

http_access deny all

Save the file and quit your editor. Then run this command:

kill -SIGHUP `cat /var/run/squid.pid`

That command looks for the PID of the Squid daemon and then sends the SIGHUP signal to it, which forces it to reread its configuration file while running. You should see a string of messages in the log window as Squid rereads its configuration files. If you now go back to Firefox and enter a new URL, you should see the Squid error page informing you that you do not have access to the requested site.

The reason you are now blocked from the proxy is because Squid reads its ACL lines in sequence, from top to bottom. If it finds a line that conclusively allows or denies a request, it stops reading and takes the appropriate action. So, in the previous lines, localhostis being denied in the first line and allowed in the second. When Squid sees localhostasking for a site, it reads the denyline first and immediately sends the error page — it does not even get to the allowline. Having a deny allline at the bottom is highly recommended so that only those you explicitly allow are able to use the proxy.

Go back to editing the configuration file and remove the deny localhostand allow localhostlines. This leaves only deny all, which blocks everyone (including the localhost) from accessing the proxy. Now we are going to add some conditional allow statements: We want to allow localhostonly if it fits certain criteria.

Defining access criteria is done with the acllines, so above the deny allline, add this:

acl newssites dstdomain news.bbc.co.uk slashdot.org

http_access allow newssites

The first line defines an access category called newssites, which contains a list of domains ( dstdomain). The domains are news.bbc.co.ukand slashdot.org, so the full line reads, "create a new access category called newssites, which should filter on domain, and contain the two domains listed." It does not say whether access should be granted or denied to that category; that comes in the next line. The line http_access allow newssitesmeans, "allow access to the category newssiteswith no further restrictions." It is not limited to localhost, which means this applies to every computer connecting to the proxy server.

Save the configuration file and rerun the kill -SIGHUPline from before to restart Squid; then go back to Firefox and try loading http://fedora.redhat.com. You should see the same error as before because that was not in the newssitescategory. Now try http://news.bbc.co.uk, and it should work. However, if you try http://www.slashdot.org, it will not work, and you might also have noticed that the images did not appear on the BBC News website either. The problem here is that specifying slashdot.orgas the website is very specific: It means that http://slashdot.org will work, whereas http://www.slashdot.org will not. The BBC News site stores its images on the site http://newsimg.bbc.co.uk, which is why they do not appear.

Go back to the configuration file, and edit the newssitesACL to this:

acl newssites dstdomain .bbc.co.uk .slashdot.org

Putting the period in front of the domains (and in the BBC's case, taking the newsoff also) means that Squid allows any subdomain of the site to work, which is usually what you will want. If you want even more vagueness, you can just specify .comto match *.comaddresses.

Moving on, you can also use time conditions for sites. For example, if you want to allow access to the news sites in the evenings, you can set up a time category using this line:

acl freetime time MTWHFAS 18:00-23:59

This time, the category is called freetimeand the condition is time, which means we need to specify what time the category should contain. The seven characters following that are the days of the week: Monday, Tuesday, Wednesday, tHursday, Friday, sAturday, and sUnday. Thursday and Saturday use capital H and A so that they do not clash with Tuesday and Sunday.

With the freetimecategory defined, you can change the http_accessline to include it, like this:

http_access allow newssites freetime

For Squid to allow access now, it must match both conditions — the request must be for either *.bbc.co.ukor slashdot.org, and during the time specified. If either condition does not match, the line is not matched and Squid continues looking for other matching rules beneath it. The times you specify here are inclusive on both sides, which means users in the freetimecategory can surf from 18:00:00 until 23:59:59.