Go back to the config editor window and add the following before the last two http_access lines:
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, localhost is being denied in the first line and allowed in the second. When Squid sees localhost asking for a site, it reads the deny line first and immediately sends the error page — it does not even get to the allow line. Having a deny all line 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 localhost and allow localhost lines. 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 localhost only if it fits certain criteria.
Defining access criteria is done with the acl lines, so above the deny all line, 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.uk and 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 newssites means, "allow access to the category newssites with 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 -SIGHUP line 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 newssites category. 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.org as 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 newssites ACL 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 news off 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 .com to match *.com addresses.
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 freetime and 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 freetime category defined, you can change the http_access line 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.uk or 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 freetime category can surf from 18:00:00 until 23:59:59.
You can add as many rules as you like, although you should be careful to try to order them so that they make sense. Keep in mind that all conditions in a line must be matched for the line to be matched. Here is a more complex example:
► You want a category newssites that contains serious websites people need for their work.
► You want a category playsites that contains websites people do not need for their work.
► You want a category worktime that stretches from 09:00 to 18:00.
► You want a category freetime that stretches from 18:00 to 20:00, when the office closes.
► You want people to be able to access the news sites, but not the play sites, during working hours.
► You want people to be able to access both the news sites and the play sites during the free time hours.
To do that, you need the following rules:
acl newssites dstdomain .bbc.co.uk .slashdot.org
acl playsites dstdomain .tomshardware.com fedora.redhat.com
acl worktime time MTWHF 9:00-18:00
acl freetime time MTWHF 18:00-20:00
http_access allow newssites worktime
http_access allow newssites freetime
http_access allow playsites freetime
The letter D is equivalent to MTWHF in meaning "all the days of the working week."
Notice that there are two http_access lines for the newssites category: one for worktime and one for freetime. All the conditions must be matched for a line to be matched. The alternative would be to write this:
http_access allow newssites worktime freetime
However, if you do that and someone visits news.bbc.co.uk at 2:30 p.m. (14:30) on a Tuesday, Squid works like this:
► Is the site in the newssites category? Yes, continue.
► Is the time within the worktime category? Yes, continue.
► Is the time within the freetime category? No; do not match rule, and continue searching for rules.
Two lines therefore are needed for the worktime category.
One particularly powerful way to filter requests is with the url_regex ACL line. This enables you to specify a regular expression that is checked against each request: If the expression matches the request, the condition matches.