Using .htaccess Redirection to Standardize Web Server Addresses

Friday, 5 May 2006

Answers to a couple of somewhat frequently-asked questions about the .htaccess tricks I’m using to standardize the domain names at Daring Fireball:

Getting Rid of ‘www’ Prefixes

I’ve always rather disliked the “www” prefix for web servers. It has become so common that everyone is accustomed to both seeing and saying it, and so it’s sort of invisible now, but it really is a goofy convention.¹

Long ago, I started using .htaccess redirection to standardize all Daring Fireball URLs on the prefix-less “daringfireball.net” address. If you attempt to load “www.daringfireball.net/anything”, you’ll be redirected to “daringfireball.net/anything”.

Here’s the applicable portion of my .htaccess file, located in the root of my web hosting directory:

RewriteEngine on
RewriteBase /

RewriteCond %{HTTP_HOST} ^www\.daringfireball\.net$
RewriteRule (.*) http://daringfireball.net/$1 [R=Permanent]

The first line turns on Apache’s mod_rewrite module. The second line sets the “base” URL to the root of the domain.

The RewriteCond line specifies a condition for the subsequent rule — i.e. the rule on the next line is only applied if the condition in this line is met. There are two parts to the condition: a test string, and a regular expression to match against the test string. The parts are separated by spaces.

The test string, “%{HTTP_HOST}”, is a reference to the HTTP_HOST header that came in with the HTTP request. It’s the name of the server the request has been sent to. The pattern, “^www\.daringfireball\.net$”, is a precise regular expression that will successfully match when the HTTP_HOST header is “www.daringfireball.net”. You could simply write:

RewriteCond %{HTTP_HOST} www.daringfireball.net

which is less precise, but still matches “www.daringfireball.net”. But why be lazy? I’m not going to explain regular expression syntax here, so look it up if you don’t understand what’s going on in the precise version.

The RewriteRule line contains the rule that is applied when the preceding RewriteCond is met. The rule consists of three parts, again separated by spaces:

• (.*) — a regular expression that is applied against the request string. The request string is everything that comes after the slash after the domain name. So if you request the URL “http://daringfireball.net/archives”, the request string that gets tested against this pattern is “archives”.

  This particular pattern, “(.*)”, will match anything. It even successfully matches “nothing” (a.k.a. the empty string), which means it will successfully match when you request the URL “http://www.daringfireball.net/”. Wrapping it in parentheses allows us to refer to it later on, in the replacement string.

• http://daringfireball.net/$1 — the replacement pattern. When the regular expression in the first part matches, this pattern describes how to rewrite the URL. $1 is a reference to the text captured by the sub-pattern within the parentheses; if we had a pattern with more than one set of parentheses, we could use $2, $3, etc. to refer to the additional parenthesized sub-patterns.

  Because the pattern in the first part always matches, this replacement pattern is always applied.

• [R=Permanent] — this is a flag that says that this redirection is permanent. That means the HTTP redirection goes out with a 301 code, which tells the client that the redirected URL is the new permanent home for the resource at the original URL. You could also write this as [R=301], but I prefer the wordy syntax.

  This flag is optional, but if you leave it off, the redirection will be marked as temporary — i.e. it’ll go out with an HTTP 302 code. If the redirection is permanent, it’s good practice to mark it so. Intelligent client software will remember this and go directly to the new URL instead of going through the redirection on subsquent requests.

So, in summary:

1. We turn on mod_rewrite processing.

2. We use a RewriteCond condition that applies only when the HTTP_HOST variable matches “www.daringfireball.net”.

3. When that condition is met, we apply a RewriteRule that takes the entire request after the “www.daringfireball.net/” part, and redirects it to “http://daringfireball.net/”.

Redirecting From ‘.com’ to ‘.net’

I can’t quite explain why I decided to use “daringfireball.net” instead of “daringfireball.com” as the canonical domain for Daring Fireball. Suffice it to say I have some sort of irrational dislike for “.com”.

But I realized early on that it was a somewhat common mistake for people to type “daringfireball.com” instead of “.net”, and the last thing I wanted was for some asshole domain squatter to register “daringfireball.com”, so I registered the “.com” version too.

But what to do with it? What I want is for any URL in the form “daringfireball.com/anything to redirect to “daringfireball.net/anything”. Sound familiar?

One solution to this problem would be to configure both domain names to point to the very same web site, much like how “www.example.com” and “example.com” almost always point to the same web site for any given domain. I’m not doing this, however, and, uh, to be honest, I forget why. If I had to guess, I’d say it’s because I had the vague notion that someday I might want to serve a different web site from .com than I do from .net. Anyway, the point is that at my web hosting account, I have “daringfireball.com” configured as an entirely separate web site than “daringfireball.net”.

I have a simple one-line .htaccess file in the root directory for “daringfireball.com”:

RedirectMatch permanent (.*) http://daringfireball.net$1

This pretty much does the same thing as the first example above, the one that gets rid of “www” prefixes, except that instead of using mod_rewrite, it uses the RedirectMatch command from Apache’s mod_alias module.

(There is absolutely no good reason that I couldn’t use mod_rewrite for this, too. mod_rewrite is much more powerful than mod_alias’s RedirectMatch command. My excuse is that I wrote this one first, and at the time, I didn’t know how to use mod_rewrite. I use mod_rewrite for the “www” truncation because I have a bunch of other, more complicated redirect rules in my main .htaccess file, and those rules require mod_rewrite. It’s also worth noting that mod_alias is available on more web hosts than mod_rewrite is; but personally I’d never use a web host who didn’t enable mod_rewrite.)

The syntax for RedirectMatch should be self-explanatory after the previous explanation regarding RewriteRule. The order of the components is different, however:

• permanent — this specifies that the redirection is permanent and should use an HTTP 301 result code.

• (.*) — regular expression to match the request string.

• http://daringfireball.net$1 — replacement pattern.

The most interesting difference is that with RedirectMatch, the request string that the regular expression is tested against contains the leading “/”. As someone who first learned to use RedirectMatch and then later switched to mod_rewrite, this has tripped me up dozens of times.

So the replacement pattern when using mod_rewrite was http://daringfireball.net/$1, but the replacement pattern when using RedirectMatch is http://daringfireball.net$1, because with RedirectMatch, the text referenced by $1 already starts with a slash.

References

• Apache mod_rewrite Documentation
• Dave Child’s mod_rewrite Cheat Sheet