Nginx rewrite non-www-prefixed domain to www-prefixed domain

Question

I see the Nginx HttpRewriteModule documentation has an example to rewrite a www-prefixed domain to a non-www-prefixed domain:

if ($host ~* www\.(.*)) {
  set $host_without_www $1;
  rewrite ^(.*)$ http://$host_without_www$1 permanent; # $1 contains '/foo', not 'www.mydomain.com/foo'
}

How can I do the reverse-- rewrite a non-www-prefixed domain to a www-prefixed domain? I thought maybe I could do something like the following but Nginx doesn't like the nested if statement.

if ($host !~* ^www\.) {                       # check if host doesn't start with www.
    if ($host ~* ([a-z0-9]+\.[a-z0-9]+)) {    # check host is of the form xxx.xxx (i.e. no subdomain)
        set $host_with_www www.$1;
        rewrite ^(.*)$ http://$host_with_www$1 permanent;
    }
}

Also I wanted this to work for any domain name without explicitly telling Nginx to rewrite domain1.com -> www.domain1.com, domain2.com -> www.domain2.com, etc. since I have a large number of domains to rewrite.

Answer 1

As noted in the Nginx documentation, you should avoid using the if directive in Nginx where possible, because as soon as you have an if in your configuration your server needs to evaluate every single request to decide whether to match that if or not.

A better solution would be multiple server directives.

server {
        listen 80;
        server_name website.com;
        return 301 $scheme://www.website.com$request_uri;
}

server {
        listen 80;
        server_name www.website.com;
        ... 
}

If you're trying to serve an SSL (HTTPS) enabled site, you got more or less three different options.

1. Set up multiple IP addresses having each server directive listening on their own IP (or different ports if that's an option for you). This options needs SSL certificates for both website.com and www.website.com, so either you have a wild card certificate, a UNI certificate (multiple domains) or just plainly two different certificates.
2. Do the rewrite in the application.
3. Use the dreaded if directive.

There is also an option to use SNI, but I'm not sure this is fully supported as of now.