apache

## Canonical redirect for Apache

# BEGIN Canonical Redirect
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.domain\.com$ [NC] ## will match any domain that's not our main domain
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]
</IfModule>
# END Canonical Redirect

lighttpd

## Canonical redirect for lighttpd

$HTTP["host"] == "domain.com" {
  url.redirect = (
    "^/(.*)" => "http://www.domain.com/$1"
  )
}


# for various domains
$HTTP["host"] =~ "(domain.com|domain1.com|domain2.com)" {
  url.redirect = (
    "^/(.*)" => "http://www.domain.com/$1"
  )
}

nginx

## Canonical redirect for ngingx

# as separate vhost rules
server {
  listen 80;
  server_name domain.com; # add other domains separated by a space as necessary
  rewrite ^/(.*)$ http://www.domain.com/$1 permanent;
}

# as a condition on an existent vhost
server {
  listen 80;
  server_name www.domain.com domain.com domain2.com www.domain2.com
  if ( $host != 'www.domain.com' ) {
   rewrite ^/(.*)$ http://www.domain.com/$1 permanent;
  }
  # ... other vhost configs
}