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I have five domains pointing to the same server/wp installation. the domain names are important and my visitors need to be able to stay on their preferred domain as they surf through my site.

how do I set up wp (or .htaccess or...) so that links all point to the domain/host specified by the visitor?

right now, if I set Dashboard/General/Site Address to site-A.com, and I load the site as site-B.com, the links still point to site-A.com instead of site-B.com; eg, load http://site-B.com/blog/this-is-my-home/. the links within the loaded page still point to site-A.com instead of site-B.com.

in my old MT system, I was able to define all links as root-relative, so the links all pointed to the current domain. is this possible with WP?


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2 Answers

I tracked the function calls in my theme (a child theme of Oenology by Chip Bennett) and wp-include that generate the links, found the home_url() function, and then wrote these functions for my site:

function gregory_make_relative($url='') {
    return preg_replace( '#^https?://[^/]+/#iu', '/', $url, 1 );
add_filter( 'home_url', 'gregory_make_relative', 11, 1 );

I intentionally grep-searched to include the / after the domain so that I could still use home_url('') to return the Blog's specified domain, and specify true canonical url's to that domain in the <head> via WordPress SEO using the following functions (i.e., the canonical links will be the same regardless of the domain used to load the page; i.e., no 'duplicate content'):

function gregory_wpseo_canonical_add_domain( $canonical ) {
    return home_url('').$canonical;
add_filter( 'wpseo_canonical', 'gregory_wpseo_canonical_add_domain', 10, 1 );

so far, it works really well but I wonder if it'll adversely affect Feeds or the e-Commerce solution I eventually implement. comments, notes, tips, warnings are welcome :-)


a simple home_url() (i.e., no path specified) is used throughout the system, so the trailing / in the grep-search couldn't be used. I had to remove it and find another way to specify the domain in the canonical url's. so, a little more research through wp-includes, and now the functions look like this:

function gregory_make_relative($url='') {
    return preg_replace( '#^https?://[^/]+#iu', '', $url, 1 );
add_filter( 'home_url', 'gregory_make_relative', 11, 1 );

function gregory_wpseo_canonical_add_domain( $canonical ) {
    // get_option() is defined in wp-includes/functions.php and is used by get_home_url()
    return get_option('home').$canonical;
add_filter( 'wpseo_canonical', 'gregory_wpseo_canonical_add_domain', 10, 1 );

Update 2

it's harder than it first appeared ;-) my code now looks like this. feeds are affected. somewhere in the chain of functions that produces the links, feeds seems to be using _SERVER['HTTP_HOST']. I'll have to examine my options there.

// define WP_SITEURL because the formula in wp-includes/functions::wp_guess_url() makes a
// false assumption and appends $_SERVER['REQUEST_URI'] to the base_url.
define('WP_SITEURL', 'http://my.domain.hk/', true );

// strip the domain
function gregory_make_relative( $url='' ) {
    return preg_replace( '#^https?://[^/]+#iu', '', $url, 1 );
add_filter( 'site_url', 'gregory_make_relative', 11, 1 );
add_filter( 'home_url', 'gregory_make_relative', 11, 1 );
add_filter( 'template_directory_uri', 'gregory_make_relative', 11, 1 );
add_filter( 'stylesheet_directory_uri', 'gregory_make_relative', 11, 1 );
add_filter( 'script_loader_src', 'gregory_make_relative', 11, 1 );

function gregory_make_stylehref_relative( $tag='' ) {
    // $wp_styles->do_item() passes this along to the filter:
    // "<link rel='$rel' id='$handle-rtl-css' $title href='$rtl_href' type='text/css' media='$media' />\n"
    $matches = array();
    if( !preg_match( '#^(.+ +href=\')(.+)(\' +type=.+)$#iu', $tag, &$matches ))
        return $tag;
    $matches[2] = gregory_make_relative($matches[2]);
    return $matches[1].$matches[2].$matches[3];
add_filter( 'style_loader_tag', 'gregory_make_stylehref_relative', 11, 1 );

function gregory_wpseo_canonical_add_domain( $canonical='' ) {
    // get_option is defined in wp-includes/functions.php and is used by get_home_url() to get the home url.
    return get_option('home').$canonical;
add_filter( 'wpseo_canonical', 'gregory_wpseo_canonical_add_domain', 10, 1 );

Update 3 -- WPSEO

there's an option in WPSEO to add text/links before and after RSS posts including the option to use placeholders. one of those placeholders is %%BLOGLINK%%. unfortunately, with the root-relative filters in place, %%BLOGLINK%% produced an empty string which was not useful in the feeds. this code fixes that problem (noting that my other choice was to simply hardcode the link in WPSEO's RSS settings, probably the smarter thing to do :-)

// a change for WPSEO's %%BLOGLINK%% code.
// I changed get_bloginfo() to get_bloginfo_rss() in wpseo/frontend/class-frontend.php to allow this.
// without these changes, %%BLOGLINK%% is printed into the rss feeds as an empty string.
function gregory_set_domain_in_rss_urls( $info, $show ) {
    // copied from wp-includes/general-template.php::get_bloginfo()
    $url = true;
    if (strpos($show, 'url') === false &&
        strpos($show, 'directory') === false &&
        strpos($show, 'home') === false)
        $url = false;

    return ( !$url || !empty($info) ? $info : '/' );
add_filter( 'get_bloginfo_rss', 'gregory_set_domain_in_rss_urls', 11, 2 );

(I have since decided to hardcode my RSS after-post message, so the above filter has been disabled in my theme.)

Update 4 -- preg_replace() becomes preg_match()

I've updated the gregory_make_stylehref_relative() function shown in Update 2 to use preg_match() instead of preg_replace(). this is the old code:

    $href = preg_replace( '#^(.+ +href=\')(.+)(\' +type=.+)$#iu', '$2', $tag );
    $href = gregory_make_relative($href);
    return preg_replace( '#^(.+ +href=\')(.+)(\' +type=.+)$#iu', '$1'.$href.'$3', $tag );

Update 5 -- get_avatar()

another filter, this time for get_avatar() so that the schema and current domain are included in the path to wp's blank.gif. without these, the gif won't be found and loaded. without the schema, Gravatar will load it's own corporate avatar instead.

function gregory_avatar_add_domain( $avatar ) {
    // look for urlencode( includes_url('images/blank.gif')) in the $avatar string.
    // if found, encode the schema and domain, insert it into the $avatar string.
    $gif = includes_url('images/blank.gif'); // from get_avatar()
    if( preg_match( '|^https?://|i', $gif ))
        // the url already includes the schema (and domain).
        return $avatar;
    $gif = urlencode($gif);
    $schema = is_ssl() ? 'https://' : 'http://'; // from wp_guess_url()
    $domain = urlencode( $schema . $_SERVER['HTTP_HOST'] );
    return str_replace( $gif, $domain.$gif, $avatar );
add_filter( 'get_avatar', 'gregory_avatar_add_domain', 11, 1 );

Update 6 -- redirect_canonical()

the filters were causing problems with incoming url's that involved query strings. the url's scheme://domain would get chopped to just ://domain. it took a few hours to pinpoint the problem, but it was in wp-includes/canonical.php::redirect_canonical() and a filter hook there made it possible to correct the problem. here's the filter:

function gregory_redirect_canonical_addScheme( $redirect_url ) {
    // redirect_canonical() requires (but doesn't need) a fully qualified url.
    // get_permalink() in the second iteration of redirect_canonical()
    // (redirect_canonical() calls itself) usually switches to the domain specified in the
    // WP Settings. but if we do the same, redirect_canonical() doesn't recognise the
    // permalink as a redirect, and WP doesn't update the url in the User Agent's Address bar.
    if( preg_match( '|^https?://|i', $redirect_url ))
        // fully qualified url. leave it alone.
        return $redirect_url;
    if( substr( $redirect_url, 0, 3 ) == '://' )
        // no scheme specified in $redirect during the
        // second pass through redirect_canonical().
        return (is_ssl() ? 'https' : 'http') . $redirect_url;
    if( substr( $redirect_url, 0, 1 ) == '/' )
        // root-relative url.
        return (is_ssl() ? 'https://' : 'http://').$_SERVER['HTTP_HOST'].$redirect_url;
    // relative url. not root-relative.
    return (is_ssl() ? 'https://' : 'http://').$_SERVER['HTTP_HOST'].'/'.$redirect_url;
add_filter( 'redirect_canonical', 'gregory_redirect_canonical_addScheme', 11, 1 );

please note that I have not marked my question as answered just yet, because I don't know what consequences these filters will have further down the road. more time and testing is required.

(WordPress 3.3.2)

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You can answer your own questions and accept your own answers, so go ahead:) –  Wyck Apr 29 '12 at 15:01
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Reverting to absolute url's

Implementing root-relative url's was pretty much doable, but complicated because WP was written to use absolute url's and expects absolute url's. I realised that all I wanted was for people visiting my site to be able to see all links using the same domain as the one they specified. This didn't mandate using root-relative url's. So I've rewritten the code to replace the domains in all relevant url's with the domain specified by the visitor. The complete code is much simpler!

// set the url's domain to that of the current host, IGNORING any specified subdomains.
// (other WordPress users may need to retain the subdomains)
function gregory_use_host_domain( $url='' ) {

    $myDomains = '(?:domain1|domain2|domain3|xn--domain4|xn--domain5)';
    $matches = array();

    // get the current host domain. ignore/remove any subdomain.
    if( 1 != preg_match( '!.*('.$myDomains.'\.hk)!iu', $_SERVER['HTTP_HOST'], &$matches )) // preg_match returns 1 if the pattern was found.
        // the current host is not one of the above specified domains! weird. this will probably never happen, but just in case...
        return $url;
    $thisDomain = ( is_ssl() ? 'https://' : 'http://' ).$matches[1];

    if( 1 == preg_match( '!^(?:https?://[^/]*'.$myDomains.'\.hk)(/.*)?$!iu', $url, &$matches ))
        // fully qualified url.
        return $thisDomain.$matches[1];
    if( substr( $url, 0, 1 ) == '/' )
        // root-relative url.
        return $thisDomain.$url;
    // relative url but not root-relative.
    return $thisDomain.'/'.$url;

add_filter( 'site_url', 'gregory_use_host_domain', 11, 1 );
add_filter( 'home_url', 'gregory_use_host_domain', 11, 1 );

 * this function directly affects the template_ and stylesheet_ directory uri results,
 * but they don't matter as far as the visitor is concerned. they're not going to directly
 * access those particular url's.
 * the Spam-Free-Wordpress plugin uses plugin_dir_url() and plugin_dir_path() to get the paths to its resources,
 * but we're not going to worry about specifying the domain for these items.
 * wp's plugin_dir_url() uses wp's plugins_url()
 * wp's plugin_dir_path() uses php's dirname()

// a filter to specify the 'domain1.hk' domain for WPSEO's canonical links.
// one specific domain for all content canonical links. no 'duplicate' content.
function gregory_wpseo_specify_canonical_domain( $canonical='' ) {

    $matches = array();
    if( 1 != preg_match( '!^(?:https?://[^/]+)(/.*)?$!iu', $canonical, &$matches ))
        return $canonical;
    // get_option is defined in wp-includes/functions.php and is used by get_home_url() to get the home url.
    return get_option('home').$matches[1];
add_filter( 'wpseo_canonical', 'gregory_wpseo_specify_canonical_domain', 10, 1 );

(WordPress 3.4.2)

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