function relTags() {

if (document.getElementsByTagName) {

var anchors = document.getElementsByTagName( "a" );

for (var loop = 0; loop < anchors.length; loop++) {

var anchor = anchors[loop];
if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") {
anchor.target = "_blank";
}
}
}
}

window.onload = function() {
relTags();
}