bagerathan/change-permalinks.php

## change-permalinks.php
<?php

//redirect based on custom field value
add_filter( 'post_link', 'catalyst_external_permalink', 10, 2 );
function catalyst_external_permalink( $link, $post ){
    $meta = get_post_meta( $post->ID, 'external_url', TRUE );
    $url  = esc_url( filter_var( $meta, FILTER_VALIDATE_URL ) );
    return $url ? $url : $link;
}
	<?php

	//redirect based on custom field value
	add_filter( 'post_link', 'catalyst_external_permalink', 10, 2 );
	function catalyst_external_permalink( $link, $post ){
	$meta = get_post_meta( $post->ID, 'external_url', TRUE );
	$url = esc_url( filter_var( $meta, FILTER_VALIDATE_URL ) );
	return $url ? $url : $link;
	}