server {
    listen 80;
    server_name www.test123.com;
    rewrite ^/(.*) http://www.test456.com/$1 permanent;
}

server
{
    listen 80 ;
    server_name 123.top 1234.com;
    if ( $host = 123.top )
    #增加判断条件，当访问域名是mxiaoqi.top的时候
    {
        rewrite /(.*)  http://1234.com/$1 permanent;
        #把mxiaoqi.top/后面的内容重新写到aaa.com/后面如果后面有多段则使用$2、$3以此类推
        #permanent是转发状态码
    }
}