dialinfo := mgo.DialInfo{
    Addrs:    {"1.2.3.45","1.2.3.56"}
    Database: Mongo1,
    Username: User,
    Password: Pwd,
    Timeout:  60 * time.Second,
    DialServer: func(addr *mgo.ServerAddr) (net.Conn, error) {
        conn, err := connectToCluster("172.12.13.145")
        if err != nil {
            fmt.Println("couldn't connect to the cluster, trying again..")
            return nil, err
        }
        remote, err := conn.Dial("tcp", addr.String())
        if err != nil {
            fmt.Println("couldn't connect to the mongodb server:", addr.String())
        }
        return remote, err
    },
}
session, err = mgo.DialWithInfo(dialinfo)

dialinfo := mgo.DialInfo{
    Addrs:    {"mongodbserver1","mongodbserver2"}
    Database: Mongo1,
    Username: User,
    Password: Pwd,
    Timeout:  60 * time.Second,
    DialServer: func(addr *mgo.ServerAddr) (net.Conn, error) {
        conn, err := connectToCluster("172.12.13.145")
        if err != nil {
            fmt.Println("couldn't connect to the cluster, trying again..")
            return nil, err
        }
        remote, err := conn.Dial("tcp", addr.String())
        if err != nil {
            fmt.Println("couldn't connect to the mongodb server:", addr.String())
        }
        return remote, err
    },
}
session, err = mgo.DialWithInfo(dialinfo)