The Domain Name System (DNS) plays a crucial role in the Internet. However, it is vulnerable to many attacks such as the cache poisoning attack and DDoS attack. Though some countermeasures have been proposed, they still have some limitations. In this paper, we propose B-DNS, a blockchain-based domain name system, which can provide a secure and efficient DNS service. B-DNS fills up two shortcomings of current blockchain-based DNS, namely computation-heavy Proof-of-Work (PoW) protocol and inefficient query, by building a Proof-of-Stake (PoS) consensus protocol and an index of domains. We propose a novel way to quantitatively compare the security of B-DNS and legacy DNS in terms of attack success rate, attack cost, and attack surface. Our experiments show that the probability of a successful attack on B-DNS is 1% of a successful attack on legacy DNS, the attack cost goes up a million times in B-DNS, and the attack surface of B-DNS is far smaller than that of legacy DNS. The query performance evaluation of B-DNS shows that B-DNS can achieve similar or even less query latency than state-of-the-art commercial DNS implementations.