DNS security

在设计 DNS 时并未考虑安全性,因此目前创建了许多类型的攻击来利用 DNS 系统中的漏洞。

学习目标

阅读本文后,您将能够:

  • 了解什么是 DNSSEC 及其工作方式
  • 熟悉最常见的 DNS 攻击
  • 区分 DNSSEC 与其他 DNS 安全解决方案

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DNS 安全为什么非常重要?

几乎所有 Web 流量都需要标准 DNS 查询,这为 DNS 攻击提供了机会,例如 DNS 劫持和在途攻击。这些攻击可将网站的入站流量重定向到伪造的站点,从而收集敏感的用户信息并使企业承担主要责任。防御 DNS 威胁的最知名方式之一是采用 DNSSEC 协议。

What is DNSSEC?

Like many Internet protocols, the DNS system was not designed with security in mind and contains several design limitations. These limitations, combined with advances in technology, have made it easy for attackers to hijack a DNS lookup for malicious purposes, such as sending a user to a fraudulent website that can distribute malware or collect personal information.

DNS Security Extensions (DNSSEC) is a security protocol created to mitigate this problem. DNSSEC protects against attacks by digitally signing data to help ensure its validity. In order to ensure a secure lookup, the signing must happen at every level in the DNS lookup process.

此签名过程类似于人们用笔签署法律文件;此人签署别人无法创建的唯一签名,并且法院专家能够查看该签名并验证文件是否由该人签署的。这些数字签名可确保数据未被篡改。

DNSSEC 在 DNS 的所有层中实施分层数字签名策略。例如,在 “google.com”查找中,根 DNS 服务器将为 .COM 域名服务器签写一个密钥,然后 .COM 域名服务器将为 google.com 的权威性域名服务器签写一个密钥。

While improved security is always preferred, DNSSEC is designed to be backwards-compatible to ensure that traditional DNS lookups still resolve correctly, albeit without the added security. DNSSEC is meant to work with other security measures like SSL/TLS as part of a holistic Internet security strategy.

DNSSEC creates a parent-child train of trust that travels all the way up to the root zone. This chain of trust cannot be compromised at any layer of DNS, or else the request will become open to an on-path attack.

To close the chain of trust, the root zone itself needs to be validated (proven to be free of tampering or fraud), and this is actually done using human intervention. Interestingly, in what’s called a Root Zone Signing Ceremony, selected individuals from around the world meet to sign the root DNSKEY RRset in a public and audited way.

以下是有关 DNSSEC 工作方式的更详细说明 >>>

涉及 DNS 的常见攻击有哪些?

DNSSEC is a powerful security protocol, but unfortunately it is not currently universally adopted. This lack of adoption coupled with other potential vulnerabilities, on top of the fact that DNS is an integral part of most Internet requests, makes DNS a prime target for malicious attacks. Attackers have found a number of ways to target and exploit DNS servers. Here are some of the most common:

DNS 欺骗/缓存中毒这是将伪造的 DNS 数据引入 DNS 解析器缓存中的攻击,其将导致解析器返回域的错误 IP 地址。流量可能会被转移到恶意计算机或攻击者想要的其他任何位置,而不是前往正确网站;通常是用于恶意目的的原始站点副本,例如分发恶意软件或收集登录信息。

DNS tunneling: This attack uses other protocols to tunnel through DNS queries and responses. Attackers can use SSH, TCP, or HTTP to pass malware or stolen information into DNS queries, undetected by most firewalls.

DNS hijacking: In DNS hijacking the attacker redirects queries to a different domain name server. This can be done either with malware or with the unauthorized modification of a DNS server. Although the result is similar to that of DNS spoofing, this is a fundamentally different attack because it targets the DNS record of the website on the nameserver, rather than a resolver’s cache.

DNS 劫持

NXDOMAIN attack: This is a type of DNS flood attack where an attacker inundates a DNS server with requests, asking for records that do not exist, in an attempt to cause a denial-of-service for legitimate traffic. This can be accomplished using sophisticated attack tools that can auto-generate unique subdomains for each request. NXDOMAIN attacks can also target a recursive resolver with the goal of filling the resolver’s cache with junk requests.

Phantom domain attack: A phantom domain attack has a similar result to an NXDOMAIN attack on a DNS resolver. The attacker sets up a bunch of ‘phantom’ domain servers that either respond to requests very slowly or not at all. The resolver is then hit with a flood of requests to these domains and the resolver gets tied up waiting for responses, leading to slow performance and denial-of-service.

Random subdomain attack: In this case, the attacker sends DNS queries for several random, nonexistent subdomains of one legitimate site. The goal is to create a denial-of-service for the domain’s authoritative nameserver, making it impossible to lookup the website from the nameserver. As a side effect, the ISP serving the attacker may also be impacted, as their recursive resolver's cache will be loaded with bad requests.

Domain lock-up attack: Attackers orchestrate this form of attack by setting up special domains and resolvers to create TCP connections with other legitimate resolvers. When the targeted resolvers send requests, these domains send back slow streams of random packets, tying up the resolver’s resources.

Botnet-based CPE attack: These attacks are carried out using CPE devices (Customer Premise Equipment; this is hardware given out by service providers for use by their customers, such as modems, routers, cable boxes, etc.). The attackers compromise the CPEs and the devices become part of a botnet, used to perform random subdomain attacks against one site or domain.

What is the best way to protect against DNS-based attacks?

In addition to DNSSEC, an operator of a DNS zone can take further measures to secure their servers. Over-provisioning infrastructure is one simple strategy to overcome DDoS attacks. Simply put, if your nameserver can handle several multiples more traffic than you expect, it is harder for a volume-based attack to overwhelm your server.

Anycast routing is another handy tool that can disrupt DDoS attacks. Anycast allows multiple servers to share a single IP address, so even if one DNS server gets shut down, there will still be others up and serving. Another popular strategy for securing DNS servers is a DNS firewall.

什么是 DNS 防火墙?

DNS firewall 是一种可为 DNS 服务器提供众多安全和性能服务的工具。DNS firewall 位于用户的递归解析器与他们正尝试访问的网站或服务的权威性域名服务器之间。防火墙可提供速率限制服务,以关闭试图淹没服务器的攻击者。如果服务器确实由于攻击或其他任何原因而停机,则 DNS firewall 可通过提供来自高速缓存的 DNS 响应来使运营商的站点或服务保持正常运行。

除其安全功能外,DNS firewall 还可为 DNS 运营商提供高性能解决方案,例如更快的 DNS 查找和更低的带宽成本。了解有关 Cloudflare DNS firewall 的更多信息。

作为安全工具的 DNS

DNS resolvers can also be configured to provide security solutions for their end users (people browsing the Internet). Some DNS resolvers provide features such as content filtering, which can block sites known to distribute malware and spam, and botnet protection, which blocks communication with known botnets. Many of these secured DNS resolvers are free to use and a user can switch to one of these recursive DNS services by changing a single setting in their local router. Cloudflare DNS has an emphasis on security.

Are DNS queries private?

Another important DNS security issue is user privacy. DNS queries are not encrypted. Even if users use a DNS resolver like 1.1.1.1 that does not track their activities, DNS queries travel over the Internet in plaintext. This means anyone who intercepts the query can see which websites the user is visiting.

This lack of privacy has an impact on security and, in some cases, human rights; if DNS queries are not private, then it becomes easier for governments to censor the Internet and for attackers to stalk users' online behavior.

DNS over TLS and DNS over HTTPS are two standards for encrypting DNS queries in order to prevent external parties from being able to read them. Cloudflare DNS supports both of these standards. Cloudflare also partners with other organizations to help improve DNS security — for example, helping Mozilla enable DNS over HTTPS in its Firefox browser in order to protect users.