DNS security

DNSはセキュリティを最優先とした設計ではないため、DNSシステムの脆弱性は多くのタイプの攻撃で悪用されています。

学習目的

この記事を読み終えると、以下のことができるようになります。

  • DNSSECとは何か、またその仕組みを理解する
  • 最も一般的なDNS攻撃について知る
  • DNSSECおよびその他DNSセキュリティソリューションの違い

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DNSセキュリティが重要な理由は?

ほとんどすべてのWebトラフィックに必要な標準DNSクエリは、DNSハイジャックや中間者攻撃などのDNSエクスプロイトの機会を作り出します。これらの攻撃は、トラフィックを偽のWebサイトのコピーに転送し、機密のユーザー情報を収集し、ビジネスは深刻な責任が問われることになります。DNSを悪用の脅威からの保護する方法として最もよく知られているのが、DNSSECプロトコルの採用です。

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.

この署名プロセスは、ペンで法律文書に署名することに似ています。署名者は、他の人には作成できない一意の署名を行い、法廷の専門家はその署名を見て、文書がその人によって署名されたことを確認できます。こうした電子署名によって、そのデータが改ざんされていないことが確認できます。

DNSSECは、DNSのすべての層で階層的デジタル署名方針を実装します。たとえば「google.com」ルックアップの場合、ルートDNSサーバーは、.COMネームサーバーのキーに署名し、.COMネームサーバーはgoogle.comの権威ネームサーバーのキーに署名します。

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.

DNSSECの仕組みの詳細な説明>>>

DNSに関連する一般的な攻撃とは?

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スプーフィング/キャッシュポイズニングこれは、偽造DNSデータがDNSリゾルバーのキャッシュに導入されることで、リゾルバーがドメインに対して誤ったIPアドレスを返す攻撃です。トラフィックは正しいWebサイトではなく悪意のあるPCや、攻撃者が意図した別の場所に転送されます。多くの場合、これは、オリジナルサイトの複製で、マルウェアを配布したり、ログイン情報を不正に盗み出すような、悪意のある目的に使われるサイトです。

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ハイジャック

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.

DNS Firewallとは?

DNS Firewallは、DNSサーバーに多くのセキュリティとパフォーマンスサービスを提供できるツールです。DNS Firewallは、ユーザーの再帰リゾルバーと、到達しようとしているWebサイトやサービスの権威ネームサーバーの間に設置されます。ファイアウォールは、Rate Limitingサービスを提供し、サーバーを大量のクエリであふれさせようとする攻撃者をシャットダウンすることができます。サーバーが攻撃の結果またはその他の理由でダウンタイムを経験した場合、DNS Firewallは、キャッシュからDNS応答を提供することにより、オペレーターのサイトまたはサービスを維持できます。

セキュリティ機能に加えて、DNS Firewallは、さらにDNSオペレーターにより高速DNSルックアップや帯域幅コスト削減など、パフォーマンス面のソリューションエンジニアリングも提供することができます。CloudflareのDNS Firewallの詳細をお読みください。

セキュリティツールとしての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.